Microbiology 204
Antigen Presentation and MHC Molecules
Art Weiss
October 17, 2014
Functions of T lymphocytes
• Primary defense against intracellular microbes
• Activation of other cells (phagocytes, B cells)
• Killing of infected cells (i.e., CTL)
Key features:
• Mediated through interactions with other cells
– Allows surface molecules, cytokines to act at short range
(enhances specificity)
• Different classes of T lymphocytes are most effective
against different types of microbes
– Phagocytosed (extracellular) microbes: CD4+ helper T
cells stimulate antibody production, macrophages
– Cytoplasmic (endogenous microbes): CD8+ CTLs kill
infected cells and eliminate reservoirs of infection
The Challenge for T Lymphocytes
• Very few lymphocytes in the body are specific for any
one microbe (or antigen)
– Specificity and diversity of antigen receptors: the immune system
recognizes and distinguishes between 107 - 109 antigens
– The frequency of antigen responsive lymphocytes is typically 1 in
105 to 106
• Lymphocytes must be able to locate microbes that enter
anywhere in the body
• Lymphocytes must respond to each microbe in ways that
are best able to eradicate that microbe
Functions of APCs
• Capture antigens and take them to the “correct” place
– To peripheral (secondary) lymphoid organs, through which
naïve lymphocytes constantly recirculate
• Display antigens in a form that can be recognized by
specific lymphocytes
– For T cells: MHC-associated peptides (cytosolic peptides to
class I, vesicular peptides to class II)
– For B cells: native antigens; APCs include macrophages,
follicular dendritic cells in germinal centers
• Provide “second signals” for T cell activation
– Costimulators and cytokines induced by microbes; ensure that
T cells respond best to microbial antigens
APCs are Required to Present Antigenic Peptide
Fragments to T Cells
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
Induction of Immune Responses
Antigens (microbes) enter through interfaces with external
environment
(portals of entry: skin, GI tract, respiratory tract)
Antigens are transported to peripheral lymphoid organs
(lymph nodes, spleen) by professional APCs
(Dendritic Cells) or in soluble form
Lymphocytes that recirculate through the same organs
locate the antigens and are activated
Antigen capture
Sites of antigen entry
Sites of
initial antigen capture
Sites of antigen
collection and capture
c Elsevier
Abbas, Lichtman and Pillai. Cellular and Molecular Immunology, 7th edition, 2011
The Capture and Presentation of
Protein Antigens by Dendritic Cells
Mannose receptors, FcR
C-type Lectin receptors
and Toll-like receptors
and expression of
CCR7, and upregulation of
MHC I & II, and coreceptors
Maturation of Dendritic Cells
Class II MHC molecules
Number
106
7 X 106
T½
~10 hr
>100 hr
Why are Dendritic Cells the Most Efficient
APCs for Initiating Immune Responses?
• Location
– At sites of microbe entry (epithelia)
– Express receptors (CCR1,2, and 5) that recognize
inflammatory chemokines
• Receptors for capturing microbes
– Such as mannose and C-type lectin receptors, Toll-like
receptors (TLRs) and intracellular RNA and DNA sensors
which function as pattern recognition receptors to activate
APCs and have very active endocytic machinery
• Migration to T cell zones of lymphoid organs
– Role of CCR7 (ligands are Mip-3b and SLC)
– Co-localize with naïve T cells
• Maturation during migration
– Increase levels of MHC molecules, induce costimulators (B7
molecules, CD40) and decrease endocytic capacity
– Conversion from cells for antigen capture into cells for
antigen presentation and T cell activation
Dendritic cell subsets
• Immature dendritic cells capture antigens from sites of entry
• Mature (activated) DCs present antigen to and activate naïve T cells
in lymphoid organs
• Distinct DC subsets may stimulate differentiation to different T cell
subsets (Th1, Th2)
• Conventional DC’s (myeloid) can be divided into epithelial (i.e.,
Langerhans in skin) or interstitial/dermal – long processes sample
antigens at epithelial surfaces
• Plasmacytoid DCs, resemble plasma cells, are located in 2o
lymphoid organs and are particularly important for Type I interferon
production in response to viruses
• Exposure to antigen on immature DC may induce tolerance
Functions of Different Antigen Presenting Cells
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
T cell Recognition of Virus is Genetically Controlled
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
Self MHC Restriction of T cells
The genes that differ between strains A and B and control T cell
recognition were found to map to a locus called the MHC
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
What is the Major Histocompatibility
Complex (MHC)?
• A genetic locus discovered on the basis of transplantation
– Different individuals express products of different MHC alleles
and reject grafts from one another
– Human MHC: HLA (human leukocyte antigens)
– Mouse MHC: H2
• The peptide display molecules of the immune system
• Highly polymorphic.
• Different alleles of MHC molecules display distinct but overlapping
sets of peptides
– Determines which protein antigens are recognized in different
individuals
• MHC molecules determine how antigens in different cellular
compartments are recognized by different classes of T cells
MHC-Restricted Antigen Recognition by T Cells
• A T lymphocyte recognizes an antigen on an antigenpresenting cell (APC) that also expresses an MHC allele
that the T cell sees as “self”
– Historically, the first indication that T cells recognize complex of
protein (peptide) antigen bound to MHC molecules
– Antigen receptors of T cells have dual specificities:
1. For peptide antigen (responsible for specificity of immune response),
and
2. For polymorphic residues of self MHC molecules (responsible for MHC
restriction)
– T cells learn self MHC restriction during maturation in the thymus:
only T cells that “see” MHC molecules in the thymus (can only be
self MHC molecules) are selected to survive
Schematic Representation of How a TCR Recognizes the
Peptide/MHC complex (pMHC)
Genes of the MHC Locus
Features of Class I and Class II MHC Molecules
Feature
Class I MHC
Class II MHC
Polypeptide chain
a (44-47 kD)
b2-microglobulin (12kD)
a (32-34 kD)
b (29-32 kD)
Polymorphic residues
a1 and a2 domains
a1 and b1 domains
Coreceptor
a3 domain binds CD8
b2 domain binds CD4
Source of peptide
Cytosol (biosyntheic pathway)
Endocytic/lysosomal
pathway
Peptides bound
8-11 residues
10-30 residues
Nomenclature
Human
Mouse
HLA-A, -B, or C
H-2K, H-2D, H-2L
HLA-DR, -DQ, -DP
I-A, I-E
Types of APCs
All nucleated Cells
DCs, Macrophages,
B cells
Structure of Class I MHC Molecule
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
MHC Molecule Structures
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
Some important properties of MHC molecules
•
MHC molecules are the immune system’s mechanism for displaying peptide
antigens to T lymphocytes:
– Highly polymorphic genes: large number of alleles in the population
– Co-dominantly expressed: each cell has six class I molecules (3 from
each parent) and 10-20 class II molecules (3 from each parent + some
hybrid molecules)
– Class I MHC molecules are expressed on all nucleated cells
– Class II MHC molecules are expressed on few cells types (specialized
APCs, e.g. dendritic cells; B lymphocytes, macrophages)
– Stable expression of MHC molecules on cell surfaces requires the
peptide cargo
– MHC molecules present foreign and self peptides
– Expression of Class II MHC molecules, in particular, is up-regulated by
activation of the innate immune response (IFNs, etc.)
Amount of MHC Molecules is Affected by Inflammation
Enhancement of Class II MHC Expression by IFNg
Similar effects
on class I MHC and
the activation of
CD8 T cells.
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
How Peptides Bind to MHC Molecules
Polymorphic residues of MHC molecules
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
Alleles of MHC Molecules are Co-dominantly Expressed
Presentation of Extracellular and Cytosolic Antigens
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
Antigen Processing
• Conversion of native antigen (globular protein) into
peptides capable of binding to MHC molecules
• Occurs in same cellular compartments as synthesis
and assembly of MHC molecules
– Determines which source of antigen generates
peptides that are displayed by class I or class II
MHC
– Uses cellular organelles that serve basic
“housekeeping” functions in most cells
Pathways of antigen processing
Protein antigen in cytosol (cytoplasm) --> class I MHC pathway
Protein antigen in vesicles --> class II MHC pathway
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
The Class I MHC Pathway of Processing
of Endogenous Cytosolic Protein Antigens
Cytoplasmic peptides are actively transported into the ER;
class I MHC molecules are available to bind peptides in the ER
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
The Immunoproteasome:
Interferon g induces new subunit proteases to alter peptides
presented
Spaapen and Neefjes, Nat. Immunol. 2012
Class I MHC Pathway of Presentation of
Cytosolic Peptide Antigens
• Cytosolic proteins are processed into peptides and presented in
association with class I molecules
• Most cytosolic peptides are derived from endogenous (e.g. viral,
tumor) proteins; some may be from phagocytosed microbes, proteins
enter cytosol
• CD8 binds to class I MHC; therefore, CD8+ T cells recognize class Idisplayed peptides
• CD8+ T cells give rise to cytotoxic T lymphocytes (CTLs) that kill
other cells that harbor infections or are transformed (all nucleated
cells express class I MHC)
• CTLs destroy cells infected with intracellular microbes and eradicate
reservoirs of infection
The Class II MHC Pathway of Processing of
Internalized Vesicular Protein Antigens
Endocytosed proteins are cleaved into peptides in vesicles; class II
MHC molecules are available to bind the peptides in the same vesicles
Class II MHC Pathway of Presentation of
Vesicular Peptide Antigens
• Proteins ingested into endosomes/lysosomes are
processed and presented in association with class II
molecules
• Most vesicular peptides are derived from extracellular
proteins
• CD4 binds to class II MHC; therefore, CD4+ T cells
recognize class II-displayed peptides (only some cell types
express class II MHC)
• CD4+ T cells are helper cells that activate B lymphocytes
and macrophages which have encountered extracellular
microbes
• Antibodies and macrophages attack and destroy
extracellular microbes
How Class I- and Class II-associated Antigen Presentation
Influence the Nature of the Host T Cell Response
Significance of MHC-associated Antigen Presentation
• MHC molecules display foreign and self peptides from the extracellular
and intracellular environment
– T cells survey the body for foreign (microbial) peptides
• Different classes of MHC molecules present cytosolic (endogenous)
and vesicular (ingested) peptides
– Helper T cells and CTLs respond to the microbes that each is best
able to combat
• T cell receptors only recognize MHC-peptide complexes, and MHC
molecules are cell surface proteins
– T cells interact with other cells and not with cell-free antigens
• Only peptides bind to MHC molecules
– T cells recognize only proteins (natural source of peptides)
• Few peptides are presented even from complex proteins
– Immunodominance: few peptides bind to any MHC molecule
Immunodominance of Peptide Epitopes
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
“Determinant Selection” - MHC alleles select best binding peptides
and thereby select which determinants will be immunogenic in an
individual.
The Problem for CD8 T cells
• Viruses and tumors may be present in any nucleated
cells; therefore, the immune system has to be able to
generate CTL responses (class I-restricted) to any
nucleated cell
• Only some APCs, particularly DCs, are able to initiate
the responses of naïve T cells
• How are antigens from virus-infected or neoplastic
non-APC cell types “transferred” to APCs?
Cross-Presentation of Antigens to CD8+ T cells
Abbas & Lichtman. Cellular and Molecular Immunology, 5th ed. W. B. Saunders 2003
Cross presentation explains how infections of non-professional APCs can
lead to the initiation of a CD8 T cell response
Potential Mechanisms for Cross Presentation
Mantegazza, et al., Traffic, 2013
Functions of APCs
• Capture antigens and take them to the “correct”
anatomic site
– Antigens are concentrated in peripheral lymphoid
organs, through which naïve lymphocytes circulate
• Display antigens in a form that can be recognized by
specific lymphocytes
– For T cells: MHC-associated peptides (cytosolic
peptides to class I, vesicular peptides to class II)
– For B cells: native antigens
• Provide “second signals” for T cell activation
APCs and Self Antigens
• Normally, APCs are constantly presenting self
antigens
– MHC molecules do not distinguish self from foreign
• If MHC molecules are bathed in self peptides, how
can they ever be free to present microbial peptides?
– Very few peptides (complexed with MHC) are
enough to activate specific T cells
– Microbes induce “second signals” on APCs
• If self peptides are always being displayed, why do
we not react against our own antigens?