Role of IL-23 in mobilization of immunoregulatory nitric oxide- or superoxide-producing Gr-1+ cells from bone marrow.

Spleens of mice injected with heat-killed Mycobacterium tuberculosis increase their Gr-1+ cell content and develop a system of interactive Ly-6G+ and Ly-6G-Gr-1+ populations or "Greg" subsets, which, upon stimulation by activated T cells, produce immunoregulatory superoxide (O2(-)) and nitric oxide (NO), respectively. The balance between immunosuppressive NO and its antagonist O2(-) regulates T cell expansion, similar to regulation of vasodilation. Reduction of NO levels by O2(-) is required for efficient T cell expansion and development of autoimmunity. We studied the source of Gr-1+ cells in bone marrow (BM), where their levels were higher than in spleen, with both Greg subsets expressing strong activity. In the spleens of primed IL-23-/- mice, Ly-6G+ cells remained at naïve levels and produced no O2(-). The complementary Ly-6G(-)Gr-1+ splenocytes and their suppressive activity were partially reduced. Surprisingly, Gr-1+ cell levels in BM of IL-23-/- mice were increased, as were their O2(-) and NO production. Transfer of primed BM cells partially restored regulatory function in the spleen of IL-23-/- recipients. The results suggest that IL-23 is involved in mobilization of O2(-)- and NO-producing Gr-1+ cells from BM, which may contribute to its widely studied role in (auto)immunity.

Mycobacteria-induced Gr-1+ subsets from distinct myeloid lineages have opposite effects on T cell expansion.

Similar to the regulation of vasodilation, the balance between NO and superoxide (O2-) regulates expansion of activated T cells in mice. Reduction of suppressive NO levels by O2- is essential for T cell expansion and development of autoimmunity. In mice primed with heat-killed Mycobacterium, a splenocyte population positive for Gr-1 (Ly-6G/C) is the exclusive source of both immunoregulatory free radicals. Distinct Gr-1+ cell subpopulations were separated according to Ly-6G expression. In culture with activated T cells, predominantly monocytic Ly-6G- Gr-1+ cells produced T cell-inhibitory NO but no O2-. However, mostly granulocytic Ly-6G+ cells produced O2- simultaneously but had no measurable effect on proliferation. Recombination of the two purified Gr-1+ subpopulations restored controlled regulation of T cell proliferation through NO and O2- interaction. Coculture of p47phox-/- and inducible NO synthase-/- Gr-1+ cells confirmed this intercellular interaction. These data suggest that bacterial products induce development of distinct Gr-1+ myeloid lineages, which upon stimulation by activated T cells, interact via their respective free radical products to modulate T cell expansion.

Extra-cellular superoxide promotes T cell expansion through inactivation of nitric oxide.

The mechanism and regulation of immunosuppression by nitric oxide (NO) is unclear. Extra-cellular superoxide (EC-O2-) production by NADPH-oxidase (phox) may prevent NO-mediated suppression of T cell proliferation. p47(phox-/-) mice are resistant to experimental allergic encephalomyelitis (EAE), coinciding with enhanced splenic NO activity, but no causal link was established. Here, we demonstrate such link, since p47(phox-/-) mice developed severe EAE by adoptive transfer, but only if NO production during ex vivo donor cell reactivation was inhibited. EC-O2- production increased during cognate T cell reactivation, while inhibition of EC-O2- by exogenous superoxide dismutase enhanced NO activity. By inhibiting NO, EC-O2- production promotes T cell expansion during peripheral immune-response activation, not during tissue inflammation.

Tissue expression of inducible nitric oxide synthase requires IFN-gamma production by infiltrating splenic T cells: more evidence for immunosuppression by nitric oxide.

The cause of increased severity of experimental allergic encephalomyelitis (EAE) in inducible nitric oxide synthase (iNOS)(-/-) or IFN-gamma(-/-) mice remains unclear. Transient chimeras were generated to examine the source of iNOS-inducing IFN-gamma in the central nervous system (CNS). IFNgamma(-/-) donor cells induced severe EAE but no iNOS expression in the CNS of wild-type recipients upon their immunization. By contrast, milder EAE, but strong iNOS expression was induced in immunized recipients of wild-type donor spleen cells. These results demonstrate that IFN-gamma secretion by infiltrating spleen cells is essential for iNOS expression during CNS inflammation. Furthermore, splenic NO is immunosuppressive.