IN-VITRO SIMULATION OF IMMUNOSUPPRESSION CAUSED BY TRYPANOSOMA-BRUCEI- ACTIVE INVOLVEMENT OF GAMMA-INTERFERON AND TUMOR-NECROSIS-FACTOR INTHE PATHWAY OF SUPPRESSION

Experimental infections of mice with the African trypanosome Trypanoso ma brucei lead to a profound state of T-cell unresponsiveness in the l ymph node cell (LNC) compartment. This suppression is mediated by macr ophage-like cells which inhibit interleukin 2 (IL-2) secretion and dow n-regulate IL-2 receptor expression (M. Sileghem, A. Darji, R. Hamers, M. Van de Winkel, and P. De Baetselier, fur. J. Immunol, 19:829-835, 1989). Similar suppressive cells can be generated in vitro by pulsing 2C11-12 macrophage hybridoma cells with opsonized T. brucei parasites (2C11-12P cells), Cocultures of 2C11-12P cells and LNCs secrete higher levels of gamma interferon (IFN-gamma), and the hyperproduction of IF N-gamma was found to be confined to CD8(+) lymphoid cells. Elimination of CD8(+) cells from cocultures of 2C11-12P cells and LNCs restores t he T-cell proliferative response. Furthermore, addition of neutralizin g anti-IFN-gamma antibodies to the cocultures reduces the level of sup pression and concomitantly restores the level of IL-2 receptor express ion. Hence, IFN-gamma plays a cardinal role in this in vitro model for T. brucei-elicited immunosuppression. Cocultures of LNCs and 2C11-12P cells in a two-chamber culture system further demonstrated that cell- cell contact is required for hyperproduction of IFN-gamma and, moreove r, that IFN-gamma cooperates with a 2C11-12P-derived diffusible factor to exert its suppressive activity. Finally, tumor necrosis factor alp ha (TNF-alpha) produced by 2C11-12P cells was found to be implicated i n the hyperproduction of IFN-gamma, since addition of neutralizing ant i-TNF-alpha antibodies to cocultures reduced the level of suppression and concomitantly abrogated the hyperproduction of IFN-gamma. Collecti vely, our findings indicate that T. brucei-elicited suppressive 2C11-1 2 macrophage cells differentially influence T-cell subpopulations: (i) CD8(+) cells are signaled via cell-cell contact to produce IFN-gamma, and TNF-alpha is implicated in this process, and (ii) locally produce d IFN-gamma, and macrophage-released factors act in concert to inhibit CD4(+) and CD8(+) T-cell proliferative responses.