Oxygenated derivatives of sterols (oxysterols), including 25-hydroxycholesterol and 27-hydroxycholesterol, have immunosuppressive effects. Oxysterols can directly induce apoptosis in immature thymocytes, cells which are inherently sensitive to induction of programmed cell death. For that reason, the metabolism of 25-hydroxycholesterol and 27-hydroxycholesterol in mouse thymus has been studied. When incubated with thymic tissue, both oxysterols were found to be 7alpha-hydroxylated with subsequent oxidation to 7alpha-hydroxy-3-oxo-delta4 steroids. A minor fraction of 27-hydroxycholesterol was also metabolised to 3beta-hydroxy-5-cholestenoic, 3beta,7alpha-dihydroxy-5-cholestenoic and 7alpha-hydroxy-3-oxo-4-cholestenoic acids. The 7alpha-hydroxylase was found to be localised to the thymic epithelial cells and the reaction was stimulated by interleukin-1beta and inhibited by metyrapone and RU486. In contrast to 25-hydroxycholesterol and 27-hydroxycholesterol, the 7alpha-hydroxylated metabolites, 7alpha,25-dihydroxycholesterol, 7alpha,25-dihydroxy-4-cholesten-3-one and 7alpha,27-dihydroxy-4-cholesten-3-one did not induce thymocyte apoptosis. The results suggest that 7alpha-hydroxylation may be of regulatory importance, possibly by protecting the developing thymocytes against toxic effects by oxysterols.