The Presence of a vpu Gene and the Lack of Nef-Mediated Downmodulation of T Cell Receptor-CD3 Are Not Always Linked in Primate Lentiviruses

Abstract

Nef is an accessory protein critical for the ability of human and simian immunodeficiency viruses (HIV and SIV) to replicate efficiently in their respective hosts. Previous analyses of members of 15 different primate lentivirus lineages revealed a link between Nef function and the presence of a vpu gene. In particular, Nef proteins of all vpu-containing viruses had lost their ability to downmodulate the T cell (TCR-CD3) receptor. Here we examined Nef proteins from eight additional SIV lineages, including SIVgor, SIVwrc, SIVolc, SIVgri, SIVdrl, SIVlho, SIVden, and SIVasc, from western lowland gorillas, western red colobus monkeys, olive colobus monkeys, grivet monkeys, drills, L'Hoest's monkeys, Dent's mona monkeys, and red-tailed monkeys, respectively. We found that except for the nef gene of SIVdrl, all of them were efficiently expressed and modulated CD4, major histocompatibility complex class I (MHC-I), CD28, CXCR4, and Ii cell surface expression and/or enhanced viral infectivity and replication. Furthermore, the Nef proteins of SIVgri, SIVlho, SIVwrc, SIVolc, and SIVgor antagonized tetherin. As expected, the Nef protein of SIVgor, which carries vpu, failed to downmodulate CD3, whereas those of SIVwrc, SIVgri, SIVlho, and SIVasc, which lack vpu, were capable of performing this function. Surprisingly, however, the Nef protein of the vpu-containing SIVden strain retained the ability to downmodulate TCR-CD3, whereas that of SIVolc, which does not contain vpu, was unable to perform this function. Although the SIVden Vpu is about 20 amino acids shorter than other Vpu proteins, it degrades CD4 and antagonizes tetherin. Our data show that there are exceptions to the link between the presence of a vpu gene and nef alleles deficient in CD3 modulation, indicating that host properties also affect the selective pressure for Nef-mediated disruption of TCR-CD3 signaling. Our results are also further evidence that tetherin antagonism is a common function of primate lentivirus Nef proteins and that the resistance of human tetherin to Nef represents a relevant barrier to cross-species transmission of SIVs to humans.

Lentiviral Nef Proteins Manipulate T Cells in a Subset-Specific Manner

“…To examine the susceptibility of different T cell subsets to HIV-1 infection, PBMCs and HLACs were exposed to R5-and X4-tropic HIV-1 IRES-eGFP constructs containing various nef genes. It has been previously shown that these HIV-1 reporter constructs replicate efficiently in infected PBMC cultures (19,21). We found that HIV-1 infected (eGFP-positive [eGFP ϩ ]) cells were readily detectable at 3 days postinfection (Fig.…”

“…Indeed, some Nef functions may help these viruses to replicate efficiently in their natural simian hosts without causing disease (14,16). Specifically, most Nef proteins block the responsiveness of virally infected T cells to stimulation by downmodulation of T cell receptor (TCR)-CD3 from the cell surface (11,(17)(18)(19). Exceptions are Nefs of HIV-1 and its vpu-containing SIV counterparts that lack this function entirely (11,19).…”

“…Thus, most lentiviral Nef proteins may help the virally infected hosts to maintain normal CD4 ϩ T cell homeostasis by suppressing the activation and apoptosis of infected CD4 ϩ T cells. The effect of Nef on cell surface expression of TCR-CD3 (11,(17)(18)(19) and the costimulatory CD28 receptor (22,23) together with the manipulation of intracellular signaling pathways may help the virus to establish levels of CD4 ϩ T cell activation high enough to allow effective virus production but also low enough to avoid cell death prior to the completion of the viral replication cycle. It is well established that different CD4 ϩ T cell subsets that are targets of HIV and SIV infection differ fundamentally in their state of activation, their susceptibility to virus infection, and their importance for maintaining critical immune functions (24)(25)(26).…”

“…Specifically, most Nef proteins block the responsiveness of virally infected T cells to stimulation by downmodulation of T cell receptor (TCR)-CD3 from the cell surface (11,(17)(18)(19). Exceptions are Nefs of HIV-1 and its vpu-containing SIV counterparts that lack this function entirely (11,19). The efficiency of Nef-mediated downmodulation of TCR-CD3 correlates with high, stable CD4 ϩ T cell counts in sooty mangabeys that are naturally infected with SIV and do not develop disease (20) and in viremic HIV-2-infected individuals (21).…”

Functions of the Lentiviral Accessory Protein Nef During the Distinct Steps of HIV and SIV Replication Cycle

“…HIV-1, SIVcpz, SIVgor, SIVgsn, SIVmus, SIVmon (Schindler et al, 2006). Exceptions to this association has been demonstrated since in SIVden strain, in which Nef retains the ability to downmodulate TCR/CD3, the vpu gene is present, whereas in SIVolc, which does not contain Vpu, Nef protein is unable to perform this function (Schmokel et al, 2011). Besides the role of Nef in TCR/CD3 downmodulation a question remains to be understood: does CD3 downregulation contribute to the nonpathogenic phenotype of natural SIV infections?…”