Download LESSON 2: THE ADAPTIVE IMMUNITY Today we will get to know

Introduction to immunology.
LESSON 2: THE ADAPTIVE IMMUNITY
Today we will get to know:
• The adaptive immunity
• T- and B-cells
• Antigens and their recognition
• How T-cells work
1
Introduction to immunology. Lesson 2
The adaptive immunity
Unlike the innate immunity, that is fast and inborn but non-specific, the adaptive immunity
ensures a specific, though slow, response to an enormous amount of pathogens. The main
features of adaptive immunity are:
Feature
Function
Specificity
Ensures that the response to a microbe (or a non microbial
antigen) is specific for that microbe (or antigen) only
Diversity
Makes the immune system able to respond to a great
variety of antigens
Memory
Ensures that the immune system is able to respond to
repeated infections by the same microbe
Clonal expansion
Increases the number of lymphocytes specific for that
particular microbe/antigen
Specialization
Generates optimal responses against a specific microbe
Homeostasis
Ensures that the immune system will be able to get back
to its normal functions and respond to a new challenge in
a short time
Self tolerance
Prevents damage of the host during infections
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Introduction to immunology. Lesson 2
The adaptive immunity – cells
The cellular components of the adaptive immunity are the T and B lymphocytes.
• B lymphocytes (or B-cells) are the only cells of our body able to produce antibodies. They
differentiate into either plasma cells (antibody-producing cells) or memory cells (cells
which are “stored” to ensure a fast antibody response against the same pathogen). They
are involved in humoral immunity.
• T lymphocytes (or T-cells) are involved in cellular immunity because they do not produce
antibodies. Conversely, they are able to recognize antigens and either attack the cells
expressing them or stimulate/suppress the immune response against the antigen. Hence,
they show a very wide differentiation ability, including:
• T helper (Th) cells. Upon antigen recognition , they will produce soluble mediators of
immunity (the cytokines) which will regulate the strength and duration of the
response.
• Cytotoxic T-cells (CTLs). Upon antigen recognition, they will kill the cells expressing
the antigens.
• Regulatory T cells (Treg). They will play a role in suppressing the immune response,
ensuring that no damage to the host occurs. Their excess/deficit also play a role in
many pathologies.
• Natural killer and natural killer T-cells (NK and NKT). Subpopulations of lymphocytes
which directly kill the cells expressing the antigens (similar to CTLs). But, they show a
very poor diversity and they are generally considered as innate immunity effectors.
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Introduction to immunology. Lesson 2
The adaptive immunity – cells
Humoral immunity
Wide
variety
of
effects mediated by
cytokines
Cellular immunity
Active regulation of
the
immune
response
Cellular immunity
Abbas et al.
4
Introduction to immunology. Lesson 2
The adaptive immunity – time-course
Note that T- and Bcells which have
never
seen
an
antigen are called
naïve. They will need
time
to
activate
mechanisms
which
will allow them to
respond
to
the
stimulus.
T- and B-cells will
eventually produce
memory cells
Abbas et al.
5
Introduction to immunology. Lesson 2
The adaptive immunity – primary and secondary reactions
The advantage of memory cells is that they
live for a long time and that they are already
“armed” against the same pathogen. So, a
second challenge with the same pathogen
will lead to faster and stronger specific
responses. Conversely, innate immunity cells
cannot turn into memory cells and need to
reactivate at every challenge.
Note that the example drawn here is for Bcells, but the same applies to T-cells.
Abbas et al.
6
Introduction to immunology. Lesson 2
The adaptive immunity – lymphoid organs and lymphocytes routes
Naïve
lymphocytes
will need to travel to
secondary lymphoid
organs to meet the
antigens and activate
Abbas et al.
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Introduction to immunology. Lesson 2
Adaptive immunity – B-cells
B-lymphocytes (B-cells) are the only cells of the body which produce antibodies. These are
soluble molecules (belonging to different classes) also known as gamma globulins or
immunoglobulins. Antibodies are very efficient players of humoral immunity.
B-cells have characteristic big nuclei, with large endoplasmic reticulum
and numerous ribosomes. These features reflect their active
transcriptional state (they produce large amounts of antibodies). They are
infrequently found in the blood (where they generally circulate as naïve B
cells), as they mainly localize in the bone marrow where they can survive
for many years.
Antibodies are produced as both transmembrane and soluble forms. In
the transmembrane form, they are the antigen receptors of B-cells (called
B-Cell Receptor, BCR). In the soluble form, they act as humoral immunity
effector molecules which bind to microbes and toxins.
B-cells are sub-grouped in B-1 and B-2 cells
Abbas et al.
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Introduction to immunology. Lesson 2
Adaptive immunity – overview of the immunoglobulins (Ig)
All Ig have a similar structure, but they
display an enormous variability in the
antigens that they can recognize.
Always present:
• Two identical heavy chains and two
identical light chains, which form a
symmetric structure.
• The variable regions of heavy and
light chains (VH and VL) participate to
antigen recognition, forming the
antigen-binding site.
• The constant regions of heavy chains
are responsible for the functions of
the antibodies. I.e., they can activate
the complement system, or be
recognized by phagocytes.
Abbas et al.
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Introduction to immunology. Lesson 2
Adaptive immunity – maturation of B-cells
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Introduction to immunology. Lesson 2
Adaptive immunity – maturation of B-cells
B-1 cells originate from fetal hematopoietic stem cells localized in the liver. They are a
subpopulation of total B-cells which produce natural antibodies.
B-2 cells originate from adult hematopoietic stem cells localized in the bone marrow. They are
the majority of total B-cells and produce specific antibodies. During their differentiation, they
transit through a short intermediate stage, called transitional B-2 cells, and then fully
differentiate into either follicular and marginal-zone B-2 cells.
Follicular B-2 cells are the
majority, circulating in
between the blood, the
lymphatic organs and the
bone
marrow.
They
produce all antibodies
isoforms.
Marginal-zone B-2 cells
are a minority, mostly
present in the spleen
and lymph nodes. They
produce antibodies with
poor diversity and also
natural antibodies
(similarly to B-1 cells)
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Introduction to immunology. Lesson 2
The adaptive immunity – T-cell maturation and selection
The name “T-cells” was originally used to identify those lymphocytes that need the thymus to
mature.
1
3
4
2
Abbas et al.
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Introduction to immunology. Lesson 2
The adaptive immunity – T-cell maturation and selection
In the thymus, many important processes take place. These will ensure that T-cells have highaffinity receptors (able to recognize antigens in a strong way) and that they’re not
autoreactive (they will discriminate self cells).
1
Immature T-cells coming from the bone marrow (pro-T) need proliferative signals to survive in
the thymus. These are provided by specialized cells in the cortical area of the thymus, but only
if pro-T are able to bind to them with their receptor. If they don’t, they will die by nutrient
deprivation (death by neglect).
2
Pro-T-cells which manage to survive in the cortical area will be tested for their ability to
discriminate normal body cells (self cells, represented by thymus epithelial cells). If T-cells bind
too tightly to those cells (meaning that they are autoreactive clones, which would attack
normal body cells), they are induced to die by apoptosis. If they survive this negative
selection, they move towards the medullar area and become pre-T-cells.
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Introduction to immunology. Lesson 2
The adaptive immunity – T-cell maturation and selection
3
Pre-T which have “good receptors” and are not autoreactive are induced to proliferate and
expand by cells of the thymus. This is the basis of positive selection, which will induce T-cells
to become CD4+ (Th) or CD8+ (CTLs) T-cells.
4
Newly-formed CD4+ and CD8+ T-cells will be thoroughly tested again to ensure that they are
not autoreactive. If they show autoreactive behavior, they will be suppressed and induced to
die by apoptosis by regulatory T-cells of the thymus.
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Introduction to immunology. Lesson 2
The adaptive immunity – molecular basis of T-cell recognition
Macromolecules able to stimulate an immune response are called antigens. T-cells recognize
only small parts of each antigen, called epitopes or determinants. Each macromolecule can
have many epitopes. Antigen recognition by T-cells always require presentation by MHC
molecules.
Antigen
Single
epitope
H
Epitope
2
MHC
T-cell
Epitope
3
Diversity
Y
Epitope-MHC
complex
Epitope
1
Specificity
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Introduction to immunology. Lesson 2
The adaptive immunity – molecular basis of T-cell recognition
Unlike PAMPs of the innate immunity, epitopes recognized by T-cells are millions. Still, they
have some limitations:
•
Must be linear peptides
•
Must be small
•
Must be on a cell, not soluble
•
Must be conjugated to MHC molecules
All of these features depend on the ability of the antigen receptor of T-cells (called the T-cell
receptor, TCR) to recognize an epitope only if conjugated to a “presenter” molecule (the MHC
molecules). This phenomenon is known as MHC restriction, and so all T-cells are MHCrestricted (apart from “strange” T-cells like NK, NKT and gd T-cells)
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Introduction to immunology. Lesson 2
The adaptive immunity – molecular basis of T-cell recognition
T-cell receptor (TCR) only gets activated if it
binds to an antigen together with some specific
parts of MHC molecules. This is why T-cells are
MHC-restricted.
The antigen-binding pocket of MHC molecules is
small and linear, and this is why T-cells epitopes
must be linear and short.
MHC molecules are only expressed on the
surface of cells. This is why T-cells can not
recognize soluble antigens.
Abbas et al.
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Introduction to immunology. Lesson 2
The adaptive immunity – the Major Histocompatibility Complex (MHC)
MHC has been discovered in mice and humans during studies to understand
the genetic basis for transplant rejection. Today we know that those
reactions depend mostly on T-cell activation.
George Snell
Major Histocompatibility Complex genes were identified at different times in
mice and humans, so they have (or had) various names:
•
•
In mice, they are still called H-2 (histocompatibility-2 genes). During the
60´s/70´s, the genes involved in the regulation of the immune response
were called Ir (immune response) genes. Later they were mapped in the
H-2 locus and the term Ir was abandoned.
In humans, they’re either called HLA (human leukocyte antigens) or MHC.
Baruj Benacerraf
MHC molecules are a set of different molecules grouped into class-I, class-II
and class-III subtypes. Only class-I and -II are involved in antigen recognition
by T-cells, while class-III includes genes for complement proteins, HSP and
cytokines.
Jean Dausset
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Introduction to immunology. Lesson 2
The adaptive immunity – HLA locus
Very big locus (E. Coli genome is c.ca
4500 kb).
MHC genes are co-dominantly
expressed. This means that each
person expresses both paternal and
maternal MHC genes at the same
time. This maximizes the number of
MHC available for antigen binding.
MHC are the most polymorphic genes
in our genome. At the moment, 3500
possible HLA isoforms have been
postulated, but they’re likely to be
much more.
The series of all the HLA alleles in
each
chromosome
is
called
haplotype.
Abbas et al.
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Introduction to immunology. Lesson 2
The adaptive immunity – MHC-I
Abbas et al.
Each MHC-I molecule:
• Is made of an a chain (44-47kD)
transcribed from HLA class-I genes
and the b2-microglobulin (12kD).
• Has polymorphic regions in a1 and
a2 domains. Their variability
depends on the class-I allele from
which they derive (HLA-A, HLA-B
and HLA-C).
• The antigen-binding pocket can
accommodate peptides made of
8-11 residues.
• Has an invariant a3 domain which
is recognized by the TCR. Only
CTLs can recognize this region.
• Binds to intracellular antigens.
VIRTUALLY ALL NUCLEATED CELLS OF THE
BODY EXPRESS MHC-I MOLECULES ON THEIR
SURFACE.
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Introduction to immunology. Lesson 2
The adaptive immunity – MHC-II
Abbas et al.
Each MHC-II molecule:
• Is made of an a chain (32-34kD)
and a b chain (29-32kD). Both a
and b chains are transcribed from
HLA class-II genes.
• Has polymorphic regions in a1 and
b1 domains. Their variability
depends on the class-II allele from
which they derive (HLA-DR, HLADQ and HLA-DP).
• The antigen-binding pocket can
accommodate peptides made of
10-30 residues.
• Has an invariant b2 domain which
is recognized by the TCR. Only Th
can recognize this region.
• Binds to extracellular antigens.
ONLY MACROPHAGES, DENDRITIC CELLS, BCELLS AND FEW OTHER EXPRESS MHC-II ON
THEIR SURFACE.
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Introduction to immunology. Lesson 2
The adaptive immunity – MHC-I and MHC-II pathways
Abbas et al.
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Introduction to immunology. Lesson 2
The adaptive immunity – T-Cell Receptor (TCR)
Abbas et al.
The TCR:
• Is made of an a and a b chain (all Th
and CTLs). A small subpopulation of
T-cells has its TCR made by a g and a d
chain.
• Has variable domains (V) in both a
and b chains. Those regions are
involved in antigen recognition and
mutations here provide diversity.
• Va and Vb have three hyper-variable
regions which recognize the antigenMHC complex. These are called
Complementarity-Determining
Regions (CDR). Vb also has another
region (CDR4) which has a role in
pathologies.
• TCR needs other proteins to activate
the T-cell response.
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Introduction to immunology. Lesson 2
The adaptive immunity – TCR and accessory molecules
TCR complex = TCR+CD3+z chains
Accessory molecules
Abbas et al.
Abbas et al.
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Introduction to immunology. Lesson 2
The adaptive immunity – T-cell diversity
Cytotoxic T cells = CD8+ T-cells
TCR
CD8+ T-cells (CTLs) recognize antigens presented by MHC-I
molecules. Since MHC-I is expressed by all nucleated cells
and captures intracellular antigens, this ensures that every
cell infected by viruses and intracellular bacteria, as well
as transformed cells expressing tumor antigens, will be
recognized and killed by CTLs.
CD8
MHC-I
All nucleated cells
CTLs will kill the cells that
express microbial antigens
presented by MHC-I
Abbas et al.
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Introduction to immunology. Lesson 2
The adaptive immunity – T-cell diversity
T helper cells = CD4+ T-cells
TCR
CD4
MHC-II
APC
CD4+ T-cells (Th) recognize antigens presented by MHC-II
molecules. Since MHC-II is expressed only by specific cells
involved in immune reactions (APC) and captures
extracellular antigens, this ensures that microbes in the
extracellular space can be ingested by APC and presented
to Th. This is a fine example of cooperation in between
innate (APCs are mostly DC and macrophages) and
adaptive immunity. Th cells will respond by differentiating
into different Th subpopulations and producing cytokines
which will boost the immune activity of other cells
(phagocytes, CTL, B-cells, etc.)
Th cells will produce
cytokines to control and
regulate the immune
reaction
26
Introduction to immunology. Lesson 2
The adaptive immunity – Antigen-Presenting Cells (APC)
Abbas et al.
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Introduction to immunology. Lesson 2
The adaptive immunity – Antigen-Presenting Cells (APC)
Regardless of the different routes that
antigens can take, they will always end
up being captured by APC, which will
present them to CTL and Th cells and
ensure an optimal adaptive response.
Abbas et al.
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Introduction to immunology. Lesson 2
The adaptive immunity – The role of co-stimulatory molecules
Co-stimulatory molecules (especially the
family of B7-1/B7-2) are essential to activate
Th cells (and also to strongly boost CTLs
activity). In the absence of co-stimulation, Tcells become anergic.
Anergic T-cells do not clonally expand after
antigen recognition. Rather, they undergo
apoptosis and die (clonal anergy). Presenting
self antigens to developing T-cells without costimulatory molecules is the strategy used in
the thymus to delete autoreactive T-cell
clones.
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Introduction to immunology. Lesson 2
The adaptive immunity – T-cell expansion and differentiation
Clonal expansion ensures
the generation of a high
number of effector Tcells from a small
number of naïve T-cells
activated by an antigen.
CD8+
T-cells
expand
approx. 50000 times
CD4+
T-cells
expand
approx. 10000 times
Abbas et al.
30
Introduction to immunology. Lesson 2
The adaptive immunity – Th differentiation
While CTL are directly part of cellular immunity, CD4+ cells (Th) respond to antigenic
stimuli releasing a wide variety of cytokines. These are paracrine mediators of
immunity, which stimulate and empower the functions of surrounding immune cells.
Upon activation and clonal expansion, CD4+ T-cells undergo a crucial step of
differentiation (sometimes called polarization) which will tailor their cytokine
production towards three major “combinations”, which will have different effects.
Actually, the situation is more
complicated, as different subtypes
can express a mix of cytokines from
different
subpopulations.
Th
subpopulations can sometimes
interconvert!
Naïve T-cell
Th1-cell
Th2-cell
Th17-cell
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Introduction to immunology. Lesson 2
The adaptive immunity – Th differentiation
Abbas et al.
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Introduction to immunology. Lesson 2
The adaptive immunity – Th1
Th1 differentiation is induced by viruses, intracellular bacteria (L. monocytogenes and
Mycobacteria, i.e.), parasites (Leishmania) and some antigens administered with
strong adjuvants. It is specific against intracellular microbes.
Intracellular microbes infect DC and macrophages
Infected APC start producing IL-12, IFN-g and other
IL-12 and IFN-g induce Th1 differentiation
Th1 cells produce mainly IFN-g, which
inhibits the growth of intracellular
microbes
IFN-g inhibits naïve T-cell differentiation
towards Th2 and Th17
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Introduction to immunology. Lesson 2
The adaptive immunity – Th2
Th2 differentiation is induced by helminthes parasites, allergens and some weak
antigens that do not stimulate Th1 and Th17 differentiation. It is specific against
worms and some persistent bacterial infections.
Granulocytes and mast cells recognize helminthes
Activated granulocytes and mast cells produce IL-4
IL-4 induces Th2 differentiation
Th2 cells produce mainly IL-4, IL-5 and
IL-13 which coordinate mucosal
defenses active against worms
IL-4 inhibits naïve T-cell differentiation
towards Th1 and Th17
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Introduction to immunology. Lesson 2
The adaptive immunity – Th17
Th17 differentiation is induced by inflammatory cytokines produced by APCs in
response to bacteria and fungi. It is not as specific as Th1 and Th2, but has a wider
range of action and is stronger.
APC recognize bacteria and fungi in the extracellular space
Activated APC produce IL-1, IL-6, IL-23, TGF-b and other
All of these cytokines together induce Th17 differentiation
Th17 cells produce mainly IL-17 and IL22, which strongly stimulate
inflammation in the mucosae
IL-23 and TGF-b (mainly) suppress Th1
and Th2 differentiation
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Introduction to immunology. Lesson 2
The adaptive immunity – Memory T-cells (Tmem)
Memory T-cells (Tmem) arise either from effector T-cells or from naïve T-cells. What
drives the differentiation into memory T-cells is largely unknown. Their features are:
•
•
•
•
Their quantity depends on the strength of the antigen stimulus. The stronger the
stimulus, the more Tmem will be generated.
They survive for many years (sometimes for the whole life) after their
differentiation. To compare, effector T-cells live for weeks or months.
They outnumber the quantity of naïve T-cells for the same antigen. Their number
is 10 - 100 times bigger than that of naïve T-cells for the same antigen.
Their responses to the antigen are faster than the responses of naïve T-cells. In
mice, naïve T-cells require 5 – 7 days to respond to an antigen, while Tmem activate
in 1 – 3 days.
Tmem cells constitute a pool which ensures SPECIFIC, RAPID and STRONG responses
to antigens (pathogens) already “met” by the immune system
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Introduction to immunology. Lesson 2
Adaptive immunity switch-off – feedback mechanisms and regulatory T-cells (Treg)
Spontaneous termination of T-cell responses occur with pathogens’ eradication: as
soon as the number of microbes decreases, the number of activated APC will decrease
too. This means that those cytokines which stimulated Th differentiation will decrease,
and that differentiated Th cells will die by apoptosis. Similarly, CTLs die by apoptosis
soon after they destroyed target cells.
Regulatory T-cells (Treg) arise spontaneously in the thymus and in peripheral tissues
from immature CD4+ naïve T-cells. Their primary function is to dominantly suppress
Th1, Th2, Th17, B-cells and innate immunity cells by releasing immunosuppressive
cytokines (like IL-10), by cell-to-cell inhibition and also by cytolysis. They are of a
crucial importance to ensure non-self reactivity and to avoid damages caused by
excessive immune responses in peripheral tissues.
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