A purified protein from Salmonella typhimurium inhibits proliferation of murine splenic anti‐CD3 antibody‐activated T‐lymphocytes

In a previous study, we observed that the purified substance Salmonella typhimurium ‐derived inhibitor of T‐cell proliferation (STI) had an immunosuppressive effect, demonstrated as the suppression of mitogenic lectin‐induced proliferation of murine spleen cells. In the present study, we confirmed the immunosuppressive effect of STI, which suppressed the proliferation of murine splenic T‐lymphocytes activated with the anti‐CD3 antibody (Ab) and phorbol 12‐myristate‐13 acetate (PMA) and this phenomenon was accompanied by augmentation of interferon‐γ (IFN‐γ) secretion and inhibition of interleukin‐2 (IL‐2) secretion. Furthermore, the augmentation of IFN‐γ secretion caused IL‐2 receptor α chain (IL‐2R α) over expression on T‐cells. However, the addition of an anti‐IFN‐γ Ab and recombinant IL‐2 (rIL‐2) did not reverse the suppressed T‐cell proliferation, although the level of IL‐2R α expression on T‐cells recovered to around normal. Furthermore, Western blotting using an anti‐phosphotyrosine Ab showed that IL‐2R‐mediated tyrosine phosphorylation of protein substrates in T‐cells was inhibited by incubation with STI for 48 h and this inhibition was not reversed by adding the anti‐IFN‐γ Ab and rIL‐2. These results suggest that STI‐induced suppression of T‐cell proliferation involves a defect in IL‐2R function and/or IL‐2 signaling pathway in T‐cells.