Positive selection of the T-cell repertoire is affected by mutations in the peptide-binding site of MHC class I molecules

Peptide-independent recognition by alloreactive cytotoxic T lymphocytes (CTL)

We have isolated several H-2K(b)-alloreactive cytotoxic T cell clones and analyzed their reactivity for several forms of H-2K(b). These cytotoxic T lymphocytes (CTL) were elicited by priming with a skin graft followed by in vitro stimulation using stimulator cells that express an H-2K(b) molecule unable to bind CD8. In contrast to most alloreactive T cells, these CTL were able to recognize H-2K(b) on the surface of the antigen processing defective cell lines RMA-S and T2. Furthermore, this reactivity was not increased by the addition of an extract containing peptides from C57BL/6 (H-2(b)) spleen cells, nor was the reactivity decreased by treating the target cells with acid to remove peptides bound to MHC molecules. The CTL were also capable of recognizing targets expressing the mutant H-2K(bm8) molecule. These findings suggested that the clones recognized determinants on H-2K(b) that were independent of peptide. Further evidence for this hypothesis was provided by experiments in which H-2K(b) produced in Drosophila melanogaster cells and immobilized on the surface of a tissue culture plate was able to stimulate hybridomas derived from these alloreactive T cells. Precursor frequency analysis demonstrated that skin graft priming, whether with skin expressing the wild-type or the mutant H-2K(b) molecule, is a strong stimulus to elicit peptide-independent CTL. Moreover, reconstitution experiments demonstrated that the peptide-independent CTL clones were capable of mediating rapid and complete rejection of H-2-incompatible skin grafts. These findings provide evidence that not all allorecognition is peptide dependent.

T cell receptor (TCR) recognition of MHC class I variants: intermolecular second-site reversion provides evidence for peptide/MHC conformational variation

We investigated mechanistic differences in antigen presentation between murine MHC class I variants H-2K(b) and H-2K(bm)8. H-2K(bm)8 differs from H-2K(b) by four residues at the floor of the peptide-binding site, affecting its B pocket which interacts with the second (P2) residue of the peptide. The rest of the molecule, including the T cell receptor (TCR)-contacting residues, is identical to H-2K(b). Due to this variation, CTLs that recognize the ovalbumin 257-264 and HSV gB 498-505 peptides on H-2K(b) cannot recognize them on H-2K(bm)8. This could be due to impaired peptide binding or an altered peptide: K(bm)8 conformation. Peptide binding studies ruled out the first explanation. Molecular modeling indicated that the most obvious consequence of amino acid variation between peptide/H-2K(b) and peptide/H-2K(bm)8 complexes would be a loss of the conserved hydrogen bond network in the B pocket of the latter. This could cause conformational variation of bound peptides. Intermolecular second-site reversion was used to test this hypothesis: P2-substituted OVA and HSV peptides, engineered to restore the hydrogen bond network of the B pocket, were the only ones which restored CTL recognition. These results provide a molecular understanding of peptide/MHC conformational variation.

Novel HLA-B alleles, B*8201, B*3515 and B*5106, add to the complexity of serologic identification of HLA types

Three class I alleles, B*8201, B*3515 and B*5106, have been described using DNA and cDNA sequencing. The B*8201 allele is most structurally related to B*5602, differing from it by 14 nucleotide substitutions resulting in 5 amino acid differences. The other two alleles, B*3515 and B*5106, differ from their most closely related HLA-B alleles by 2-3 nucleotide substitutions resulting in 1-2 amino acid substitutions, respectively. The majority of nucleotide substitutions marking these new alleles are observed in other HLA-B alleles suggesting that gene conversion and/or reciprocal recombination have created this diversity. All of the amino acid substitutions are predicted to alter the antigen binding site of the HLA-B molecule. The newly defined HLA-B allelic products were originally defined by their unusual serologic reactivity patterns. The B*8201 allelic product is serologically typed as a B "blank" or as a variant of B22 or B45. These patterns and the serologic reactivity of the other newly described allelic products are consistent with the protein sequence homology among specific HLA-B molecules. While serology remains a powerful tool for detecting HLA diversity, alleles generated by events resulting in the sharing of HLA sequence polymorphisms among alleles at a locus will continue to create complexity in the interpretation of typing results.