Selection of the T cell repertoire during ontogeny: limiting dilution analysis.

The relative frequencies of cytotoxic T lymphocyte precursors (CTL-P) specific for minor histocompatibility antigens that are restricted to either k or b major histocompatibility complex antigens in normal B6 mice and in B6 leads to (B6 x CBA)F1 radiation chimeras were estimated. The ratio of k-restricted to b-restricted CTL-P determined for the radiation chimeras served as a base value to which the ratio of k: b-restricted CTL from normal B6 mice can be compared. The frequencies of allorestricted CTL-P in B6 mice were determined after the B6 cells had been depleted of k-reactive cells by filtration through irradiated B6.H-2k mice. Lymphocytes from immunized animals were used to obtain frequency estimates of CTL-P specific for minor H antigens. The cultures contained nu/nu spleen cells syngeneic to the responder, but no exogenous interleukin 2 (T cell growth factor) since a high degree of nonspecific killing was found in IL2-supplemented cultures. It was found that the CTL-P frequency of B6 mice to BALB.B antigens in the lymph node cells of normal or immune B6 mice were 1 in 300 000 and 1 in 8000, respectively. In B6 leads to (B6 x CBA)F1 radiation chimeras, the ratio of BALB.B/BALB.K-specific CTL-P was 3:1; this ratio was 17:1 in normal B6 mice. Twenty-five percent of CTL clones from normal B6 mice lysing BALB.K target also lysed BALB.B target whereas less than 1% of CTL clones lytic on BALB.K target from the chimeras showed this cross-reaction The results are compatible with the idea of complete positive selection of self MHC-restricted CTL-P in the absence of priming effects by foreign antigen.