Current strategies to induce selective killing of HIV-1-infected cells

Abstract

Although combination antiretroviral therapy (ART) has led to significant HIV-1 suppression and improvement in immune function, persistent viral reservoirs remain that are refractory to intensified ART. ART poses many challenges such as adherence to drug regimens, the emergence of resistant virus, and cumulative toxicity resulting from long-term therapy. Moreover, latent HIV-1 reservoir cells can be stochastically activated to produce viral particles despite effective ART and contribute to the rapid viral rebound that typically occurs within 2 weeks of ART interruption; thus, lifelong ART is required for continued viral suppression. Several strategies have been proposed to address the HIV-1 reservoir such as reactivation of HIV-1 transcription using latency reactivating agents with a combination of ART, host immune clearance and HIV-1-cytotoxicity to purge the infected cells-a "shock and kill" strategy. However, these approaches do not take into account the multiple transcriptional and translational blocks that contribute to HIV-1 latency or the complex heterogeneity of the HIV-1 reservoir, and clinical trials have thus far failed to produce the desired results. Here, we describe alternative strategies being pursued that are designed to kill selectively HIV-1-infected cells while sparing uninfected cells in the absence of enhanced humoral or adaptive immune responses.

Keywords: HIV-1; IAP; SMAC mimetics; apoptosis; autophagy; autosis; cell death.

FIGURE 1

Selective killing of HIV‐1‐infected cells. (A) The “shock and kill” strategy aims to decrease the HIV‐1 reservoir through the reactivation of proviral transcription using latency reversing agents (LRA), leading to the elimination of infected cells by CTLs and HIV‐1 cytopathogenesis. (B) The “prime, shock, and kill” is similar to the “shock and kill” strategy with the addition of pharmacologic agents to overcome the latent reservoir cells inherent resistance to cell death. (C) This strategy simultaneously reactivates the expression of viral RNA while also increasing the infected cells sensitization to innate antiviral signaling pathways. (D) IAP inhibitors, autosis inducing peptides, and TREM1 antagonists can all induce death in HIV‐1‐infected cells without the need for reactivation of the virus with minimal toxicity to uninfected cells

FIGURE 2

Regulation of apoptosis. The initiation of apoptosis is a multistep process that can be either extrinsic or intrinsic. In the extrinsic pathway, cognate ligand binding to TNFRSF members can either promote the direct recruitment of TRADD (as is the case for TNFRSF25 or TNFR1) or FADD (as is the case for FAS and TNFRSF10A [TRAILR1]). In the case of TRADD recruitment, this either promotes the recruitment of FADD leading to a caspase signaling cascade resulting in the activation of caspase‐3 and apoptosis, or TRADD recruits RIPK1 and TRAF2, triggering NF‐κB activation, cell survival and a proinflammatory response. In both cases, the extrinsic pathway can be either mitochondria independent (type I death receptor pathway) or converge with the intrinsic pathway and be mitochondria dependent (type II death receptor pathway) when BID is activated by caspase‐8, which then oligomerizes BAK1 leading to mitochondrial outer membrane permeabilization (MOMP) and the release of cytochrome c from the mitochondrial intermembrane space. The key event in the intrinsic pathway is the formation of the apoptosome and subsequent activation of caspase‐9 after MOMP. Also released during MOMP are DIABLO/SMAC, which promotes apoptosis indirectly by inhibiting IAPs, and AIF, which promotes parthanatos, a caspase‐independent form of cell death (not shown)

FIGURE 3

Regulation of autophagy. The initiation of autophagy is a multistep process, the main regulators of which are MTOR, an inhibitor, and AMPK an activator. MTORC1 inhibition drives the formation of the phagophore through the formation of the ULK1 complex that directly activates the PIK3C3 complex. This complex translocates to endoplasmic reticulum sites and produces PI3P, which recruits WIPI2 and ZFYVE1. In the ATG12 conjugation system, ATG12 is covalently attached to ATG5, which is then attached to ATG16L1. WIPI2b acts immediately upstream of ATG16L1 and recruits ATG12–ATG5‐ATG16L1 to PI3P‐tagged phagophores. The ATG12–ATG5‐ATG16L1 complex then promotes conjugation of ATG8 proteins with phosphatidylethanolamine leading to their incorporation into the phagophore membranes where they interact with cargo receptors harboring LC3‐interacting motifs that recruit and incorporate ubiquitin‐decorated cargoes into the nascent autophagosome. After detachment of ATG factors, the phagophore closes through scission forming the autophagosome, which then fuse with lysosomes resulting in the degradation of the engulfed components

FIGURE 4

DIABO/SMAC mimetic mediated apoptosis. HIV‐1‐infected cells have increased expression of IAPs. Upon binding of TNF to TNFR1, RIPK1 is recruited to the receptor and is ubiquitinated by BIRC2 and BIRC3 and the LUBAC complex leading to the activation of MAPK signaling, inflammation, and cell survival. Separately, IAPs inhibit autophagy induction through the inhibition of MDM2 and the ATG12–ATG5‐ATG16L1 complex. Treatment of latent HIV‐infected cells with DIABLO/SMAC mimetics induces the degradation of IAPS. This results in the deubiquitination of RIPK1 and the induction of autophagy leading to the formation of a death inducing signaling complex on preautophagosome membranes, the activation of caspase‐8 and subsequent cell death by apoptosis