Download BSC-4-1-Immunology

C-9: IMMUNOLOGY
Syllabus UNIT-I
Immune Response - An overview, components of mammalian immune system, molecular
structure of Immuno-globulins or Antibodies, Humoral & Cellular immune responses, Tlymphocytes & immune response (cytotoxic T-cell, helper T-cell, suppressor T-cells), T-cell
receptors, genome rearrangements during B-lymphocyte differentiation, Antibody affinity
maturation class switching, assembly of T-cell receptor genes by somatic recombination.
NOTES
(Part-I)
Immune Response - An overview
The B and T-lymphocytes that are yet to encounter an antigen are called naive B and naive Tcells. The naive B-cells that encounter the antigen, proliferate and differentiate into two types
of cells: the antibody-secreting plasma cells and the memory B-cells.
The plasma cells form the basis of primary immune response, which is the response mounted
by the immune system to an antigen that the animal encounters for the first time. The primary
response has a characteristic lag phase, during which naive B-cells proliferate and differentiate
into plasma cells and memory cells.
Following this, serum antibody level increases logarithmically, reaches the peak at about day
14, remains at a plateaus for some time, then begins to drop off as the plasma cells begin to
die. The memory cells remain in G0 phase, and have a much longer life than plasma cells; some
memory cells persist for the life of the individual.
Therefore, when the animal encounters the same antigen a second time, the population of
memory cells responds rapidly to begin antibody secretion. The antibody levels peak in about
7 days, and the level of antibody is about 100 to 1,000-fold higher than that in the primary
response.
The immune response mounted by the animal to an antigen, which it encounters a second time
is called secondary immune response. The population of memory B-cells specific for a given
antigen is considerably larger than the population of corresponding naive B-cells; this accounts
for some of the differences between primary and secondary immune responses (Fig. 41.4).
In a similar manner, the recognition of an antigen-MHC complex by a specific mature Tlymphocyte induces its proliferation and differentiation into TH cells and CTLs (the effector
cells) and into memory cells.
The effector cells bring about the primary immune response, which is relatively slower; it takes
about 10-14 days in mouse for rejection of a skin graft in the first instance. But when skin tissue
from the same source is grafted the second time, it is rejected in about 7-9 days due to the faster
action of memory T-cells.
Components of immune system
Organs of immune system
Primary or Central Lymphoid Organs:
Immature lymphocytes generated in hematopoiesis, the process of formation and development
of blood cells, mature and become committed to a particular antigenic specificity within the
primary lymphoid organs, namely, thymus, bursa of Fabricius (in birds) and bone marrow (in
mammals). A lymphocyte becomes immuno-competent, i.e., capable of mounting an immune
response only after it matures within a primary lymphoid organ.
1. Thymus:
Thymus is a greyish, flat, bilobed lymphoid organ situated above the heart and extending into
the neck on the front and sites of trachea. It develops from the epithelium of third and fourth
pharyngeal pouches and, on maturity, acts as the site of development and maturation of
lymphocytes named thymus-derived lymphocytes or T-lymphocytes or T-cells.
The thymus reaches peak activity in childhood and attains its largest size at puberty. Thereafter,
the thymus begins to atrophy without any apparent effect on T-lymphocyte function and is
extremely small in old age.
For convenience, the average weight of the thymus is 70 g in infants and its age- dependent
involution leaves the thymus with an average weight of 3 g in the old age. This is probably due
to the fact that T-lymphocytes are very long-lived and can circulate in the resting state for long
periods of time.
Each lobe of thymus is surrounded by a capsule and is divided into a series of lobules, which
are separated from each other by strands of connective tissue called trabeculae. Each lobule is
organized into two compartments-outer and inner. The outer component is called cortex,
whereas the inner component is called medulla (Fig. 42.2).
The cortex is densely packed with thymocytes, whereas the medulla is sparsely populated with
thymocytes. Thymocytes develop from prothymocytes. The latter are produced in bone
marrow, migrate through blood stream, enter the cortex of the thymus, and act as thymocytes.
Thymocytes divide rapidly in the cortex and give rise to T-lymphocytes.
Of the T-lymphocytes produced in thymus only 5% leave the thymus as viable cells. Though
the reason for this apparent wasteful process is not known, some believe that it is the
elimination of T-lymphocyte clones that react against self.
Both the cortex and the medulla of the thymus are criss-crossed by a three dimensional network
consisting of epithelial cells, dendritic cells, and macrophages, which make up the framework
of the organ and contribute to the growth and maturation, of thymocytes.
Some epithelial cells of the outer cortex possess long membrane extensions that surround as
many as 50 thymocytes. These cells are called nurse cells. Other epithelial cells of the cortex
have long interconnecting cytoplasmic extensions that form a network and have been found to
interact with many of the thymocytes when they traverse the cortex.
The function of the thymus is to generate T-lymphocytes and to confer immunological
competence on to them during their stay in the organ. T-lymphocytes so educated in the thymus
become capable of mounting cell-mediated immune response against appropriate antigen.
This is effected under the influence of the thymic microenvironment and several hormones
such as thymosin and thymopentin produced by the epithelial cells of the thymus. The
competent T-lymphocytes immediately move from thymus to the secondary or peripheral
lymphoid organs.
Surgical removal of bursa (bursectomy) from newly hatched chickens destroys their subsequent
ability to produce antibodies. The B-cells mature, proliferate, and differentiate into bursa and
then migrate from it and reach outer or superficial cortex of the germinal follicles and
medullary cords of peripheral lymph nodes and lymphoid follicles of spleen where, following
appropriate antigenic stimulation, transform into plasma cells and secrete antibodies. Like
thymus, the bursal of Fabricius starts to shrink or atrophy at puberty.
3. Bone Marrow:
Bone marrow is the site of origin and development of B-lymphocytes or B-cells (bone marrow
derived lymphocytes) in mammals particularly in humans and mice after birth. Before birth,
the yolk sac, foetal lever, and total bone marrow are the major sites of B-lymphocyte
maturation. Bone marrow, therefore, is the mammalian equivalent of the bursa of Fabricius in
birds.
Development of B-lymphocytes (B-cells) begins with the differentiation of lymphoid stem
cells into the earliest distinctive progenitor B cells (pro-B cell), which proliferate within the
bone marrow filling the extravascular spaces between large sinusoids in the shaft of a bone.
Proliferation and differentiation of pro-B cells into precursor B cells (pre-B cells) requires the
microenvironment provided by the bone marrow stromal cells.
The stromal cells within the bone marrow:
(1) Interact directly with the pro-B and pre-B cells and
(2) Secrete various cytokines that are required for development.
Bone marrow is not the site of origin and development of B-lymphocytes (B-cells) in all
mammals. In cattle and sheep, the fietal spleen is the primary lymphoid tissue wherein the
maturation, proliferation, and diversification of B-cells take place during early gestation.
During later gestation this function is performed by ideal Peyer’s patch, a patch of tissue
embedded in the wall of the intestine. In rabbit, gut-associated tissues (e.g.. appendix) act as
primary lymphoid tissue for maturation, proliferation, and diversification of B-cells.
Secondary or Peripheral Lymphoid Organs:
As stated earlier, the lymphocytes mature, proliferate, and differentiate in the primary or central
lymphoid organs. These lymphocytes migrate therefrom via circulation to the secondary or
peripheral lymphoid organs. Here they bind appropriate antigens and undergo further antigendependent differentiation.
Once in the secondary lymphoid organs, the lymphocytes do not remain there but move from
one lymphoid organ to another through the blood and lymphatic’s. The passage of lymphocytes
facilitates the induction of an immune response. Lymph nodes and the spleen are the most
highly organized secondary or peripheral lymphoid organs, whereas mucosa-associated
lymphoid tissue (MALT) is the less organized lymphoid tissue.
1. Lymph Nodes:
Lymph nodes are small, encapsulated, bean-shaped structures clustered at junctions of the
lymphatic vessels which are distributed throughout the body. Lymph nodes contain a reticular
network packed with lymphocytes, macrophages and dendritic cells, and filter out pathogenic
microorganisms and antigens from the lymph.
As the lymph percolates through a lymph node, any pathogen or antigen that is brought in with
the lymph is trapped by the phagocytic cells and dendritic cells.
A lymph node consists of three regions: the cortex, the paracortex, and the medulla (Fig. 42.3).
Cortex is the outermost region and contains several rounded aggregates of lymphocytes (mostly
B-lymphocytes), macrophages, and follicular dendritic cells arranged in primary follicles. Each
follicle has a pale-staining germinal centre surrounded by small dark-staining lymphocytes.
The deeper region lying beneath the cortex is the paracortex. It is the zone between the cortex
and the medulla. Paracortex possesses large number of T-lymphocytes and also contains interdigitating dendritic cells thought to have migrated from tissues to the lymph node.
Because of the presence of large number of T-lymphocytes in it. the Para-cortex is also referred
to as a thymus-dependent area in contrast to the cortex which is a thymus-independent area.
Medulla, the inner most region of lymph node, is more sparsely populated with lymphoidlineage cells. Of the lymphoid-lineage cells present, many are plasma cells actively secreting
antibody molecules.
Each lymph node has a number of lymph vessels called afferent lymphatic vessels, which
pierce the capsule of a lymph node at numerous sites and empty lymph into the sub-capsular
sinus. The lymph now percolates slowly inward through the cortex, paracortex, and medulla,
allowing phagocytic cells and dendritic cells to trap pathogens and antigens carried by the
lymph.
The lymph then is drained into a single large lymph vessels called efferent lymphatic vessel
that carries the lymph to the thoracic duct, which empties into a large vein in the neck.
2. Spleen:
The spleen, which is about 5 inches long and 200 g in weight in adults, is an ovoid encapsulated,
and the largest secondary or peripheral lymphoid organ. Spleen is specialized for trapping
blood-borne antigens and is present high in the left abdominal cavity and being encapsulated,
its capsule extends a number of projections, called trabeculae, into the interior resulting in the
formation of compartments.
These compartments are filled by two types of tissues, the red pulp and white pulp, which are
separated by a diffuse marginal zone (Fig. 42.4). The red pulp consists of a network of sinusoids
populated by large number of erythrocytes (red blood cells) and macrophages and few
lymphocytes.
In fact, red pulp is the region where old and defective erythrocytes are destroyed and
eliminated. The white pulp consist of the branches of the splenic artery that make a
periarteriolar lymphoid sheath (PALS) populated heavily by T-lymphocytes.
Periarteriolar lymphoid sheath (PALS) is attached with primary lymphoid follicles that are rich
in B-lymphocytes. The marginal zone separating the red pulp from white pulp is populated by
lymphocytes and macrophages.
When the blood-borne antigens enter the spleen the B- and T-lymphocytes present in
periarteriolar lymphoid sheath (PALS) are initially activated. Here interdigitating dendritic
cells capture antigen and present it combined with class II MHC molecules (major
histocompatibility molecules) to TH cells (T helper cells). Once activated, these TH cells can
then activate B- lymphocytes (B-cells).
The activated B-lymphocytes, together with some TH cells then migrate to primary follicles in
the marginal zone. When the primary follicles are challenged by antigen, they differentiate into
characteristic secondary follicles.
The latter contain germinal centres (similar to those occurring in lymph nodes) where rapidly
dividing B-lymphocytes and plasma cells are surrounded by dense clusters of concentrically
arranged lymphocytes.
3. Mucosal-Associated Lymphoid Tissue (MALT):
The mucous membranes lining the alimentary, respiratory, and genitourinary systems have a
very large combined surface area (about 400 m2; nearly the size of a basketball court), which
is constantly exposed to numerous antigens and is the major site of entry for most pathogens.
These vulnerable membrane surfaces possess a group of organized lymphoid tissues which
defend it from pathogens and antigens. The group of organized lymphoid tissues is known
collectively as mucosal-associated lymphoid tissue (MALT).
There are several types of MALT; the most studied one is the gut-associated lymphoid tissue
(GALT) which includes tonsils, Peyer’s patch, appendix, and loosely organised clusters of
lymphoid cells in the lamina propria of intestinal villi.
Mucosal-associated lymphoid, tissue (MALT) is functionally very significant in immune
system of the body because of the presence of large number of antibody-producing plasma cells
in it. The number of plasma cells in MALT for exceeds that of the total of the number of plasma
cells present in spleen, lymph nodes, and bone marrow.
(i) Tonsils:
(a) Palatine tonsils are largest sized tonsils present on either side of oropharynx.
(b) Pharyngeal tonsils are present on the posterior wall of the pharynx.
(c) Lingual tonsils are present on the dorsum of the posterior part of tongue.(Fig. 42.5).
All the aforesaid tonsil groups are nodule-like and consist of a meshwork of reticular cells and
fibres interspersed with lymphocytes, macrophages, granulocytes, and mast cells.
The B-lymphocytes are organised into follicles and germinal centres. The germinal centres are
surrounded by regions showing T-lymphocyte activity. However, the tonsils protect against
antigens that enter through the nausal and oral epithelial routes.
(ii) Peyer’s Patch:
Peyer’s patches occur in the sub-mucosal layer present beneath the lamina propria lying under
the epithelial layer of intestinal villi. Each Peyer’s patch is a nodule of 30-40 lymphoid follicles.
Like lymphoid follicles in other sites, those that compose Peyer’s patches can develop into
secondary follicles with germinal centres (Fig. 42.6).
(iii) Lamina Propria:
Lamina propria occurs under the epithelial layer of intestinal villi (Fig. 42.6). It is populated
with large number of plasma cells, macrophages, activated T helper cells (activated TH cells)
in loose clusters. More than 15,000 lymphoid follicles have beer, reported within the lamina
propria of a healthy child.
Cells of immune system
Cell # 1. Hematopoietic Stem Cell:
All blood cells arise from a type of cell called hematopoietic stem cell (HSC) (or stem cell).
The stem cells are self-renewing, maintain their population by cell division, and differentiate
into other cell types. This process of formation and development of blood cells (red and white
blood cells) is called hematopoiesis.
It is remarkable that every functionally specialised, mature blood cell is derived from the same
type of hematopoietic stem cell. In contrast to a unipotent cell, which differentiates into a single
cell type, a hematopoietic stem cell is multi-potent or pluripotent as it is able to differentiate in
various ways and thereby gives rise to various type of blood cells.
In humans, the formation and development of blood cells begins in the embryonic yolk sac
during the first weeks of development. The hematopoietic stem cells differentiate into primitive
erythroid cells that contain embryonic haemoglobin. In the third month of gestation,
hematopoietic stem cells migrate from the yolk sac to the foetal liver and then to the spleen.
Liver and spleen play major roles in hematopoiesis from the third to the seventh months of
gestation. In later months, hematopoietic stem cells differentiate in the bone marrow and play
major role in hematopoiesis, and by birth there is little or no hematopoiesis in the liver and
spleen.
Multi-potent hematopoietic stem cell (or stem cell) in the bone marrow differentiates to
form two lineages:
(1) Common-lymphoid progenitor cell and
(2) Common myeloid progenitor cell (Fig. 42.7).
The progenitor cells, unlike hematopoietic stem cell that is self-renewing, loss the capacity for
self-renewal, and are committed to their specific cell linkage.
The common lymphoid progenitor cells give rise to B-lymphocytes (B-cells) that differentiate
into antibody secreting plasma cells. T-lymphocytes (T-cells) that become activated T-cells.
natural killer (NK) cells, and some dentritic cells.
The common myeloid progenitor cells give rise to erythroblasts that produce erythrocytes (red
blood cells), megakaryoblasts that produce platelets (thrombocytes), myeloblasts that produce
granulocytes (eosinophils, basophils, neutrophils), monoblasts that differentiate into
monocytes which give rise to macrophages and dendritic cells, and an unknown precursor that
produces mast cells.
However, B-lymphocytes (B-cells) T-lymphocytes (T-cells) and natural killer (NK) cells
produced by lymphoid progenitor cell lineage and eosinophils, basophils, neutrophils,
macrophages, and dendritic cells produced by myeloid progenitor cell lineage are collectively
called white blood cells or leucocytes (Gk. leucos = white, kytos = cell). White blood cells or
leucocytes are the cells that are responsible for nonspecific and specific immunity in the body.
Cell # 2. Lymphocytes:
Lymphocytes (L. lympha = water, cyte = cell) are the most important effector cells of many
cells involved in specific immune response. These cells are small, round and 5-15 μm in
diameter. They are found in peripheral blood, lymph, lymphoid organs, and in many other
tissues. Lymphocytes constitute 20% – 40% of the white blood cell (leucocyte) population in
the body and 99% of the cells in the lymph.
They may be small (5-8 μm), medium (8-12 μm). and large (12-15 μm). The small lymphocytes
are more numerous and may be short-lived with a life-span about two weeks or long-lived with
a life-span of three years or more or even for life.
Short-lived lymphocytes act as effector cells in immune response, while long-lived ones
function as memory cells. Long-lived lymphocytes are mainly thymus derived. The formation
and development of lymphocytes, i.e.. lymphopoiesis takes place in bone marrow, primary or
central lymphoid organs, and secondary or peripheral lymphoid organs.
Lymphocytes are approximately 1011 in number in a human body; their number ranges from
1010 to 1012 depending on body size and age. Lymphocytes can be broadly subdivided into
three populations: B-lymphocytes or B-cells, T-lymphocytes or T-cells, and null cells (natural
killer cells or NK cells are included in this group).
1. B-Lymphocytes or B-Cells:
B-lymphocytes or B-cells derive their name from their site of maturation. They are so named
since they were first detected in the bursa of Fabricius of birds and later from bone marrow of
a number of mammalian species, including humans and mice. In birds, the multi-potent
hematopoietic stem cells originating in the bone marrow migrate to the bursa of Fabricius and
differentiate there into antibody synthesizing cells.
The bursa is a small pouch-like organ in the embryonic hind-gut of birds and is absent in
mammals. In a number of mammalian species including humans and mice, the B-cells originate
in the foetal lever and later migrate to the bone marrow which becomes the site for production
of B-cells after embryonic life.
B-lymphocytes do not have the ability to synthesize antibody molecule during undifferentiated
stage. During differentiation, each lymphocyte acquires the ability to synthesize antibody
molecules when provoked by antigens.
2. T-Lymphocytes or T-Cells:
T-Lymphocytes or T-cells derive their name from their site of maturation in the thymus. They
are major players in the cell-mediated immune response and also have an important role in Bcell activation. T-cells themselves do not secrete antibodies (immunoglobulin) like B-cells.
They are immunologically specific and are directly involved in cell-mediated immune
responses, can carry a vast repertoire of immunologic memory, and can function in a variety
of effector and regulatory way.
The main effector functions include tuberculin reaction (delay-ed hypersensitivity response),
destruction of tissue grafts, secretion of soluble chemical mediators called lymphokines and
their ability to perform killer functions of other cells.
The regulatory functions involve their cooperation with B-lymphocytes to produce antibodies.
In additions to these functions, some subpopulations of T-cells contribute immune responses
such as cytotoxicity, suppression, and killer properties.
Like B-lymphocytes, T-lymphocytes have specific receptors on the plasma membrane surface
for antigen. The receptors on T-cell membrane are called T-cell receptors (TCRs).
Although T-cell receptor (TCR) is structurally distinct from immuno-globulin (the membrane
receptor of B-lymphocyte), it does share some common structural features with the
immunoglobulin molecule, most notably in the structure of its antigen- binding site.
Unlike the membrane-bound antibody on B-cells that recognise free antigen, the T-cell receptor
(TCR) does not recognize free antigen instead it recognizes the hound one to particular class
of a self-molecule (e.g., major histocompatibility complex molecule or MHC molecule)
displayed on self-cells (e.g., antigen presenting cells or APCs, virus-infected cells, cancer cells,
and grafts). It is the T-cell system that helps eliminating these altered self-cells that threaten
the normal functioning of the body.
Cell # 3. Monocytes:
Monocytes (G. monos = single; cyte = cell) are mononuclear phagocytic leucocytes possessing
an oval or kidney-shaped nucleus and granules in the cytoplasm that stain grey-blue (Fig. 42.8).
Monocytes are produced in bone marrow. During hematopoiesis in bone marrow, granulocytemonocyte progenitor cells differentiate into pro-monocytes, which-leave the bone marrow and
enter the blood where they further differentiate into mature monocytes.
Mature monocytes circulate in the blood stream for about eight hours, enlarge in size, migrate
into the tissues, and differentiate into specific tissue macrophages or into myeloid dendritic
cells.
Cell # 4. Macrophages:
Macrophages (G. macros = large; phagein = to eat), as noted above, are differentiated from
monocytes into the tissues of the body.
Differentiation of a monocyte into a tissue macrophage (Fig. 42.9) involves a number of
changes:
(i) The monocyte enlarges five- to ten-fold,
(ii) Its intracellular organelles increase in both number (especially lysosomes and
phagolysosomes) and complexity,
(iii) The cell acquires increased phagocytic ability,
(iv) Produces higher levels of hydrolytic enzymes,
(v) Begins to secrete a variety of soluble factors, and
(vi) Develops ruffles or microvilli on the surface of its plasma membrane.
Macrophages are transported throughout the body. Some macrophages reside in particular
tissues and become fix macrophages. Others remain motile by amoeboid movement throughout
the body and are called free or wondering macrophages.
Macrophages serve different functions i different tissues and are named according to their
tissue location, e.g., histiocytes in connective tissues, osteoclasts in bone, microglial cells in
the brain, alveolar macrophages in the lung, kupffer cells in the liver, and mesangial cells in
the kidney.
Macrophages normally remain in a resting state and are activated for effective functioning.
They are activated by a variety of stimuli such as interferon gamma (IFN-γ) secreted by
activated T helper (TH) cells, mediators of inflammatory response, components of bacterial cell
walls, etc.
Activated macrophages secrete different types of cytotoxic proteins that help them eliminate
large number of pathogens including vims-infected cells, malignant cells, and intracellular
bacteria.
Activated macrophages also display class II MHC molecules that allow them to act more
effectively as antigen-presenting cells (APCs). Thus, macrophages and T helper (TH) cells
facilitate each other’s activation during the immune response.
Macrophages are highly phagocytic and they are capable of ingesting and digesting exogenous
antigens (e.g., whole microorganisms and insoluble particles) and exogenous matter (e.g.,
injured or dead host cells, cellular debris, activated clotting factors).
Cell # 5. Granulocytes:
Granulocytes (Fig. 42.10) are those white blood cells (leucocytes) which have irregular-shaped
nuclei with two to five lobes and granulated cytoplasmic matrix.
Granules of cytoplasmic matrix contain reactive substances that kill microorganisms and
enhance inflammation. Granulocytes are also called polymorphonuclear leucocytes (PMNs).
Three types of granulocytes are recognised in the body and they are: basophils, eosinophils,
and neutrophils.
1. Basophils:
Basophils (G. basis = base; philein = to love) possess bi-lobed irregular-shaped nucleus and
cytoplasmic matrix granules that stain bluish-black with basic dyes (e.g., methylene blue).
These granulocytes are non-phagocytic cell that function by releasing pharmacologically active
substances (e.g., histamine, prostaglandins, serotonin, and leucotrienes) from their cytoplasmic
granules upon appropriate stimulation.
Since these pharmacologically active substances influence the tone and diameter of blood
vessels, they are collectively termed vasoactive mediators. Basophils possess high-affinity
receptors for immunoglobulin-E (IgE) antibody and thereby become coated with these
antibodies.
Once coated, antigens trigger the basophil cells to secrete vasoactive mediators which are
inflammatory and play a major role in certain allergic responses (e.g., eczema, hay fever, and
asthma). Basophils, however, comprise less than 1 % of white blood cells, are non-motile, and
remain confined to the blood stream.
2. Eosinophils:
Eosinophils (G. eos = dawn; philein = to love) have a bi-lobed nucleus connected by a slender
thread of chromatin and prominent acidophilic granules in cytoplasmic matrix. Eosinophils,
like neutrophils, are motile cells that migrate from bloodstream into tissue spaces.
These granulocytes are considered to play a role in the defence against parasitic organisms
(protozoans and helminth parasites) by phagocytosis.
They release mainly cationic proteins and reactive oxygen metabolites into the extracellular
fluid. These substances damage the plasma membrane of the parasite. Eosinophils constitute
only 3-5% of the white blood cells and their acidophilic granules stain red with acidic dyes.
3. Neutrophils:
Neutrophils (L. neuter – neither; philein = to love) possess a three- to five-lobed nucleus
connected by slender threads of chromatin, and contain fine primary and secondary granules
in cytoplasmic matrix. Neutrophils, like eosinophils, are motile cells that migrate from
bloodstream into the tissue.
These granulocytes circulate in the bloodstream for 7 to 10 hours before their migration into
the tissues where they enjoy a life span of only a few days. Approximately 60% of the
circulating white blood cells (leucocytes) in human are the neutrophils. Like macrophages, the’
primary function of neutrophils is phagocytosis of foreign or dead cells and pinnocytosis of
pathological immune complexes.
Phagocytosis by neutrophils is similar to that operated by macrophages except that the lytic
enzymes and bactericidal substances in neutrophils are contained within primary and secondary
granules instead of lysosomes in macrophages. The primary granules are larger and denser and
contain peroxidase, lysozyme, and various hydrolytic enzymes.
The secondary granules are smaller and contain collagenase, lactoferrin, and lysozyme. Both
primary and secondary granules fuse with phagosome, whose contents are then digested and
the remains excreted much as they are in macrophages.
Neutrophils, like macrophages, also use oxygen-dependent and oxygen-independent pathways
to generate antimicrobial substances and defensins to kill ingested microorganisms.
Neutrophils generate more reactive oxygen intermediates and reactive nitrogen intermediates
and express higher levels of defensins than macrophages do.
Cell # 6. Dendritic Cells:
Dendritic cells constitute only 0.2% of WBCs (leucocytes) in the blood and are present in even
smaller numbers in skin and mucous membranes of the nose, lungs, and intestines. They derive
their name due to long membrane extensions resembling the dendrites of nerve cells.
Dendritic cells arise from hematopoietic stem cells in the bone marrow via different pathways
and in different locations (Fig. 42.11); they descend through both the myeloid and lymphoid
lineages. Stem cell-originated dendritic cells are of four types: Langerhans cells, interstitial
dendritic cells, myeloid dendritic cells, and lymphoid dendritic cells.
Despite differences, all the stem cell-originated mature dendritic cells perform the same major
function of presenting antigen to T helper (TH) cells by expressing high levels of both class II
MHC molecules and members of the co-stimulatory B-7 family, and thereby play an important
accessory role in the specific immune response.
This pattern of functioning makes dendritic cells more potent antigen-presenting cells (APCs)
than macrophages and B-lymphocytes, both of which need to be activated before they can
function as antigen-presenting cells (APCs).
In addition to dendritic cells originated in bone marrow, there are another type of dendritic
cells, the follicular dendritic cells, that do not arise in bone marrow and perform their function
in a different ways as they do not express class II MHC molecules and do not act as antigenpresenting cells (APCs).
Follicular dendritic cells express high levels of membrane receptors for antibody; which allows
the binding of antibody complexes. The interaction of B-lymphocytes with this bound antigen
can have important effects on B-lymphocyte responses.
Cell # 7. Mast Cells:
Mast cell precursors originate in the bone marrow and are released into the blood as
undifferentiated cells. Mast cells are not differentiated from their precursors until the latter
leave the blood and enter the tissues. Mast cells are found in a variety of tissues including the
skin, connective tissues of various organs, and mucosal epithelial tissue of the respiratory,
genitourinary, and digestive tracts.
These cells, like basophils, possess large numbers of granules in cytoplasmic matrix. The
granules in cytoplasm contain histamine and other pharmacologically active substances that
contribute to the inflammatory response. Mast cells, together with basophils, play an important
role in the development of allergies and hypersensitivities.
Cell #8.Natural Killer Cells:
These cells are mostly derived from the large granular lymphocytes. Most surface antigen of
NK are shared with T cells or cells of the myelomonocytic series. NK cells are able to kill
certain tumour cells and are also cytotoxic for virus infected cells.
NK cells may also release interferon-7 and other cytokines (immunological mediators) which
may be important in the regulation of hemopoiesis and immune responses. NK cells have other
important surface molecules which are common to all leucocytes. These surface molecules are
important for cell adhesion and intercellular communication.
Structure of Antibody (Ab) Molecule
Polypeptide chains:
Antibody molecules have a common structure of four polypeptide chains, having two different
sizes. These are a pair of identical high molecular weight chains called Heavy chains (H-chains)
and a pair of identical low molecular weight chains called Light chains (L-chains). Each light
and heavy chain may be subdivided into homologous regions termed domains.
H-chains:
H-chains have a molecules weight of 50-55 kd. Each H-chain has disulfide linkages (-S-S-)
and also contains carbohydrate molecule attached to its asparagine residue. Each H-chain has
N-terminal (NH3+) and C-terminal (Coo–) respectively.
L-chains:
L-chains have a molecular weight of 20-25kd. L-chains are covalently linked with H-chains by
disulfide bridge (-S-S-). Each L-chain also has N-terminal and C-terminal respectively.
Hinge region:
Electron microscopy of purified immunoglobulin (IgG) molecules, after negative staining
reveals the Y-shape of the molecules indicating a flexible “hinge region” at about the middle
of the H-chains where two H-chains are connected by disulfide bridge (-S-S-).
Antigen combinding site (ACS):
One variable region of a heavy chain (VH) and one variable domain of light chain (VL) together
constitute an antigen combinding site (ACS) or in other words determine the antibody
specificity.
Structure of Immunoglobulin Molecule (Ab) based on Amino Acid sequencing Studies:
Polypeptide chains:
Amino acid sequencing study reveals that each heavy and light chain in an immunoglobulin
molecule contains an amino terminal variable (V) regions (VH → variable region of H-chain;
VL → → variable region of L-chain) that consist of 100-110 amino acids and differ from one
antibody to the next.
The remainder of the molecule—the constant region (CH → constant region of Heavy-chain;
CL → constant region of Light-chain) exhibits limited variation that defines two Light chain
subtypes (k & λ) and five Heavy chain subtypes (γ, α, μ, δ, or ε).
Heavy and Light chains are folded into domain, each containing about 110 amino acid residues
and an inter-chain disulfide bonds that forms a 60 amino acids loop.
Hinge region:
Amino acid analysis of the hinge region has indicated an unusual feature—a large no. of proline
residues present.
Because of its structure, proline prevents the peptide chains from assuming an α-helix
conformation and thus the hinge region remains extended.
And the peptide bonds of the hinge region are accessible to the proteolytic enzymes shown
below:
Antigen binding site:
Within the variable region of both Light and Heavy chain (VL and VH), amino acids at reversal
positions are often substituted and at certain points, the substitution is at a notably higher frequency. These high frequency-regions are termed as “hyper-variable regions” or “hot spots”.
Three regions on the Light (VL) chain and three on the Heavy (VH) chain lie relatively close to
each other to form the antigen binding site.
Each hyper-variable region consists of five to ten amino acid residues. As the hyper-variable
regions determine the combinding sites for antigenic determinant, that’s why they are also
termed as complementarity-determining regions (CDRs). The variable sequences on either side
of the hyper-variable regions termed as framework regions (FRs). Thus each variable (V)
region consists of three CDRs and four FRs.
Humoral immune response
The immune system protects the body from potentially harmful substances by recognizing
and responding to antigens. Antigens are foreign particles, normally large or small molecules
on the surface of cells, viruses, fungi or bacteria. Some non-living substances such as toxins,
chemicals, drugs and foreign particles can also be antigens. Substances that contain these
antigens are recognized and destroyed by the immune system.
One of the most important immune response is humoral effector response. The effector functions in humoral immunity are mainly mediated by secreted antibodies. It protects body from
extra-cellular pathogenic agents by combining with them to form antigen-antibody complex
and gradually leads towards their elimination.
Humoral immunity combats extracellular bacteria, fungi and even obligate intra-cellular
microbes e.g. viruses; before they infect their target T-cells. Any defect in humoral immunity
results in increased susceptibility to infection with bacteria and fungi.
Ways involved in Humoral Immunity:
Humoral effector functions facilitate effective elimination of foreign pathogens from a host
animal in a variety of ways.
Antibodies play vital role in elimination of antigenic agents:
(i) The antibody can bind to the surface epitopes of the antigen making it more susceptible to
phagocytosis—known as opsonization.
(ii) The antibody molecule can bind to the antigen forming an antigen-antibody complex,
which then combines with the complement in a step-wise manner to initiate and facilitate
phagocytosis of the antigen.
(iii) The antibody can bind to toxin molecules elaborated by microbes making them nontoxic.
(iv) Antibodies can inactivate free virus particles by combining with the epitopes on viral
particles to make them incapable of attachment to host cell membranes.
(v) Binding to potential pathogens at mucous membrane surfaces, preventing colonization.
(vi) Binding to Fc (fragment crystalized) receptors on NK cells or macrophages in antibody
dependent cell mediated cytotoxicity (ADCC), confirming specificity for antigen.
Humoral Immune Responses:
Most defenses that are mediated by antibody present in the plasma, lymph and tissue fluids
are called humoral immune responses. It protects against extra-cellular bacteria and foreign
macromolecules. Transfer of antibodies confers this type of immunity on the recipient.
Humoral immune responses have an activation phase and an effector phase.
These phases occur as follows (Fig. 10.1):
1. The antigen is taken up by phagocytosis and degraded in a lysosome in an APC, such as a
macrophage.
2. A T-cell receptor recognizes processed antigen bound to a class II MHC protein on the
macrophage.
3. Cytokines released by the TH cell and IL-1 released by macrophage stimulate the TH cell to
produce a clone of differentiated cells capable of interacting with B-cells.
Activation phase occurs in lymphatic tissue.
4. B-cells are also antigen presenting cells. Binding of antigen to a specific IgM receptor
triggers receptor mediated endocytosis, degradation and display of the processed antigen on
class II MHC proteins.
5. When a TH cell receptor binds to the displayed antigen—MHC II complex on the B cell, it
releases cytokines.
6. These cytokines cause the B-cell to produce a clone of B-cells.
7. Now, these B-cells produce antibody secreting plasma cells.
5 Major Classes of Immunoglobulin | Immunology
Class # 1. IgG:
(a) It constitutes 75% of the total serum immunoglobulin in human.
(b) During the secondary immune response, it is the major Ig to be synthesized. Hence, it
plays a vital role in the defense against infection.:
(c) It is the only Ig class that can cross the placenta. Hence it is responsible for the protection
of the neonate during the first few months of life.
(d) It diffuses readily into extra-vascular spaces and hence provides a major defense against
bacterial toxins and other blood born infectious agents.
(e) Organisms coated with IgG attract macrophages via their FC region receptors thus
enhancing phagocytosis.
(f) The complement binding site on the IgG molecule appears to be on the CH-II region.
(g) Ig is unable to bind onto the mast cells but has the ability to bind guinea-pig skin— the
significance remains unclear.
(h) The property of Fc portion of IgG to bind to protein-A on the surface of Staphylococcus
aureus has been greatly exploited for use in diagnosis and research.
Class # 2. IgA:
(a) It is actively secreted by mucosal associated lymphoid tissue (MALT).
(b) It appears selectively in sero-mucous secretions such as saliva, tears, nasal fluids and in
the secretions of the lung and also in GI tracts and in UG system.
(c) It is present in fluids as a dimer stabilized against proteolysis by combination with another
protein, the secretory compound (J-chain) which is synthesized by local epithelial cells and
has a single peptide chain of MW. 60 kd.
(d) The IgA chain is synthesized locally by plasma cells and dimerized intra-cellularly before
secretion with the help of J-chain.
(e) It is actively endocytosed and transported within the endocytic vacuole and the mucosal
surface. Cleavage of the receptor releases the IgA, still attached to the part of receptor termed
the secretory piece, into sero-mucous secretion.
(f) IgA is the most abundant in body secretions. It performs the role of defending the exposed
external surfaces of the body against the attack of the microorganisms.
(g) IgA activates complements by the alternative pathway.
(h) It, the prime functional units of MALT, facilitates passage through epithelial cells and
protects the secretory molecule from proteolytic degestion.
Class # 3. IgD:
(a) It is present in serum in trace amounts.
(b) Because of an extended hinge region it is relatively liable to degradation by proteolytic
enzymes.
(c) The main functions of IgD has not yet been determined with IgM, it is found abundantly
on the surface of B-lymphocytes. It has been suggested that they may operate as antigen
receptors and in the control of lymphocytic activation and suppression.
Class # 4. IgM:
(a) It is the largest Ig in science and exists as a pentamer of the basic 4-chain subunit, held
together by disulfide bond.
(b) A relatively small molecule, the J-chain participates in the polymerization of IgM via SH
residue near the C-terminal.
(c) The heavy-chain of IgM are designated as μ-chains.
(d) Electron microscopic study reveals that it is shaped like a star but when it is touched to a
bacterium, its antigen binding sites (Fab) are bound to the bacterial surface. This changes the
appearance of IgM to a crab like form, and causes cross linking of the different antigenic
determinants (epitopes) on the bacterial cell-surface by polyvalent IgM molecule.
(e) IgMs appear early in response to injection and because of their size are largely confined
to their blood stream.
(f) They are an important defense mechanism against bacteria.
(g) The size and valency of IgM makes it a very effective, agglutinating and cytolytic agent.
(h) Since it does not cross the placenta its presence in blood vessels indicates active foetal
injection.
(i) Since IgM response is short lived, its presence may be helpful in establishing an acute
injection.
Class # 5. IgE:
(a) It is the present in very low concentration in serum.
(b) IgE antibody has a very high affinity for mast cells and binding occurs via Fc portion of
the Ig molecule.
(c) On contact with specific antigens called allergens, the mast cells undergo degranulation
with release of vasoactive amines (Histamine). This process is responsible for skin reaction in
allergy for the symptoms of Hay fever and an extrinsic asthma.
(d) IgE also have the ability to attach to human skin where they probably bound to mast cells.
(e) It is found mainly in lining of the respiratory and GI tracts where they form constituents
of MALT.
(f) The main physiological role of IgE appears to be the protection of external mucosal
surface.
(g) Infection agents penetrating the IgA defenses, combined with specific IgE on the mast
cell surface to trigger the release of vasoactive agents and other factors, chemotactic for
granulocyte.
(h) It is possible that IgE acts in the way as a defense against helminth infection which is
characterised by an extra-aggregated IgE response.
Cell-Mediated Immune Responses
The cell-mediated or cellular immunity is that where the T-lymphocytes destroy other cells
having antigens on their surface without any mediation by antibodies. The precursors of Tlymphocytes produced by stem cells of bone marrow pass through liver and spleen before
reaching the thymus where they are processed, hence called thymus-dependent (T)
lymphocytes.
These lymphocytes come under the influence of the hormone “thymosin” and become
immunologically competent and are called lymplioblasts. When stimulated by an appropriate
antigen, the lymphoblasts divide and differentiate into cytotoxic T-lymphocyte (killer TIymphocytes), helper T-cells, and suppressor T-cells.
The cytotoxic T-lymphocytes, in addition with other T-lymphocytes, release biologically active
soluble factors collectively called lymphokines which act as a biochemical mediators of
cellular immunity.
Unlike B-lymphocytes which are normally stimulated by free antigens in the circulatory system
of the body, the cytotoxic T-lymphocytes possess specific cell surface proteins, called T-cell
receptors, on their surface and respond to only major histocompatibility complex antigens
(MHC-antigens) bound to the surface of other cells.
After the interaction between T-cell receptor and MHC-antigen is established and the cytotoxic
T-lymphocyte cells binds the MHC-antigen containing cell, the latter undergoes lysis and is
phagocytised (Fig. 41.2).
The cell-mediated immunity is important in controlling those infections where the pathogens
are intracellular and reproduce within the infected cells (e.g., viruses, rickettsia, chlamydia,
some protozoans like Trypanosomes, etc.).
In such infections the antibodies (hence the antibody-mediated or humoral immunity) prove to
be ineffective because the antibodies are unable to penetrate and attack intracellular pathogens
multiplying within the host cells.
In addition, the cellular immunity is considered to play an important role in monitoring and
regulating the proliferation of abnormal type of cells, (e.g., would be tumor cells), and thus,
inhibiting the tumor development.
Types of T cell
T cells are grouped into a series of subsets based on their function. CD4 and CD8 T cells are
selected in the thymus, but undergo further differentiation in the periphery to specialized cells
which have different functions. T cell subsets were initially defined by function, but also have
associated gene or protein expression patterns.
Depiction of the various key subsets of CD4-positive T cells with corresponding associated
cytokines and transcription factors.
Helper CD4+ T cells
T helper cells (TH cells) assist other lymphocytes, including maturation of B cells into plasma
cells and memory B cells, and activation of cytotoxic T cells and macrophages. These cells are
also known as CD4+ T cells as they express the CD4 on their surfaces. Helper T cells become
activated when they are presented with peptide antigens by MHC class II molecules, which are
expressed on the surface of antigen-presenting cells (APCs). Once activated, they divide
rapidly and secrete cytokines that regulate or assist the immune response. These cells can
differentiate into one of several subtypes, which have different roles. Cytokines direct T cells
into particular subtypes.
CD4+ Helper T cell subsets
Cell
type
Cytokines
Produced
Key
Transcription
Factor
Role in immune defence
Role
in
autoimmunity
Th1
IFNγ
Tbet
Produce an inflammatory
response, key for defense MS,
Type
against intracellular bacteria, diabetes
viruses and cancer.
Th2
IL-4
GATA-3
Aid the differentiation and
Asthma and other
antibody production by B
allergic diseases
cells
1
Th17
IL-17
RORγt
Defense
against
gut
Rheumatoid
pathogens and at mucosal
Arthritis, Psoriasis
barriers
Th9
IL-9
IRF4, PU.1
Defense against helminths
Multiple Sclerosis
(parasitic worms)
Tfh
IL-21, IL-4
Bcl-6
Help B
antibody
cells
produce Asthma and other
allergic diseases
Cytotoxic CD8+ T cells
Cytotoxic T cells (TC cells, CTLs, T-killer cells, killer T cells) destroy virus-infected cells and
tumor cells, and are also implicated in transplant rejection. These cells are defined by the
expression of CD8+ on the cell surface. These cells recognize their targets by binding to short
peptides (8-11AA) associated with MHC class I molecules, present on the surface of all
nucleated cells. CD8+ T cells also produce the key cytokines IL-2 and IFNγ, which influence
the effector functions of other cells, in particular macrophages and NK cells.
Memory T cells
Antigen-naïve T cells expand and differentiate into memory and effector T cells, after they
encounter their cognate antigen within the context of an MHC molecule on the surface of a
professional antigen presenting cell (e.g. a dendritic cell). Appropriate co-stimulation must be
present at the time of antigen encounter for this process to occur. Historically, memory T cells
were thought to belong to either the effector or central memory subtypes, each with their own
distinguishing set of cell surface markers (see below). Subsequently, numerous new
populations of memory T cells were discovered including tissue-resident memory T (Trm)
cells, stem memory TSCM cells, and virtual memory T cells. The single unifying theme for
all memory T cell subtypes is that they are long-lived and can quickly expand to large numbers
of effector T cells upon re-exposure to their cognate antigen. By this mechanism they provide
the immune system with "memory" against previously encountered pathogens. Memory T cells
may be either CD4+ or CD8+ and usually express CD45RO.
Memory T cell subtypes:


Central memory T cells (TCM cells) express CD45RO, C-C chemokine receptor type
7 (CCR7), and L-selectin (CD62L). Central memory T cells also have intermediate to high
expression of CD44. This memory subpopulation is commonly found in the lymph
nodes and in the peripheral circulation. (Note- CD44 expression is usually used to
distinguish murine naive from memory T cells).
Effector memory T cells (TEM cells and TEMRA cells) express CD45RO but lack expression
of CCR7 and L-selectin. They also have intermediate to high expression of CD44. These
memory T cells lack lymph node-homing receptors and are thus found in the peripheral
circulation and tissues. TEMRA stands for terminally differentiated effector memory cells reexpressing CD45RA, which is a marker usually found on naive T cells.


Tissue resident memory T cells (TRM) occupy tissues (skin, lung, etc..) without
recirculating. One cell surface marker that has been associated with TRM is the intern αeβ7,
also known as CD103.
Virtual memory T cells differ from the other memory subsets in that they do not originate
following a strong clonal expansion event. Thus, although this population as a whole is
abundant within the peripheral circulation, individual virtual memory T cell clones reside
at relatively low frequencies. One theory is that homeostatic proliferation gives rise to this
T cell population. Although CD8 virtual memory T cells were the first to be described, it
is now known that CD4 virtual memory cells also exist.
Regulatory CD4+ T cells
Regulatory T cells are crucial for the maintenance of immunological tolerance. Their major
role is to shut down T cell-mediated immunity toward the end of an immune reaction and to
suppress autoreactive T cells that escaped the process of negative selection in the thymus.
Two major classes of CD4+ Treg cells have been described — FOXP3+ Treg cells and
FOXP3− Treg cells.
Regulatory T cells can develop either during normal development in the thymus, and are then
known as thymic Treg cells, or can be induced peripherally and are called peripherally derived
Treg cells. These two subsets were previously called "naturally occurring", and "adaptive" or
"induced", respectively.[18] Both subsets require the expression of the transcription
factor FOXP3 which can be used to identify the cells. Mutations of the FOXP3 gene can
prevent regulatory T cell development, causing the fatal autoimmune disease IPEX.
Several other types of T cell have suppressive activity, but do not express FOXP3. These
include Tr1 cells and Th3 cells, which are thought to originate during an immune response and
act by producing suppressive molecules. Tr1 cells are associated with IL-10, and Th3 cells are
associated with TGF-beta. Recently, Treg17 cells have been added to this list.
Natural killer T cell
Natural killer T cells (NKT cells – not to be confused with natural killer cells of the innate
immune system) bridge the adaptive immune system with the innate immune system. Unlike
conventional T cells that recognize peptide antigens presented by major histocompatibility
complex (MHC) molecules, NKT cells recognize glycolipid antigen presented by CD1d. Once
activated, these cells can perform functions ascribed to both Th and Tc cells (i.e., cytokine
production and release of cytolytic/cell killing molecules). They are also able to recognize and
eliminate some tumor cells and cells infected with herpes viruses.
Mucosal associated invariant
MAIT cells display innate, effector-like qualities. In humans, MAIT cells are found in the
blood, liver, lungs, and mucosa, defending against microbial activity and infection. The MHC
class I-like protein, MR1, is responsible for presenting bacterially-produced vitamin
B metabolites to MAIT cells. After the presentation of foreign antigen by MR1, MAIT cells
secretes pro-inflammatory cytokines and are capable of lysing bacterially-infected
cells. MAIT cells can also be activated through MR1-independent signaling. In addition to
possessing innate-like functions, this T cell subset supports the adaptive immune response and
has a memory-like phenotype. Furthermore, MAIT cells are thought to play a role
in autoimmune diseases, such as multiple sclerosis, arthritis and inflammatory bowel
disease, although definitive evidence is yet to be published.
Gamma delta T cells
Gamma delta T cells (γδ T cells) represent a small subset of T cells which possess a γδ TCR
rather than the αβ TCR on the cell surface. The majority of T cells express αβ TCR chains.
This group of T cells is much less common in humans and mice (about 2% of total T cells) and
are found mostly in the gut mucosa, within a population of intraepithelial lymphocytes. In
rabbits, sheep, and chickens, the number of γδ T cells can be as high as 60% of total T cells.
The antigenic molecules that activate γδ T cells are still mostly unknown. However, γδ T cells
are not MHC-restricted and seem to be able to recognize whole proteins rather than requiring
peptides to be presented by MHC molecules on APCs. Some murine γδ T cells recognize MHC
class IB molecules. Human γδ T cells which use the Vγ9 and Vδ2 gene fragments constitute
the major γδ T cell population in peripheral blood, and are unique in that they specifically and
rapidly respond to a set of nonpeptidic phosphorylated isoprenoid precursors, collectively
named phospho antigens, which are produced by virtually all living cells. The most common
phosphor antigens from animal and human cells (including cancer cells) are isopentenyl
pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMPP). Many microbes
produce the highly active compound hydroxy-DMAPP (HMB-PP) and corresponding
mononucleotide conjugates, in addition to IPP and DMAPP. Plant cells produce both types of
phosphor antigens. Drugs activating human Vγ9/Vδ2 T cells comprise synthetic phosphor
antigens and amino bisphosphonates, which upregulate endogenous IPP/DMAPP.
T-Cell Receptor
T lymphocytes or T cells respond only to peptide fragments of protein antigens that are
displayed by self-MHC molecules (major histocompatibility complex). T cell receptor or TCR
differs from the B cell receptor in two important ways.
First, the T cell receptor is membrane bound and does not appear in a soluble form as the B
cell receptor does; second, the T cell receptor is specific not for antigen alone but for antigen
combined with a molecule encoded by MHC.
Further, the T cell receptor remains associated on the membrane with a signal-transducing
complex CD3 which is non-covalently linked to the receptor to form the TCR complex. The
TCR is a clonally distributed receptor, meaning that clones of T cells with different specificities
express different TCRs.
The biochemical signals that are triggered in T cells by antigen recognition are transduced not
by the TCR itself but by TCR complex. T cells also express other membrane receptors that do
not recognise antigen but participate in responses to antigens; these are collectively called
accessory molecules. These molecules deliver signals to the T cell that function in concert with
signals from the TCR complex to fully activate the cells.
Antigen recognition by T cells is specific not only for antigen but also for an MHC molecule.
T cells were shown to recognise antigen only when presented on the membrane of APC
(antigen presenting cell) by a self-MHC molecule. This attribute called self-MHC restriction,
distinguishes of antigen recognition by T cells from that by B cells.
Two models were proposed to explain the MHC restriction of T cell receptor. The dual-receptor
model proposed that a T cell has two separate receptors, one for antigen and one for class I or
class II MHC molecules. The altered self-model proposed that T cell possesses a single receptor
capable of recognising foreign antigen, bound to a self-MHC molecule.
However, the elegant experiments by J. Kappler and P. Marrack provided supports for the
altered self-model. Unlike the dual-receptor model, in which an antigen and MHC molecule
are recognised separately, the altered self-model predicts that a single receptor recognises an
alteration in self-MHC molecules induced by their association with foreign antigens.
Structure of T Cell Receptors:
The antigen receptor of MHC restricted CD4+ helper T cells and CD8+ cytotoxic T cells is a
heterodimer consisting of two trans-membrane polypeptide chains. These chains are designated
as α and β which are covalently linked to each other by disulfide bonds (Fig. 6.58). Another
group of TCR, found on a small subset of T cells, has y and δ chains.
In the amino terminal, each a chain and β chain consists of the Ig-like variable domain (V), one
constant domain (C), hydrophobic trans-membrane region and a short cytoplasmic region.
Thus, the extracellular portion of αβ heterodimer in TCR is structurally similar to the antigen
binding fragment (Fab) of an Ig molecule.
The α and β chains of V regions of TCR contain short stretches of amino acids where the hypervariable or complementarity determining regions (CDRs) are located. Three such CDRs in the
α chain are juxtaposed to three similar regions in the β chain to form the part of the TCR that
specifically recognises peptide-MHC complexes.
The P chain V domain contains a fourth hyper-variable region, which does not appear to
participate in antigen recognition but is the binding site for microbial products called super
antigens. The C regions of both α and β chains continue into short hinge regions, which contain
cysteine residues that contribute to a disulfide bond linking the two chains.
The hinge is followed by a hydrophobic trans membrane portion of 21 or 22 amino acids. In a
chain, positively charged amino acid residues like lysine and in (3 chain lysine or arginine
residues are present in the trans membrane portion. Both α and β chains have carboxyl terminal
cytoplasmic tails that are 5 to 12 amino acids long.
Each TCR chain, like Ig heavy and light chains, is encoded by multiple gene segments that
undergo somatic rearrangements during the maturation of the T lymphocytes. In α and β chains
of the TCR, the third hyper-variable regions (CDR3) are composed of sequences encoded by
V and J (joining) gene segments (in the α chain) or V, D (diversity) and J segments (in the β
chain).
The CDR3 regions also contain sequences that are not present in the genome but are encoded
by different types of nucleotide additions, so-called N regions and P nucleotides. Thus, most
of the sequence variability in TCRs is concentrated at CDR3.
CDRs of T Cell Receptor and their Role in MHC-associated Peptide Recognition:
Different experimental studies have established that both α and β chains of TCR form a single
hetero-dimeric receptor that is responsible for both antigen specificity and MHC restriction of
a T cell. The antigen binding site of the TCR is a flat surface formed by the CDRs of α and β
chains.
The TCR contacts the peptide-MHC complex in a diagonal orientation, fitting between the high
points of MHC α helices. In general, the CDRl loop of the TCR α and β chains are positioned
over the ends of the bound peptide.
The CDR2 loops are over the helices of the MHC molecule and the CDR3 loop is positioned
over the centre of the MHC-associated peptide. In fact, the side chain of only one or two amino
acid residues of the MHC bound peptide make contact with the TCR. T cells have very
remarkable ability to distinguish among diverse antigens on the basis of very few amino acid
differences.
The affinity of the TCR for peptide-MHC complex is very low. Such low affinity of specific
antigen binding is the likely reason that accessory molecules are needed to stabilise the
adhesion of T cells to APCs. The TCR and accessory molecules in the T cell plasma membrane
move coordinately with their ligands in the APC membrane to form a transient supra-molecular
structure that is referred to as immunological synapse.
This structure regulates the TCR-mediated signal transduction. Virtually all αβ TCRexpressing T cells are MHC restricted and express either the CD4 or the CD8 co-receptors. A
small population of T cells also expresses markers that are found on NK (natural killer) cells;
these are called NK-T cells.
T Cell Receptor Complex: TCR CD3:
Experiments by J. P. Allison and L. Lanier and others demonstrated that T cell receptor and
another protein CD3 are located quite close together in the T cell plasma membrane (within 11.5 nm of each other).
The expression of CD3 molecule is required for membrane expression of αβ and yδ T cell
receptors; thus each heterodimer forms a complex with CD3 on the T cell membrane. Loss of
genes encoding either CD3 or TCR chains results in the loss of the entire molecular complex
from the membrane.
CD3 is a complex of five invariant polypeptide chains that associate to form three dimers: a
heterodimer of gamma and epsilon chains (yɛ), a homodimer of delta and epsilon chains (δɛ),
and a heterodimer of two zeta chains (ϚϚ) or a heterodimer of zeta and eta chain (Ϛƞ) (Fig.
6.59).
The Ϛ and ƞ chains, though encoded by the same gene, may differ in their carboxyl terminal
ends because of differences in RNA splicing of the primary transcript. About 90% of the CD3
complexes examined to date have ϚϚ homodimer than heterodimer Ϛƞ as possessed by rest
10% of CD3 complexes.
In CD3 complex, the y, δ and ɛ chains belong to immunoglobulin superfamily, each containing
an extracellular domain followed by a trans membrane region and a cytoplasmic domain of
more than 40 amino acid residues.
The zeta and eta (Ϛ and ƞ) chains have quite different structure, each with a very short
extracellular region of only 9 amino acids, a trans membrane region and a long cytoplasmic
tail of 113 amino acids in Ϛ chain and 155 amino acids in ƞ chain.
The trans membrane segment of all CD3 polypeptide chains contains a negatively charged
amino acid residue of aspartic acid. Such residues enable the CD3 complex to interact with one
or two positively charged amino acid residues in the trans membrane segment of each TCR
chain.
The cytoplasmic domains of CD3 Ƴ, ɛ and δ chains contain one copy of a conserved sequence
motif called the immuno-receptor tyrosine-based activation motif (ITAM) in each chain. An
ITAM contains two copies of the sequence tyrosine-X-X-leucine (X is an unspecified amino
acid) separated by six to eight residues.
ITAM plays a central role in signaling by TCR complex. They are also found in Ϛ chain of the
TCR complex, Iga and IgP proteins associated with membrane Ig molecules of B cells.
Functions of CD3 complex:
The CD3 and Ϛ chains link antigen recognition by the TCR to the biochemical events that lead
to functional activation of the T cells. The earliest intracellular event that occurs in T cells after
antigen recognition is the phosphorylation of tyrosine residues within the ITAMs in the
cytoplasmic tails of CD3 and Ϛ proteins.
This phosphotyrosines then become the docking sites for adapter proteins and for tyrosine
kinase with a kinase called ZAP-70 that binds to the Ϛ chain. Another kinase that also docks at
phosphotyrosine is Fyn that binds to CD3. Subsequent activation of these kinases triggers
signal transduction pathway that ultimately lead to changes in gene expression in the T cells.
yδ TCR:
In a few T cell populations, a second type of diverse, disulfide linked heterodimer yδ TCR
receptor is expressed instead of αβ TCR. This receptor also remains associated with CD3 and
Ϛ proteins. (This yδ TCR should not be confused with the y and δ chains of CD3 complex).
The TCR y and δ chains contain extracellular V and C domains, short connective or hinge
regions, hydrophobic trans-membrane segments and short cytoplasmic tails.
The constituents of these segments are almost similar to those of α and β chains. Furthermore,
TCR – mediated signaling events typical of αβ expressing T cells are also observed in yδ T
cells. However, majority of yδ T cells do not express CD4 or CD8.
The percentages of yδ TCR expressing T cells vary widely in different tissues and species, but
overall, less than 5% of all T cells express this receptor. T cells with yδ TCR are a lineage
distinct from the αβ-expressing MHC- restricted T cells.
Many δy T cells present in different organs, may have different V regions, indicating that these
subsets may be specific for different ligands. One intriguing feature of yδ T cells is their
abundance in epithelial tissue of certain species (e.g., small bowel mucosa of mice and
chicken). In human only about 10% of intestinal intra-epithelial T cells express the yδ
receptors.
The function of the yδ T cells is not clear. yδ T cells do not recognise MHC-associated peptide
antigens and are not MHC restricted. Some can recognise small phosphorylated molecules,
alkyl amines or lipids that are commonly found in association with “non-classical” class I
MHC-like molecules in mycobacteria and other microbes.
Others may recognise protein or nonprotein antigens that do not require processing or any
particular type of APCs for their presentation. Some suggest that they may initiate immune
responses to a small number of common microbes that frequently encounter at epithelial
boundaries between the host and the external environment.
Accessory Molecules of T Cell Receptor:
T cells express several integral membrane proteins that play important role in antigen
recognition and T cell activation. Some of these molecules strengthen the interaction between
T cells and antigen presenting cells or target cells; some act in signal transduction and some do
both. These protein molecules are often collectively called accessory molecules.
(i) CD4 and CD8:
Mature αβ T cells express either CD4 or CD8 membrane protein, but not both. CD4 and CD8
interact with class II and class I MHC molecules, respectively, when the antigen receptor of T
cells specifically recognise peptide-MHC complexes on APCs.
Both CD4 and CD8 are trans membrane glycoprotein members of the Ig superfamily, with
similar functions but different structures. CD4 is a 55-kDa monomeric protein that contains
four extracellular domains (D1-D4), a hydrophobic trans membrane segment and a long
cytoplasmic tail of 38 basic amino acids (Fig. 6.60). It binds through its two N-terminal
domains to non-polymorphic β2 domain of the class II MHC molecule.
Most CD8 molecules exist as disulfide-linked heterodimers composed of two related chains
called CD8α and CD8β. Both the a and 8 chains have a single extracellular domain, a
hydrophobic trans membrane region and a highly basic cytoplasmic tail of about 25 amino
acids long (Fig. 6.60).
The extracellular domain of CD8 binds to the non-polymorphic α3 domain of class I MHC
.molecules. Some T cells express CD8αα homodimers, but this form appears to function like
the more common CD8αβ heterodimers.
Functions of CD4 and CD8:
CD4 binds to class II MHC molecules and is expressed on T cells whose TCRs recognise
complexes of peptide and class II MHC molecules. Most CD4+ class II-restricted T cells are
cytokine-producing helper cells (TH cells) and function in host defence against extracellular
microbes. CD8 binds to class I MHC molecules and is expressed on T cells whose TCRs
recognise complexes of peptide and class I MHC molecules.
Most CD8+ class I-restricted T cells are CTLs (cytotoxic T lymphocytes) which serve to
eradicate infections by intracellular microbes. However, in humans, some CD4+ T cells may
function as CTLs, but even these are class II restricted. Thus, expression of CD4 or CDS
determines the MHC restriction of the T cells and not their functional capabilities.
CD4 and CD8 participate in the early signal transduction events that occur after T cell
recognition of peptide MHC complexes on APCs. This signal transduction is mediated by a T
cell specific Src family tyrosine kinase called Lak that is non-covalently but tightly associated
with cytoplasmic tails of both CD4 and CD8. This kinase is also required for T cell maturation
and activation.
CD4 and CDS promote the adhesion of MHC-restricted T cells to APCs or target cells
expressing peptide MHC complexes. However, in such strengthening function, both coreceptors need the help of other accessory molecules.
The CD4 act as a receptor for the human immunodeficiency virus.
(ii) CD28 and CTLA-4:
CD28 is a homodimer membrane protein that is expressed on more than 90% of CD4+ T cells
and on 50% of CD8+ T cells in humans. Binding of B7 molecules on APCs to CD28 delivers
signals to the T cells that induce the expression of anti-apoptotic proteins, stimulate production
of growth factors and other cytokines and thus promote T cell proliferation and differentiation.
Thus CD28 is the principal receptor for delivering signals for T cell activation.
A second receptor for B7 molecule was discovered later and called CTLA-4. It is structurally
homologous to CD28 but is expressed on recently activated CD4+ and CD8+ T cells. It inhibits
T cell activation by counteracting signals delivered by CD28. Thus, CTLA-4 is involved in
terminating T cell responses.
(iii) CD45, CD2:
CD45, a cell surface glycoprotein is believed to play a role in T cell activation. Various forms
of CD45 are expressed on immature and mature leucocytes like T and B cells, thymocytes,
mononuclear phagocytes and polymorphonuclear leucocytes.
CD2 is a glycoprotein present on more than 90% of mature T cells, 50% of thymocytes and on
NK cells. It functions both as an intercellular adhesion molecule and as a signal transducer.
(iv) Adhesion molecules:
Some accessory molecules of T cells function as intercellular adhesion molecules and play
important roles in the interactions of T cells with APCs. These molecules also help in the
migration of T cells to sites of infection and inflammation. The major adhesion molecules of T
cells include integrins, selectins and CD44.
The major functions of integrins are to mediate adhesion of T cells to APCs, endothelial cells
and extracellular matrix proteins. The T cell selecting mediates the migration of naive T cells
into lymph nodes, where antigens are concentrated and immune responses are initiated.
Selectins also help in the migration of effector and memory T cells to sites of inflammation.
CD44 is responsible for the retention of T cells in extravascular tissues at sites of infection. It
also causes rapid binding of activated and memory T cells to endothelium at sites of
inflammation and in mucosal tissues.