Suppressor macrophages in African trypanosomiasis inhibit T cell proliferative responses by nitric oxide and prostaglandins.

Suppression of host T cell responses is one of the hallmarks of infection with the African trypanosomes. The cellular basis for immunosuppression includes the generation of suppressor macrophages that down-regulate T cell proliferative but not necessarily cytokine responses to both mitogen and trypanosome Ag. Since macrophages from infected animals display activation characteristics, we have asked whether products of activated cells, specifically nitric oxide (NO) and PG, may mediate the suppressor cell effects and immunosuppression observed. We demonstrate that cells isolated from B10.BR mice infected with Trypanosoma brucei rhodesiense exhibited transcriptional up-regulation of inducible NO synthase and released significant amounts of NO. The levels of NO released were elevated further after stimulation of cells with T cell mitogens or specific parasite Ag; antibody blocking experiments demonstrated that this up-regulation of NO synthesis was at least partially dependent upon IFN-gamma and TNF-alpha. The addition of inducible NO synthase substrate analogues such as NG-monomethyl-L-arginine to cell cultures inhibited NO release and also partially reversed the suppressor cell activity and immunosuppression displayed by such cultures. PG levels also were elevated in cell cultures from infected mice, but the PG inhibitor indomethacin had no effect on suppressor cells or suppression when added alone to the cultures. However, the concurrent inhibition of NO and PG synthesis by the addition of both NG-monomethyl-L-arginine and indomethacin completely blocked suppressor cell activity associated with infected macrophages and also resulted in further recovery of infected cells from immunosuppression, thus revealing an epistatic effect between these two mediators. We conclude that macrophage activation in trypanosomiasis induces the release of reactive nitrogen intermediates and PG, which down-regulate proliferative responses by T cells during infection.

Characterization of T helper cell responses to the trypanosome variant surface glycoprotein.

T cell responses to the variant surface glycoprotein (VSG) previously have not been detected in animals infected with the African trypanosomes despite the fact that such animals make strong T-dependent B cell responses to VSG molecules displayed by the parasites. In the present study, we have examined B 10.BR mice for VSG-specific Th cell responses at different times after infection with Trypanosoma brucei rhodesiense clone LouTat 1. T cell populations derived from different tissues were tested for their ability to proliferate and secrete cytokines when stimulated with purified LouTat 1 VSG. Furthermore, VSG-specific T cell lines and clones were derived from immunized mice and examined for their phenotypic and functional profiles in comparison with T cell responses of infected mice. The results of this study show that VSG-specific T cells were not consistently detected in the peripheral lymphoid tissues such as spleen or lymph nodes of infected animals. In contrast, VSG Ag-specific T cells were detectable principally in the peritoneal T cell populations of infected mice. Peritoneal T cells did not proliferate in response to VSG, yet produced substantial cytokine responses when stimulated; the cytokines produced were IFN-gamma and IL-2, without detectable IL-4. The cellular phenotype of VSG-responsive T cells was that of classical Th cells in that all cells were CD4-positive and expressed the CD3 alpha/beta TCR membrane complex. Thus, the VSG appears to preferentially stimulate a Th1 cell subset response during infection. Intrinsic molecular characteristics of the VSG molecule did not induce mice to make this response, however, since VSG-specific T cell lines derived from VSG-immunized mice displayed cytokine profiles characteristic of both Th1 and Th2 cells. Isolation of Th1 clones from selected lines demonstrated that these cells displayed the same membrane-phenotypic characteristics and cytokine profiles as the T cells from infected mice. Furthermore, all Th clones were VSG type-specific, APC-dependent, and I-Ak-restricted in their responses. In summary, these experiments provide the first direct evidence for VSG-specific responses at the T cell level. T cell responses to the VSG molecule during infection appear to be anatomically compartmentalized and exhibit evidence of clonal maturation (cytokine production) but not clonal expansion (proliferation) after antigenic stimulation. The cellular phenotype and cytokine profiles predict that infection predisposes the animals to mount Th1 cell subset responses to VSG. The results of this study, including the T clones generated, provide an experimental basis for examining the regulation of VSG-specific immune responses during infection.