Hla

1. HLAHLA Dr Mesfin Hunegnaw,Dr Mesfin Hunegnaw, Consultant dermatologist and venerologist,Consultant dermatologist and venerologist, AAU, Medical faculty, Dept. ofAAU, Medical faculty, Dept. of DermatovenerologyDermatovenerology

2. THE HLA COMPLEX AND ITS PRODUCTSTHE HLA COMPLEX AND ITS PRODUCTS The human major histocompatibility complex (MHC),The human major histocompatibility complex (MHC), commonly called the human leukocyte antigen (HLA)commonly called the human leukocyte antigen (HLA) complex, is a 4-megabase (Mb) region on chromosome 6complex, is a 4-megabase (Mb) region on chromosome 6 (6p21.3) that is densely packed with expressed genes.(6p21.3) that is densely packed with expressed genes. The best known of these genes are the HLA class I and classThe best known of these genes are the HLA class I and class II genes,II genes, whose products are critical for immunologicwhose products are critical for immunologic specificity and transplantation histocompatibility, and theyspecificity and transplantation histocompatibility, and they play a major role in susceptibility to a number ofplay a major role in susceptibility to a number of autoimmune diseases.autoimmune diseases.

3. THE HLATHE HLA Many other genes in the HLA region are also essential to theMany other genes in the HLA region are also essential to the innate and antigen-specific functioning of the immune system.innate and antigen-specific functioning of the immune system. The HLA region shows extensive conservation with the MHCThe HLA region shows extensive conservation with the MHC of other mammals in terms of genomic organization, geneof other mammals in terms of genomic organization, gene sequence, and protein structure and function.sequence, and protein structure and function. The HLA class I genes are located in a 2-Mb stretch of DNAThe HLA class I genes are located in a 2-Mb stretch of DNA at the telomeric end of the HLA region.at the telomeric end of the HLA region.

4. The classic (MHC class Ia) HLA-A, -B, and -C loci, theThe classic (MHC class Ia) HLA-A, -B, and -C loci, the products of which are integral participants in the immuneproducts of which are integral participants in the immune response to intracellular infections, tumors, and allografts, areresponse to intracellular infections, tumors, and allografts, are expressed in all nucleated cells and are highly polymorphic inexpressed in all nucleated cells and are highly polymorphic in the population.the population.

5. Polymorphism refers to a high degree of allelic variationPolymorphism refers to a high degree of allelic variation within a genetic locus that leads to extensive variationwithin a genetic locus that leads to extensive variation between different individuals expressing different alleles.between different individuals expressing different alleles. Over 260 alleles at HLA-A, 500 at HLA-B, and 125 at HLA-COver 260 alleles at HLA-A, 500 at HLA-B, and 125 at HLA-C have been identified in different human populations, makinghave been identified in different human populations, making this the most highly polymorphic segment known within thethis the most highly polymorphic segment known within the human genome.human genome.

6. Although HLA-C is considered a classic class I molecule, itsAlthough HLA-C is considered a classic class I molecule, its degree of polymorphism and level of surface expression aredegree of polymorphism and level of surface expression are significantly lower than those of HLA-A and HLA-B.significantly lower than those of HLA-A and HLA-B. Moreover, unlike HLA-A and -B molecules, which functionMoreover, unlike HLA-A and -B molecules, which function primarily by presenting antigen to CD8+ T cells expressing primarily by presenting antigen to CD8+ T cells expressing T cell receptors,T cell receptors, the primary function of HLA-C moleculesthe primary function of HLA-C molecules appears to be to serve as targets of NK cell recognition.appears to be to serve as targets of NK cell recognition.

7. Each of the alleles at these loci encodes a heavy chain (alsoEach of the alleles at these loci encodes a heavy chain (also called an chain) that associates noncovalently with thecalled an chain) that associates noncovalently with the nonpolymorphic light chain nonpolymorphic light chain 22-microglobulin, encoded on-microglobulin, encoded on chromosome 15.chromosome 15.

8. Two characteristic structural features in particular define theTwo characteristic structural features in particular define the functional properties of class I and class II HLA molecules:functional properties of class I and class II HLA molecules: First is the peptide-binding groove that enables these molecules toFirst is the peptide-binding groove that enables these molecules to form highly stable complexes with a wide array of peptideform highly stable complexes with a wide array of peptide sequences that can be recognized as antigens by T cells,sequences that can be recognized as antigens by T cells, Second is a site for binding either the CD8 (in the case of class I HLASecond is a site for binding either the CD8 (in the case of class I HLA molecules) or the CD4 (in the case of HLA class II) molecules, whichmolecules) or the CD4 (in the case of HLA class II) molecules, which are expressed on mature T lymphocytes.are expressed on mature T lymphocytes.

9. vv In the case of class I molecules, peptide binding provides a display onIn the case of class I molecules, peptide binding provides a display on the cell surface of peptides derived from intracellular proteins and thusthe cell surface of peptides derived from intracellular proteins and thus serve as a readout to CD8+ T cells of the proteins being producedserve as a readout to CD8+ T cells of the proteins being produced within somatic cells.within somatic cells. The polymorphism at the loci encoding these molecules predominantlyThe polymorphism at the loci encoding these molecules predominantly affects the amino acid residues that make up the peptide-binding groove,affects the amino acid residues that make up the peptide-binding groove, further amplifying the array of peptides that can be bound by different HLAfurther amplifying the array of peptides that can be bound by different HLA molecules and generating important functional immune differences and transplantationmolecules and generating important functional immune differences and transplantation incompatibility among different individuals.incompatibility among different individuals.

10. The HLA class III region is a name given to a cluster of genes betweenThe HLA class III region is a name given to a cluster of genes between the class I and class II complexes, which includes genes for the twothe class I and class II complexes, which includes genes for the two closely related cytokinesclosely related cytokines tumor necrosis factor (TNF)- andtumor necrosis factor (TNF)- and lymphotoxin (TNF-); the complement components C2, C4, andlymphotoxin (TNF-); the complement components C2, C4, and Bf; heat shock protein (HSP)-70; and the enzyme 21-hydroxylase.Bf; heat shock protein (HSP)-70; and the enzyme 21-hydroxylase.

11. The class I genes HLA-A, -B, and -C are expressed in allThe class I genes HLA-A, -B, and -C are expressed in all nucleated cells, although generally to a higher degree onnucleated cells, although generally to a higher degree on leukocytes than on nonleukocytes.leukocytes than on nonleukocytes. In contrast, the class II genes show a more restrictedIn contrast, the class II genes show a more restricted distribution: HLA-DR and HLA-DP genes are constitutivelydistribution: HLA-DR and HLA-DP genes are constitutively expressed on most cells of the myeloid cell lineage,expressed on most cells of the myeloid cell lineage,

12. Whereas all three class II gene families (HLA-DR, -DQ, andWhereas all three class II gene families (HLA-DR, -DQ, and -DP) are inducible by certain stimuli provided by-DP) are inducible by certain stimuli provided by inflammatory cytokines such as interferon .inflammatory cytokines such as interferon . Within the lymphoid lineage, expression of these class II genesWithin the lymphoid lineage, expression of these class II genes is constitutive on B cells and inducible on human T cells.is constitutive on B cells and inducible on human T cells. Most endothelial and epithelial cells in the body, including theMost endothelial and epithelial cells in the body, including the vascular endothelium and the intestinal epithelium, are alsovascular endothelium and the intestinal epithelium, are also inducible for class II gene expression.inducible for class II gene expression.

13. Thus, while these somatic tissues normally express onlyThus, while these somatic tissues normally express only class I and not class II genes, during times of localclass I and not class II genes, during times of local inflammation they are recruited by cytokine stimuli toinflammation they are recruited by cytokine stimuli to express class II genes as well, thereby becoming activeexpress class II genes as well, thereby becoming active participants in ongoing immune responses.participants in ongoing immune responses.

14. Class II expression is controlled largely at the transcriptionalClass II expression is controlled largely at the transcriptional level through a conserved set of promoter elements thatlevel through a conserved set of promoter elements that interact with a protein known as CIITA.interact with a protein known as CIITA. Cytokine-mediated induction of CIITA is a principal methodCytokine-mediated induction of CIITA is a principal method by which tissue-specific expression of HLA gene expression isby which tissue-specific expression of HLA gene expression is controlled.controlled. Other HLA genes involved in the immune response, such asOther HLA genes involved in the immune response, such as TAP and LMP, are also susceptible to upregulation by signalsTAP and LMP, are also susceptible to upregulation by signals such as interferon .such as interferon .

15. MHC STRUCTURE AND FUNCTIONMHC STRUCTURE AND FUNCTION Class I and class II molecules display a distinctive structuralClass I and class II molecules display a distinctive structural architecture, which contains specialized functional domainsarchitecture, which contains specialized functional domains responsible for the unique genetic and immunologicresponsible for the unique genetic and immunologic properties of the HLA complex.properties of the HLA complex. The principal known function of both class I and class IIThe principal known function of both class I and class II HLA molecules is toHLA molecules is to bind antigenic peptides in order tobind antigenic peptides in order to present antigen to an appropriate T cell.present antigen to an appropriate T cell.

16. MHC STRUCTUREMHC STRUCTURE The ability of a particular peptide to satisfactorily bind to an individualThe ability of a particular peptide to satisfactorily bind to an individual HLA molecule is a direct function of the molecular fit between theHLA molecule is a direct function of the molecular fit between the amino acid residues on the peptide with respect to the amino acidamino acid residues on the peptide with respect to the amino acid residues of the HLA molecule.residues of the HLA molecule. The bound peptide forms a tertiary structure called the MHC-peptideThe bound peptide forms a tertiary structure called the MHC-peptide complex, which communicates with T lymphocytes through binding tocomplex, which communicates with T lymphocytes through binding to the T cell receptor (TCR) molecule.the T cell receptor (TCR) molecule.

17. The first site of TCR-MHC-peptide interaction in the life of a T cellThe first site of TCR-MHC-peptide interaction in the life of a T cell occurs in the thymus, where self-peptides are presented to developingoccurs in the thymus, where self-peptides are presented to developing thymocytes by MHC molecules expressed on thymic epithelium andthymocytes by MHC molecules expressed on thymic epithelium and hematopoietically derived antigen-presenting cells, which are primarilyhematopoietically derived antigen-presenting cells, which are primarily responsible for positive & negative selection, respectively.responsible for positive & negative selection, respectively. Mature T cells encounter MHC molecules in the periphery both in theMature T cells encounter MHC molecules in the periphery both in the maintenance of tolerance and in the initiation of immune responses.maintenance of tolerance and in the initiation of immune responses.

18. Because most antibody responses and all T cell responses areBecause most antibody responses and all T cell responses are T cell dependent, the MHC-peptide-TCR interaction is theT cell dependent, the MHC-peptide-TCR interaction is the central event in the initiation of most antigen-specific immunecentral event in the initiation of most antigen-specific immune responses,responses, since it is the event that actually confers thesince it is the event that actually confers the specificity.specificity. Thus, the population of MHCT cell complexes expressed inThus, the population of MHCT cell complexes expressed in the thymus shapes the TCR repertoire.the thymus shapes the TCR repertoire.

19. For potentially immunogenetic peptides, the ability of a given peptide toFor potentially immunogenetic peptides, the ability of a given peptide to be generated and bound by an HLA molecule is a primary determinantbe generated and bound by an HLA molecule is a primary determinant of whether or not an immune response to that peptide can be generated,of whether or not an immune response to that peptide can be generated, and the repertoire of peptides that a particular individual's HLAand the repertoire of peptides that a particular individual's HLA molecules can bind exerts a major influence over the specificity of thatmolecules can bind exerts a major influence over the specificity of that individual's immune response.individual's immune response.

20. When a TCR molecule binds to an HLA-peptide complex, itWhen a TCR molecule binds to an HLA-peptide complex, it forms intermolecular contacts with both the antigenic peptideforms intermolecular contacts with both the antigenic peptide and with the HLA molecule itself.and with the HLA molecule itself. The outcome of this recognition event depends on the densityThe outcome of this recognition event depends on the density and duration of the binding interaction, accounting for a dualand duration of the binding interaction, accounting for a dual specificity requirement for activation of the T cell.specificity requirement for activation of the T cell. That is, the TCR must be specific both for the antigenicThat is, the TCR must be specific both for the antigenic peptide and for the HLA molecule.peptide and for the HLA molecule.

21. The polymorphic nature of the presenting molecules, and theThe polymorphic nature of the presenting molecules, and the influence that this exerts on the peptide repertoire of eachinfluence that this exerts on the peptide repertoire of each molecule, results in the phenomenon of MHC restriction ofmolecule, results in the phenomenon of MHC restriction of the T cell specificity for a given peptide.the T cell specificity for a given peptide. The binding of CD8 or CD4 molecules to the class I or classThe binding of CD8 or CD4 molecules to the class I or class II molecule, respectively, also contributes to the interactionII molecule, respectively, also contributes to the interaction b/n T cell & the HLA-peptide complex, by providing for theb/n T cell & the HLA-peptide complex, by providing for the selective activation of appropriate T cell.selective activation of appropriate T cell.

22. The rejection of tissue transplants between twoThe rejection of tissue transplants between two incompatible members of the same species isincompatible members of the same species is caused by an immunologic reaction againstcaused by an immunologic reaction against antigens expressed on the surface of cells;antigens expressed on the surface of cells; histocompatibility antigenshistocompatibility antigens..

23. HLA: Organization and Function:HLA: Organization and Function: In humans, this complex, which is highly polymorphicIn humans, this complex, which is highly polymorphic (having more than one form or allele, of the genes), is(having more than one form or allele, of the genes), is called thecalled the human leukocyte antigen (HLA)human leukocyte antigen (HLA) system.system. HLAs are glycoproteins on the surface of mostHLAs are glycoproteins on the surface of most nucleated cells, and their manifestations on the cellnucleated cells, and their manifestations on the cell surface designate self or non self.surface designate self or non self. HLA molecules are receptors for foreign antigens, whichHLA molecules are receptors for foreign antigens, which they then present to selected T cells.they then present to selected T cells.

24. HLAs are the gene products of polymorphous allelesHLAs are the gene products of polymorphous alleles found on a single major chromosomal locus, the HLAfound on a single major chromosomal locus, the HLA locus on the short arm (p) of chromosome 6.locus on the short arm (p) of chromosome 6. It is very rare for these genes to be separated, but inIt is very rare for these genes to be separated, but in about 1 % of the population, genetic recombinationabout 1 % of the population, genetic recombination (crossover) occurs.(crossover) occurs.

25. Class I genes encode surface antigens manifested onClass I genes encode surface antigens manifested on most nucleated cells and blood platelets; they aremost nucleated cells and blood platelets; they are generally absent from red blood cells.generally absent from red blood cells. Class II genes encode surface antigens (also called BClass II genes encode surface antigens (also called B cell alloantigens or Ia antigens) that are expressed onlycell alloantigens or Ia antigens) that are expressed only on immunocompetent cells: B lymphocytes,on immunocompetent cells: B lymphocytes, macrophages, dendritic cells, and Langerhans cells inmacrophages, dendritic cells, and Langerhans cells in the skin.the skin.

26. Class I molecules present endogenously processedClass I molecules present endogenously processed antigens to the T cell antigen receptor onantigens to the T cell antigen receptor on cytotoxic/suppressor (CD8) T cells.cytotoxic/suppressor (CD8) T cells. Class II molecules present processed antigens to the TClass II molecules present processed antigens to the T cell antigen receptor on helper T cells (CD4).cell antigen receptor on helper T cells (CD4).

27. Class III genes are not involved withClass III genes are not involved with histocompatibility antigens but control somehistocompatibility antigens but control some complement constituents.complement constituents. C2 and C4 are the second and fourth components ofC2 and C4 are the second and fourth components of the classical complement pathway, while factor B (Bf) isthe classical complement pathway, while factor B (Bf) is part of the alternate complement pathway.part of the alternate complement pathway. Tumor necrosis factor (Tumor necrosis factor ( andand ) & the steroid hormone) & the steroid hormone 21-hydroxylase are also encoded by HLA class III21-hydroxylase are also encoded by HLA class III genes.genes.

28. Each person has two antigens (alleles) for each locusEach person has two antigens (alleles) for each locus (A, B, C, D).(A, B, C, D). A haplotype consists of the complete set of A, B, C, andA haplotype consists of the complete set of A, B, C, and D (-DR, -DQ, and -DP) antigens present on oneD (-DR, -DQ, and -DP) antigens present on one chromosome, and each parent has two haplotypes.chromosome, and each parent has two haplotypes. The child inherits (according to Mendelian laws) oneThe child inherits (according to Mendelian laws) one haplotype from each parent.haplotype from each parent.

29. These cell surface glycoproteins direct protein antigensThese cell surface glycoproteins direct protein antigens to T cells, and the type of immune response isto T cells, and the type of immune response is determined by the nucleotide sequence polymorphismdetermined by the nucleotide sequence polymorphism of the variable regions of the MHC genes.of the variable regions of the MHC genes. The polymorphism also correlates with the generationThe polymorphism also correlates with the generation of autoimmune disease; certain class II polymorphismsof autoimmune disease; certain class II polymorphisms are associated with susceptibility to specific diseasesare associated with susceptibility to specific diseases (e.g., insulin-dependent diabetes mellitus, rheumatoid(e.g., insulin-dependent diabetes mellitus, rheumatoid arthritis, and pemphigus vulgaris)arthritis, and pemphigus vulgaris)

30. The Trimolecular ComplexThe Trimolecular Complex It is now generally believed that helper T cells areIt is now generally believed that helper T cells are involved in the onset and maintenance of autoimmuneinvolved in the onset and maintenance of autoimmune disease.disease. Most immune responses begin with the activation ofMost immune responses begin with the activation of CD4+ helper T cells in coordination with MHC moleculesCD4+ helper T cells in coordination with MHC molecules and peptide.and peptide. Therefore, T cells are largely responsible for the controlTherefore, T cells are largely responsible for the control of self-nonself differentiation.of self-nonself differentiation.

31. TrimolecularTrimolecular Although the failure of the immune system to allowAlthough the failure of the immune system to allow certain tissues to be approved as self can lead tocertain tissues to be approved as self can lead to autoimmune disease, some environmental injuryautoimmune disease, some environmental injury (toxin, viral disease, or bacterial pathogen) may(toxin, viral disease, or bacterial pathogen) may precede or trigger the event.precede or trigger the event.

32. Proteolytic enzymes aid in splitting cellular proteins intoProteolytic enzymes aid in splitting cellular proteins into small polypeptides (8 or 9 amino acids that bind tosmall polypeptides (8 or 9 amino acids that bind to MHC class I molecules and 12 to 15 that bind to MHCMHC class I molecules and 12 to 15 that bind to MHC class II molecules), forming an antigenic peptide thatclass II molecules), forming an antigenic peptide that binds to the cleft or groove of the MHC molecule.binds to the cleft or groove of the MHC molecule. During this process, it is not clear whether MHCDuring this process, it is not clear whether MHC molecules are involved in the cleavage events, but themolecules are involved in the cleavage events, but the result is a peptide/MHC class I or II complex thatresult is a peptide/MHC class I or II complex that appears on the surface of an antigen-presenting cell.appears on the surface of an antigen-presenting cell.

33. T lymphocytes recognize antigenic peptides that areT lymphocytes recognize antigenic peptides that are part of a trimolecular complex.part of a trimolecular complex. The other two elements in the triad areThe other two elements in the triad are the MHC molecule, to which the antigen is attached,the MHC molecule, to which the antigen is attached, and the antigen-specificand the antigen-specific T cell receptor (TCR)T cell receptor (TCR).. The presence or absence of costimulatory moleculesThe presence or absence of costimulatory molecules (B7, CD28) on T cells and antigen-presenting cells is(B7, CD28) on T cells and antigen-presenting cells is also critical in determining whether a T cell becomesalso critical in determining whether a T cell becomes activated or remains quiet (tolerance).activated or remains quiet (tolerance).

Hla

Technology

dependent

alternate

complement

amino acid

highly polymorphic

tumor necrosis

nk cell recognition

heat shock