In the present study we used hydrocortisone (HC) treatment in vivo as a probe to analyze two different in vitro systems for the regeneration of cytotoxic T lymphocyte (CTL), namely the antifibroblast reaction (AFR) and the mixed lymphocyte culture (MLC) system. We found that cells remaining in the thymus after HC treatment had increased reactivity in these two systems. However, the same treatment in the spleen severely depressed the MLC reactivity in both the proliferative and the cytolytic phases, while markedly increasing the AFR reactivity. These findings demonstrate heterogeneity of CTL precursors and/or their pathways of differentiation into effector cells. In addition, MLC-reactive cells in the thymus appear to be distinct from such cells in the spleen, as judged from their differential sensitivity to HC.

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