Characterization of the immunoregulatory action of saikosaponin-d.

The immunoregulatory action of saikosaponin-d (SSd), which was isolated from the root of Bupleurum falcatum L. and has a steroid-like structure, was examined on splenic T lymphocytes of C57BL/6 mice. SSd displayed a definite action in vitro to bidirectionally control the growth response of T lymphocytes stimulated by concanavalin A, anti-CD3 monoclonal antibody, and calcium ionophore A23187 plus phorbol 12-myristate 13-acetate. Low concentrations (1-3 micrograms/ml) of SSd upregulated the responses to suboptimum stimuli of agonists, particularly during the relatively late stage of the responses, whereas it downregulated the responses to supraoptimal stimuli. Under appropriate experimental conditions, SSd promoted interleukin-2 (IL-2) production and IL-2 receptor expression. It also accelerated c-fos gene transcription, but it did not modulate the level of tyrosine phosphorylation of cellular proteins. We concluded from these results that SSd uniquely modulates T lymphocyte function and that at least one target of the action of SSd is located at or before the step of c-fos gene transcription and after T-cell receptor/CD3-mediated protein tyrosine kinase activation.