Specificity and precursor frequency of cytotoxic T cells in the response to minor H antigens.

The cytotoxic T cell response to minor histocompatibility antigens (MIHA) has been characterized in terms of the specificity repertoire, Lyt phenotype and precursor frequency. In the response of primed BALB/c spleen cells to B10.D2 or DBA/2 MIHA, the cytotoxic T lymphocyte (CTL) generated bear Thy-1.2, and are Lyt-1+2+. In the response to intact stimulator cells we have been able to distinguish at least three different specificities of anti-MIHA CTL in the response of primed BALB/c T cells to DBA/2 MIHA. These subsets exhibit differential inductive requirements and are derived from CTL precursors (CTLp) present at different frequencies in primed spleen. As reported by others, at high responder cell numbers (10(7)) the CTL generated see MIHA shared between DBA/2 and B10.D2 in H-2-restricted fashion. However, at lower numbers (3 X 10(5)), the CTL generated are only weakly crossreactive between DBA/2 and B10.D2 as defined by cold target competition. These CTL show a higher activity for DBA/2 than for B10.D2 regardless of the hot target used in the competition assay. CTLp are depleted by absorption on DBA/2 but not B10.D2 monolayers. These CTLp are present at higher frequency than are the CTLp responsive to shared MIHA--yet are undetectable at high responder cell numbers, suggesting strong clonal dominance effects. At low cell numbers (3 X 10(4] in the response to intact stimulators, the CTL are specific for MIHA unique to DBA/2, and crossreactive CTL able to lyse B10.D2 are no longer detectable. Addition of growth factors to cultures restores the crossreactive CTL, suggesting a requirement for higher levels of help and indicating that these two subsets derive from different CTLp. Finally the frequency of CTLp responsive to MIHA on intact cells was found to be 40-fold higher than the frequency of CTLp responsive to MIHA on membrane fragments. These data are discussed in terms of strong immunodominance mechanisms that appear to operate in the response to MIHA.