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 (RT1f), CD8 T cellsutilizing the V alpha 8.2 segment are 10-fold overselected during thymicdevelopment, resulting in V alpha 8.2 expression by 14% of mature CD8 Tcells as compared to 1-2% in MHC congenic strains. In the alloreactiveresponses of CD8 T cells from RT1f-negative rats against RT1f, V alpha 8.2+CD8 T cells are also preferentially expanded. Neither overselection noralloreactivity of V alpha 8.2+ TCR require selective V beta pairing.However, RT1f alloreactive V alpha 8.2+ TCR preferentially use a relatedset of J alpha segments which contribute short homogeneous CDR3 alphaloops, with features suggesting peptide promiscuity, and little Nadditions. In contrast, only few overselected V alpha 8.2+ CD8 T cellsshowed an imprint of positive selection on J usage or CDR3 composition. Theresults demonstrate that a single V alpha segment can promote both MHCallele-specific positive selection and alloreactivity, and that the latteris more dependent on an additional contribution of CDR3 alpha, possibly bypromoting reactivity with a diverse set of MHC-bound peptides or byproviding additional MHC contacts.