microbiology 204 antigen presentation and mhc molecules art weiss october 17, 2014

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 ofinitial antigen capture

Sites of antigencollection and capture

Abbas, Lichtman and Pillai. Cellular and Molecular Immunology, 7th edition, 2011 c Elsevier

and expression ofCCR7, and upregulation of MHC I & II, and coreceptors

Mannose receptors, FcRC-type Lectin receptorsand Toll-like receptors

The Capture and Presentation ofProtein Antigens by Dendritic Cells

Maturation of Dendritic Cells

Class II MHC molecules

Number T ½

106 ~10 hr

7 X 106 >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-3 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


T cell Recognition of Virus is Genetically Controlled



Self MHC Restriction of T cells

The genes that differ between strains A and B and control T cellrecognition were found to map to a locus called the MHC

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 antigen-presenting 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 thePeptide/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 (44-47 kD) (32-34 kD)2-microglobulin (12kD) (29-32 kD)

Polymorphic residues 1 and 2 domains 1 and 1 domains

Coreceptor 3 domain binds CD8 2 domain binds CD4

Source of peptide Cytosol (biosyntheic pathway) Endocytic/lysosomal pathway

Peptides bound 8-11 residues 10-30 residues

NomenclatureHuman HLA-A, -B, or C HLA-DR, -DQ, -DPMouse H-2K, H-2D, H-2L I-A, I-E

Types of APCs All nucleated Cells DCs, Macrophages, B cells

Structure of Class I MHC Molecule



MHC Molecule Structures

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 InflammationEnhancement of Class II MHC Expression by IFN

Similar effectson class I MHC and the activation of CD8 T cells.


How Peptides Bind to MHC Molecules

Polymorphic residues of MHC molecules


Alleles of MHC Molecules are Co-dominantly Expressed

Presentation of Extracellular and Cytosolic Antigens


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

Protein antigen in cytosol (cytoplasm) --> class I MHC pathway Protein antigen in vesicles --> class II MHC pathway

Pathways of antigen processing


The Class I MHC Pathway of Processingof Endogenous Cytosolic Protein Antigens

Cytoplasmic peptides are actively transported into the ER; class I MHC molecules are available to bind peptides in the ER


The Immunoproteasome:Interferon induces new subunit proteases to alter peptides

presentedSpaapen 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 I-displayed 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 ofInternalized 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 PresentationInfluence 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

“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

Cross presentation explains how infections of non-professional APCs canlead 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?

microbiology 204 antigen presentation and mhc molecules art weiss october 17, 2014

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