QUANTITATIVE-ANALYSIS OF THE T-CELL REPERTOIRE SELECTED BY A SINGLE PEPTIDE-MAJOR HISTOCOMPATIBILITY COMPLEX

The positive selection of CD4(+) T cells requires the expression oi ma jor histocompatibility complex (MHC) class II molecules in the thymus, but the role of self-peptides complexed to class II molecules is stil l a matter of debate. Recently, it was observed that transgenic mice e xpressing a single peptide-MHC class II complex positively select sign ificant numbers of diverse CD4(+) T cells in the thymus. However, the number of selected T cell specificities has not been evaluated so far. Here, we have sequenced 700 junctional complementarity determining re gions 3 (CDR3) from T cell receptors (TCRs) carrying V beta 11-J beta 1.1.1 or V beta 12-J beta 1.1 rearrangements. We found that a single p eptide-MHC class II complex positively selects at least 10(5) differen t V beta rearrangements. Our data yield a first evaluation of the size of the T cell repertoire. In addition, they provide evidence that the single E alpha 52-68-I-A(b) complex skews the amino acid frequency in the TCR CDR3 loop of positively selected T cells. A detailed analysis of CDR3 sequences indicates that a fraction of the beta chain reperto ire bears the imprint of the selecting self-peptide.