Thalidomide prolonged graft survival in a rat cardiac transplant model but had no inhibitory effect on lymphocyte function in vitro.

The effects of thalidomide on in vitro interleukin 2 (IL-2) production and thymidine uptake by human peripheral blood lymphocytes or rat splenocytes were investigated. Phytohaemagglutinin-stimulated human lymphocytes were incubated in the presence of thalidomide added at culture initiation. No immunosuppressive effect of thalidomide was observed in these experiments. Primary human mixed lymphocyte cultures treated with thalidomide for 6 days were also unaffected. A microsomal rabbit liver homogenate was prepared for metabolizing thalidomide. Stimulated lymphocytes secreted significantly more IL-2 in the presence of microsomal-treated thalidomide than did controls. The effect of thalidomide was then studied either as single therapy or in combination with cyclosporin A (CyA) in a rat allograft cardiac transplantation model. In addition, T cell subsets were analysed by flow cytometry in untransplanted rats treated with thalidomide. Treatment was given as induction therapy from the day of transplantation until day 9. Graft survival in rats treated with thalidomide was significantly prolonged compared to the untreated group. No difference in graft survival was detected between rats treated with thalidomide or CyA only. Graft survival was found to be slightly prolonged in rats given thalidomide and CyA in combination compared to rats treated with CyA alone. In untransplanted rats given thalidomide a decrease of CD4 positive T cells was detected on days 3 and 5. The T helper/cytotoxic-suppressor cell ratio was significantly diminished but, after 1 week of treatment, values for T cell subsets had almost returned to baseline levels. No inhibitory effect was obtained when phytohaemagglutinin-stimulated rat splenocytes were cultured with metabolized thalidomide. In summary, the ability of thalidomide to improve allograft survival in a solid organ transplant model was verified. The occurrence of thalidomide-induced changes in T cell subset ratios was demonstrated. In in vitro studies, however, there was no decrease but an increase in IL-2 production, and no change in thymidine uptake. The mechanism responsible for the immunosuppressive effect of thalidomide remains to be elucidated.