HIV ENV Glycoprotein-mediated Bystander Apoptosis Depends on Expression of the CCR5 Co-receptor at the Cell Surface and ENV Fusogenic Activity

Abstract

HIV-1 infections lead to a progressive depletion of CD4 cells culminating in AIDS. The coreceptor usage by HIV varies from CCR5 (R5) tropic early in infection to CXCR4 (X4) tropic in later infections. Although the coreceptor switch from R5 to X4 tropic HIV is well associated with progression to AIDS, the role of CCR5 in disease progression especially in patients infected exclusively with R5 isolates throughout the disease remains enigmatic. To better understand the role of CCR5 and R5 tropic HIV envelope in AIDS pathogenesis, we asked whether the levels of CCR5 and/or HIV Env-mediated fusion determine apoptosis of bystander cells. We generated CD4 ؉ T cell lines expressing varying levels of CCR5 on the cell surface to show that CCR5 expression levels correlate with bystander apoptosis induction. The mechanism of apoptosis involved caspase-3 activation and mitochondrial depolarization and was dependent on gp41 fusion activity as confirmed by fusion-restricted gp41 point mutants and use of the fusion inhibitor T20. Interestingly, lower levels of CCR5 were able to support virus replication in the absence of bystander apoptosis. Our findings suggest that R5 HIV-1-mediated bystander apoptosis is dependent on both CCR5 expression levels as well as fusogenic activity of the Env glycoprotein.For viral entry, HIV-1 utilizes CD4 as receptor and one of the chemokine receptors as coreceptors, the most common ones being CXCR4 (X4) and CCR5 (R5) (1-3). It is well known that R5 tropic viruses predominate during the initial establishment of infection as they are transmitted with greater efficiency (4 -6), whereas the X4 viruses emerge later during the disease and are associated with rapid progression to AIDS (7,8). The progressive increase in virulence in vivo during the late stages of disease is attributed in many cases to a coreceptor switch from CCR5 tropic to CXCR4 tropic HIV (9 -11). Although in approximately half of the HIV infections the virus switches coreceptor usage to CXCR4, and there are several hypotheses to explain this phenomenon (12-15), the mechanism by which R5 tropic HIV isolates lead to AIDS remains poorly defined especially in patients that remain solely infected with R5 isolates throughout the disease.Although HIV selectively infects CD4 ϩ cells, the relatively few infected cells in vivo do not account for the extensive depletion of CD4 cells. This has led to the idea that the virus is able to kill uninfected bystander CD4 ϩ cells (16) (24,25). In support of this observation, we and others showed that the fusion process mediated by the gp41 subunit of HIV envelope may be critical in bystander killing (26 -28). Furthermore, through mutagenesis studies, we demonstrated that hemifusion (incomplete fusion accompanied by partial mixing of apposing cell membranes) induced by gp41 subunit is the major mechanism for HIV Env-mediated bystander T cell apoptosis (29). The fusion of biological membranes mediated by HIV Env, specifically in the case of R5 tropic viruses, is dependent on the expression level of CCR5 cor...

ERK1-Based Pathway as a New Selective Mechanism To Modulate CCR5 with Natural Antibodies

“…The mechanisms underlying this unconventional and atypical long-lasting downregulation of CCR5 required further investigation. Accordingly, in-depth analysis of the early events subsequent to the incubation of CCR5-specific Abs (CCR5 Ab pos) was performed with the T lymphoblastoid R5-SupT1-L23 cell line, which has cell surface levels very similar to those expressed in human CD4 + T lymphocytes (23). Where indicated, the majority of the key experiments were confirmed in human CD4 + T lymphocytes, which represent the natural target of HIV.…”

“…To this end, we used R5-SupT1-M10 to immunoprecipitate both ERK1/2 and CCR5. R5-SupT1-M10 cells displayed a higher level of CCR5 expression than did R5-SupT1-L23 cells (23). R5-SupT1-M10 or mock cells (SupT1 not transduced with CCR5) were treated with CCR5 Ab pos for 150 min.…”

Point Mutations Associated with HIV-1 Drug Resistance, Evasion of the Immune Response and AIDS Pathogenesis

“…Four point mutations (A835W, A838W, A838I, and I842R) in the cytoplasm domain of the gp41 subunit of the envelope protein reduces HIV-induced apoptosis of target cells (Micoli et al, 2006). A recent study has shown that induction of bystander apoptosis in CCR5 expressing cells requires the fusogenic activity of gp41; and that V2E and V38A/E point mutation in the second amino acid and the heptad repeat 1 region of gp41 respectively, abrogated fusion activity and hence apoptosis induced by an R5 virus (Joshi et al, 2011). In an earlier study, the same group showed that a V513E point mutation in the fusion domain of gp41 resulted in a fusion-defective envelope protein that failed to induce apoptosis and that a G547D mutation in the gp41 N-terminal helix also reduced cell fusion capacity and apoptosis induced by an X4 virus (Garg et al, 2007).…”

“…J. Li et al, 1995;Zhu et al, 2011). HIV-induced apoptosis is triggered by a number of HIV-1 proteins, notably the envelope protein (Herbein et al, 1998;Ishikawa et al, 1998;Joshi et al, 2011;Micoli et al, 2006;Micoli et al, 2000). Four point mutations (A835W, A838W, A838I, and I842R) in the cytoplasm domain of the gp41 subunit of the envelope protein reduces HIV-induced apoptosis of target cells (Micoli et al, 2006).…”

“…However in general, progression from acute infection is often accompanied by depletion of the CD4 + lymphocytes and this depletion is a major component of the eventual failure of the immune system and hence AIDS pathogenesis. Apoptosis is one mechanism that contributes to depletion of HIV-1 infected and uninfected cells, deterioration of the immune system and progression to AIDS, respectively (Cotton et al, 1997;Gougeon & Montagnier, 1993;Herbein et al, 1998;Joshi et al, 2011;C. J. Li et al, 1995;Zhu et al, 2011).…”