HIV cell-to-cell transmission: effects on pathogenesis and antiretroviral therapy

Abstract

HIV spreads more efficiently in vitro when infected cells directly contact uninfected cells to form virological synapses. A hallmark of virological synapses is that viruses can be transmitted at a higher multiplicity of infection (MOI) that, in vitro, results in a higher number of proviruses. Whether HIV also spreads by cell-cell contact in vivo is a matter of debate. Here we discuss recent data that suggest that contact-mediated transmission largely manifests itself in vivo as CD4+ T cell depletion. The assault of a cell by a large number of incoming particles is likely to be efficiently sensed by the innate cellular surveillance to trigger cell death. The large number of particles transferred across virological synapses has also been implicated in reduced efficacy of antiretroviral therapies. Thus, antiretroviral therapies must remain effective against the high MOI observed during cell-to-cell transmission to inhibit both viral replication and the pathogenesis associated with HIV infection.

Keywords: antiretroviral therapy; cell-to-cell transmission; human immunodeficiency virus; virological synapse.

Figure 1. Virological synapses are characterized by the polarization of viral assembly and the accumulation of viral particles at the site of cell-cell contact

An HIV-infected CD4+ T cell (green) accumulates HIV Gag-GFP at the sites of cell-cell contact (arrows) with uninfected target CD4+ T cells (red). The size bar corresponds to 17 μm.

Figure 2. Potential pathways for innate immune sensing of HIV during the early stages of the virus life cycle

In contrast to cell-free virus infection, the virological synapse can promote the entry of multiple HIV virions into target cells [1, 2, 18, 37]. Virus fusion events can be sensed as danger signals (depicted here as red stars) due to the release of reactive oxygen species (ROS) and the activation of phospholipase C gamma 1-phosphoinositide 3-kinase pathway (PLC-γ–PI(3)K pathway), which stimulates the release of Ca²⁺ culminating in stimulator of interferon genes (STING)-dependent Type I interferon (IFN) production [45]. *While the contribution of endocytosis to productive HIV entry remains highly controversial [–98], it has been best documented in studies of HIV transmitted via cell-cell contacts [, , –101]. Endocytosis of virions, their transfer to lysosome related organelles and subsequent lysis, releases viral RNA contents that can be sensed by TLR7 and TLR8 [102]. TLR7- and TLR8-sensing results in the synthesis of IFN and the inflammatory cytokine IL-1β respectively [46, 47, 103]. Fusion at the plasma membrane or from mature endosomes releases viral capsids into the host cytosol [97, 104]. The released capsids can also be delivered to the lysosome-related organelle by autophagy to be sensed by TLR7/8 [44]. In addition, TRIM5α in the host cytosol senses viral capsid as ‘foreign’, inducing IFN and accelerating capsid disassembly [50, 105]. In contrast, cyclophilinA (CypA) and cleavage and polyadenylation specific factor 6 (CPSF6) may act as ‘cloaking’ factors to prevent premature exposure and sensing of viral RNA and reverse transcription (RT) products [58]. Premature disassembly of viral capsids can release the RNA contents of the virus and can be potentially sensed by the cytoplasmic RNA sensor retinoic acid inducible gene-1 (RIG-I) to elicit IFN production [52]. The next step of reverse transcription of viral RNA is repressed by APOBEC3G (apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3G) packed within the incoming virions or SAM domain and HD domain-containing protein 1 (SAMHD1) present in the host cytosol [106, 107]. SAMHD1 is thought to deplete the nucleotide pool and elevates the generation of incomplete RT products. SAMHD1 was also recently reported to have direct RNase activity against incoming HIV RNA [56]. The DNA sensor, IFI16 can sense abortive RT products to induce IFN as well as activate caspase-1 and IL-1β, triggering pyroptosis and cell death [55, 59, 60]. In contrast, the exonuclease TREX1 can degrade excess RT products and prevent sensing of viral DNA [54]. Viral DNA can also be sensed by cyclic GMP-AMP synthase (cGAS) to generate cyclic guanosine monophosphate–adenosine monophosphate (cGAMP) that activates the STING-dependent IFN pathway [53]. Pre-integration complexes (PICs) generated at the end of RT reaction are transported into the nucleus through nuclear pore complex, a step that is inhibited by IFN-inducible factor MX2 [108, 109]. PICs mediate integration of HIV DNA into the host chromosome. Multiple double stranded DNA breaks and strand transfer reactions into the host chromosomal DNA can elicit a DNA damage response pathway that leads to phosphorylation of gamma histone 2AX (H2AX) and activation of p53 and DNA-dependent protein kinase (DNA-PK) to trigger apoptosis and cell death [57].

Figure 3. Mechanisms of how HIV cell-to-cell transmission can overcome inhibition by antiretroviral inhibitors

(A) The number of particles that infect a target cell during cell-free HIV infection is small and is inhibited by antiretroviral inhibitors. (B) However, during cell-to-cell transmission via virological synapses, the number of particles can be high, thereby overwhelming the drug concentration.

Figure 4. Mechanisms of how RT nucleotide excision and HIV cell-to-cell transmission can overcome inhibition by antiretroviral inhibitors

(A) A simplified model of nucleotide excision by HIV RT. When HIV RT incorporates a nucleotide analog (NRTI) in the growing reverse transcript, RT can return the analog from the chain-terminated primer site back to the nucleotide binding site (shown as green and yellow ovals labeled P and N respectively). Once in the N site, a reaction thought to be pyrophosphate or ATP-dependent (red A), mediates the release of the NRTI from the reverse transcript. Upon freeing the N site, RT can resume normal reverse transcription. (B, C) While cell-free spread of HIV may not benefit sufficiently from nucleotide excision activity, delivery of large numbers of viruses across virological synapses can overwhelm the active concentration of intracellular NRTIs, thus increasing the likelihood of reverse transcripts escaping inhibition. (D) This resistance to individual NRTIs can be prevented by combining NRTIs, by excision-resistant NRTIs or by mutations in HIV RT.