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T Cells Recognize “Processed” Antigen Via Their T cell - B cell collaboration Antigen Presentation to T cells: Major Histocompatibility Complex (MHC) Antigen recognition strategies of B versus T cells Discovery of MHC: Mouse genetics (inbred strains, congenic strains) T cell functional assays human leukocyte antigens (HLA) Organization of MHC genes •Required for many antibody responses: esp. protein antigens •Requires direct, physical B-T interaction Structure of MHC proteins (class I and II) •Involves multiple cell surface receptors on T and B cells MHC nomenclature (polymorphism, alleles, and haplotypes) •Both B and T cell must recognize antigen (but usually not the same epitope). (teaching evaluations) •Both B and T cells need signal 1 (through antigen receptor) and 1 signal 2 (co-stimulation) 2 During T-B collaboration antigen is bound both by Antigen Recognition by B and T lymphocytes BCR on the B cell, and the TCR on the T cells MHC TCR B Cell MHC Antigen Presenting Cell (B cell, dendritic cells, etc) The BCR binds intact antigen. T Cell APC The TCR binds a fragment of antigen bound to MHC proteins on the surface of the B cell. 3 4 Antigen recognition by B cells vs. T cells T cells recognize “processed” Both form their antigen receptors by V(D)J recombination. antigen via their TCR Ig (alias: antibody, BCR) for B cells, TCR for T cells Ag B cells can bind intact protein antigen in solution. Phagocytosis/ •Antigen presenting cell ingests Ag pinocytosis (Receptor can be cell surface or secreted.) •Ag is degraded into peptides MHC class II •Peptides form complex with MHC T cells bind peptides displayed on the surface of another cell : an class II molecules Digestion •Peptide-MHC complexes transported “antigen presenting cell” (dendritic cell, macrophage, or B cell). into peptides to cell surface (T cell antigen receptor exists only in cell surface form.) APC 5 6 1 T cells recognize “processed” Humoral immune response: antigen via their TCR Immunoglobulins produced by B-cell interact with pathogens and their toxic products in the Phagocytosis/ pinocytosis extracellular spaces of the body. MHC class II T-cell mediated immunity: T Cell Digestion TCR T cells only recognize antigen as a small into peptides peptide fragment bound to an MHC molecule CD4 and displayed at the cell surface. APC 7 8 Inbred strains of mice Major Histocompatibility Complex (MHC) Review: antigen recognition strategies of B versus T cells Discovery of MHC: Mouse genetics (inbred strains, congenic strains) T cell functional assays human leukocyte antigens (HLA) Organization of MHC genes Structure of MHC proteins (class I and II) Originally bred as pets and selected for docility and coat color: MHC nomenclature (polymorphism, alleles, and haplotypes) “fancy mice” Starting around 1910 inbred strains were developed for research Generated by repeated brother-sister matings 9 10 Genetically identical (homozygous) at all loci Progeny of 2 different inbred strains (F1 generation) Skin transplants require MHC identity Parental recipient are heterozygous at all genetic loci Skin graft donor F1 hybrid recipient Inbred strain Inbred strain b/b McDevitt’s and H-2b H-2k Gorer & Snell’s b/b b/k experiments Parent k/k mapping the genetics of transplantation led b/b to the discovery of F1 progeny k/k b/k the MHC (heterozygous at all loci) k/k Parent H-2b/k The MHC locus is called “H-2” in mouse. Different inbred strains of mice differ at their MHC locus (different allelic form of the b/b locus). This is designated by lower case superscripts: H-2b and H-2k b/k b/k for inbred parents, and H-2b/k for F1 progeny. 11 k/k progeny 12 2 cross “Syngenic” mice are genetically identical at all In vitro assay for T cell function F loci (inbred). 1 Inter- breeding “Congenic” mice are genetically identical except •Immunize mice with PPD (extract from heat-killed tuberculosis for a small genomic region bacteria). F2 •F1 hybrid between inbred strains Select for b/b •F2 progeny tested for ability to •Harvest splenic T cells from immunized mice and re-stimulate in at H-2 complex reject skin graft from A mice vitro (recall response) with antigen (PPD) and antigen presenting •Selected progeny backcrossed to cell (APC). backcross strain A mouse •Progeny of backcross tested for ability to reject skin graft from A •Measure T cell response (proliferation: incorporation of mice radioactive thymidine). •Repeat cycle >10 times •Congenic strain containing all loci Interbreed, select, and from strain A except MHC which backcross > 10 genr. is from strain B 13 14 F10 Strain A.B APC must be from same inbred mouse strain as primed T cell T cells responses depend on both the antigen and something expressed by the Zinkernagel & Doherty Macrophage Antigen (Nobel Prize, 1996) (APC) APC. (PPD) APC Proliferation? The strain difference that controls T cell - Primed T cell APC co-operation maps to the MHC locus: (from immunized mouse) the same locus that controls skin graft Macrophage Primed T cell Antigen T cell prolif. none strain 1 no 100 cpm rejection. none strain 1 yes 300 cpm strain 1 strain 1 no 800 cpm strain 1 strain 1 yes 119,000 cpm strain 5 strain 1 no 2000 cpm T cell responses show “MHC restriction” strain 5 strain 1 yes 2200 cpm 15 16 Inbred strains of mice differ in their ability to respond to simple antigens: MHC= Major Histocompatibility Locus Immunize different inbred mouse strains with polypeptides consisting of repeats of the same amino acids (ie poly-alanine). Measure antibody production in serum. • Maps to small region on chromosome 17. • Controls skin graft and tumor rejection. • Controls transplant survival. • Controls T-B collaboration. • Controls immune responses to simple antigens. This differences also map to MHC locus! 17 18 3 Human Leukocyte Antigens (HLA) MHC locus Patients that receive ABO-matched blood transfusions generate antibodies against the donor white blood cells (leukocytes). also controls The antigens recognized by these anti-leukocyte antibodies are called human leukocyte antigens or HLA. transplant You can determine a person’s “HLA” type by testing their survival in white blood cells for reactivity to a panel of anti-leukocyte humans antibodies. HLA are encoded by the human MHC locus!! Anti-HLA antibodies provide a useful tool for determining whether individuals have same or different MHC genes. 19 20 MHC: a complex loci encoding several genes related to antigen presentation. Major Histocompatibility Complex (MHC) Gene structure of the human MHC: HLA Review: antigen recognition strategies of B versus T cells Discovery of MHC: Mouse genetics (inbred strains, congenic strains) T cell functional assays human leukocyte antigens (HLA) Organization of MHC genes Gene structure of the mouse MHC: H-2 Structure of MHC proteins (class I and II) MHC nomenclature (polymorphism, alleles, and haplotypes) 21 22 MHC molecules MHC= Major Histocompatibility Locus • Class I: associates with β2-microglobulin, expressed on all tissues except brain and RBCs. Present antigen to CD8 “killer: T cells. • Also called HLA (human leukocyte antigen) locus in humans • Class II: heterodimers of α and β chains, expressed only on “professional” antigen presenting cells (APCs: B • Also called H-2 in mice cells, macrophage, dendritic cells). Present antigen to CD4 “helper” T cells. • Complex loci containing three classes of genes-- I, II, III • Class III: Generally encode various secreted proteins that have immune functions, including components of the complement system and molecules involved in 23 inflammation. 24 4 Tissue distribution of MHC molecules Major Histocompatibility Complex (MHC) Review: antigen recognition strategies of B versus T cells Discovery of MHC: Mouse genetics (inbred strains, congenic strains) human leukocyte antigens (HLA) T cell functional assays Organization of MHC genes Structure of MHC proteins (class I and II) MHC nomenclature (polymorphism, alleles, and haplotypes) 25 26 MHC class II molecules resemble MHC class I molecules Structure of Class I and Class II in overall 3D structure MHC molecules MHC-II MHC-I 27 28 Genetic polymorphism: many alternative forms of Major Histocompatibility Complex (MHC) the same gene, or alleles are present in a population- - “normal” variants Review: antigen recognition strategies of B versus T cells Discovery of MHC: Mutation: an alteration in the sequence of a gene Mouse genetics (inbred strains, congenic strains) as compared to the sequence of the predominant human leukocyte antigens (HLA) allele in a population (e.g., β-globin mutation in T cell functional assays sickle-cell anemia) Organization of MHC genes Structure of MHC proteins (class I and II) The genetic loci encoding MHC genes are MHC nomenclature (polymorphism, alleles, and haplotypes) extremely polymorphic: many different alleles of each locus are present in the population 29 30 5 haplotype MHC Alleles Show Extensive Polymorphism • The set of genes within a complex locus which are inherited as a group because they are linked on a chromosome # of distinct alleles in population 31 32 Mouse MHC nomenclature The MHC locus in mouse (H-2) encodes 3 MHC-class I proteins (K, D, L) and 2 MHC-class II proteins (I-A and I-E). (NotMe tHhCat individual mouse MHC genes are designated by upper hcapselo lteyttpeers.) and The MHC locus is polymorphic: each inbred strain haas ldleisletisnct versions (allelic forms) of the MHC genes. These are designated by lower case superscripts. (For example, Kb would indicate the “b” H-2 haplotypes of some common mouse strains allelic version of the “K” MHC gene). The MHC haplotype of CBA mice is H-2k, and one of its class II MHC H-2 alleles genes is IAk. The entire locus containing all the MHC genes is usually inherited as The MHC haplotype of C57BL/10 mice is H-2b, and one of its class I a group.
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