CONTROL OF TCR V-ALPHA-MEDIATED POSITIVE REPERTOIRE SELECTION AND ALLOREACTIVITY BY DIFFERENTIAL J(ALPHA) USAGE AND CDR3-ALPHA COMPOSITION

In rats expressing the f allele of the rat MHC (RT1(f)), CD8 T cells u tilizing the V(alpha)8.2 segment are 10-fold overselected during thymi c development, resulting in V(alpha)8.2 expression by 14% of mature CD 8 T cells as compared to 1-2% in MHC congenic strains, In the alloreac tive response of CD8 T cells from RT1(f)-negative rats against RT1(f), V(alpha)8.2(+) CD8 T cells are also preferentially expanded, Neither overselection nor alloreactivity of V(alpha)8.2(+) TCR require selecti ve V-beta pairing. However, RT1(f) alloreactive V(alpha)8.2(+) TCR pre ferentially use a related set of J(alpha) segments which contribute sh ort homogeneous CDR3 alpha loops, with features suggesting peptide pro miscuity, and little N additions, In contrast, only few overselected V (alpha)8.2(+) CD8 T cells showed an imprint of positive selection on J usage or CDR3 composition, The results demonstrate that a single V-al pha segment can promote both MHC allele-specific positive selection an d alloreactivity, and that the latter is more dependent on an addition al contribution of CDR3 alpha, possibly by promoting reactivity with a diverse set of MHC-bound peptides or by providing additional MHC cont acts.