Dynamics of Cytotoxic T Lymphocyte Precursors in Vivo Assessed by Change in the Radiation Sensitivity: Evidence for Development of Radiation‐Sensitive Memory Cells without Clonal Expansion

The dynamics of cytotoxic T lymphocyte precursors (CTL‐p) in mice injected with allogeneic spleen cells (SC) was studied with special reference to changes in their radiation sensitivity. Whole‐body 400 rad X‐ray irradiation of allo‐SC‐primed and unprimed mice virtually abolished the capacity of their SC to proliferate and to generate CTL in primary or secondary mixed leucocyte culture (MLC). However, the impaired ability of SC to generate CTL in the primary MLC was restored by interleukin 2 (IL‐2). This showed that helper cells whose activity was replaceable with IL‐2 (IL‐2‐producing cells) were functionally more radiation‐sensitive than CTL‐p in unprimed mice. In contrast, the radiation‐impaired activity in secondary MLC was not restored by IL‐2, suggesting that memory CTL‐p in allo‐SC‐primed mice were unexpectedly sensitive to radiation. The D₃₇ values determined from the percentage of residual CTL‐p activity of SC in bulk cultures 1 day after irradiation were 525 rad for virgin CTL‐p and 75 rad for memory CTL‐p. Further studies demonstrated that the radiation‐sensitive memory CTL‐p were generated from relatively radiation‐resistant precursors, largely independent of radiation‐sensitive IL‐2‐producing cells and of cellular proliferation. The mean frequency of CTL‐p in SC measured by limiting dilution assay was not significantly increased by the priming. This supports our conclusion that the development of the memory CTL‐p activity in allo‐SC‐primed mice did not depend on clonal expansion. Whole‐body 400 rad‐irradiation reduced the frequency of CTL‐p in SC from unprimed‐mice 1/2–1/3 and that in SC from allo‐SC‐primed mice to 1/8–1/15. This supports the view that the majority of radiation‐resistant virgin CTL‐p functionally mature to radiation‐sensitive memory CTL‐p without cellular proliferation in allo‐SC‐primed mice.