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Macrophages and NK cells help provide innate immunity against infection by intracellular parasites and communicate with each other to regulate this process. When stimulated, macrophages secrete the cytokine IL-12 which is essential for activation of the cytotoxic activity of natural killer (NK) cells. IL-12 stimulates this NK cell response through activation of a JAK/STAT signaling pathway. Binding of IL-12 to its receptor on NK cells causes tyrosine phosphorylation and activation of JAK2 and another JAK kinase, Tyk2. Tyk2 in turn phosphorylates the transcription factor STAT4, which can then translocate to the nucleus to activate genes, including the expression of interferon-gamma. IFN-gamma and IL-12 induce the differentiation of TH1 helper T cells that activate macrophages through interferon-gamma. A key modulator of NK cell activation by IL-12 is nitric oxide, NO, produced from arginine by the inducible nitric oxide synthase NOS2. Inactivation of the NOS2 gene in mice impairs the initial innate response to infection, including the activation of NK cells. This phenotype is similar to that of mice lacking interferon-gamma or IL-12 and a similar effect can be produced by inhibiting NOS2 with L-N6-iminoethyl-lysine (L-NIL) during infection, blocking the activation of NK cells by IL-12. NO production does not affect the activation of JAK2 by IL-12, but is required for Tyk2 activity, the downstream activation of STAT-4 and the production of interferon-gamma by NK cells. The role of NO in Tyk2 activation is not yet known, but does not seem to involve changing the phosphorylation status of Tyk2. Curiously, activation of T cells by IL-12 does not appear to involve NO production, indicating a distinct mechanism is involved in NK cells. NO also plays a role as a feedback inhibitor of IL-12 production by macrophages, helping to prevent over-activation of the Th1 cells by IL-12. NO appears to make an important contribution to the early innate response to infections before specific immunity is active. (This definition may be outdated - see the DesignNote.)
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