Genetic control of peripheral TCRAV usage by representation in the preselection repertoire and MHC allele-specific overselection

TCRAV segments contribute significantly to MHC restriction as illustrated b y their general preference for either the CD4 or CD8 T cell subset and addi tional, MHC allele-specific overselection during T cell differentiation. Th e 10-fold over-representation of the TCRAV8S2 (VA8S2) segment in CD8 over C D4 T cells by the RT1(f) haplotype of LEW,1F rats provides the most strikin g example of MHC allele-specific overselection of a VA segment reported so far. Also in alloreactivity, VA8S2(+) CD8 cells from RT1(f-) rats are prefe rentially expanded by RTf+ stimulators. We have identified the class I mole cule, A(f), mediating VA8S2 overselection and report that it differs only i n four amino acids at the MHC-TCR interface from the class I molecule Aa, w hich is neutral with regard to selection of VA8S2. We also provide an exten sive survey of the TCRAV8 family and show that among 14 functional VA8 segm ents in LEW rats, the dramatic Af-dependent overselection is unique for VA8 S2, Surprisingly, VA8S2 expression in CD8 T cells of RT1(f+) rats derived f rom a Sprague-Dawley stock was only 3% as compared to the 12% observed in L EW.1F. The VA8S2 segment of Sprague-Dawley (VA8S2(SD)) differs from VA8S2 o f the LEW background (VA8S2(I)) in only two amino acids, one of which is lo cated in CDR2 and could thus participate in allele-specific recognition of A(f). However, analysis of the pre- and postselection thymic repertoires of Sprague-Dawley and LEW.1F rats and of the repertoire of CD8 cells from bot h strains expanded in the alloreactive response to RT1(f) revealed that the difference in VA8S2 representation between the two backgrounds is explaine d by differential availability in the preselection repertoires and not by a difference in overselection, Sequence comparisons of A(f) and Aa and of bo th VA8S2 segments suggest a predominant role of CDR1 in hyper-reactivity to Af, Thus, the VA composition of the mature Ton repertoire is influenced by Tcra locus polymorphisms at two levels: the regulation of VA usage in the preselection repertoire and the composition of structural elements which co ntribute to specific VA-MHC interactions during thymic selection.