BIOLOGY 151 LEC 7Major Histocompatibility Complex (MHC)

Parungao-Balolong 2011

Discovery of the Major Histocompatibility Complex

Transplantation experiments in Mice

Serologic Studies in Humans

Structure and Function of MHC Molecules

Class I and Class II

Expression and Regulation of MHC Molecules

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• MAJOR HISTOCOMPATIBILITY COMPLEX (MHC)

• a region of highly polymorphic gene whose products are expressed on the surfaces of a variety of cells

• discovered in the 1940s via an artificial transplantation experiments

• principal determinants of graft rejection

• THUS: individuals who express the same MHC molecules accept tissue grafts from one another, and, individuals who differ at their MHC loci vigorously rejects such grafts

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INTRO...

MHC: ROLE IN IMMUNE RESPONSE???

1960s: Benacerraf et al

demonstrated that different inbred strains of guinea pigs and mice did or did not produce antibodies in response to immunization with simple polypeptide antigen

this immune responsiveness was an autosomal dominant trait mapped to the MHC region

genes that controlled such immune response = Ir genes or immune response genes

controlled the activation of helper T lymphocytes

necessary for antibody response to protein antigens

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INTRO...

• MHC: ROLE IN IMMUNE RESPONSE???

• 1970s : central role of MHC genes in immune response to protein antigens was explained

• demonstrated that antigen specific T lymphocytes do not recognize antigens in free or soluble form but recognize portions of protein antigens that are non-covalently bound to MHC gene products

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INTRO...

MHC: TYPES OF GENE PRODUCTS

MHC Class I molecules

MHC Class II molecules

any given T cell recognizes foreign antigen bound to only one specific class (I or II)

THUS : MHC molecules are integral components of the ligands that T cell recognize

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INTRO...

IMPORTANCE: specificity of T-lymphocytes for self MHC associated antigens?

1. MHC molecules are membrane - associated and not secreted : T-lymphocytes can recognize foreign antigens only when bound to surfaces of other cells

This limits T-cell activation such that T cells interact most effectively with other cells that bear MHC-associated antigens and not with soluble antigens (i.e antigen presentation)

The recognition of antigen on a cell surface also serves to localize the effector functions of the activated T cell to the anatomic site of antigen presentation

NOTE: In contrast, antibodies can function in the circulation by binding to and neutralizing soluble antigens

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IMPORTANCE

IMPORTANCE: specificity of T-lymphocytes for self MHC associated antigens?

2. the patterns of antigen association with class I or II MHC molecules determine the kinds of T cells that are stimulated by different forms of antigens

peptide fragments derived from extracellular proteins = binds to class II

endogenously synthesized peptides = associates with class I

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IMPORTANCE

IMPORTANCE: specificity of T-lymphocytes for self MHC associated antigens?

3. the immune response to a foreign protein is determined by the presence or absence of MHC molecules that can bind and present fragments of that proteins to T cells

since MHC genes are polymorphic, many different alleles exist within a population and these alleles differ in their ability to bind and present different antigenic determinants of proteins

this is how MHC genes control immune responses to protein antigens

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IMPORTANCE

IMPORTANCE: specificity of T-lymphocytes for self MHC associated antigens?

4. Mature T cells in any individual recognize and respond to foreign antigens but are responsive to self proteins

this antigen recognition is shaped by the selection of foreign antigen-specific T cells from developing lymphocytes based on their recognition of self MHC molecules with or without bound peptide antigens

THUS, a second means by which MHC can influence immune responses to particular antigens is through the role of MHC molecules in shaping the repertoire of mature T cells

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IMPORTANCE

MURINE MHC

George Snell and colleagues

used classical genetic techniques to analyze rejection of transplanted tumors and other tissues (grafts)

examined the outcome of skin grafts between individual animals using inbred strains of laboratory mice

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DISCOVERY!

THE EXPERIMENT

principle: RECALL....

non-polymorphic: some genes are represented by only one normal nucleic acid sequence

variant nucleic acid sequence is an uncommon mutation and may result in a disease state

polymorphic: genes may vary at relatively high frequency among normal individuals in the populations polymorphic

any individual animal can have the same allele at a genetic locus on both chromosome of the pair (homozygous) or two different alleles one on each chromosome (heterozygous)

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DISCOVERY!

THE EXPERIMENT

inbred mouse strains: produced by repetitive matings of siblings (> 20 generations)

every individual animal of a given inbred mouse strain will have identical nucleic acid sequences at all locations on both members of each pair of chromosomes

completely homozygous at every genetic locus

genetically completely identical to every mouse of the same strain = syngeneic

THUS, when a tissue or organ, such as patch of skin, is grafted from one animal to another, two possible outcomes may ensue:

grafted skin survives and functions as normal skin or;

immune system destroys the graft (graft rejection)

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DISCOVERY!

THE EXPERIMENT: genetic basis of graft rejection among inbred mice

1. grafts of skin from one animal to itself (isogeneic or isografts) or grafts between animals of the same inbred strain (syngeneic grafts of syngrafts) are usually NEVER rejected

2. grafts between animals of different inbred strains or between outbred mice (allogeneic grafts or allografts) are almost ALWAYS rejected

Distinguishes the grafts as FOREIGN: the genes responsible for causing a grafted tissue to be perceived as similar to one’s own tissue or as foreign as called histocompatibility genes

differences between foreign and self were attributed to genetic polymorphisms among diffrent histocompatibility alleles

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DISCOVERY!

THE CONGENEIC MOUSE STRAINS EXPERIMENT

differed only by genes responsible for graft rejection (MHC)

NOTE: although several different genes could contribute to rejection, a single genetic region is responsible for most rejection phenomena

this gene encodes a polymorphic blood group antigen called antigen II or histocompatibility-2 (H-2)

JUSTIFICATION

initially, MHC congeneic strains were thought to differ at a single locus

occasional recombination events occurred within the MHC during interbreeding of different strains, suggesting that the MHC actually contained several different genes, each involved in graft rejection

H-2 region is now known to be homologous to genes that determine the fate of grafted tissues in other species (Major Histocompatibility Complex)

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DISCOVERY!

GENETICS OF GRAFT REJECTION

indicated that the products of MHC genes are co-dominantly expressed

alleles on both chromosomes of a pair are expressed

as a consequence, each parent of a genetic cross between two different strains can reject a graft from the offspring by recognizing MHC alleles inherited from the other parent

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DISCOVERY!

SEROLOGIC STUDIES IN HUMANS (Dausett et al)

development of allogeneic blood transfusion and allogeneic organ transplantation in clinical medicine provided ways to detect and define genes that control rejection in humans

OBSERVATIONS: patients who rejected kidneys or had transfusion reactions to WBC often develops circulating antibodies reactive with antigens on the WBC of the blood or organ donor

in the presence of complement, the recipient’s serum would lyse lymphocytes obtained from the donor and also lyse lymphocytes obtained from some but not all third parties (individuals other than the blood or organ donor or the recipient)

this sera which react against cells of allogeneic individuals are called alloantisera or allosera

said to contain alloantibodies whose molecular targets are alloantigens (HLA)

HLA or human leukocyte antigens: products of polymorphic genes that distinguish foreign tissues from self

HLAs = H-2 in mice = MHC

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DISCOVERY!

Larger chain: alpha 1, 2 and 3 domain

alpha 3-domain is an immunoglobulin fold and is the attachment point to the membrane

alpha 1 and 2 domain forms an 8-stranded β-sheet that serves as a platform for peptide binding.

Edges of the peptide binding site are defined by long a -helices, one from a 1 and one from a 2

alpha 3 is paired with β2 microglobulin, which also has a typical Ig fold

β2 microglobulin is essential for stability and peptide binding

CD8 on TC cells binds to the alpha domainParungao-Balolong 2011

Structure and Function

FUNCTION and PRODUCTION

PRODUCTION

The peptides are mainly generated in the cytosol by the proteasome

proteasome degrades intracellular proteins into small peptides that are then released into the cytosol

The peptides have to be translocated from the cytosol into the (ER) to meet the MHC class I molecule, whose peptide-binding site is in the lumen of the ER

FUNCTION

display fragments of proteins from within the cell to T cells

healthy cells will be ignored while cells containing foreign proteins will be attacked by the immune system

Because MHC class I molecules present peptides derived from cytosolic proteins, the pathway of MHC class I presentation is often called the cytosolic or endogenous pathway

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TRANSLOCATION

The peptide translocation from the cytosol into the lumen of the ER is accomplished by the transporter associated with antigen processing (TAP) = TAP 1 and 2

The two subunits form a peptide binding site and two ATP binding sites that face the lumen of the cytosol

TAP binds peptides on the cytoplasmic site and translocates them under ATP consumption into the lumen of the ER

The MHC class I molecule is then in turn loaded with peptides in the lumen of the ER

The peptide-loading process involves several other molecules that form a large multimeric complex consisting of TAP, tapasin, calreticulin, calnexin, and ERP57

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TRANSLOCATIONOnce the peptide is loaded onto the MHC class I molecule, it leaves the ER through the secretory pathway to reach the cell surface

The transport of the MHC class I molecules through the secretory pathway involves several post-translational modifications of the MHC molecule

example: change to the N-glycan regions of the protein, followed by extensive changes to the N-glycans in the Golgi apparatus

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GENES AND ISOTYPES

Very polymorphic

HLA-A (HLA-A)

HLA-B (HLA-B)

HLA-C (HLA-C)

Less polymorphic

HLA-E (HLA-E)

HLA-F (HLA-F)

HLA-G (HLA-G)

HLA-K

HLA-L

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Almost identically sized a and b chains

Each chain is divided into two segments, e.g. alpha 1 and 2; beta 1 and 2

alpha 2 and beta 2 are immunoglobulin domains that pair with each other

alpha 2 and beta 2 are the point of membrane attachment

alpha 1 and beta 1 form the peptide binding domain, conformation quite similar to Class I MHC, except, the ends are open allowing the binding of longer peptides

CD4 on TH cells binds to beta 2 domain

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Structure and Function

FUNCTIONfound only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells (APCs)

The peptides presented by class II molecules are derived from extracellular proteins (not cytosolic as in class I)

MHC class II-dependent pathway of antigen presentation is called the endocytic or exogenous pathway

Loading of class II molecules: extracellular proteins are endocytosed, digested in lysosomes, and bound by the class II MHC molecule prior to the molecule's migration to the plasma membrane

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SYNTHESISresult of dimerization of α and β chains, with the assistance of an invariant chain = a special polypeptide involved in the formation and deliverance of MHC class II protein

The nascent MHC class II protein in the rough ER has its peptide-binding cleft blocked by the invariant chain (Ii; a trimer) to prevent it from binding cellular peptides or peptides from the endogenous pathway

The invariant chain also facilitates MHC class II's export from the ER in a vesicle which fuses with a late endosome containing the endocytosed, degraded proteins

It is then broken down in stages, leaving only a small fragment called CLIP which still blocks the peptide binding cleft

An MHC class II-like structure, HLA-DM, removes CLIP and replaces it with a peptide from the endosome

The stable MHC class-II is then presented on the cell surfaceParungao-Balolong 2011

GENESAlpha Beta

HLA-DM HLA-DMA HLA-DMB

HLA-DO HLA-DOA HLA-DOB

HLA-DP HLA-DPA1 HLA-DPB1

HLA-DQHLA-DQA1, HLA-DQA2

HLA-DQB1, HLA-DQB2

HLA-DR HLA-DRAHLA-DRB1, HLA-DRB3, HLA-DRB4, HLA-DRB5

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Characteristic MHC-I pathway MHC-II pathway

Composition of the stable peptide-MHC complex

Polymorphic chain α and β2 microglobulin, peptide bound to α

chain

Polymorphic chains α and β, peptide binds to both

Types of antigen presenting cells (APC) All nucleated cells

Dendritic cells, mononuclear phagocytes, B lymphocytes, some

endothelial cells, epithelium of thymus

T lymphocytes able to respond

Cytotoxic T lymphocytes (CD8+)

Helper T lymphocytes (CD4+)

Origin of antigenic proteins

cytosolic proteins (mostly synthetized by the cell; may also enter from the

extracellular medium via phagosomes)

Proteins present in endosomes or lysosomes (mostly internalized from

extracellular medium)

Enzymes responsible for peptide generation Cytosolic proteasome Proteases from endosomes and

lysosomes (for instance, cathepsin)

Location of loading the peptide on the MHC molecule Endoplasmic reticulum Specialized vesicular

compartment

Molecules implicated in transporting the peptides and

loading them on the MHC molecules

TAP (transporter associated with antigen processing) DM, invariant chain

IMPORTANT ASPECTS OF THE MHC

• High polymorphism in MHC for a species

• Alleles for MHC genes are co-dominant

• Each MHC gene product is expressed on surface of individual cell

• Each MHC has ONE peptide binding site but each MHC can bind many different peptide one at a time (Peptide binding is “degenerate”)

• NO recombination mechanisms for creating diversity in MHC

• Peptide must bind with individual’s MHC to induce immune response

• MHC molecules are membrane-bound

• Recognition by Ts requires cell-cell contact

• Mature Ts must have TCR that recognizes particular MHC

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T-CELL RECEPTOR (TCR)

• role in immune response

• Surface molecule on Ts

• Recognize Ag presented in MHC context

• Similar to Immunoglobulin

• Two types of TCR

• α β: predominant in lymphoid tissues

• γ δ: enriched at mucosal surfacesParungao-Balolong 2011

IMPORTANT ASPECTS OF TCR

• Each T cell has TCR of only ONE specificity

• Allelic exclusion

• αβ TCR recognizes Ag only in the context of cell-cell interaction and in correct MHC context

• γδ TCR recognizes Ag in MHC-independent manner

• Response to certain viral and bacterial Ag

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GENETIC BASIS FOR RECEPTOR

GENERATION

• Accomplished by recombination of V, D and J gene segments

• TCR β chain genes have V, D, and J

• TCR α chain genes have V and J

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TCR AND CD3• TCR is closely associated

with CD3 complex

• Group of 5 proteins

• Commonly called “invariant” chains of TCR

• Role of CD3 complex

• CD3 necessary for cell surface expression of TCR

• transduces signal after Ag interaction with TCR

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READING ASSIGNMENT

• The IMMUNOLOGICAL SYNAPSE

• T-CELL ACTIVATION

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NICE TO KNOW (from wikipedia

though...)

• MHC and Sexual Selection

• MHC plays a role in the selection of potential mates, via olfaction

• MHC genes make molecules that enable the immune system to recognize invaders; in general, the more diverse the MHC genes of the parents the stronger the immune system of the offspring

• It would be beneficial, therefore, to have evolved systems of recognizing individuals with different MHC genes and preferentially selecting them to breed with

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NICE TO KNOW (from wikipedia

though...)• MHC and Sexual Selection

• Yamazaki et al. (1976) showed this to be the case for male mice, which show a preference for females of different MHC; similar results have been obtained with fish

• Claus Wedekind (1995) determined MHC-dissimilar mate selection tendencies in humans

• group of female college students smelled t-shirts that had been worn by male students for two nights, without deodorant, cologne, or scented soaps

• An overwhelming number of women preferred the odors of men with dissimilar MHCs to their own

• preference was reversed if they were taking oral contraceptives

• Rates of early pregnancy loss are lower in couples with dissimilar MHC genes

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ASSIGNMENT: WORK IN

PAIRS

• Describe the immune response to:

• A bacterial infection in your arm

• A bacterial infection from your intestinal tract

• A viral infection

• A cancerous cell in your body

• Which MHC type is most likely to be involved?

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NEXT MEETING: ANTIGEN

PRESENTATION