Direct effect of cyclosporin A on CD8+ T lymphocytes in a primary mixed lymphocyte reaction

Cyclosporin A (CsA) blocks the development of cytotoxic T lymphocytes (CTL) in a primary mixed lymphocyte reaction (MLR) when added during the first few days of culture. We found that addition of recombinant IL-2 (rIL-2) or an IL-2-containing supernatant (2 degrees MLR SN) either alone or in combination was unable to reverse the suppression. Purified CD8+ responder cells were inhibited as effectively by CsA as unfractionated cells, suggesting that CsA has a direct, lymphokine-independent effect on CD8+ cells. The frequency of CTL precursors (CTLp) responding in a primary MLR was measured by limiting dilution in the presence and absence of CsA. Using either unfractionated cells or cell-sorter-purified CD8+ responder cells, no suppression was seen at very low cell numbers but increased markedly as the cell number per culture increased. These results imply that either a CD8+ regulatory/suppressor cell is being diluted out at low cell numbers, or that low cell-density culture conditions might override the inhibitory effects of CsA.