HIV-1 Env induces pexophagy and an oxidative stress leading to uninfected CD4+ T cell death.

The immunodeficiency observed in HIV-1-infected patients is mainly due to uninfected bystander CD4+ T lymphocyte cell death. The viral envelope glycoproteins (Env), expressed at the surface of infected cells, play a key role in this process. Env triggers macroautophagy/autophagy, a process necessary for subsequent apoptosis, and the production of reactive oxygen species (ROS) in bystander CD4+ T cells. Here, we demonstrate that Env-induced oxidative stress is responsible for their death by apoptosis. Moreover, we report that peroxisomes, organelles involved in the control of oxidative stress, are targeted by Env-mediated autophagy. Indeed, we observe a selective autophagy-dependent decrease in the expression of peroxisomal proteins, CAT and PEX14, upon Env exposure; the downregulation of either BECN1 or SQSTM1/p62 restores their expression levels. Fluorescence studies allowed us to conclude that Env-mediated autophagy degrades these entire organelles and specifically the mature ones. Together, our results on Env-induced pexophagy provide new clues on HIV-1-induced immunodeficiency.Abbreviations: Ab: antibodies; AF: auranofin; AP: anti-proteases; ART: antiretroviral therapy; BafA1: bafilomycin A1; BECN1: beclin 1; CAT: catalase; CD4: CD4 molecule; CXCR4: C-X-C motif chemokine receptor 4; DHR123: dihydrorhodamine 123; Env: HIV-1 envelope glycoproteins; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GFP: green fluorescent protein; GFP-SKL: GFP-serine-lysine-leucine; HEK: human embryonic kidney; HIV-1: type 1 human immunodeficiency virus; HTRF: homogeneous time resolved fluorescence; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; NAC: N-acetyl-cysteine; PARP: poly(ADP-ribose) polymerase; PEX: peroxin; ROS: reactive oxygen species; siRNA: small interfering ribonucleic acid; SQSTM1/p62: sequestosome 1.

Autophagy restricts HIV-1 infection by selectively degrading Tat in CD4+ T lymphocytes.

UNLABELLED: Autophagy is a ubiquitous mechanism involved in the lysosomal-mediated degradation of cellular components when they are engulfed in vacuoles called autophagosomes. Autophagy is also recognized as an important regulator of the innate and adaptive immune responses against numerous pathogens, which have, therefore, developed strategies to block or use the autophagy machinery to their own benefit. Upon human immunodeficiency virus type 1 (HIV-1) infection, viral envelope (Env) glycoproteins induce autophagy-dependent apoptosis of uninfected bystander CD4(+) T lymphocytes, a mechanism likely contributing to the loss of CD4(+) T cells. In contrast, in productively infected CD4(+) T cells, HIV-1 is able to block Env-induced autophagy in order to avoid its antiviral effect. To date, nothing is known about how autophagy restricts HIV-1 infection in CD4(+) T lymphocytes. Here, we report that autophagy selectively degrades the HIV-1 transactivator Tat, a protein essential for viral transcription and virion production. We demonstrated that this selective autophagy-mediated degradation of Tat relies on its ubiquitin-independent interaction with the p62/SQSTM1 adaptor. Taken together, our results provide evidence that the anti-HIV effect of autophagy is specifically due to the degradation of the viral transactivator Tat but that this process is rapidly counteracted by the virus to favor its replication and spread. IMPORTANCE: Autophagy is recognized as one of the most ancient and conserved mechanisms of cellular defense against invading pathogens. Cross talk between HIV-1 and autophagy has been demonstrated depending on the virally challenged cell type, and HIV-1 has evolved strategies to block this process to replicate efficiently. However, the mechanisms by which autophagy restricts HIV-1 infection remain to be elucidated. Here, we report that the HIV-1 transactivator Tat, a protein essential for viral replication, is specifically degraded by autophagy in CD4(+) T lymphocytes. Both Tat present in infected cells and incoming Tat secreted from infected cells are targeted for autophagy degradation through a ubiquitin-independent interaction with the autophagy receptor p62/SQSTM1. This study is the first to demonstrate that selective autophagy can be an antiviral process by degrading a viral transactivator. In addition, the results could help in the design of new therapies against HIV-1 by specifically targeting this mechanism.

Inhibition of HIV-1 replication with stable RNAi-mediated knockdown of autophagy factors.

Autophagy is a cellular process leading to the degradation of cytoplasmic components such as organelles and intracellular pathogens. It has been shown that HIV-1 relies on several components of the autophagy pathway for its replication, but the virus also blocks late steps of autophagy to prevent its degradation. We generated stable knockdown T cell lines for 12 autophagy factors and analyzed the impact on HIV-1 replication. RNAi-mediated knockdown of 5 autophagy factors resulted in inhibition of HIV-1 replication. Autophagy analysis confirmed a specific defect in the autophagy pathway for 4 of these 5 factors. We also scored the impact on cell viability, but no gross effects were observed. Upon simultaneous knockdown of 2 autophagy factors (Atg16 and Atg5), an additive inhibitory effect was scored on HIV-1 replication. Stable knockdown of several autophagy factors inhibit HIV-1 replication without any apparent cytotoxicity. We therefore propose that targeting of the autophagy pathway can be a novel therapeutic approach against HIV-1.

Differential role of autophagy in CD4 T cells and macrophages during X4 and R5 HIV-1 infection.

BACKGROUND: HIV-1 can infect and replicate in both CD4 T cells and macrophages. In these cell types, HIV-1 entry is mediated by the binding of envelope glycoproteins (gp120 and gp41, Env) to the receptor CD4 and a coreceptor, principally CCR5 or CXCR4, depending on the viral strain (R5 or X4, respectively). Uninfected CD4 T cells undergo X4 Env-mediated autophagy, leading to their apoptosis, a mechanism now recognized as central to immunodeficiency. METHODOLOGY/PRINCIPAL FINDINGS: We demonstrate here that autophagy and cell death are also induced in the uninfected CD4 T cells by HIV-1 R5 Env, while autophagy is inhibited in productively X4 or R5-infected CD4 T cells. In contrast, uninfected macrophages, a preserved cell population during HIV-1 infection, do not undergo X4 or R5 Env-mediated autophagy. Autophagosomes, however, are present in macrophages exposed to infectious HIV-1 particles, independently of coreceptor use. Interestingly, we observed two populations of autophagic cells: one highly autophagic and the other weakly autophagic. Surprisingly, viruses could be detected in the weakly autophagic cells but not in the highly autophagic cells. In addition, we show that the triggering of autophagy in macrophages is necessary for viral replication but addition of Bafilomycin A1, which blocks the final stages of autophagy, strongly increases productive infection. CONCLUSIONS/SIGNIFICANCE: Taken together, our data suggest that autophagy plays a complex, but essential, role in HIV pathology by regulating both viral replication and the fate of the target cells.

HIV-1 gp41 fusogenic function triggers autophagy in uninfected cells.

Cell-expressed HIV-1 envelope glycoproteins (gp120 and gp41, called Env) induce autophagy in uninfected CD4 T cells, leading to their apoptosis, a mechanism most likely contributing to immunodeficiency. The presence of CD4 and CXCR4 on target cells is required for this process, but Env-induced autophagy is independent of CD4 signaling. Here we demonstrate that CXCR4-mediated signaling pathways are not directly involved in autophagy and cell death triggering. Indeed, cells stably expressing mutated forms of CXCR4, unable to transduce different Gi-dependent and -independent signals, still undergo autophagy and cell death after coculture with effector cells expressing Env. After gp120 binding to CD4 and CXCR4, the N terminus fusion peptide (FP) of gp41 is inserted into the target membrane, and gp41 adopts a trimeric extended pre-hairpin intermediate conformation, target of HIV fusion inhibitors such as T20 and C34, before formation of a stable six-helix bundle structure and cell-to-cell fusion. Interestingly, Env-mediated autophagy is triggered in both single cells (hemifusion) and syncytia (complete fusion), and prevented by T20 and C34. The gp41 fusion activity is responsible for Env-mediated autophagy since the Val2Glu mutation in the gp41 FP totally blocks this process. On the contrary, deletion of the C-terminal part of gp41 enhances Env-induced autophagy. These results underline the major role of gp41 in inducing autophagy in the uninfected cells and indicate that the entire process leading to HIV entry into target cells through binding of Env to its receptors, CD4 and CXCR4, is responsible for autophagy and death in the uninfected, bystander cells.