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Trends Immunol. 2003 Jun;24(6):302-6.

L-arginine metabolism in myeloid cells controls T-lymphocyte functions.

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Department of Oncology and Surgical Sciences, Via Gattamelata 64, 35128, Padua, Italy.


Although current attention has focused on regulatory T lymphocytes as suppressors of autoimmune responses, powerful immunosuppression is also mediated by a subset of myeloid cells that enter the lymphoid organs and peripheral tissues during times of immune stress. If these myeloid suppressor cells (MSCs) receive signals from activated T lymphocytes in the lymphoid organs, they block T-cell proliferation. MSCs use two enzymes involved in arginine metabolism to control T-cell responses: inducible nitric oxide synthase (NOS2), which generates nitric oxide (NO) and arginase 1 (Arg1), which depletes the milieu of arginine. Th1 cytokines induce NOS2, whereas Th2 cytokines upregulate Arg1. Induction of either enzyme alone results in a reversible block in T-cell proliferation. When both enzymes are induced together, peroxynitrites, generated by NOS2 under conditions of limiting arginine, cause activated T lymphocytes to undergo apoptosis. Thus, NOS2 and Arg1 might act separately or synergistically in vivo to control specific types of T-cell responses, and selective antagonists of these enzymes might prove beneficial in fighting diseases in which T-cell responses are inappropriately suppressed. This Opinion is the second in a series on the regulation of the immune system by metabolic pathways.

[Indexed for MEDLINE]

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