Nitric oxide was found to trigger mitochondrial biogenesis in cells as diverse as brown adipocytes and 3T3-L1, U937, and HeLa cells. This effect of nitric oxide was dependent on guanosine 3',5'-monophosphate (cGMP) and was mediated by the induction of peroxisome proliferator-activated receptor gamma coactivator 1alpha, a master regulator of mitochondrial biogenesis. Moreover, the mitochondrial biogenesis induced by exposure to cold was markedly reduced in brown adipose tissue of endothelial nitric oxide synthase null-mutant (eNOS-/-) mice, which had a reduced metabolic rate and accelerated weight gain as compared to wild-type mice. Thus, a nitric oxide-cGMP-dependent pathway controls mitochondrial biogenesis and body energy balance.

Human herpesvirus 6 (HHV-6) is a potentially immunosuppressive CD4؉ -T-lymphotropic betaherpesvirus that causes severe human thymocyte depletion in heterochimeric SCID-hu thy/liv mice and has been implicated as a potential cofactor in the progression of AIDS. However, the mechanisms of HHV-6-mediated immunosuppression have not yet been fully elucidated. We investigated the phenotypic and functional alterations induced by HHV-6 on peripheral blood-derived human dendritic cells (DC). The infection of DC with HHV-6 A or B was nonproductive, as revealed by calibrated real-time PCR measuring the accumulation of viral genome equivalents over time. Nevertheless, preexposure to HHV-6 markedly impaired the maturation of DC driven by gamma interferon and lipopolysaccharide, as shown by the reduced surface expression of major histocompatibility complex class I molecules, HLA-DR, CD40, and CD80. Moreover, HHV-6, but not the closely related betaherpesvirus HHV-7, dramatically suppressed the secretion of interleukin-12 (IL-12) p70 by DC, while the production of other cytokines that influence DC maturation, i.e., IL-10 and tumor necrosis factor alpha, was not significantly modified. Likewise, the secretion of the CC chemokines macrophage inflammatory protein 1␤ and RANTES was unaltered. Functionally, a pretreatment with HHV-6 impaired the ability of DC to stimulate allogeneic T-cell proliferation. Altogether, these data identify interference with the functional maturation of DC as a potential mechanism of HHV-6-mediated immunosuppression.

Tumor necrosis factor-␣ (TNF␣)-induced maturation of dendritic cells (DC), with down-regulation of their endocytic ability, has been reported to be mediated by the accumulation of the lipid messenger ceramide. We have now studied the effects and mechanisms of action of NO on endocytosis, investigated with fluorescein isothiocyanate-labeled dextran using human monocyte-derived DC, both immature and after treatment with TNF␣. Exposure of DC to NO, released by either bystander phagocytes or NO donors, reversed the inhibition of endocytosis induced by TNF␣. The intracellular accumulation of ceramide induced by TNF␣ was also inhibited by NO. In addition, NO was found to exert an inhibitory effect downstream of the TNF␣-triggered ceramide accumulation, because NO donors reversed the inhibition of endocytosis induced by the cell-permeant C 2 -ceramide. These effects of NO were mimicked by the membrane-permeant cyclic GMP analogue, 8-Br cyclic GMP, and prevented by inhibition of the soluble guanylyl cyclase. At variance with rodents, the inducible isoform of the NO synthase was expressed neither in immature human DC nor after cell treatment with TNF␣, interferon-␥, and lipopolysaccharide, suggesting that regulation of these cells depends on exogenous NO. NO, working through cyclic GMP, might therefore prolong the ability of human DC to internalize antigens at the site of inflammation and thus modulate the initial steps leading to antigen-specific immune responses.

Nitric oxide (NO), generated by phagocytes at inflammation sites, contributes to regulate immune responses through autocrine and paracrine actions on bystander cells. Among the latter are dendritic cells (DCs). Little is known about regulation of DC function by NO, especially in the human system. We exposed human monocyte-derived DCs to the NO donor (z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino] diazen-1-ium-1,2 diolate (DETA-NO) during their maturation process induced by treatment with tumor necrosis factor alpha or lipopolysaccharide or by CD40 activation. We report here that after exposure to DETA-NO, DCs exhibit a significantly increased ability to activate T lymphocytes stimulated by mycobacterial antigens, Staphylococcus aureus Cowen strain B, allo-antigens, or cross-linking of the CD3-T cell receptor complex. This effect persists after removal of DETA-NO, depends on the generation of cyclic guanosine 5'-monophosphate, and is a result of enhanced release by DCs of soluble factors, in particular interleukin (IL)-12. This modulation of DC function is a result of a synergism between NO and the various maturation stimuli, as neither enhanced T cell activation nor IL-12 release was observed after DC exposure to DETA-NO only. These results provide the first evidence that NO acts as a cosignaling molecule regulating human DC response to maturation stimuli.

Vgamma9/Vdelta2+ T cells specifically recognize Mycobacterium tuberculosis in vitro and are precociously recruited in early mycobacterial lesions. Even if gammadelta T cells are only fortuitously detected in granulomas or bronchoalveolar lavages of patients with active pulmonary tuberculosis, a role in shaping the mature alphabeta T cell response against M. tuberculosis is substantiated. Here we provide a molecular explanation for this paradox: the engagement of the gammadelta TCR by mycobacterial antigens induced the expression of CD95 ligand (CD95L) by chronically activated CD95+/CD95L- gammadelta T lymphocytes. The receptor was functional, as CD95/CD95L interaction triggered the bystander death of CD95+ cells by apoptosis. Cell death was abolished by CD95-blocking antibodies. The transient accumulation at the site of infection of CD95L+ gammadelta lymphocytes, capable of interacting with CD95+ leukocytes attracted by the response towards the pathogen, may determine the characteristics of the ensuing granulomatous disease.