Role of HIV-1 Nef expression in activation pathways in CD4+ T cells

Intracellular overexpression of HIV-1 Nef impairs differentiation and maturation of monocytic precursors towards dendritic cells

Nef functions as an immunosuppressive factor critical for HIV-1 replication, survival and development of AIDS following HIV-1 infection. What effects Nef exerts on differentiation and maturation of monocytes towards dendritic cells (DCs) remains greatly controversial. In this study, we used THP-1 (human monocytic leukemia cell line) as monocytic DC precursors to investigate how overexpression of HIV-1 Nef influences the processes of differentiation and maturation of dendritic cells. In striking contrast to negative controls, our results showed that morphological and phenotypical changes (CD11c, CD14, CD40, CD80, CD83, CD86, and HLA-DR) occurred on recombinant THP-1 expressing HIV-1 Nef (short for Nef) upon co-stimulation of GM-CSF/IL-4 or GM-CSF/IL-4/TNF-α/ionomycin. Moreover, CD4, CCR5, and CXCR4 were also down-regulated on Nef. It might be hypothesized that Nef prevents superinfection and signal transduction in HIV-1 infected monocytes. Collectively, our study demonstrates that long-lasting expression of Nef at high levels indeed retards differentiation and maturation of dendritic cells in terms of phenotype and morphology. We are hopeful that potentially, stable expression of intracellular Nef in vivo may function as a subtle mode to support long-lasting HIV-1 existence.

CD4+ T cells from CD4C/HIVNef transgenic mice show enhanced activation in vivo with impaired proliferation in vitro but are dispensable for the development of a severe AIDS-like organ disease

The cellular and molecular mechanisms of dysfunction and depletion of CD4+ T lymphocytes over the course of human immunodeficiency virus type 1 (HIV-1) infection are still incompletely understood, but chronic immune activation is thought to play an important role in disease progression. We studied CD4+ T-cell biology in CD4C/HIV transgenic (Tg) mice, in which Nef expression is sufficient to induce a severe AIDS-like disease including a preferential decrease of CD4+ T cells. We show here that Nef-expressing Tg CD4+ T cells exhibit an activated/memory-like phenotype which appears to be independent of antigenic stimulation, as documented in experiments involving breeding with AD10 TcR Tg mice. In addition, in vivo bromodeoxyuridine incorporation showed that a larger proportion of Tg than non-Tg CD4+ T cells entered the S phase. However, in vitro, Tg CD4+ T cells were found to have a very limited capacity to divide in response to stimulation with anti-CD3 and anti-CD28 or in allogeneic mixed leukocyte reactions. Interestingly, despite these observations, the deletion of Tg CD4+ T cells had little impact on the development of other AIDS-like organ phenotypes. Thus, the Nef-induced chronic activation of CD4+ T cells may exhaust the T-cell pool and may contribute to the thymic atrophy and the low number of CD4+ T cells observed in these Tg mice.

HIV-1 Nef interacts with inositol trisphosphate receptor to activate calcium signaling in T cells

HIV-1 pathogenicity factor Nef has been shown to modulate calcium signaling in host cells, but the underlying molecular mechanisms have remained unclear. Here we show that calcium/calcineurin-dependent activation of nuclear factor of activated T cells (NFAT) by Nef in Jurkat T cells requires the endoplasmic reticulum-resident inositol trisphosphate receptor (IP(3)R), but yet does not involve increase in phospholipase-C gamma 1 (PLC gamma 1)-catalyzed production of IP(3) or depletion of IP(3)-regulated intracellular calcium stores. Nef could be coprecipitated with endogenous IP(3)R type-1 (IP(3)R1) from Nef-transfected Jurkat T cells as well as from HIV-infected primary human peripheral mononuclear cells. Thus, the Nef/IP(3)R1-interaction defines a novel T cell receptor-independent mechanism by which Nef can promote T cell activation, and appears to involve atypical IP(3)R-triggered activation of plasma membrane calcium influx channels in a manner that is uncoupled from depletion of intracellular calcium stores.

HIV-1 Nef increases T cell activation in a stimulus-dependent manner

Lentiviral Nef increases viral replication in vivo, plays a direct role in pathogenesis, and increases viral particle infectivity. We now find that HIV Nef also increases the activation of T cells, a cellular state required for optimal viral replication. This enhancement is stimulant-dependent. As defined by IL-2 generation, activation of T cells stimulated with classical mitogens [phorbol 12-myristate 13-acetate (PMA) + anti-CD3, PMA + phytohemagglutinin, and PMA + ionomycin] is unaffected by the expression of Nef. However, Nef increases IL-2 secretion when cells are stimulated through the T cell receptor and the costimulus receptor (CD28). This increase in activation, which depends on Nef myristylation, is caused by an increase in the number of cells reaching full activation and not by an increase in the amount of IL-2 secreted per cell. These findings demonstrate that Nef lowers the threshold of the dual-receptor T cell activation pathway. The capacity of Nef to increase T cell activity may be very important in vivo when Nef is the predominant or the only viral gene product expressed.

The Nef protein of primate lentiviruses

The Nef protein of primate lentiviruses acts as an important virulence factor in vivo both in monkeys and in humans. Among a human cohort of long-term non-progressors, several Nef defective HIV1 viruses have been isolated, indicating that Nef may accelerate HIV progression and disease in humans. Additionally, a Nef-deleted SIV virus has low titres in rhesus monkeys and the animals develop AIDS at a much slower rate. In vitro, Nef can exert at least three kinds of effects: it downregulates CD4 and MHC class I, it stimulates virion infectivity and it alters signal transduction pathways. To accomplish this, Nef interacts with a series of cellular partners including CD4, components of the adaptor complexes AP-1 and AP-2, and several protein kinases, Nef often functioning as a connector between targets and effectors. The high degree of understanding of at least some aspects of Nef action, as well as the importance of this viral gene product for disease induction, identify Nef as a valuable target for the development of novel antiviral therapies. Moreover, the possibility of developing vaccines using attenuated viruses with deletions in nef and other crucial genes raises the possibility that the AIDS epidemic might one day be restrained.

Human immunodeficiency virus type 1 Nef does not alter T-cell sensitivity to antigen-specific stimulation

We have developed an in vitro model to study the influence that human immunodeficiency virus type 1 (HIV-1) may have on the ability of T cells to respond to antigenic challenge. We have examined consequences of HIV-1 gene expression on T-cell activation in antigen-dependent T cells that have stably integrated copies of replication-defective proviral HIV-1. Virus production by HIV-infected, antigen-dependent T cells was induced in response to antigenic stimulation and then decreased as infected cells returned to a state of quiescence. Contrary to the predictions of models proposing that Nef alters signal transduction pathways in T lymphocytes and thereby alters cellular activation, Nef expression in antigen-dependent T-cell clones did not influence their proliferative responses to low or intermediate concentrations of antigen and did not affect other measures of T-cell activation, such as induction of interleukin 2 receptor alpha-chain expression and cytokine production. In addition, we found no evidence for alteration of T-cell responsiveness to antigen by the gag, pol, vif, tat, or rev gene of HIV-1.

Priming of human CD4+ antigen-specific T cells to undergo apoptosis by HIV-infected monocytes. A two-step mechanism involving the gp120 molecule

The study of the pathology of HIV-1 infection in chimpanzees supports the idea of the crucial role of HIV-infected monocytes in the pathogenesis of AIDS, although viral mechanisms that lead to T cell dysfunction and deletion during HIV infection are still unclear. We show here that HIV-1-infected antigen-presenting monocytes (APCs) are able to prime in vitro non-HIV-infected antigen-specific CD4+ T cell lines or peripheral blood CD4+ T cells to undergo apoptosis after antigen-specific restimulation. The priming of T cells for apoptosis occurs in the absence of HIV replication in the T cells. Priming for apoptosis required two concomitant signals present on the same APC, an antigenic stimulus and a second signal provided by the HIV gp120 protein as demonstrated by the use as APCs of EBV-LCLs infected with different recombinant deleted proviruses or transfected with different HIV proteins. These results provide a mechanism for the priming for apoptosis of T cells in HIV-infected patients, implicating a role for HIV-infected APCs in the induction of T cell dysfunction and depletion in AIDS.