The foreign antigen binding site and T cell recognition regions of class I histocompatibility antigens

Most of the polymorphic amino acids of the class I histocompatibility antigen, HLA-A2, are clustered on top of the molecule in a large groove identified as the recognition site for processed foreign antigens. Many residues critical for T-cell recognition of HLA are located in this site, in positions allowing them to serve as ligands to processed antigens. These findings have implications for how the products of the major histocompatibility complex (MHC) recognize foreign antigens.     

Differential function of major histocompatibility complex antigens in T-lymphocyte activation.

We have emphasised the functional dichotomy of MHC LD and LD antigens as well as the differences in cellular responses to these antigens. Perhaps in so doing we have failed to stress adequately the similarities that exist. But while the similarities (for example skin graft rejection associated with both K and I region differences) are so very clear, the differences have best allowed our progressive understanding of MHC induced cellular responses from the perspective stressed in this article. Of greatest importance to our understanding of these transplantation antigens are the potentially differential roles for the LD and SD antigens in the complex series of events that are collectively referred to as the "allograft reaction". It has been suggested that these differences may be "merely quantitative". This possibility has been discussed repeatedly in our previous reports on the distinction of LD and SD. In fact, the great bulk of biological phenomena can be reduced to quantitative differences. It would seem to us that sufficient evidence for such differential activity exists to make the LD-SD dichotomy model an heuristically valuable one for purposes of designing future experiments. We have discussed the clinical relevance of this model elsewhere. Many authors have speculated and evidence has been gathered to suggest, that cell surface antigens associated with the MHC are important in developmental and other cell interactions. Some studies have directly addressed the question of the need for MHC compatibility to allow cell interaction to proceed optimally. It thus seems most appropriate that the genetic complex with which we are dealing has been termed the major histocompatibility complex; allowing for the literal interpretation of this term this may be the genetic region that by its influence on "tissue compatibility" may control critical cellular interactions in addition to those observed in allograft reactions. It is the simple good fortune for those whose attention was focused on this complex by transplantation problems to find themselves with a panorama of biological phenomena that require extensive experimental probing and integration, hopefully ultimately leading to an understanding of the MHC in a broader context than has to date been possible.     

Interaction of peptide antigens and class II major histocompatibility complex antigens

T lymphocytes require a foreign antigen to be presented on a cell surface in association with a self-transplantation antigen before they can recognize it effectively. This phenomenon is known as major histocompatibility complex (MHC) restriction. It is not clear how an incalculably large number of foreign proteins form unique complexes with a very limited number of MHC molecules. We studied the recognition properties of T cells specific for a peptide derived from bacteriophage lambda cI protein. Analogues of this peptide, as well as peptides derived from other unrelated antigens which can be presented in the context of the same MHC molecule, can competitively inhibit activation of these T cells by the cI peptide. Furthermore, these unrelated antigens can stimulate cI-specific T cells if certain specific amino-acid residues are replaced. Here we suggest a model in which all antigens give rise to peptides that can bind to the same site on the MHC molecule. T-cell recognition of this site (which is presumed to be polymorphic) with or without antigen bound can explain self-selection in the thymus and MHC restriction.     

Identification of classical minor histocompatibility antigen as cell-derived peptide.

Histocompatibility antigens expressed on tissue grafted between individuals are recognized by host T cells, which reject the graft. The major histocompatibility complex (MHC) antigens have been identified on the molecular level, whereas the molecules representing the remaining ones, the minor histocompatibility antigens, are unknown, apart from some exceptions. The cytotoxic T lymphocyte (CTL) response against minor histocompatibility antigens shares many aspects with that against virus-infected cells. Virus-specific CTL recognize peptides derived from viral proteins produced in the infected cell. These peptides are presented by MHC class I molecules, as indicated by functional and crystallographic data. By analogy, minor histocompatibility antigens have been postulated to be peptides derived from normal cellular proteins presented by MHC class I molecules. Here we report that peptides derived from normal cellular proteins can indeed be recognized by CTL raised in the classical minor histoincompatible mouse strain combination, C57BL/6 against BALB.B. Thus, we have proven the above postulate, and isolated one of the minor histocompatibility molecules elusive for several decades.     

Tolerance of class I histocompatibility antigens expressed extrathymically

Although convincing evidence has been obtained for the imposition of self-tolerance by the intrathymic deletion of self-reactive T cells, the development of tolerance to antigens which are expressed only in the periphery is not so well understood. We have approached this question by creating transgenic mice which carry a class I major histocompatibility complex (MHC) gene (H-2Kb) linked to the rat insulin promoter. Mice expressing the transgene develop diabetes, but do not appear to mount an immune response against the transgene-expressing pancreatic beta-cells, even when the transgene is allogeneic with respect to the endogenous host H-2 antigens. We have now explored the mechanism of this tolerance further. We find that spleen cells from pre-diabetic transgenic (RIP-Kb) mice do not kill targets bearing H-2Kb, whereas thymus cells from the same mice do. The unresponsiveness of these spleen cells can be reversed in vitro by providing recombinant interleukin-2 (rIL-2). In older, diabetic mice, responsiveness develops as the pancreatic beta-cells are lost. Our results point to an extrathymic mechanism of tolerance induction, dependent on the continuous presence of antigen and the lack of IL-2 in the local environment of potentially reactive T cells.     

Tumour-specific phenylalanine tRNA contains two supernumerary methylated bases.

Every malignant tumour examined contains aberrant tRNA methyltransferases and a few tRNAs which are absent from the normal tissue of origin. To determine whether tumour-specific tRNAs have different modifications from those in normal tissue, we purified the most frequently occurring tumour-specific isoaccepting tRNA from two malignant tissues. The isoaccepting phenylalanine tRNA from Novikoff hepatoma and Ehrlich ascites cells both contain two supernumerary methylated bases. One of these l-methylguanine, is absent from the phenylalanine tRNA of normal rat, mouse, rabbit and calf liver. An increase in the levels of 5-methylcytidine and dihydrouridine was also detected.     

Expression of class I major histocompatibility antigens switched off by highly oncogenic adenovirus 12 in transformed rat cells

Rat cells transformed by the highly oncogenic adenovirus 12 lack at least two cellular proteins which are present in cells transformed by the non-oncogenic adenovirus 5 and in untransformed cells. One protein has been identified as the heavy chain of the rat class I major histocompatibility complex. This finding may explain the difference in oncogenicity between adenoviral species.     

The heavy chain of human histocompatibility antigen HLA-B7 contains an immunoglobulin-like region.

The 88-residue fragment (ac-2) containing the second disulphide loop from HLA-B7 heavy chain is shown to have statistically significant homology with Ig constant domains, including both matches at invariant positions and conservative substitutions of structurally important residues. Thus, both the light chain (beta 2m) and a segment of the heavy chain of HLA antigens may be structurally and evolutionarily related to immunoglobulins.     

An Fc receptor structurally related to MHC class I antigens

Maternal immunoglobulin G transmitted to the fetus or newborn provides humoral immunity for the first weeks of mammalian life. Fc receptors on intestinal epithelial cells of the neonatal rat (FcRn) mediate the uptake of IgG from milk. Affinity-purified FcRn is resolved by SDS-PAGE into components of relative molecular masses 45,000-53,000 (p51) and about 14,000 (p14). We report the identification of the smaller component as beta 2-microglobulin. Association of beta 2-microglobulin with p51 was confirmed by crosslinking in intestinal epithelial cell brush borders. We have cloned a cDNA encoding the presumptive Fc-binding subunit, p51, and its predicted primary structure has three extracellular domains and a transmembrane region which are all homologous to the corresponding domains of class I major histocompatibility complex (MHC) antigens. This is the first time a function has been assigned to an MHC antigen-related molecule.