Algeciras, Alicia

Persistent human immunodeficiency virus (HIV) infection of human monocytes and macrophages increases I kappa B alpha degradation, resulting in the activation of NF-kappa B, a key transcription factor in the regulation of the HIV long terminal repeat. The signal transduction pathways leading to NF-kappa B activation in cells of the monocytic lineage, especially those regulated by HIV infection, and their relevance in regulating viral persistence remain unknown. Both p21ras and its downstream Raf-1 kinase participate in the transduction of signals initiated from a variety of cell surface receptors and in the regulation of transcription factors. We have studied whether the Ras-Raf pathway is functional and participates in HIV-mediated NF-kappa B activation in monocytic cells. Constitutively active p21ras (v-H-Ras) activated NF- kappa B-dependent transcription and induces the nuclear translocation of a bona fide p65/p50 heterodimer by targeting I kappa B alpha. In addition, the constitutively active form of Raf (RafBXB) also increases the NF-kappa B-dependent transcriptional activity. Because of the similarity between HIV and Ras-Raf-induced NF-kappa B activation in monocytic cells, we next tested whether HIV-induced NF-kappa B activation was mediated by the Ras-Raf signal transduction pathway. Negative dominant forms of both Ras (Ras N17) and Raf (Raf 301) decreased the HIV- but not lipopolysaccharide-dependent NF-kappa B activation in U937 cells. Moreover, Raf-1 kinase activity was greater in HIV-infected than uninfected monocytic cells in in vitro kinase assays. Altogether, these results indicate that the Ras-Raf pathway is unregulated in HIV monocytic cells and participates in the virus-induced activation of NF-kappa B.

The current knowledge of CD4 function is limited to its role as a necessary coreceptor in TCR-initiated signaling. We have investigated whether CD4 regulates additional T cell functions. Using human primary resting CD4+ T cells, we demonstrate that CD4 activation is sufficient to induce lymphocyte death. Immediately after CD4 cross-linking, CD4+ T cells are rendered susceptible to apoptosis mediated by TNF or FasL. This, together with the concomitant induction of FasL within the same population, results in significant CD4+ T cell death in vitro. The CD4-dependent induction of susceptibility to apoptosis that is mediated by TNF or FasL is protein synthesis independent but phosphorylation dependent. After CD4 activation, PKC regulates susceptibility to apoptosis mediated by FasL but not the induction of susceptibility to TNF-dependent apoptosis. Moreover, significant differences between CD3 and CD4 activation were observed with regards to the kinetics of induction of CD4+ T cell susceptibility to FasL- and TNF-mediated apoptosis. Altogether, these results provide a model with which to study the molecular mechanisms regulating lymphocyte survival after CD4 activation, and highlight the potential role of CD4 in controlling lymphocyte apoptosis under physiological conditions or in disease states such as HIV infection.