Adaptive Immunology
Overview
W. Robert Fleischmann, Ph.D.
Department of Urologic Surgery
University of Minnesota Medical School
rfleisch@umn.edu
(612) 626-5034
Objectives
• Provide an overview of immunological
principles
• Provide a framework for future lectures
• Introduce immunological terminology
Adaptive Immunity Differs From Innate
Immunity In Important Ways
• Innate Immunity
– Pre-existing defenses that are non-specific
– Pre-existing defenses that do not change
with repeated exposure
• Adaptive Immunity
– Reactive defenses that are specific
– Reactive defenses that have memory and
can give a greater level of 2° response
Cells of the
Adaptive Immune System
• B and T Lymphocytes
• Monocytes/Macrophages/Dendritic
Cells
• Basophils and Mast Cells
• Eosinophils
Lymphoid Organs
• B cells and T cells are produced in the bone
marrow.
• B cells are released from the bone marrow as
mature cells, while T cells must pass through the
thymus to become mature cells.
• Mature B cells and T cells can be in the blood or
resident in the lymph nodes and spleen.
– The spleen filters the blood
– The lymph nodes filter the lymph
• Mature B cells and T cells can travel from one
lymph node to another and to and from the
spleen (trafficking or homing).
There Are Two Arms To Adaptive
Immunity
• Cell-Mediated Immunity
Cytotoxic T cells
• Humoral Immunity
Antibodies
Adaptive Immunity - Theory
• Specific response for each antigen
– Can recognize 107 to 109 different antigenic
sequences
– Only a few T and B lymphocytes recognize any
given antigenic sequence
• Must be induced
– Requires 7-10 days for activation because rare B
and T lymphocytes with identical antigen
recognition sequences must find each other
• Generates memory
– Takes 1-5 days additional days for development of
memory
Adaptive Immunity - Overview
• Antigen is phagocytosed and processed by
professional antigen-presenting cells, such
as macrophages and dendritic cells.
• An epitope of the antigen is bound to an
MHC class II molecule and presented to
the helper T cell.
• The helper T cell produces cytokines and
stimulates the activation of cytotoxic T cells
and B cells.
Functions of the Thymus
• Immature T cells migrate from the bone marrow to
the thymus.
• The T cells mature to become either CD4+ helper T
cells or CD8+ cytotoxic T cells.
• During the maturation process selection occurs.
– Positive selection: T cells must recognize MHC class I or
MHC class II molecules in order to be stimulated to mature
(self-restricted).
– Negative selection: T cells that recognize self-antigens
bound to MHC class I or MHC class II on thymus epithelial
cells are driven to apoptose (tolerant to self-antigens.
• Mature T cells that are self-MHC restricted and
tolerant to self-antigens leave the thymus to settle in
lymph nodes or the spleen.
Functions of the Cells
Involved in Adaptive Immunity
Antigen Presentation
• The first step in responding to an antigen is to
be able to “see” it.
• Macrophages and dendritic cells are
professional antigen-presenting cells.
– Macrophages are found in the tissues and lymph
nodes.
• Monocytes are undifferentiated macrophages that are
found in the blood.
– Dendritic cells are found in the tissues and lymph
nodes.
• Macrophages and dendritic cells produce
important cytokines and lymphokines that
activate T and B cells.
Antigen Presentation
• Antigens are presented as peptides
(epitopes) bound to antigen-presenting
molecules.
• In the mouse, the antigen-presenting
molecules are called major
histocompatibility antigens or MHC.
• In man, the antigen-presenting molecules
are called human leukocyte antigens or
HLA.
What Is an Epitope?
• Epitopes are regions on
an antigen that can be
recognized by an
antibody or by T cell
receptor.
• Epitopes are also called
antigenic determinants.
• In the picture, epitopes
on a whole poliovirus
virion and on the
isolated poliovirus VP1
protein are shown in
white.
Two Types of HLA Molecules Are Used
for Antigen Presentation
• HLA Class I Antigen Presentation
– Antigens that are synthesized within a cell
– Self-antigens or antigens from cell infection
– Recognized by CD8+ cytotoxic T cells
• HLA Class II Antigen Presentation
– Antigens that are products of phagocytosis
are expressed on HLA Class II antigens
– Recognized by CD4+ helper T cells
Antigen Presentation
What Cells Express HLA?
• HLA Class I
– All cells except RBCs
– Lack of expression on RBCs may play a role in
persistence of malarial parasite (Plasmodium)
• HLA Class II
–
–
–
–
Monocytes/macrophages
Dendritic cells
B cells
Epithelial cells of thymus
Class I versus Class II HLA
• Class I (one unique  chain plus common  chain [2microglobulin])
– Expresses epitopes of antigens (8-10 aa) that have been digested
in the endoplasmic reticulum (endogenous antigen); 8-10 aa
bound to 1 and 2 domains
– Recognized by cytotoxic T cells
– Important for killing virus-infected cells and for tumor surveillance
– Mediates transplant rejection
• Class II (two unique chains:  chain plus  chain)
– Expresses epitopes of antigens (12-28 aa) that have been
digested in phagolysosomes (exogenous antigen); 12-28 aa
bound to 1 and 1 domains
– Recognized by helper T cells to trigger adaptive immunity
T Lymphocytes
• T lymphocytes (T cells)
– One subset expresses CD4 on their surface
• CD4 = for something = help
• CD4+ helper T cells play the central role in
adaptive immunity as activators of both cellmediated and humoral immunity
– One subset expresses CD8
• CD8 = ate
• CD8+ cytotoxic T cells are effectors of cellmediated immunity that are HLA-restricted (see
antigen in context of HLA)
Characteristics of T Cells
• All T cells express CD3.
• All T cells express a T cell receptor that
recognizes their cognate antigen epitope.
• T cell receptors are created through DNA
rearrangement; 1010 to 1012 paratopes.
• There are two main subtypes of T cells.
– CD4+ Helper T cell
– CD8+ Cytotoxic T cell
Activation of Helper T Cells
• Signal 1
– Requires T cell receptor recognition of HLA bound
antigen
– Requires CD4+ binding to Class II or CD8+ binding to
Class I
• Signal 2
– B7-1 or B7-2 on the antigen presenting cells binds to T
cell surface protein CD28
• Other co-stimulation molecules
– CD2 binding to leukocyte functional antigen-3 (LFA-3)
– Intercellular adhesion molecule (ICAM) binding to LFA-1
• Cytokine signals
Cytokine Signals That Activate
Helper T Cells
• IL-2 and IL-15 are general activators of
T cells.
• IL-12 and IFN- drive T helper cells to
Th1 helper cell subtype.
• IL-4 drives T helper cells to Th2 helper
cell subtype.
• IL-10 down-regulates Th1 and TGF-
down-regulate Th1 and Th2 subtypes.
T Cell: APC Contacts
Helper T Lymphocytes
• CD4+ Helper T cells mature in the thymus.
• CD4+ Helper T cells are activated by
macrophage-produced or dendritic cellproduced cytokines.
• Differential cytokine production by
macrophages induces different types of
helper T cells.
– Th1 cells are induced by exposure of naïve CD4+
T cells to IL-1 + IL-12.
– Th2 cells are induced by exposure of naïve CD4+
T cells to IL-1.
Th1 Helper T Lymphocytes
• CD4+ Th1 Helper T cells
– Activated by IL-1 + IL-12
– Produce IL-2, IFN-
– Activate CD8+ cytoxic T cells to divide
and differentiate
– Some CD4+ Th1 helper T cells become
memory cells
Th2 Helper T Lymphocytes
• CD4+ Th2 Helper T cells
– Activated by IL-1
– Produce IL-4, IL-5, IL-6, IL-9, IL-10, IL-13
– Activate B cells to divide and differentiate to
become plasma cells that produce antibodies
– Some CD4+ Th2 helper T cells become
memory cells
Regulatory T Lymphocytes
• CD4+ Regulatory T cells (Tregs)
– Produce IL-10 and TGF-
– Down-regulate Th1 and Th2 cells functions
– Some CD4+ Treg cells become memory
cells
Cytotoxic T Lymphocytes
• CD8+ T cells mature in the thymus.
• CD8+ T cells recognize target cells by
antigen bound on MHC class I molecules.
• CD8+ T cells kill target cells by several
different mechanisms.
• Some CD8+ T cells become memory
cells.
Killing by Cytotoxic T Lymphocytes
• Kill by FAS - FAS ligand interaction
– T cells expressing FAS ligand bind to FAS, a protein
on the target cell, inducing caspase activation and
apoptosis
• Kill by secreting toxic agents
– TNF, a cytokine, binds to TNF receptor on the target
cell, inducing caspase activation and apoptosis
– Perforin, a pore-forming protein, related to
complement C9 is inserted into the target cell
membrane causing lysis
– Granzymes, serine esterases that activate apoptosis
in a manner similar to that of caspases, are passed
to target cells
Complement Pores Versus Perforin Pores
B Lymphocytes
• B lymphocytes (B cells)
– Express CD19, CD20, and CD22 on their surface
– Differentiate to plasma cells that produce
antibodies
– Some B cells become memory B cells
B Lymphocytes
• Rearrange their DNA as they mature in bone
marrow
– Each B cell has a unique VDJ rearrangement (idiotype)
– Antibody repertoire permits recognition of “all” antigens
• When activated, divide and differentiate to become
plasma cells
– Undergo class switching from IgM or IgD by
recombination
• Change from IgM or IgD isotype production to IgG, IgE, or IgA
production
– Express increased affinity for antigen
• Undergo somatic cell mutations as they differentiate
• Undergo sloppy recombination during isotype switching
Antibody Structure
Antibody
Rearrangement
Terms Related to Antibodies
• Epitope: part of antigen that binds to antibody
• Paratope: part of antibody that binds antigen
• Idiotype: unique antigen-binding site on each
antibody; antibodies with same antigen
binding sites
• Isotypes: antibodies with different types of Fc
regions
B Cell Proliferation
• Each B cell expresses specific antibody on its surface that
recognizes a specific antigen.
• B cells are triggered to proliferate and differentiate to
plasma cells by exposure to that specific antigen.
• Triggering this proliferation also requires help by exposure
to CD4+ Th2 cytokines.
• Plasma cells then secrete this specific antibody.
• For memory B cells, exposure to antibody alone is
sufficient.
Types of Antibodies
IgD: Membrane-associated Ig that is found on mature but
unactivated B cells
IgM: Pentameric Ig that is found on mature but unactivated
B cells; first secreted Ig made; found in blood
IgG: Monomeric Ig that is most abundant Ig in blood and
most important Ig for memory response
IgE: Monomeric Ig that is associated with mast cells and
basophils; mediator of allergic reactions
IgA: Dimeric Ig that is associated with J chain and
secretory component; most important secretory Ig
Stylized
Structures
Of Different
Antibody
Classes
Summary of Acquired Immunity
M
IL-1
IL-12
IL-1
CD4 Th1
CD4 Th2
IL-2
IL-3
IFN-
IL-4
IL-5
IL-6
IL-9
IL10
IL-13
CD8 T cell
Cytolysis:
FAS/FAS ligand
TNF
Granzyme B
Perforin
B cell
Antibody
Additional Topics
• Most important disorders of the adaptive
immune system
• Hypersensitivities
• Tolerance
B Cell Immunodeficiencies
• Disorders of B cells
– Bruton’s agammaglobulinemia (x-linked
agammaglobulinemia)
– Transient hypogammaglobulinemia of
infancy
– Common variable immunodeficiency
(actually defect in T cell signalling of B cell)
– IgA and IgG subclass deficiencies
T Cell Immunodeficiencies
• Disorders of T cells
– Severe combined immunodeficiency syndrome (SCID)
(common  chain lack of IL-7 signalling causes failure
to mature)
– Purine nucleoside phosphorylase deficiency (causes
severe combined immunodeficiency)
– Adenosine deaminase deficiency (causes severe
combined immunodeficiency)
– MHC class II deficiency
– DiGeorge’s syndrome (congenital thymic aplasia)
– Wiscott-Aldrich Syndrome (x-linked, few platelets,
cytotoxic T cell malfunction)
– Ataxia-telangiectasia (wobbly gait, T cell deficiency)
Deficiencies of Innate Immunity
•
Congenital neutropenia
– Lack of GM-CSF
– Frequent bacterial infections
•
Glucose-6-phosphate dehydrogenase deficiency (G6PD)
– Unable to produce NADPH by pentose phosphate pathway, buildup of reduced
glutathione
– RBC denaturation and hemolysis
•
Chronic granulomatous disease
– Inability to produce hydrogen peroxide and hypochlorous acid
– Inability to kill phagocytosed bacteria
•
Leukocyte adhesion deficiency (LAD)
– Lack of integrin subunit, the common  chain
– Inability to recruit innate immune cells to site of inflammation
– Increased susceptibility to bacterial, fungal, and viral infections.
•
Complement defects
– Increased susceptibility to bacterial infections
– Reduced ability to remove immunocomplexes
•
Chediak-Higashi Syndrome
– Defect in gene LYST, a lysosomal trafficking gene that affects lysosomes and
melanosomes
– Increased susceptibility to bacterial infections.
Hypersensitivities
• Type I Hypersensitivity
– Allergy
– Mediated by IgE bound to mast cells and to
basophils causes degranulation (early
phase)
– Release of histamine
– Synthesis of prostaglandins and
leukotrienes
– Recruitment of Th1 cells and basophils
(late phase)
Hypersensitivities
• Type II Hypersensitivity
– Antibodies against cell surface and/or
extracellular matrix components
– Causes complement activation
– Examples
• hemolytic disease of the newborn (maternal
antibodies to fetal blood group antigens cross the
placenta to destroy the fetal RBCs)
• Myasthenia gravis (antibodies to acetylcholine
receptors cause problems with nerve conduction)
• Goodpasture’s syndrome (antibodies to basement
membranes causes nephritis)
Hypersensitivities
• Type III Hypersensitivity
– Immune complex disease
– Deposition of antigen:antibody complexes
• Kidney, joints, lungs, arteries, skin
• Damage occurs by activation of complement
– Examples
• Post-streptococcal glomerulonephritis
• Autoimmunity such as Systemic Lupus
Erythrematosus
Hypersensitivities
• Type IV Hypersensitivity
– Cell-mediated hypersensitivity caused by activated CD4
cells
– Examples
• Contact hypersensitivity such as reaction to nickel
– Dendritic Langerhans cells react recruiting CD4 cells that
ultimately mediate this hypersensitivity
• Tuberculin reaction
– Macrophages react recruiting CD4 cells (2/3) and CD8 cells (1/3)
that ultimately mediate this hypersensitivity
• Granulomatous hypersensitivity
– Macrophages wall off mycobacterium, undergo changes to
become epitheloid (that may form giant cells), and recruit CD4
cells.
– Crohn’s disease = granulomas containing macrophages and CD4
cells in the ileum and colon
Tolerance
• Central Tolerance
– Self-reactive T cells recognize antigens presented by
thymus epithelial cells and are deleted in the thymus
by negative selection
– Self-reactive B cells are deleted in the bone marrow
• Peripheral Tolerance
– Not all self-antigens can be presented in the thymus
– Thus, tolerance must be established in mature T and
B cells
Possible Mechanisms of Peripheral Tolerance
• Clonal exhaustion
– High levels of self-antigen expression trigger a rapid
proliferative response in responding T cells that cannot
be sustained in the absence of cytokines and results in
apoptosis of the responding T cells (no cytokinemediated activation of T cells)
• Clonal anergy
– Non-professional antigen presenting cells or resting
(non-activated) antigen presenting cells provide only
signal one but not signal two (B7 molecules), so no
proliferation (no cytokine-mediated activation by APCs)
• Regulatory T cells
– CD4+ T cells that also are CD25+ inhibit Th1 and Th2
cells (IL-10- and TGF--mediated inhibition)