Evidence for intact CD28 signaling in T cell hyporesponsiveness induced by the HIV‐1 nef gene

Abstract

Infection by human immunodeficiency virus (HIV)-1 is associated with quantitative and qualitative T cell alterations that severely impair the host's immune defense system. The molecular basis for this immunosuppression remains unclear. Peripheral blood mononuclear cells (PBMC) isolated from patients show markedly decreased interleukin (IL)-2 secretion but unaffected or even increased T helper (Th)2 cytokine production. T cell functional defects were recently reported to correlate more with T cell receptor (TcR) signaling, whereas signals provided by ligation of co-receptors CD27 and CD28 appeared to be preserved. Among the various mechanisms proposed to be involved in HIV-1-induced T cell dysfunction, we and others have reported that the nef gene product exhibited significant immunosuppressive activity. By using an inducible stably integrated nef gene, we demonstrated that Nef specifically down-regulated IL-2 and interferon (IFN)-gama produced upon TcR triggering. Here, using the same experimental system, we extended our initial observations to additional mitogenic signals, and investigated the co-stimulatory function of CD28. Nef down-regulated IL-2, but not IL-4 produced upon induction by combinations of mitogens that mimicked TcR signals together with CD28 mAb or CD28's natural ligand (CD80 and CD86). However, the co-signals provided by CD28 to up-regulate IL-2 induction were unaffected by Nef, since IL-2 produced by nef-transfected cells was proportionally enhanced to the same extent as that of control cells, either upon stimulation by the CD28 mAb or CD80 and CD86. In addition, phosphatidylinositol-3 kinase recruitment induced upon CD28 triggering was also found to be unaltered by nef expression. Together with the observation that similar levels of the Nef protein were detected in nef-transfected cells and upon infection of PBMC, these data suggest a selective 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 from HIV-infected individuals.

HIV-1 Nef and T-Cell Activation: A History of Contradictions

“…Yet another series of studies indicated that Nef had no effect on NF-κB: Collette et al . observed that CD28 signaling did not contribute to Nef-mediated modulation of IL-2 [75], and Yoon et al . reported that there was no influence of Nef on NF-κB or AP-1 [65].…”

“…One related study found that stimulation with anti-CD28 did not affect Nef’s inhibitory role [75]. By contrast, later studies have employed anti-CD3 and anti-CD28 that are either cross-linked [39,40,42,44,53], presented on beads [45,66,159] or bound to coverglass to mimic the immune synapse [86]; however, the stimulation dose in these assays may not reflect biological interactions with APC.…”

The Nef protein of HIV-1 associates with rafts and primes T cells for activation

“…Finally, simian immunodeficiency virus strains with Nef variants harboring amino acid sequences that resemble immunoreceptor tyrosinebased activation motifs could replicate to high levels in peripheral blood mononuclear cells without a need for exogenous stimulation (9). Contrasting with these results, other reports have suggested that Nef inhibits lymphocyte activation (10)(11)(12)(13)(14)(15)(16)(17)(18)(19): for instance, in Jurkat cell clones stably expressing HIV-1 nef (10)(11)(12)(13)(14) or electroporated with recombinant nonmyristoylated Nef (15,16). However, interpretation is complicated by the possible selection of cells tolerized for Nef (10,14) or the improper subcellular localization of nonmyristoylated Nef (15,16).…”

“…However, interpretation is complicated by the possible selection of cells tolerized for Nef (10,14) or the improper subcellular localization of nonmyristoylated Nef (15,16). Two other studies relied on an inducible system, but the inducing agent was phorbol myristate acetate (PMA), itself a major modulator of T cell activation (17,18).…”

Design and In Vivo Immunogenicity of a Polyvalent Vaccine Based on SIVmac Regulatory Genes

“…Both the HIV-1 Tat and Nef have been demonstrated to inhibit proliferation of T-cells (Viscidi et al, 1989;Collette et al, 1996b;Wrenger et al, 1997). To assess whether this would also be the case for the related SIVmac Tat and Nef proteins, PBMC of vaccinated (691 and 3152) or infected (22M and 25M) macaques known to have positive proliferative responses to p27 Gag were tested in a proliferation assay with the p27 Gag in the presence or absence of native or heat-inactivated Tat and Nef proteins.…”