Lack of inducible nitric oxide synthase activity in T cell clones and T lymphocytes from naive and Leishmania major-infected mice.

Nitric oxide (NO) generated by the inducible isoform of nitric oxide synthase (iNOS) is implicated in a number of immunological processes including killing of intracellular parasites, suppression of T cell proliferation, production of cytokines and destruction of tissue in autoimmune diseases. Considering that cytokine-activated mouse macrophages, fibroblasts and endothelial cells are potent producers of NO, we investigated whether T cells, as central participants in immune responses, can also be activated for the release of NO. Neither thymocytes nor type 1 or type 2 T helper cell clones generated significant amounts of nitrite (the stable end product of NO in culture supernatants) when stimulated by T cell mitogens, cytokines or antigen in the presence of irradiated antigen-presenting cells. Similarly, T cells freshly isolated from mice acutely infected with the intracellular pathogen Leishmania major did not produce NO upon restimulation in vitro. The lack of NO production was not due to the expression of enzymatically inactive iNOS, as we were unable to detect any iNOS protein in activated T helper clones or in freshly isolated T cells from infected mice by Western (protein) blot analysis. Finally, we tested whether iNOS expression in T cells might be restricted to a minor subpopulation and therefore only detectable on a single cell level. After immunofluorescence staining of lymph node or spleen cells from infected mice with antibodies against iNOS, F4/80- or Thy-1-antigen, macrophages, but no T cells, were found to express iNOS. Thus, we have no evidence that activated T helper cell clones or T cells from L. major-infected mice are high producers of NO.