PEPTIDE BINDING TO MIXED ISOTYPE A-BETA-E-D-ALPHA(D) CLASS-II HISTOCOMPATIBILITY MOLECULES

Previous studies have demonstrated that mixed isotype A beta(d)E alpha (d) molecules are expressed in transfected cell lines and that the lev el of expression is very low in normal B cells from H-2(d) mice. T-cel l responses restricted by A beta(d)E alpha(d) are induced in H-2(d) mi ce immunized with the synthetic peptides YL2 and FL2 or with sperm wha le myoglobin, despite the low concentration of mixed isotype molecules expressed on antigen-presenting cells. In the present study, the pept ide binding behavior of A beta(d)E alpha(d) was investigated. A peptid e from the cytoplasmic domain of invariant chain, I(1-18), was observe d to bind with high affinity to purified A beta(d)E alpha(d). Binding was optimal at pH 5, indicating that these molecules prefer to bind pe ptide in the acidic environment of endosomal compartments similar to o ther murine class II proteins. YL2 and FL2 bind to A beta(d)E alpha(d) with slightly lower affinity. The selective restriction of YL2- and F L2-specific T cells to mixed isotype molecules was accounted for by th e observation that these peptides do not bind to either I-E-d or I-A(d ). By contrast, myoglobin peptides bind to both parental and mixed iso type molecules. None of the A beta(d)E alpha(d)-restricted peptide det erminants bind to A beta(d)E alpha(d) with extremely high affinity. Th us it is unlikely that these peptides occupy an unusually high fractio n of mixed isotype molecules during antigen presentation in vivo. It i s more likely that the presence of a subpopulation of high-affinity T cells capable of being stimulated by very low concentrations of A beta (d)E alpha(d)/peptide complexes is responsible for the unusual A beta( d)E alpha(d)-restricted response observed with some antigens. (C) 1997 Elsevier Science Ltd.