Download 03.Lecture (3) Lymphoid system Hazem KSU 2008

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REST
Lymphoid System
Hazem M. Abu-Eisha
Ph.D , MD
Assistant Professor of Immunology
Pathology Department
King Saud University
King Khaled University Hospital
E - mail : [email protected]
Outlines
1.
General considerations about Haematopoiesis
2.
Organs , cells & tissues of the lymphoid system
3.
Development of T & B lymphocytes
4.
T & B cell receptors ( TCR & BCR )
5.
T & B lymphocytes subsets
6.
Locations of T & B lymphocytes in secondary
lymphoid organs & their interactions
Haematopoiesis
•
All blood cells including lymphocytes arise from one cell
type called : Haematopoietic Stem Cell ( HSC ) that originate
in the bone marrow ( BM ) & gives rise to two cell lineages [
Common Myeloid Progenitor ( CMP ) & Common Lymphoid
Progenitor ( CLP ) ]
•
In comparison to unipotent cell which can differentiate into
a single cell type, HSC is a pluripotent stem cell [ i.e.
it has the ability to differentiate into various types of cells
including { Erythrocytes , granulocytes , monocytes , mast cells , lymphocytes
& megakaryocytes } ]
•
HSCs are few in number [ There is one HSC for every 5
x 104 cells in the bone marrow ]
All cells shown here arise from the hematopoietic stem cell. Platelets produced by megakaryocytes are released
into the circulation. Polymorphonuclear granulocytes and monocytes pass from the circulation into the tissues.
Mast cells are identifiable in all tissues. B cells mature in the fetal liver and bone marrow in mammals, whereas T
cells mature in the thymus. The origin of the large granular lymphocytes with natural killer (NK) activity is probably
the bone marrow. Lymphocytes recirculate through secondary lymphoid tissues. Interdigitating cells and dendritic
cells act as antigen-presenting cells (APCs) in secondary lymphoid tissues.
Organs of the
Lymphoid
System
Central
( Primary )
Microenvironment for
development & maturation
of lymphocytes to develop
immuno-competent cells
Peripheral
( Secondary )
Trap Ags from tissues &
vascular spaces where
mature lymphocytes
interact with Ag
1. Spleen
1. Thymus
2. Bone Marrow ( BM )
2. Lymph Node
3. Mucosal - Associated lymphoid
tissues ( MALT )
Primary Lymphoid Organs
1. Thymus
•
•
It is the main site for T - cell
maturation
Flat , bilobed organ
•
Each lobe is surrounded by a
capsule & is divided into lobules
•
Each lobule is organized into two
compartments
: Outer called
Cortex ; densely packed with
immature T - cells
called
Thymocytes & Inner called :
Medulla ; sparsely populated with
thymocytes
Role Of Thymus In The
Immune Function
Experimental studies in mice with neonatal
thymectomy & DiGeorge’s syndrome ( Congenital
thymic hypoplasia ) show :
• Dramatic
in circulating lymphocytes
• Absence of cell - mediated immunity
Primary Lymphoid Organs
2. Bone Marrow
•
The primary site for B - cell maturation
•
The bone marrow contains connective tissue, blood vessels, fat, and cells
•
Among these structures are the hematopoietic stem cells which are capable of
giving rise to the stem cells of the myeloid, granuloid, erythroid, and lymphoid cells
•
Unlike lymphoid cells that are destined to differentiate into T - cells, those
committed to the B - cell lineage remain within the bone marrow for further
development & maturation
Secondary Lymphoid
Organs
1. Spleen
o
Large , ovoid organ
o
Its main function is : Filtering blood and trapping blood-borne
antigens
o
Blood borne antigens
through splenic artery
o
It is formed of compartmentalized structure which consists
mainly of two pulps separated by diffuse marginal zone :
1. The splenic red pulp
2. The splenic white pulp
& lymphocytes are carried out to it
•
The red pulp consisted of
sinusoids
populated
with
macrophages & red blood cells
( RBCs ) where old & defective
RBCs are destroyed and
removed
•
The white pulp surrounds the
branches of
splenic artery
forming a periarteriolar lymphoid
sheath ( PALS ) populated
mainly by T - lymphocytes
•
The marginal zone is located
peripheral to the PALS , it is rich
in B - lymphocytes
•
In the marginal zone ,
antigens & lymphocytes
are
trapped by DCs that carry it to
PALS
•
•
Initial activation of T & B lymphocytes occur in PALS where
DCs capture Ags & present it in combination with MHC class II
to TH cells
Activated T- cells activate B - cells
•
Activated B - cells with some activated TH cells migrate to the
primary follicles in the marginal zone
•
Antigenic challenge
Primary follicles develop to
secondary follicles where rapidly dividing B - cells and plasma cells
are formed [ Antibody - forming cells ( AFCs ) ]
•
Effect of splenectomy :
Depends on the age
1. In children :
incidence of bacterial sepsis
2. In adults : Less serious but leads to blood-borne bacterial infection
Secondary Lymphoid Organs
2. Lymph Node
•
Histologically , LN is divided into 3 regions:
Cortex , Paracortex & Medulla
•
Cortex : Contains B - lymphocytes ,
macrophages & follicular DCs arranged in primary
follicles which enlarge after antigenic challenge to
become secondary follicles each contain a germinal
center ( GC )
•
Paracortex
: is populated mainly by T lymphocytes & DCs which express high level of
MHC class II needed for antigen presentation for TH
cells
•
Medulla : is populated mainly with plasma cells
that are actively secreting ABs
Lymphoid Follicle
•
Aggregates of lymphoid & non-lymphoid cells
surrounded by a network of draining lymphatic
capillaries
•
Until it is activated by antigen , lymphoid follicle is
called PRIMARY FOLLICLE that contains
mainly small resting B - cells and DCs
•
After antigenic challenge , it enlarges to become
a SECONDARY FOLLICLE
which
contains a ringed concentrically packed B - cells
surrounding a center [ GERMINAL CENTER
(GC) ] i.e., focus of proliferating B - cells with area
of non-dividing B - cells & some TH cells ,
macrophages & follicular DCs
Secondary Lymphoid Organs
3. Mucosal-associated lymphoid tissues ( MALT )
Mucosal-associated lymphoid tissues ( MALT ):
Are anatomical sites placed at strategic areas of
potential microbial entry
1. Tonsils : In the nasopharynx at the portal
entry of respiratory & gastrointestinal systems
2. Peyer’s patches : Lymphoid accumulations
lying underneath the villi of the small intestine
that protect the bowel from microbial invasion
Cells Of The Lymphoid System
1. T - cell lineage
• αβ T cells
• γδ T cells
• NKT cells
2.
• B cell
B - cell lineage
Lymphatic Circulatory System
Leukocytes and their products use two circulatory
systems :
1.
One,
is the cardiovascular system which is
responsible for the circulation of blood (both its soluble
and cellular components) throughout the body
2.
The other system, is the lymphatic circulatory
system. It is an extensive capillary network that collects
lymph, a watery clear fluid containing leukocytes and
cellular debris, from various organs and tissues
Lymphatic vessels within small intestine villi, contain a
milk-white fluid, chyle, produced by digestion. The
lymphatic capillaries drain into large lymphatic vessels
that drain into lymph nodes for filtration
Ultimately, the lymphatic trunk vessels join to form the
thoracic duct that conveys lymph into the subclavian
vein
Lymphatics, lymphoid organs, and
tissues. The lymphatics serve as a
drainage system to remove cellular
debris and microbes from the body's
tissues to the lymph nodes.
Lymphatic trunk vessels join to form
the thoracic duct, which returns fluid
(lymph)
to
the
cardiovascular
circulation.
T - Lymphocytes
•
They arise in the bone marrow , but migrate to the thymus for complete
maturation
•
During T - cell maturation , they express a unique antigen - binding
molecule called :
T - cell receptor ( TCR )
•
B - cells recognize antigen alone while T - cells recognize antigens
bound to cell membrane proteins called :
MAJOR HISTOCOMPATABILITY COMPLEX ( MHC )
•
When a naϊve T- cell encounters antigen combined with MHC , T - cell
proliferate & differentiate into :
1. Effector T - cells [ T - helper ( TH ) & Cytotoxic T - cells ( TC ) ]
2. Memory T - cells
•
T cells can be differentiated by their antigen
receptors ( TCR )
•
T - cell either expresses :
Two - disulfide heterodimers linked polypeptides ( α
&β)
OR
Structurally similar heterodimer consisting of ( γ & δ
polypeptides )
•
About 90 - 95 % of blood T - cells are α β & the
remaining 5 - 10 % are γδ T - cells
Development of αβ T - cells
•
Cortical thymocytes are called double negative (DN) cells
•
They soon begin to generate and express αβ TCRs, associated CD3 complex, and both CD4 and CD8
molecules & these immature thymocytes are called double positive (DP) cells
•
In a process known as positive selection, DP thymocytes die within three to four days unless they
recognize and bind to major histocompatability complex (MHC) molecules expressed by cortical epithelial
cells. This process eliminates thymocytes that are incapable of recognizing self MHC
•
Cells that pass the positive selection test are allowed to enter the medulla; those that fail the test die. DP
cells whose CD8 molecules have engaged (p) MHC I then stop the expression of CD4 molecules and
become single positive (SP) CD8+ cells. Likewise, those that are bound to (p)MHC II stop expression of
CD8, becoming SP CD4+ cells
•
Survivors of positive selection then run a second test called negative selection when they arrive at the
corticomedullary junction. There, they meet and interact with a second set of epithelial reticular cells
(antigen presenting cells such as dendritic cells and macrophages). Those that efficiently bind to self
peptides of the pMHC I or pMHC II on these APCs are potentially autoreactive and undergo apoptotic
death. Thymocytes that pass both positive and negative selection tests “graduate” from the thymus,
entering the circulation through the medullary postcapillary venules as mature T- cells
Development of γδ T - cells
•
•
The thymus is also the differentiation site for thymocytes that express γδ TCRs
and CD3 complex molecules.
Many of these cells fail to express CD4 and/or CD8. Consequently, they do not
undergo the same positive and negative selective processes as αβ TCR-bearing
thymocytes
•
γδ cells are thought to be a transitional cell type that may represent a bridge
between the innate and adaptive immune systems.
•
γδ T cells develop early in embryogenesis before many αβ T cells and migrate
preferentially to the respiratory organs, the skin, and the peritoneal cavity.
•
They use a very limited in their recognition repertoire than are αβ T cells. They
respond more quickly than do αβ T cells, but they do so without generating
memory.
Natural Killer T (NKT) cells
•
Natural killer T (NKT) cells are a distinctive subset of T - cells that share
some characteristics with NK cells
•
They express several surface markers and receptors found on natural killer (
NK ) cells [ CD16 & CD56 ] , but unlike NK cells, they undergo some
development in the thymus
•
NKT cells express TCRs that are extremely limited in repertoire and are
predominantly specific for lipids, glycolipids, and a few specialized types of
peptides
•
Their TCRs have an unusual restriction pattern. Although they may be either
CD4+ or CD4+CD8+, they specifically recognize epitopes presented by a
“nonclassical” MHC class I molecule called CD1
Terminal deoxynucleotidyl transferase (TdT) is an enzyme in thymic stem cells. It decreases in stage II and is lost altogether in the medulla. Several surface
glycoproteins appear during differentiation. CD1 is present on stage II cortical thymocytes and is lost in the medulla. CD2 and CD7 (the pan-T marker) appear
very early in differentiation and are maintained through to the mature T cell stage. CD5 appears at an early stage and persists on mature T cells. CD3 is
expressed first in the cytoplasm in stage I cells (cyto), and then on the surface simultaneously with the T cell receptor (TCR). In most stage II cells, both surface
CD3 and the αβ TCR are expressed at low density, but these markers are present at high density on stage III cells. CD4 and CD8 are co-expressed on stage II
cells (double positives). One of these molecules is lost during differentiation into mature stage III cells (single positives).
B - Lymphocytes
• They originate from HSC & mature in the BM
• They express a unique antigen binding receptor on its membrane
therefore , B - cell receptor is a membrane - bound antibody
molecule
• When a naïve B - cell encounters the antigen for the first time ,
this binding leads to rapid division of naïve B - cell & their
progeny will differentiate into :
1. Effector B - cell ( PLASMA CELLS ) which produce secreted
Abs { Effector molecules of HUMORAL IMMUNITY }
2. Memory B - cells { They have a longer life span than naïve cells &
continue to express same membrane - bound antibody }
B - Cell Surface Markers
A. Immunoglobulin ( Chiefly IgM )
1. It is the primary surface marker on the B - cell membrane
2. Surface IgM (sIgM) functions as antigen recognition site
that binds specific epitopes
3. Binding initiates
activation , proliferation
&
differentiation into plasma cells
B. Other markers
1. CD antigens ( CD10, 19, 20, 21, 22 & 23 )
2. Complement components ( C3b & C3D )
3. MHC class I & II molecules
Values of CD molecules
Allow lymphocytes to :
•
Communicate with their environment
( Cell Trafficking , Adhesion & Activation )
•
Isolation from each other that allow them
to
be
identified
using
fluorescent
antibodies as probes ( Flow cytometry )
Development of B - cells
•
In humans, progenitors of immunoglobulin-producing cells are found in
the yolk sac by the third week, in the fetal liver by the eighth week, and in
the bone marrow by approximately the twelfth week of gestation.
•
B - cells are defined as cells that synthesize immunoglobulin (
antibodies ) and display it on their cell surfaces as B - cell receptors
BCRs.
•
These cells are called bone marrow-derived lymphocytes or B - cells
• Pro - B lymphocyte represents the early stage of B - cell development in
which V , D & J gene segments combine to form an immunoglobulin μ
heavy chain
• Maturation to pre - B cell starts with cytoplasmic expression of a
surrogate light chain ( early stage ) followed by surface expression of
both μ and surrogate light chain
• Immature B - cells starts to express IgMκ or IgMλ on their surfaces
• Mature B - cells express both IgM & IgD on their cell surfaces
• Plasma cells (AFCs ) represent the end stage of differentiation of B - cell
lineage
•
B cell development reflects the stages of immunoglobulin heavy and light chain rearrangement and surface
expression
•
Arising from a common lymphoid progenitor (CLP), the earliest identifiable cell committed to the B cell lineage is
the pre-pro-B cell (Fraction A), within which the cell begins to express Igα and Igβ BCR accessory molecules
•
Immunoglobulin DJ gene joining and cytoplasmic expression of surrogate light chain (SLC) occurs at the early
pro-B cell (Fraction B) stage followed by VDJ gene joining and cytoplasmic SLC expression at the late pro-B cell
(Fraction C) stage
•
The early pre-B cell (Fraction C-prime or C') stage is characterized by the surface expression of pseudo-IgM
(rearranged µ heavy chains plus SLC) and is accompanied by a burst of cellular proliferation
•
In the late pre-B cell (Fraction D) stage, immunoglobulin light chain kappa (κ) or lambda (λ) genes rearrange, and
their products (κ or λ light chains) replace the SLCs
•
Immature B cells (Fraction E) express µ heavy chains plus κ or λ light chains on their cell surfaces
•
Mature B cells (Fraction F) coexpress IgM and IgD on their cell surfaces
•
B cell progenitors, like thymocytes, express molecules and receptors necessary for migration and interaction with
other cells
•
Some attributes (e.g., DNA recombinase expression) are lost by the time cells reach the immature B cell (Fraction E)
stage. If the IgM on the developing cells should bind to epitopes they encounter in the bone marrow, such cells
undergo apoptotic death to prevent production of autoreactive B cells
Generation , Maturation &
Differentiation of B - cells
1.
Generation of mature, immunocopmpetent B - cells ( Maturation )
2.
Activation of mature B - cells
3.
Differentiation of activated B - cells into plasma cells & memory cells
• Generation of B - cells occurs in BM in absence of Ag
( Antigen - independent phase of B - cell development )
• B - cell activation & differentiation is Ag dependent
( Antigen - dependent phase of B - cell development )
1
Antigen
independent
Bone
Marrow
s
2
Antigen
dependent
Peripheral
Lymphoid
Organs
Are you tired like this ?
Have a rest !!!!!!!
T - cell receptors ( TCR )
•
T - cell receptors are located on the surface of T cells & play a critical role in the adaptive immune
response
•
T - cells recognize fragments of degraded proteins (
peptides ) that should be bound to antigen - presenting
molecules
called
MAJOR
COMPLEX ( MHC ) molecules
HISTOCOMPATABILITY
TCR are of two varieties termed αβ & γδ
Both molecules :

Resemble immunoglobulins & made up of heterodimers
( i.e. Either α & β or γ & δ subunits )

Integral membrane proteins with large extracellular domains (
Responsible for antigen -
recognition ) and short
tails ( Responsible signal transduction )
cytoplasmic
T - cell Receptor Complex :
TCR - CD3
•
The αβ & γδ heterodimers must associate with
a series of polypeptide chains termed CD3
complex for antigen - binding domains of TCR
to form a functional receptor stably expressed
at the cell surface capable of transmitting a
signal upon binding to the antigen
•
The TCR - CD3 complex is formed of four
dimers :
1. The αβ or γδ TCR heterodimer
AND
2. The CD3 dimers ( γε, δε & ζζ or ζη ) required
for signal transduction
B - cell Receptor ( BCR )

How membrane immunoglobulin
( mIg ) mediates an activating signal
after contact with antigen ? AS IT HAS
A SHORT CYTOPLASMIC TAIL ?

The answer is : mIg does not constitute
the entire antigen - binding receptor on B cells

B cell receptor ( BCR ) is a
transmembrane protein complex composed
of mIg and disulfide - linked heterodimers
called Ig-α/Ig-β

The long cytoplasmic tails of both Igα/Ig-β enable them to interact with
intracellular signaling molecules
ITAMs : Immunoreceptor Tyrosine
- Based Activating Motifs
Major subpopulation of T - cells

About 90 - 95 % of T - cell are αβ which are divided into two distinct
populations
1. One carries CD4 marker ( CD4+ T helper - cells ) & mainly help or induce
the immune response & recognize antigens with MHC class II
molecules . CD4 TH - cells can be subdivided into two subsets depending
on cytokines profiles they produce :
CD4 TH1 - cells : Secrete IL-2 & IFN-γ
CD4 TH2 - cells : Secrete IL-4, IL-5, IL-6 & IL-10
2. The other set carries CD8 marker (CD8+ T cytotoxic - cells ) & mainly
cytotoxic that recognize antigens with MHC class I molecules

A small percentage of αβ T - cells neither express CD4 nor CD8
molecules ( DOUBLE NEGATIVE T - cells ) that may have a regulatory
function
( Regulatory T - cells & AUTOIMMUNE DISEASES )

In comparison , most circulating γδ are DOBLE NEGATIVE & most
γδ T - cells in tissues express CD8 marker
B - cell subsets

There are 2 subsets of B cells : B - 1 B & B - 2 B cells ( Major
group of B cells in Human & Mice ).

B - 1 B cells appear during fetal life , express IgM with little or no
IgD & display CD5.

B - 1 B cells are minor population
especially in secondary
lymphoid organs but it is the major population in peritoneum.

Up till now , there is no definite function for B - 1 B cells but they
may respond poorly to protein antigens but much better to CHO
Ags.

Mostly are IgM bearing cells with less class switching than B - 2
B cells.
Lymphoid Organ
1. Thymus
2. Spleen
Anatomical Site
Cell Type
Cortex
Densely
packed
with
immature T - cells (
Thymocytes )
Medulla
Sparsely populated
thymocytes
White pulp
with
T - lymphocytes
Marginal zone B - lymphocytes
3. Lymph
Node
Cortex
B - lymphocytes
Paracortex
T - lymphocytes
Medulla
Plasma cells that are actively
secreting ABs
T dependent ( TD ) & T
independent ( TI ) Antigens
 Naïve mature B - cells are free to leave BM & migrate to the periphery
 If they do not encounter Ag, they die within few weeks by apoptosis
 If however , encounter the specific Ag , they undergo activation,
proliferation & differentiation leading to generation of plasma cells ( AFCs ) &
memory B - cells ( Bm )
 Immune response to most Ags depends on T & B cells recognizing Ags
in a linked fashion which is called T- dependent ( TD ) antigens . However , a
small number of Ags can activate B cells without MHC class II restricted T cell help & they are referred as T- independent ( TI ) antigens that include [
Lipopolysaccharides (LPS) , Ficoll & Dextran ] and characterized by induction
of poor memory response
Direct interaction between B & T cells

Antigen-specific T - cells can be obtained by growing T - cells with
antigens , APCs and IL-2 which leads to :
1.
Direct visualization of B - cells & T - cell clusters interacting in vitro
2.
Polarization of T - cells
3.
Polarization of B - cells with expression of MHC class II

This interaction leads to important events in B - cells :
1.
Induction of proliferation & differentiation into antibody - forming cells
( AFCs ) & memory cells
2.
Processing of TD antigen & presentation to T - cells

Interaction between B & T cells is a two way process where :
1.
B - cells present antigens to T - cells
2.
Receive signals from T cells for division & differentiation
Antigen is presented to naive T cells by APCs
such as dendritic cells. B cells also take up
antigen and present it to the T cells, receiving
signals from the T cells to divide and
differentiate into antibody-forming cells (AFCs)
and memory B cells (BM).
Cytokine secretion from T - cells controls B
- cell proliferation & differentiation
Following B - cell T - cell interaction CD4+ T cells
can
be
divided
into
different
subsets
depending on their cytokine profile :

TH1 cells produce IL-2 & IFN-γ

TH2 cells produce IL-4 , IL-5 , IL-10 & IL-13 (
Very efficient helper cells
in production of
IgG1 & IgE

TH0 produce cytokines of TH1 & TH2
IFN-γ & IL-4
B cell development is influenced by cytokines from T cells and
APCs, and by direct interactions with TH2 cells. IL-4 is most
important in promoting division, and a variety of cytokines
including IL-4, IL-5, IL-6, IL-10, and IFN-γ influence
development into antibody-forming cells (AFCs), and affect the
especially isotype of antibody that will be produced.
End result of B - cell & T - cell interaction
•
The question is :
The answer is :
If this interaction usually leads to activation ?
NO
because this is not the rule ; as there is two opposing results namely
1. ACTIVATION
2. INACTIVATION
nergy ) (Clonal Anergy )
Clonal anergy & other forms of tolerance in the peripheral lymphoid organs are important for silencing
these potentially damaging clones but the molecular details of this process is still not clear.
•
Following activation, antigen-specific B - cells can follow two separate developmental pathways:
1. Proliferation & differentiation into AFCs in the LNs or PALS of the spleen . These AFCs rapidly clear
antigens and the vast majority of these cells die via apoptosis within two weeks
2. The second pathway , some of these expanded B - cells migrate into nearby follicles to form
germinal centers before differentiating into memory cells
TEXT BOOK FOR
FURTHER READING
KUBY
IMMUNOLOGY
4 th edition
Richard A. Goldsby , Thomas J. Kindt
and
Barbara A. Osborne
ISBN
0 - 7167 - 3331 - 5
THANK YOU
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