Melatonin antagonises the cyclosporine A immunosuppressive effects in rat thymuses.

Cyclosporine A (CsA) is a powerful immunosuppressant inducing marked involution of the thymus with disappearance of interdigitating cells (IDCs) and reducing the number of macrophages (Mphi). Melatonin (Mel) is a potent antioxidant agent that could be useful in attempting to reduce CsA toxicity. Moreover, Mel has been found to exert positive influence on lymphocytes, spleen and thymus. In this study, we evaluated the effects of Mel on the thymic involution induced by CsA. CsA caused profound changes in thymic cytoarchitecture, i.e. the complete disappearance of thymic medulla and reduced the number of acid phosphatase positive macrophages. There was no difference between thymic morphology of Mel-treated rats and the control animals injected only with olive oil. The simultaneous Mel-CsA administration prevented the involution of the thymic medulla and the reduction of the number of macrophages. These findings suggested that Mel probably antagonized the immunosuppressive CsA effects.