Beschreibung:
<jats:title>Abstract</jats:title><jats:p>At least two subsets of CD4<jats:sup>+</jats:sup> T helper cell lymphocytes termed T<jats:sub>h</jats:sub>1 and T <jats:sub>h</jats:sub>, 2 exist in the mouse and probably in humans. They are characterized by the secretion of different lymphokines and by their functional behavior. Dysregulated expansion of one or the other subset may be one reason for the development of certain diseases. Thus, it is of importance to define the signals involved in the differentiation and activation of the two T<jats:sub>h</jats:sub> cell subsets. It is known and has been confirmed in this report that the cytokine interleukin (IL)‐1 acts onT<jats:sub>h</jats:sub>2 cells but not on T<jats:sub>h</jats:sub>1 cells. We now report that a previously identified cytokine which was provisionally termed T cell stimulating factor is identical with IL‐12 and exhibits a reciprocal behaviour to IL‐1. IL‐12 has several effects on T<jats:sub>h</jats:sub>1 cells. It can induce the proliferation of certain T<jats:sub>h</jats:sub>1 cells in combination with IL‐2. Synthesis of interferon (IFN)‐γ by T<jats:sub>h</jats:sub>1 cells can be triggered by IL‐2 plus IL‐12. In contrast to the IFN‐γ production observed after T cell receptor (TcR) CD3 stimulation of T<jats:sub>h</jats:sub>1 cells with lectin Concanavalin A the IFN‐γ production induced by IL‐12+IL‐2 is insensitive to the immunosuppressive drug cyclosporin A. Furthermore, IL‐12 enhances the TcR/CD3‐induced synthesis of IFN‐γ of several T<jats:sub>h</jats:sub>1 clones. Finally, IL‐12 (+ IL‐2) induces homotypic cell aggregation of T<jats:sub>h</jats:sub>1 clones. This type of cell aggregation depends on the participation of LFA‐1 and ICAM‐1 molecules. In all activation systems with T<jats:sub>h</jats:sub>1 cells no effect of IL‐1 was demonstrable. In contrast, only IL‐1 but not IL‐12 served as a co‐stimulatory signal for several T<jats:sub>h</jats:sub>2 cell lines activated via the TcR/CD3 complex.</jats:p>