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The superantigen staphylococcal enterotoxin A (SEA) binds to major histocompatibility complex (MHC) class II molecules at two sites on either side of the peptide groove. Two separate but cooperative interactions to the human class II molecule HLA-DR1 were detected. The first high affinity interaction to the DR1 beta chain is mediated by a zinc atom coordinated by H187, H225, and D227 in SEA and H81 in the polymorphic DR1 beta chain. The second low affinity site is to the DR1 alpha chain analogous to SEB binding and is mediated by residue F47 in SEA. Binding of one SEA to the DR1 beta chain enhances the binding of a second SEA molecule to the DR1 alpha chain. The zinc site is on the opposite side of the SEA molecule from residue F47 so that one SEA molecule can readily bind two class II molecules. Both binding sites on SEA are required for maximal activity. Thus, unlike, SEB, SEA requires two separate binding sites for optimal activity, which may allow it to stabilize SEA interaction with T cell receptors, as well as to activate the antigen-presenting cell by cross-linking MHC class II.

The germline repertoire of variable genes for the TCR-gamma/delta is limited. This, together with the availability of several V delta-specific and a C delta-specific mAbs, has made it possible to assess differences in the TCR-gamma/delta repertoire in man. TCR-gamma/delta cells expressing particular V gene segments have been previously shown to be localized in different anatomical sites. In this study, analysis of TCR-gamma/delta V gene segment usage performed on subjects from the time of birth through adulthood revealed striking age-related changes in the TCR-gamma/delta repertoire in peripheral blood. V delta 1+ gamma/delta T cells predominated in thymus as well as in peripheral blood at birth and then persisted as a relatively constant proportion of CD3+ PBL. However, V delta 2+ gamma/delta T cells that constitute a small proportion of the CD3+ cells in thymus and in peripheral blood at birth, then expand and account for the major population of gamma/delta T cells in PBL in adults. No parallel postnatal expansion of V delta 2+ cells in the thymus was observed, even when paired thymus-peripheral blood specimens were obtained on subjects between the ages of 3 d and 8 yr. The subset of V delta 2+ lymphocytes that was expanded in peripheral blood expressed high levels of CD45RO suggesting prior activation of these cells, consistent with the possibility that their expansion might have resulted from exposure to foreign antigens or superantigens. In contrast, V delta 1+ T cells in PBL showed no comparable increase in relative numbers and were either negative or expressed only low levels of CD45RO. Consistent with evidence for extrathymic peripheral expansion of selective TCR-gamma/delta subsets, no link between MHC haplotype and differences in the TCR-gamma/delta V gene usage between individuals was apparent, and identical twins displayed TCR-gamma/delta variable gene segment phenotypes that were strikingly different from one another. The elements that determine the TCR-gamma/delta repertoire in individuals are not known. It is possible that both thymic selection and extrathymic factors may influence the peripheral repertoire. Recently, TCR-gamma/delta+ lymphocytes have been shown to expand markedly in peripheral lymphoid tissues and infectious lesions in response to mycobacterial antigens, and a correlation between mycobacterial responses and TCR-gamma/delta V gene usage has been shown in mice. The data presented here demonstrated peripheral age-related changes in the gamma/delta repertoire and point to the importance of extrathymic expansion of specific gamma/delta subsets in generating the human TCR-gamma/delta repertoire.

We describe here a monoclonal antibody, 131, which appears to recognize a determinant shared by HLA-A locus-encoded gene products. Isoelectric focusing analysis demonstrates that 131 reacts with the products of at least seven different HLA-A alleles but none of the five HLA-B allelic products tested. Together with evidence provided by other studies, this finding indicates the existence of A-unique and B-unique determinants, which may have different biological functions. Monoclonal antibody probes, such as the one described here, specific for shared intralocus determinants, may be valuable for assessing these possible functional differences.