Major histocompatibility complex class II polymorphisms in primates

Summary: In the past decade, the major histocompatibility complex (MHC) class II region of several primate species has been investigated extensively. Here we will discuss the similarities and differences found in the MHC class II repertoires of primate species including humans, chimpanzees, rhesus macaques, cotton-top tamarins and common marmosets. Such types of comparisons shed light on the evolutionary stability of MHC class II alleles, lineages and loci as well as on the evolutionary origin and biological significance of haplotype configurations.     

A novel,alternative pathway of apoptosis triggered through class II major histocompatibility complex molecules

Major histocompatibility complex class II (MHC-II) molecules, in addition to their role of presenting antigen to T lymphocytes, can serve as receptors triggering programmed cell death. MHC-II induced cell death affects activated/tumour transformed cells selectively, and it proceeds without the involvement of caspases, the major proteases of classical apoptosis. Caspase-independent programmed cell death can also be triggered, albeit less effectively, via a series of other cell surface molecules. Here, we discuss the major characteristics, physiological significance, and clinical relevance of caspase-independent apoptotic pathways with particular emphasis on the one induced by MHC-II ligation.Abbreviations AIF Apoptosis-inducing factor - mab Monoclonal antibody - MHC Major histocompatibility complex - PKC Protein kinase C Zoltan A. Nagy received his D.V.M. degree from the University of Budapest, and his Ph.D. in immunology from the Hungarian Academy of Sciences, Budapest, Hungary. He is presently Vice President of GPC-Biotech AG, Munich, Germany. His research interests include T cell immunology, autoimmunity, and the biotherapy of cancer. Nuala A. Mooney received her Ph.D. in pathology from the University of London, UK. She is currently a Principal Investigator (Directeur de Recherche) in INSERM Unite 396 Immunogénétique Humaine, Paris, France. Her research interests include signal transduction via MHC molecules and microdomain organization of MHC molecules.     

Brucella abortus inhibits major histocompatibility complex class II expression and antigen processing through interleukin-6 secretion via Toll-like receptor 2

The strategies that allow Brucella abortus to survive inside macrophages for prolonged periods and to avoid the immunological surveillance of major histocompatibility complex class II (MHC-II)-restricted gamma interferon (IFN-gamma)-producing CD4+ T lymphocytes are poorly understood. We report here that infection of THP-1 cells with B. abortus inhibited expression of MHC-II molecules and antigen (Ag) processing. Heat-killed B. abortus (HKBA) also induced both these phenomena, indicating the independence of bacterial viability and involvement of a structural component of the bacterium. Accordingly, outer membrane protein 19 (Omp19), a prototypical B. abortus lipoprotein, inhibited both MHC-II expression and Ag processing to the same extent as HKBA. Moreover, a synthetic lipohexapeptide that mimics the structure of the protein lipid moiety also inhibited MHC-II expression, indicating that any Brucella lipoprotein could down-modulate MHC-II expression and Ag processing. Inhibition of MHC-II expression and Ag processing by either HKBA or lipidated Omp19 (L-Omp19) depended on Toll-like receptor 2 and was mediated by interleukin-6. HKBA or L-Omp19 also inhibited MHC-II expression and Ag processing of human monocytes. In addition, exposure to the synthetic lipohexapeptide inhibited Ag-specific T-cell proliferation and IFN-gamma production of peripheral blood mononuclear cells from Brucella-infected patients. Together, these results indicate that there is a mechanism by which B. abortus may prevent recognition by T cells to evade host immunity and establish a chronic infection.     

Major histocompatibility complex class I-dependent skewing of the natural killer cell Ly49 receptor reportoire

Subsets of mouse natural killer (NK) cells express receptors encoded by the Ly49 gene family that recognize allelic determinants on major histocompatibility complex (MHC) class I molecules. Recognition of self class I molecules typically inhibits NK cell lytic function. The presence of NK cell subsets expressing receptors which are able to discriminate class I alleles raises the possibility that there exist mechanisms to coordinate the NK cell receptor repertoire with the class I molecules of the host. In the present study, we determined the effects of class I gene expression on the frequencies of NK cells expressing three different Ly49 receptors defined by monoclonal antibodies. We show here an MHC-dependent skewing of NK cell subsets expressing multiple Ly49 receptors with specificity for self MHC. The results provide the first evidence that the frequencies of NK cells expressing different Ly49 receptors are determined by the host's MHC molecules. The results also extend previous findings that MHC class I expression influences the cell surface levels of each Ly49 receptor, suggesting an additional mechanism by which MHC molecules may influence the effective specificity of NK cells. Models to account for self tolerance and MHC-controlled repertoire differences are discussed.     

Major histocompatibility complex class I-mediated inhibition of neurite outgrowth from peripheral nerves

Studies of mice deficient in classical major histocompatability complex class I (MHCI) revealed that MHCI plays an important role in neurodevelopment in the central nervous system. We previously studied the effects of recombinant MHCI molecules on wildtype retina explants and observed that MHCI can inhibit retina neurite outgrowth, with self-MHCI molecules having greater inhibitory effect than non-self MHCI molecules. Here, we examined classical MHCI's effects on axon outgrowth from neurons of the peripheral nervous system (PNS). We used the embryonic dorsal root ganglia (DRG) explant model since their neurons express MHCI and because DRG explants have been widely used to assess the effects of molecules on axonal outgrowth from PNS neurons. We observed that picomolar levels of a recombinant self-MHCI molecule, but not non-self MHCI molecules, inhibited axon outgrowth from DRG explants. This differential sensitivity to self- vs. non-self MHCI suggests that early in development, self-MHCI may "educate" PNS neurons to express appropriate MHCI receptors, as occurs during natural killer cell development. Furthermore, we observed that a MHCI tetramer stained embryonic DRG neurons, indicating the expression of classical MHCI receptors. These results suggest that MHCI and MHCI receptors play roles during early stages of PNS development and may provide new targets of therapeutic strategies to promote neuronal outgrowth after PNS injury.     

Major histocompatibility complex class II antigen expression in B and T cell non-Hodgkin''s lymphoma.

An immunohistochemical study of 46 B and T cell non-Hodgkin's lymphomas, using monoclonal antibodies to the products of the major histocompatibility complex (MHC) class II antigen subregions, DP, DQ, and DR, showed that most B and T cell lymphomas express these antigens. Both coordinate and non-coordinate expression of MHC class II antigens was observed, but this did not correlate with immunological phenotype, morphological grade, or proliferation index as determined by flow cytometry.     

Major histocompatibility complex class II association and induction of T cell responses by carbohydrates and glycopeptides

Conclusions In the following study we have investigated whether the inability to generate T cell immunity against carbohydrate antigens is the result of a failure of carbohydrates to form a complex with class II MHC molecules or a deficiency of carbohydratespecific T cells in the immune repertoire. These possibilities were examined by, first, determining the MHC binding capacity of pure carbohydrate molecules and, second, by assessing whether carbohydrate-specific T cells could be induced against a glycosylated T cell peptide epitope which had the capacity to interact with class II MHC restriction elements.In the first set of experiments, a group of synthetic and natural oligosaccharides and glycolipids were tested for their ability to bind IA^d class II molecules. Of the 26 carbohydrates analyzed, none were found to bind significantly to IA^d. Although only a small number of carbohydrate molecules of limited structural heterogeneity was tested for MHC binding, it appears that pure carbohydrates and glycolipids do not have the appropriate structures necessary to bind class II MHC molecules.Since carbohydrates may inherently lack MHC binding activity, we next addressed whether glycopeptides having MHC binding activity could induce T cells specific for the carbohydrate structure. A series of analogs of a well-characterized T cell determinant, the chicken OVA 323–339 peptide, was synthesized with a N-acetyl glucosamine substitution placed within the peptide molecule. Experiments performed with the glycosylated OVA peptides indicated three general types of response. In the first type of response, the addition of GlcNAc-Asn to the OVA peptide completely destroyed MHC binding capacity and, as a consequence, these peptides were nonimmunogenic for T cells in vivo. Peptides substituted at A329, A330, and A332 all showed this pattern. The Ala in position 332 was previously shown to be involved in interacting with the MHC molecule; thus, residues with bulky side chain groups would be expected to interfere with binding [22]. For positions A329 and A330, the lack of MHC binding could not be explained by the bulkiness of the GlcNAc side chain. For these peptides, the sugar molecule may have caused an undefined perturbation in peptide structure which completely ablated MHC binding.In the second category of response, substitution of GlcNAc-Asn appeared to be well tolerated since such analogs bound class II MHC molecules and primed T cells in vivo. This pattern was observed when the N-acetyl glucosamine was placed at an MHC contact site (e.g., at V327) or at a location sufficiently outside the core region where it was less likely to affect MHC binding and interact with the T cell receptor (e.g., at positions Q325 and N335). The cells generated against this set of peptides were highly cross-reactive for the nonglycosylated analog, further emphasizing that T cell recognition was not directed at the carbohydrate residue.A third type of response was observed when GlcNAc-Asn was placed near or within the core region of the peptide in positions A326, H328, and H331. The MHC binding capacity of these peptides was substantially but not completely diminished and the residual amount of binding present after glycosylation was still sufficient for the glycopeptide to be immunogenic in H-2^d mice. Interestingly, T cells raised against these glycopeptides failed to cross-react with its respective Asn-substituted analog, suggesting that the carbohydrate structure on the peptide was an integral part of the antigenic determinant recognized by the T lymphocyte.Collectively, results obtained in the present study indicate that the lack of T cell immunity against carbohydrate molecules is not due to a hole in the T cell repertoire, but is more likely due to the inability of carbohydrates to associate with MHC molecules with sufficient affinity. The failure of carbohydrate molecules to prime T cells may be circumvented by substituting sugar residues at strategic locations within an immunogenic T cell epitope in such a way that MHC binding is not drastically affected and T cell receptor recognition is possible. Indeed, under these conditions, our studies demonstrate that T cells with specificity for the carbohydrate moiety can be generated. It may be of interest to determine the effect of such glycopeptides on the humoral immune response of an otherwise T-independent antigen and to further examine the MHC interaction of natural and synthetic glycopeptides containing sugar residues of higher complexity in conjunction with the fine specificity of T cells induced with such carbohydrate molecules.     

Major histocompatibility complex class II invariant chain expression in non-antigen-presenting cells.

In contrast to the generally accepted belief, the major histocompatibility complex (MHC) class II invariant chain (Ii) is commonly expressed intracellularly in cells that do not present exogenous antigens. Such cells include resting peripheral blood T cells and natural killer (NK) cells. In T cells, the Ii is associated with a 77 000 molecular-weight molecule (p77) that has yet to be identified. This molecule is co-precipitated with the anti-Ii monoclonal antibody (mAb) VCD-1, but not with mAb BU-45. This suggests that in the p77-Ii complex, the extracellular epitope of Ii recognized by BU-45 is hidden, whereas the Ii epitope for VCD-1 remains exposed. In antigen-presenting cells (APCs), p77 association with the Ii was minimal, if detectable. The p77-Ii association in non-professional APCs suggests that the Ii may have another, more general, function other than the one accepted in antigen presentation.     

Sheep lymphocyte antigens (OLA). II. Major histocompatibility complex class II molecules

A panel of monoclonal antibodies have been produced which recognize monomorphic determinants of sheep MHC Class II antigens, including an allogenically derived murine monoclonal antibody specific for the I-E gene product. Immunoprecipitation and SDS-PAGE analyses indicates that these monoclonal antibodies recognize a non-covalently associated glycoprotein complex of molecular weight 30-32 kDa (alpha chain) and 24-26 kDa (beta chain). One and two colour immunofluorescence was used to measure the distribution of these 'Ia-like' antigens on mononuclear cells from various lymphoid organs. They were found almost exclusively on lymphocytes expressing surface immunoglobulin (B lymphocytes) and on a small population of surface immunoglobulin negative cells. Most thymocytes were negative for Class II molecules while thymic epithelial cells were positive. The tissue distribution of Class II molecules was found to be similar to that described in man. Individual monoclonal antibodies displayed no variations in reactivity with the different tissues studied.     

Major histocompatibility complex class I molecules of nonhuman primates

The usefulness of nonhuman primates in immunologically relevant research has until now been limited by difficulties in characterizing the major histocompatibility (MHC) gene products of these species. We have now biochemically characterized the MHC-encoded class I molecules from four different species of nonhuman primates using antibodies directed against human MHC class I structures and one-dimensional isoelectric focusing (1-D IEF). We demonstrated the functional relevancy of this technique of MHC typing by generating virus-specific cytotoxic T cells and assaying their cytotoxic activity against a panel of virus-transformed cells that expressed the same or differing class I structures. Only virus-infected cell lines expressing MHC class I antigens identical to those of the cytotoxic T lymphocyte population were lysed. This simple method of MHC class I typing using 1-D IEF will be useful in immunological research involving nonhuman primates and in nonhuman primate colony management.     

A single T cell receptor bound to major histocompatibility complex class I and class II glycoproteins reveals switchable TCR conformers

Major histocompatibility complex class I (MHCI) and MHCII proteins differ in structure and sequence. To understand how T?cell receptors (TCRs) can use the same set of variable regions to bind both proteins, we have presented a comparison of a single TCR bound to both MHCI and MHCII ligands. The TCR adopts similar orientations on both ligands with TCR amino acids thought to be evolutionarily conserved for MHC interaction occupying similar positions on the MHCI and MHCII helices. However, the TCR antigen-binding loops use different conformations when interacting with each ligand. Most importantly, we observed alternate TCR core conformations. When bound to MHCI, but not MHCII, Vα disengages from the Jα β strand, switching Vα's position relative to Vβ. In several other structures, either Vα or Vβ undergoes this same modification. Thus, both TCR V-domains can switch among alternate conformations, perhaps extending their ability to react with different MHC-peptide ligands.     

Major histocompatibility complex class I-binding peptides are recycled to the cell surface after internalization

Cytotoxic T lymphocytes (CTL) recognize target antigens as short, processed peptides bound to major histocompatibility complex class I (MHC-I) heavy and light chains (β₂-microglobuhn; β₂ m).The heavy chain, which comprise the actual peptide binding α-1 and α-2 domains, can exist at the cell surface in different forms, either free, bound to β₂m or as a ternary complex with β₂m and peptides. MHC-I chains are also known to internalize, and recycle to the cell surface, and this has been suggested to be important in peptide presentation. Whether MHC-I-bound peptides also can recycle is not known. We have investigated this by using both peptide transporter mutant RMA-S cells and EL4 cells loaded with D^b-binding peptides, by two different approaches. First, peptides were covalently linked with galabiose (Galα4Gal) at a position which did not interfere with D^b binding or immunogenicity, and peptide recycling tested with Gal₂-specific monoclonal antibodies. By flow cytometry, a return of Gal₂ epitopes to the cell surface was found, after cellular internalization and cell surface clearance by pronase treatment. This peptide recycling could be discriminated from free fluid-phase uptake and was inhibited by methylamine, chloroquine and low temperature (18°C) but not by leupeptin. Second, specific CTL were reacted with peptide-loaded target cells after complete removal of surface D^b molecules by pronase, and after different times of incubation at 37C to allow reexpression. By this procedure, reappearance of target cell susceptibility was confirmed. The results are in agreement with a model for optimizing peptide presentation by recycling through an intracellular compartment similar to early endosomes in certain antigen-presenting cells.     

T cell receptor recognition of a 'super-bulged' major histocompatibility complex class I-bound peptide

Unusually long major histocompatibility complex (MHC) class I-restricted epitopes are important in immunity, but their 'bulged' conformation represents a potential obstacle to alphabeta T cell receptor (TCR)-MHC class I docking. To elucidate how such recognition is achieved while still preserving MHC restriction, we have determined here the structure of a TCR in complex with HLA-B(*)3508 presenting a peptide 13 amino acids in length. This complex was atypical of TCR-peptide-MHC class I interactions, being dominated at the interface by peptide-mediated interactions. The TCR assumed two distinct orientations, swiveling on top of the centrally bulged, rigid peptide such that only limited contacts were made with MHC class I. Although the TCR-peptide recognition resembled an antibody-antigen interaction, the TCR-MHC class I contacts defined a minimal 'generic footprint' of MHC-restriction. Thus our findings simultaneously demonstrate the considerable adaptability of the TCR and the 'shape' of MHC restriction.     

Major histocompatibility complex class II binding site for streptococcal pyrogenic (erythrogenic) toxin A

Streptococcal pyrogenic exotoxin A (SPEA) is an important pathogenicity factor of group A streptococci. It is a member of the family of „superantigens” produced by Staphylococcus aureus and Streptococcus pyogenes and its T lymphocyte stimulating activity is involved into the pathogenesis of certain diseases caused by pyogenic streptococci. In this study we have produced and characterized recombinant SPEA molecules in Escherichia coli. These molecules are indistinguishable from natural SPEA in both T cell stimulatory and HLA class II binding activities. Human class II molecules are more efficient than mouse class II molecules in presenting SPEA to T cells. In binding tests to major histocompatibility complex class II-positive cells SPEA competes with staphylococcal enterotoxin B and A but not with toxic shock syndrome toxin-1.     

Rabbit major histocompatibility complex. IV. Expression of major histocompatibility complex class II genes

The rabbit MHC class II DP, DQ, and DR alpha and beta chain genes were transfected into murine B lymphoma cells. The transfected cells expressed R-DQ and R-DR molecules on the cell surface but they did not express the R-DP genes either on the cell surface or at the level of mRNA. Northern blot analyses showed that the R-DP genes were expressed, albeit at low levels, in rabbit spleen. Similar analyses showed that the R-DQ and R-DR genes were expressed at high levels in rabbit spleen. A new monoclonal anti-rabbit class II antibody, RDR34, has been developed and shown to react with the R-DR transfected cells and not with the R-DQ transfected cells. The previously described monoclonal anti-rabbit class II antibody, 2C4, reacted with the R-DQ transfected cells and not with the R-DR transfected cells. Thus, 2C4 and RDR34 MAb's are specific for the R-DQ and R-DR molecules, respectively. Each of the antibodies reacted with approximately 50% of rabbit spleen cells as shown by immunofluorescent antibody studies.     

Assembly of an abundant endogenous major histocompatibility complex class II/peptide complex in class II compartments

To identify the intracellular site(s) of formation of an endogenous class II/peptide complex in a human B cell line, we employed kinetic pulse-chase labeling experiments followed by subcellular fractionation by Percoll density gradient centrifugation and immunogold labeling on ultrathin cryosections. For direct demonstration of assembly of such complexes, we used the monoclonal antibody YAe, which detects an endogenous complex of the mouse class II molecule I-A^b with a 17-amino acid peptide derived from the α chain of HLA-DR (DRα52–68). We show that in human B lymphocytes, these class II/peptide complexes assemble and transiently accumulate in major histocompatibility complex class II-enriched compartments before reaching the cell surface.