EVIDENCE FOR INTACT CD28 SIGNALING IN T-CELL HYPORESPONSIVENESS INDUCED BY THE HIV-1 NEF GENE

Infection by human immunodeficiency virus (HIV)-1 is associated with q uantitative and qualitative T cell alterations that severely impair th e host's immune defense system. The molecular basis for this immunosup pression remains unclear. Peripheral blood mononuclear cells (PBMC) is olated from patients show markedly decreased interleukin (IL)-2 secret ion but unaffected or even increased T helper (Th)2 cytokine productio n. T cell functional defects were recently reported to correlate more with T cell receptor (TcR) signaling, whereas signals provided by liga tion of co-receptors CD27 and CD28 appeared to be preserved. Among the various mechanisms proposed to be involved in HIV-1-induced T cell dy sfunction, we and others have reported that the nef gene product exhib ited significant immunosuppressive activity. By using an inducible sta bly integrated nef gene, we demonstrated that Nef specifically downreg ulated IL-2 and interferon (IFN)-gamma produced upon TcR triggering. H ere, using the same experimental system, we extended our initial obser vations to additional mitogenic signals, and investigated the co-stimu latory function of CD28. Nef down-regulated IL-2, but not IL-4 produce d upon induction by combinations of mitogens that mimicked TcR signals together with CD28 mAb or CD28's natural ligand (CD80 and CD86). Howe ver, the co-signals provided by CD28 to up-regulate IL-2 induction wer e unaffected by Nef, since IL-2 produced by nef-transfected cells was proportionally enhanced to the same extent as that of control cells, t ither upon stimulation by the CD25 mAb or CD80 and CD86. In addition, phosphatidylinositol-3 kinase recruitement induced upon CD28 triggerin g was also found to be unaltered by nef expression. Together with the observation that similar levels of the Nef protein were detected in ne f-transfected cells and upon infection of PBMC, these data suggest a s elective immunosuppression induced by nef in human T cells by altering TcR signaling without detectable impact on CD28 co-receptor function. These data agree with the T cell defects observed in PBMC isolated fr om HIV-infected individuals.