The suppression of 1,2-dimethylhydrazine (DMH)-induced colon carcinogenesis by melatonin was previously demonstrated. The objective of the present work was to evaluate histologically and immunohistochemically the splenic immune response to the induced cancer and to melatonin. Spleens from rats, either untreated, injected with DMH, fed with melatonin or treated with both carcinogen and melatonin, were studied. The exposure to the carcinogen and the consequential carcinogenesis resulted in splenic changes that reflected the insufficiency of the immune response, as manifested in significant reduction of the white pulp and the simultaneous expansion of the red pulp. The effects of melatonin on most splenic components were inverse to those of DMH. The anti-carcinogenic properties of melatonin were evidenced from the reversal of the inhibitory effects of DMH, especially when the densities of lymphocytes in different parts of the spleen were compared. The combined treatment of the rats with DMH and melatonin resulted in the expansion of the splenic zones by 106% to 125%, compared to those from DMH-treated rats, and the numbers of CD8+ lymphocytes and Fas-positive cells increased sharply. Therefore we conclude that anti-carcinogenic effects of melatonin are related to activation of several elements of the host's lymphatic system.

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