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.     

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.     

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.     

Progress in crystallization of major histocompatibility complex class I in vertebrates

The major histocompatibility complex(MHC)of proteins that exists in all vertebrates is encoded by a cluster of genes associated with the immune response and related functions.MHC is divided into MHC I,II,and III;MHC I is involved in antigenic presentation,binding T cell receptors,and leading ultimately to specific cellular immune responses.The complicated functions of MHC I are determined by the nature of the complex.The crystal structure of MHC I has been solved for many animals,revealing the relationship between spatial structure and function.MHC I consists of an a heavy chain and a b2m light chain,both ligated non-covalently to a complex when a peptide is bound to the antigenic-binding groove.The a heavy chain is divided into an extracellular domain,a transmembrane domain,and an intracellular domain.The extracellular domain consists of sub-regions a1,a2,and a3.The a1 and a2 together form the antigenic-binding groove and bind antigenic peptides with 8–10 amino acid residues.MHC I can form a stable spatial structure;however,it should be noted that there are differences in the structure of MHC I among animal species,including anchored amino acids in binding peptides,binding sites,molecular distance,crystallization conditions,etc.Here,progress in determination of the crystal structure of human,mouse,chicken,non-human primate,and swine MHC I is described in detail.     

Structure of the human class I histocompatibility antigen, HLA-A2

The class I histocompatibility antigen from human cell membranes has two structural motifs: the membrane-proximal end of the glycoprotein contains two domains with immunoglobulin-folds that are paired in a novel manner, and the region distal from the membrane is a platform of eight antiparallel beta-strands topped by alpha-helices. A large groove between the alpha-helices provides a binding site for processed foreign antigens. An unknown 'antigen' is found in this site in crystals of purified HLA-A2.     

Cellular peptide composition governed by major histocompatibility complex class I molecules

Major histocompatibility complex (MHC) class I molecules present peptides derived from cellular proteins to cytotoxic T lymphocytes (CTLs), which check these peptides for abnormal features. How such peptides arise in the cell is not known. Here we show that the MHC molecules themselves are substantially involved in determining which peptides occur intracellularly: normal mouse spleen cells identical at all genes but MHC class I express different patterns of peptides derived from cellular non-MHC proteins. We suggest several models to explain this influence of MHC class I molecules on cellular peptide composition.     

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.     

Licensing of natural killer cells by host major histocompatibility complex class I molecules

Self versus non-self discrimination is a central theme in biology from plants to vertebrates, and is particularly relevant for lymphocytes that express receptors capable of recognizing self-tissues and foreign invaders. Comprising the third largest lymphocyte population, natural killer (NK) cells recognize and kill cellular targets and produce pro-inflammatory cytokines. These potentially self-destructive effector functions can be controlled by inhibitory receptors for the polymorphic major histocompatibility complex (MHC) class I molecules that are ubiquitously expressed on target cells. However, inhibitory receptors are not uniformly expressed on NK cells, and are germline-encoded by a set of polymorphic genes that segregate independently from MHC genes. Therefore, how NK-cell self-tolerance arises in vivo is poorly understood. Here we demonstrate that NK cells acquire functional competence through 'licensing' by self-MHC molecules. Licensing involves a positive role for MHC-specific inhibitory receptors and requires the cytoplasmic inhibitory motif originally identified in effector responses. This process results in two types of self-tolerant NK cells--licensed or unlicensed--and may provide new insights for exploiting NK cells in immunotherapy. This self-tolerance mechanism may be more broadly applicable within the vertebrate immune system because related germline-encoded inhibitory receptors are widely expressed on other immune cells.     

Major histocompatibility complex class I molecules internalized via coated pits in T lymphocytes

Endocytosis of the major histocompatibility complex (MHC)-encoded class I and class II molecules has been the subject of recent investigations. Class I molecules, which are key elements in T cell-mediated cytotoxicity, are differentially endocytosed by different cell types. Fibroblasts internalize their class I molecules via uncoated cell surface vesicles and tubular invaginations when these molecules are cross-linked with multivalent ligands. T lymphocytes internalize their class I molecules spontaneously, but B lymphocytes do not internalize them at all. Here we describe a morphological investigation of the mechanism by which class I molecules are endocytosed by T lymphocytes. We show that, unlike fibroblasts, T lymphocytes spontaneously internalize 20-40% of their class I molecules in a process involving coated pits and coated vesicles. Thus, the endocytic pathway of class I molecules in T lymphocytes is similar to those of other more classical cell-surface receptors involved in receptor-mediated endocytosis. In contrast, the same class I molecules remained on the cell surface in B lymphocytes. These data show that class I molecules are differentially regulated in T and B lymphocytes and fibroblasts.     

Pattern of nucleotide substitution at major histocompatibility complex class I loci reveals overdominant selection

The major histocompatibility complex (MHC) loci are known to be highly polymorphic in humans, mice and certain other mammals, with heterozygosity as high as 80-90% (ref. 1). Four different hypotheses have been proposed to explain this high degree of polymorphism: (1) a high mutation rate, (2) gene conversion or interlocus genetic exchange, (3) over dominant selection and (4) frequency-dependent selection. In an attempt to establish which of these hypotheses is correct, we examined the pattern of nucleotide substitution between polymorphic alleles in the region of the antigen recognition site (ARS) and other regions of human and mouse class I MHC genes. The results indicate that in ARS the rate of nonsynonymous (amino acid altering) substitution is significantly higher than that of synonymous substitution in both humans and mice, whereas in other regions the reverse is true. This observation, together with a theoretical study and other considerations, supports the hypothesis of overdominant selection (heterozygote advantage).     

A gene in the human major histocompatibility complex class II region controlling the class I antigen presentation pathway

Major histocompatibility complex (MHC) class I molecules export peptides to the cell surface for surveillance by cytotoxic T lymphocytes. Intracellular peptide binding is critical for the proper assembly and transport of class I molecules. This mechanism is impaired as a result of a non-functional peptide supply factor gene (PSF) in several human mutant cell lines with genomic lesions in the MHC. We have now identified PSF in the MHC class II region by deletion mapping in mutants and chromosome-walking. PSF is homologous to mammalian and bacterial ATP-dependent transport proteins, suggesting that it operates in the intracellular transport of peptides.     

Dysmyelination in transgenic mice resulting from expression of class I histocompatibility molecules in oligodendrocytes.

Major histocompatibility complex (MHC) molecules are not normally expressed in the central nervous system (CNS). However, aberrant expression has been observed in multiple sclerosis lesions and could contribute to the destruction of myelin or the myelinating cells known as oligodendrocytes. The mechanism of cell damage associated with aberrant MHC molecule expression is unclear: for example, overexpression of class I and class II MHC molecules in pancreatic beta cells in transgenic mice leads to nonimmune destruction of the cells and insulin-dependent diabetes mellitus. We have generated transgenic mice that express class I H-2Kb MHC molecules, under the control of the myelin basic protein promoter, specifically in oligodendrocytes. Homozygous transgenic mice have a shivering phenotype, develop tonic seizures and die at 15-22 days. This phenotype, which we term 'wonky', is due to hypomyelination in the CNS, and not to involvement of the immune system. The primary defect appears to be a shortage of myelinating oligodendrocytes resulting from overexpression of the class I MHC molecules.     

Domain interactions of H-2 class I antigens alter cytotoxic T-cell recognition sites

H-2 class I antigens appear to direct the recognition of virus-infected and neoplastic transformed cells by cytotoxic T lymphocytes (CTLs). Here, to identify the regions of class I antigens involved in CTL recognition, four hybrid class I genes were constructed in which exons were exchanged between the H-2Kb and H-2Db genes. These class I genes were expressed in mouse L cells and recognition of the hybrid Kb/Db antigens by CTLs and monoclonal antibodies specific for either Kb or Db was investigated. The pattern of CTL and monoclonal antibody recognition obtained indicates three correlations between structure and function of class I antigens. First, most CTL recognition sites and alloantigenic determinants are located on domains 1 and 2 of the antigen molecule. Second, these CTL recognition sites and alloantigenic determinants are not influenced by interaction of domains 1 and 2 with polymorphic regions of domain 3. Third, in contrast, interaction between domains 1 and 2 alters these CTL recognition sites and alloantigenic determinants. The alteration of CTL recognition sites by interaction between domains 1 and 2 suggests that a CTL site may be formed by amino acids from both domains 1 and 2, or that the conformation of amino acids at a CTL site may be altered by interactions between domains 1 and 2. Through these two features, the conformation of CTL recognition sites on H-2 class I antigens may be sensitive to alteration by interaction of either domain 1 or 2 with viral antigens.     

Restored expression of major histocompatibility class I molecules by gene transfer of a putative peptide transporter

Cytotoxic T lymphocytes recognize antigen-derived peptides bound to major histocompatibility complex (MHC) class I molecules with which they assemble in the endoplasmic reticulum or in an undefined subcompartment. There is genetic evidence that the peptides that are products of cytosolic protein degradation are transported into this compartment by a peptide supply factor (PSF), encoded in the MHC class II region. Like the corresponding genes RING4, HAM1 and mtp1, PSF is related to the multidrug-resistance family of transporters and may be a peptide pump, as translocation of peptides across membranes must occur independently of the secretory pathway. There is, however, no functional evidence for this role so far. Here we report gene transfer experiments showing that expression of PSF complementary DNA in the human lymphoblastoid cell line mutant 721.134 restores normal levels of surface HLA-A2 and -B5. No similar effect was observed in 721.174 mutant cells, in which a homozygous deletion includes PSF among several other closely linked genes. At least one of these genes may therefore also be required for PSF function.     

Diabetes in transgenic mice resulting from over-expression of class I histocompatibility molecules in pancreatic beta cells

A class I histocompatibility gene, H-2Kb, linked to the rat insulin promoter, is overexpressed in the pancreatic beta cells of transgenic mice. The mice, whether syngeneic or allogeneic to the transgene, develop insulin dependent diabetes without detectable T cell infiltration, suggesting a direct, non-immune role for the transgenic class I molecules in the disease process.