HLA and antigen presentation

Institute of Immunology University Hospital Motol

Structure and function of HLA

• history• structure of HLA gens and molecules• function of HLA moleculs• nomenclature of HLA system• HLA association with disease• antigen presentation

HLA - MHC• HLA molecules are responsible for the compatibility of

the tissues of genetically different individuals• combination of HLA molecules is unique in each

individual • monozygotic twins have the same histocompatibility

molecules on their cells• HLA molecules are responsible for the rejection of transplant • HLA molecules present antigens and provoke immune reaction

leading to the transplant rejection and destruction • histocompatibility molecules are called histocompatibility antigens

or transplantation antigens.

HLA - immune system development• Histocompatibility molecules are glycoproteins expressed at

the surface of all vertebrate cells.

• HLA molecules represent the latest stage of the development of the immune system.

structure of HLA molecules

• Two classes of HLA molecules are recognized and designed HLA class I. and HLA class II.

• HLA molecules of both classes are glycoproteins, heterodimers, composed from two chains. At least one of the chains is polymorphic.

• Structure of HLA molecules of both classes enables antigen binding and contact with T cell receptors.

HLA class I. molecules Class I molecules consist of three parts

a transmembrane protein, called the heavy chain. The outermost domains contain two segments of polymorphic alpha helix that form two ridges with a groove between them called 1and 2, 3 is nonpolymorphic domain close to the membrane..

a molecule of beta-2 microglobulin (ß2m), which is attached noncovalently to the heavy chain

peptide bound to the groove formed by1and 2 domaines

structure of HLA class I. molecules

• HLA class I. molecules are composed from heavy chain -2 microglobulin peptide

HLA A,B,C• Humans synthesize three different types of class I. molecules

designated HLA-A, HLA-B, and HLA-C. • These differ only in their heavy chain, all sharing the same type of

beta-2 microglobulin. • HLA class I. molecules are expressed on all nucleated cells of the

body. • HLA class I. molecules are not expressed on red blood cells and

only minor level of expression is detected on the cells of central nervous system.

• The genes encoding the different heavy chains are clustered on chromosome 6 in the major histocompatibility complex.

HLA class II. molecules

• Class II molecules consist of two transmembrane polypeptides: an alpha chain and a beta chain. Both chains are polymorphic.

• The alpha and beta chains are encoded by clusters of loci in the region of chromosome 6 designated HLA-D.

• HLA class II. molecules form 3 types, designed HLA DR, DP and DQ.

• Class II molecules are not as widely expressed in the body as the class I molecules are. However, cells where inflammation is occurring express class II strongly and provide a powerful stimulus to the immune system.

Structure of HLA class II. molecules

• Class II molecules consist of two transmembrane polypeptides and bound peptide.

HLA class I. HLA class II.

Structure of HLA class I. and II. molecules

HLA and peptides

• antigenic peptids in the binding sites of HLA molecules

• class I.

• class II.

•

HLA genes

• HLA class III. are soluble molecules as complement, TNF, HSP • HLA DR chain is not polymorphic

HLA region

D HLA class III. B C E A G F

DP DQ DR

chromozom 6

HLA haplotypes• Haplotype is combination of allelic forms of HLA molecules on

one chromosome.• We inherit 3 types of heavy chains for HLA class I. molecules

from each parent .• Everybody expresses 6 different types of HLA class I. molecules

unless honmozygous status for some of the types was inherited.

A B C A B C

A B C A B C

A

A

B

B

C

C

HLA polymorfism• Genes for HLA are the most polymorhic structures from all known

systems. • The diversity of alleles in the population makes possible

thousands of different combinations. In a study of 1000 blood and organ donors in San Francisco that were typed for HLA-A and HLA-B,

Over half the group had a combination that was unique.

Another 111 donors had a set of these molecules that they shared with only one other person in the group.

The most frequent phenotype (HLA-A1, HLA-A3, HLA-B7, and HLA-B8) was found in 11 donors.

HLA typing

The MHC of humans is now completely sequenced! A consortium of laboratories reports in the 28 October 1999 issue of Nature that they have determined the sequence of 3,673,800 nucleotides on chromosome 6 that encode the genes of the MHC as well as many other genes involved in immune function.

HLA nomenclature

HLA AllelesAssigned July 2012There are currently 8159 HLA alleles described by the HLA nomenclature and included in the IMGT/HLA Database.

HLA nomenclature

HLA and antigen presentation

• Although histocompatibility molecules were discovered because of the crucial role they play in graft rejection, clearly evolution did not give vertebrates these molecules exclusively for that function.

• The main function of HLA molecules is the process called antigen presentation to T lymphocytes.

HLA and antigens • Antigens presented to the immune system come from

different sources. The main division of the antigens recognizes extracellular antigens and intracellular antigens.

• The nature of extracellular and intrcellular antigens differs markedly and both induce different type of immune reaction.

• Intracellular antigens are presented in connection with HLA class I.

• Extracellular antigens are presented in connection with HLA class II.

Antigen presentation• Antigens are molecules able to induce immune response. • Antigens are mostly proteins, glycoproteins or polysaccharides. • Extracellular antigens enter the body from the environment. They

are inhaled or ingested macromolecules or molecules that are introduced beneath the skin .

Intracellular antigens are antigens that are generated within the cells of the body; these would include proteins encoded by the genes of viruses that have infected a cell or aberrant proteins that are encoded by mutant genes such as mutated genes in cancer cells.

Antigen presenting cells and T lymphocytes

• The recognition of antigen by T cells is necessary for induction of the immune response.

• The nature of the outcome of the immune response is directed according to the nature of presented antigen.

– exogenic antigen presentation

HLA class II.antigenTCR

CD4

T lymfocyte

APClysozom

ER, Golgi

Antigen presenting cells and T lymphocytes

• The recognition of antigen by T cells is necessary for induction of the immune response.

• The nature of the outcome of the immune response is directed according to the nature of presented antigen.

– endogenous antigen presentation

HLA class I.antigenTCR

CD8

T lymfocyte

APCER, Golgi

Exogenous antigens• Exogenous antigens (inhaled, ingested, or injected) are taken up

by "professional" antigen-presenting cells

• These include: • phagocytic cells like macrophages and dendritic cells • B lymphocytes which are responsible

for producing antibodies against the antigen.• professional antigen-presenting cells • All these cells expressed HLA class II. molecules

Exogenous antigens and cells

Antigen presenting cells engulf the antigen by endocytosis degrade it into fragments (e.g. break an

antigenic protein into short peptides) display these peptides at the surface of the cell

nestled within a class II. histocompatibility molecule here they may be recognized by CD4+ T cells

Exogenous pathway

Steps of the exogenous antigen presentation: – The two chains of the class II molecule are inserted into the membrane of

the RER.

– They bind (in their groove) one molecule of invariant chain.

– This trimolecular complex is transported through

the Golgi apparatus and into vesicles called lysosomes.

– Invariant chain is actively deleted by HLA DM.

– Invariant chain is replaced by antigen.

– Complex of HLA and antigens moves to the surface of the cell.

– Complex HLA-antigen is recognized by CD4 positive lymfocyte.

CD4+

APC

ER

Golgi

endosome

Endogenous antigensAntigens that are generated within a cell are degraded into fragments (e.g., peptides) within the cell in

proteasome

selected antigens are transported to ER with active action of TAP (transport associated protein)

stable complex of HLA class I. and antigen is formed in ER, moves to Golgi

displayed at the surface

Here they may be recognized by CD8+ T cells.

Most CD8+ T cells are cytotoxic.

They have the machinery to destroy the infected cell (often before it is able to release a fresh crop of viruses to spread the infection).

Endogenous pathway

• HLA antigens class I. are expressed on all nucleated cells

• There are three subunits in each class I histocompatibility molecule:

the transmembrane polypeptide - heavy chain the antigenic peptide beta-2 microglobulin complex on the cell surface meets CD8+ T

lymphocyte

TAP• Transport associated protein - TAP is responsible

for the peptide transport from cytoplasm to ER.• Proteins are degraded to peptide in proteasome.• The peptides are picked up by TAP proteins and

transported from the cytosol into the RER where they assemble with – the transmembrane polypeptide and beta-2 microglobulin.

– this trimolecular complex then moves through the Golgi apparatus and is inserted in the plasma membrane

proteazome

TAP

Antigen presentation pathways

exogenous antigens, HLA class II.endogenous antigens, HLA class I.

B lymfocytes

• B lymphocytes process antigen by the class II pathway. However, antigen processing by B cells differs from that of phagocytic cells like macrophages in crucial ways. CD4

T lymfocyte

B lymfocyte

BCR B cells engulf antigen by receptor-mediated

endocytosis The affinity of B cells for an epitope on an antigen

may be so high that the B cell can process the antigen when it is present in body fluids in concentrations thousands of times smaller than a macrophage would need The remaining steps of antigen processing occur by the

same class II pathway

The B cell grows into a clone of plasma cells under the influence of cytokines produces by CD4+ lymohocyte

The antibodies are secreted into the surroundings

B lymfocyte

Antibodies

antibody production

CD4

T lymfocyte

B lymfocyte

plasma cell

cytokines

CD3

ion channels

signal pathways

protein phosphorylation

protein kinases

APC

T lymfocyte

Signal pathways, cell activation

Summary -antigen presentation pathways

CD4

T lymfocyte

B lymfocyteHLA class I.antigenTCR

CD8

T lymfocyte

APCER, Golgi

HLA class II.antigenTCR

CD4

T lymfocyte

APClysozome

ER, Golgi

HLA class II.antigen

TCR

endogenous exogenous B lymfocytes cell destruction immune response antibody production

Antigen presentation and T lymphocyts

Polarisation of T lymphocytes

Polarisation of T lymphocytes

Presentation of antigen and transplantation

Presentation of antigen and infection

Presentation of antigen and autoimmunity

Presentation of antigen and tumours

Presentation of antigen and vaccines

Presentation of antigen - summary

Presentation of antigen - summary

defective ribosomal products (DRiPs) mature proteins (retirees) ER aminopeptidases (ERAP) Peptides that fail to bind to MHC class I molecules are removed by the translocon SEC61 and enter the cytoplasm

Making sense of mass destruction: quantitating MHC class I antigen presentationJonathan W. Yewdell, Eric Reits & Jacques NeefjesNature Reviews Immunology 3, 952-961 (December 2003)

Presentation of antigen - summary

Prezentace antigenu v obraně organismu

Antigen presenting cell and lymphocyte

Antigen presenting cell and lymphocyte