The origin of cytotoxic T lymphocyte precursors (CTL-P): MHC-restricted and alloreactive CTL-P in the spleen during regeneration after a sublethal dose of cyclophosphamide (Cy).

The regenerating spleen 8 days after an injection of a sublethal dose (300 mg/kg) of cyclophosphamide (Cy) had a defective capacity to give rise to cytotoxic T lymphocytes (CTL) in response against allogeneic cells, whereas the cytotoxicity against 2,4,6-trinitrophenyl-(TNP) modified syngeneic cells was at the normal level. Alloresponse was first obtained 2 wk after the Cy treatment. The limiting dilution analysis showed this at the clonal level: the frequency of anti-TNP-specific CTL precursors (CTL-P) in the spleen treated with Cy 8 days previously was the same as the frequency in the normal spleen. The defective alloresponse was due to a decreased number of allospecific CTL-P that was later increasing. The regenerative capacity was not abolished by adult thymectomy or treatment of the mice with a bone marrow-seeking isotope, 89Sr, suggesting that these CTL-P are derived from Cy-resistant splenic precursors rather than from the thymus or bone marrow. These precursors have probably been under thymic education: the dominance of H-2k-restricted CTL over H-2d-restricted CTL in the response of (H-2k X H-2d)F1 mice to TNP-self is known to be influenced by the H-2 genotype of the thymus, and this dominance was also demonstrated with anti-TNP CTL derived from these F1 mice pretreated with Cy. The CTL-P in the regenerating spleen (day 8) were not hydrocortisone sensitive, and nylon wool-purified T cells from this stage had a lactate dehydrogenase (LDH) isoenzyme pattern of the mature T cell type (rather than of the thymocyte type). Thus, in these aspects the T cells of the regenerating spleen resembled normal splenic cells. These data suggest that the Cy-resistant spleen population contains cells that can give rise to CTL-P that have a defective specificity repertoire at the beginning of the regeneration, but later mature to a normally alloreactive population.