Selective suppression by prodigiosin of the mitogenic response of murine splenocytes

In the course of screening for immunomodulating substances among microbial metabolites using a triple mitogen assay system, we detected an immunosuppressive activity which is similar to cyclosporin A. Two active components were isolated as red pigments from the fermentation broth of Streptomyces hiroshimensis and they were identified with prodigiosin 25-C and metacycloprodigiosin respectively. These compounds inhibited T lymphocyte proliferation which was induced by plant lectins, concanavalin A (Con A) and phytohemagglutinin (PHA), much more extensively than B lymphocyte proliferation which was induced by lipopolysaccharide (LPS). Prodigiosin 25-C completely inhibited induction of cytotoxic T cells in a mixed lymphocyte reaction (MLR) at 4 ng/ml.