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Interferons play a critical role in regulating host immune responses to SARS-CoV-2, but the interferon (IFN)-stimulated gene (ISG) effectors that inhibit SARS-CoV-2 are not well characterized. The IFN-inducible short isoform of human nuclear receptor coactivator 7 (NCOA7) inhibits endocytic virus entry, interacts with the vacuolar ATPase, and promotes endo-lysosomal vesicle acidification and lysosomal protease activity. Here, we used ectopic expression and gene knockout to demonstrate that NCOA7 inhibits infection by SARS-CoV-2 as well as by lentivirus particles pseudotyped with SARS-CoV-2 Spike in lung epithelial cells. Infection with the highly pathogenic, SARS-CoV-1 and MERS-CoV, or seasonal, HCoV-229E and HCoV-NL63, coronavirus Spike-pseudotyped viruses was also inhibited by NCOA7. Importantly, either overexpression of TMPRSS2, which promotes plasma membrane fusion versus endosomal fusion of SARS-CoV-2, or removal of Spike’s polybasic furin cleavage site rendered SARS-CoV-2 less sensitive to NCOA7 restriction. Collectively, our data indicate that furin cleavage sensitizes SARS-CoV-2 Spike to the antiviral consequences of endosomal acidification by NCOA7, and suggest that the acquisition of furin cleavage may have favoured the co-option of cell surface TMPRSS proteases as a strategy to evade the suppressive effects of IFN-induced endo-lysosomal dysregulation on virus infection.

HIV-1 must replicate in cells that are equipped to defend themselves from infection through intracellular innate immune systems. HIV-1 evades innate immune sensing through encapsidated DNA synthesis and encodes accessory genes that antagonize specific antiviral effectors. Here, we show that both particle associated, and expressed HIV-1 Vpr, antagonize the stimulatory effect of a variety of pathogen associated molecular patterns by inhibiting IRF3 and NF-κB nuclear transport. Phosphorylation of IRF3 at S396, but not S386, was also inhibited. We propose that, rather than promoting HIV-1 nuclear import, Vpr interacts with karyopherins to disturb their import of IRF3 and NF-κB to promote replication in macrophages. Concordantly, we demonstrate Vpr-dependent rescue of HIV-1 replication in human macrophages from inhibition by cGAMP, the product of activated cGAS. We propose a model that unifies Vpr manipulation of nuclear import and inhibition of innate immune activation to promote HIV-1 replication and transmission.

The antiviral cytokine interferon activates expression of interferon-stimulated genes to establish an antiviral state. Myxovirus resistance 2 (MX2, also known as MxB) is an interferon-stimulated gene that inhibits the nuclear import of HIV-1 and interacts with the viral capsid and cellular nuclear transport machinery. Here, we identified the myosin light chain phosphatase (MLCP) subunits myosin phosphatase target subunit 1 (MYPT1) and protein phosphatase 1 catalytic subunit-β (PPP1CB) as positively-acting regulators of MX2, interacting with its amino-terminal domain. We demonstrated that serine phosphorylation of the N-terminal domain at positions 14, 17 and 18 suppresses MX2 antiviral function, prevents interactions with the HIV-1 capsid and nuclear transport factors, and is reversed by MLCP. Notably, serine phosphorylation of the N-terminal domain also impedes MX2-mediated inhibition of nuclear import of cellular karyophilic cargo. We also found that interferon treatment reduces levels of phosphorylation at these serine residues and outline a homeostatic regulatory mechanism in which repression of MX2 by phosphorylation, together with MLCP-mediated dephosphorylation, balances the deleterious effects of MX2 on normal cell function with innate immunity against HIV-1. Dual functions of MX2 in innate immunity against HIV-1 and nuclear import of cellular proteins are regulated by phosphorylation.

HIV-1 is the single most important sexually transmitted disease in humans from a global health perspective. Among human lentiviruses, HIV-1 M group has uniquely achieved pandemic levels of human-to-human transmission. The requirement to transmit between hosts likely provides the strongest selective forces on a virus, as without transmission, there can be no new infections within a host population. Our perspective is that evolution of all of the virus-host interactions, which are inherited and perpetuated from host-to-host, must be consistent with transmission. For example, CXCR4 use, which often evolves late in infection, does not favor transmission and is therefore lost when a virus transmits to a new host. Thus, transmission inevitably influences all aspects of virus biology, including interactions with the innate immune system, and dictates the biological niche in which the virus exists in the host. A viable viral niche typically does not select features that disfavor transmission. The innate immune response represents a significant selective pressure during the transmission process. In fact, all viruses must antagonize and/or evade the mechanisms of the host innate and adaptive immune systems that they encounter. We believe that viewing host-virus interactions from a transmission perspective helps us understand the mechanistic details of antiviral immunity and viral escape. This is particularly true for the innate immune system, which typically acts from the very earliest stages of the host-virus interaction, and must be bypassed to achieve successful infection. With this in mind, here we review the innate sensing of HIV, the consequent downstream signaling cascades and the viral restriction that results. The centrality of these mechanisms to host defense is illustrated by the array of countermeasures that HIV deploys to escape them, despite the coding constraint of a 10 kb genome. We consider evasion strategies in detail, in particular the role of the HIV capsid and the viral accessory proteins highlighting important unanswered questions and discussing future perspectives.

The role of the HIV-1 accessory protein Vpr has been obscure. Recent studies suggested that HIV-1 is sensitive to type-I Interferon stimulated by activation of cytoplasmic DNA sensor cGAS. Given that Vpr is packaged into HIV-1 particles and present during early stages of the viral lifecycle when its DNA is prone to detection by cGAS, it was hypothesised that Vpr may antagonise cGAS activation of innate immune responses. Consistent with this hypothesis, HIV-1 replication was Vpr dependent in macrophages activated with cGAMP, a product of activated cGAS. High dose infection of THP-1 cells by HIV-1 triggered a Vpr sensitive ISG response, which depended on cGAS but not MAVS. Vpr expression inhibited interferon stimulated genes (ISGs) mRNA and protein expression stimulated by cGAMP. Vpr mutants revealed that this activity required interaction with the DCAF1 E3 ubiquitin ligase complex and importin-α but is independent of Vpr cell cycle arrest function. DCAF1 requirement was further confirmed by DCAF1 depletion. Surprisingly, Vpr expression also inhibited LPS or Sendai virus activated ISG expression suggesting that Vpr targets a conserved step downstream of several innate immune sensors. Indeed, Vpr potently inhibited nuclear translocation of IRF3 without affecting IRF3 phosphorylation at serine386 which is necessary and sufficient for IRF3 activation. In addition to IRF3, Vpr also inhibited NF-ĸB nuclear translocation downstream of DNA sensing. Immunofluorescence analysis of Vpr correlated antagonism of immune signalling with localisation of Vpr to the nuclear envelope, suggesting that Vpr may target nuclear translocation of IRF3 and NF-ĸB at the nuclear pore. In parallel, investigation of Vpr in HEK293T cells revealed that Vpr inhibits mRNA expression from various promoters except the ubiquitin or EF1α promoter which lack NFĸB binding sites. This function correlated with Vpr localisation to the nuclear envelope and was independent of the cell cycle arrest function of Vpr. Interestingly, Vpr did not inhibit HIV-1 gene expression or infectivity. Moreover, nucleofection or integration of a reporter overcame the Vpr-mediated block to expression, suggesting that Vpr may inhibit nuclear import of co-transfected plasmids. In conclusion, I propose that during infection Vpr acts to suppress cGAS activation induced by inappropriately exposed HIV-1 DNA in infected cells and Vpr mediated block to expression from the co-transfected plasmids is a consequence of Vpr inhibition of IRF3 and NF-ĸB nuclear import.

Collectin-11 is a soluble C-type lectin produced at epithelial surfaces to initiate pathogen elimination by complement. Given the respiratory epithelium is a source of CL-11 and downstream complement-pathway components, we investigated the potential of CL-11 to impact the pathogenicity of SARS-CoV-2. While the SARS-CoV-2 spike trimer could bind CL-11 and trigger complement activation followed by MAC formation, the virus was resistant to lysis. Surprisingly, virus production by infected respiratory epithelial cells was enhanced by CL-11 opsonisation of virus but this effect was fully inhibited by sugar-blockade of CL-11. Moreover, SARS-CoV-2 spike protein expressed at the bronchial epithelial cell surface was associated with increased CL-11 binding and MAC formation. We propose that SARS-CoV-2 pathogenicity is exacerbated both by resistance to complement and CL-11 driven respiratory cell invasion and injury at the portal of entry. Contrary to expectation, CL-11 blockade could offer a novel approach to limit the pathogenicity of SARS-CoV-2.Competing Interest StatementThe authors have declared no competing interest.

The interaction of the SARS-CoV-2 Spike receptor binding domain (RBD) with the ACE2 receptor on host cells is essential for viral entry. RBD is the dominant target for neutralizing antibodies and several neutralizing epitopes on RBD have been molecularly characterized. Analysis of circulating SARS-CoV-2 variants has revealed mutations arising in the RBD, the N-terminal domain (NTD) and S2 subunits of Spike. To fully understand how these mutations affect the antigenicity of Spike, we have isolated and characterized neutralizing antibodies targeting epitopes beyond the already identified RBD epitopes. Using recombinant Spike as a sorting bait, we isolated >100 Spike-reactive monoclonal antibodies from SARS-CoV-2 infected individuals. ≈45% showed neutralizing activity of which ≈20% were NTD-specific. None of the S2-specific antibodies showed neutralizing activity. Competition ELISA revealed that NTD-specific mAbs formed two distinct groups: the first group was highly potent against infectious virus, whereas the second was less potent and displayed glycan-dependant neutralization activity. Importantly, mutations present in B.1.1.7 Spike frequently conferred resistance to neutralization by the NTD-specific neutralizing antibodies. This work demonstrates that neutralizing antibodies targeting subdominant epitopes need to be considered when investigating antigenic drift in emerging variants.

HIV-1 must replicate in cells that are equipped to defend themselves from infection through intracellular innate immune systems. HIV-1 evades innate immune sensing through encapsidated DNA synthesis and encodes accessory genes that antagonize specific antiviral effectors. Here we show that both particle associated, and expressed HIV-1 Vpr, antagonize the stimulatory effect of a variety of pathogen associated molecular patterns by inhibiting IRF3 and NF-κB nuclear transport. Phosphorylation of IRF3 at S396, but not S386, was also inhibited. We propose that, rather than promoting HIV-1 nuclear import, Vpr interacts with karyopherins to disturb their import of IRF3 and NF-κB to promote replication in macrophages. Concordantly, we demonstrate Vpr dependent rescue of HIV-1 replication in human macrophages from inhibition by cGAMP, the product of activated cGAS. We propose a model that unifies Vpr manipulation of nuclear import and inhibition of innate immune activation to promote HIV-1 replication and transmission.