Irreversible suppression of CD8 expression in CD4-CD8+ thymocytes upon in vitro stimulation

CD8 (Ly-2) expression was suppressed in purified murine CD4-CD8+ thymocytes at the mRNA level upon continuous stimulation with PMA and ionomycin in the presence of rIL-2. The level of CD8 expression on CD4-CD8+ thymocytes was reduced gradually during the culture and a majority of them turned into CD4-CD8- cells after 48 hr. This suppression was not transient, since CD8 expression was not recovered on these cells in additional 48 hr of culture without PMA and ionomycin. The suppression was dependent on the concentrations of PMA and ionomycin, and inhibited by adding an immunosuppressant, CsA to the culture. Treatment with either PMA or ionomycin alone did not induce suppression of CD8. Crosslinking of CD3-epsilon chains also induced suppression of CD8 for a part of CD4-CD8+ thymocytes. Interestingly, CD8 expression was hardly suppressed in CD4-CD8+ peripheral T lymphocytes, suggesting that the mechanisms of suppression of CD8 is developmentally regulated. We propose that the suppression of CD8 expression at CD4-CD8+ stage involves an additional mechanism of negative selection of thymic T cells.