HIV-1 Nef regulates the release of superoxide anions from human macrophages

The NADPH oxidase enzymatic complex participates in the oxidative burst by producing ROS (reactive oxygen species). Altered levels of ROS production may have pathogenetic implications due to the loss of some innate immune functions such as oxidative burst and phagocytosis. Considering that HIV-1 Nef protein plays a primary role in AIDS pathogenesis, by affecting the immune system, we sought to dissect possible effects of Nef on the release of superoxide anions. We show here that the inducible expression of Nef in human phagocytic cells modulates the superoxide release in a biphasic manner. In particular, an early Nef-induced increase of the superoxide release was followed by a dramatic decrease starting from 10 h after the Nef induction. This was observed whatever the presence of cell activators such as GM-CSF (granulocyte/macrophage colony-stimulating factor) or fMLP (N-formyl-L-methionyl-L-leucyl-L-phenylalanine). Whereas the early increase in superoxide release is probably the result of the already described Nef-dependent activation of PAK-2 (p21-activated kinase 2)-Rac2, we were interested in investigating the mechanisms underlying the late inhibition of superoxide release observed originally. In this regard, we individuated at least three independent requirements for the Nef-induced blockade of superoxide release: (i) the active protein synthesis; (ii) both the membrane localization and the interaction with endocytotic machinery of Nef; and (iii) the release of soluble factor(s). Moreover, we observed that IL-10 (interleukin-10) inhibits superoxide release, whereas its depletion restored NADPH oxidase activity. We propose that the cell membrane-to-lysosome Nef transit leads to the synthesis and release of soluble factor(s) and, among them, IL-10 might significantly contribute to the inhibition of NAPDH oxidase activity.