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HIV-1 accesses the nuclear DNA of interphase cells via a poorly defined process involving functional interactions between the capsid protein (CA) and nucleoporins (Nups). Here, we show that HIV-1 CA can bind multiple Nups, and that both natural and manipulated variation in Nup levels impacts HIV-1 infection in a manner that is strikingly dependent on cell-type, cell-cycle, and cyclophilin A (CypA). We also show that Nups mediate the function of the antiviral protein MX2, and that MX2 can variably inhibit non-viral NLS function. Remarkably, both enhancing and inhibiting effects of cyclophilin A and MX2 on various HIV-1 CA mutants could be induced or abolished by manipulating levels of the Nup93 subcomplex, the Nup62 subcomplex, NUP88, NUP214, RANBP2, or NUP153. Our findings suggest that several Nup-dependent ‘pathways’ are variably exploited by HIV-1 to target host DNA in a cell-type, cell-cycle, CypA and CA-sequence dependent manner, and are differentially inhibited by MX2.

Viruses employ a variety of strategies to escape or counteract immune responses, including depletion of cell surface major histocompatibility complex class I (MHC-I), that would ordinarily present viral peptides to CD8⁺ cytotoxic T cells. As part of a screen to elucidate biological activities associated with individual severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) viral proteins, we found that ORF7a reduced cell surface MHC-I levels by approximately fivefold. Nevertheless, in cells infected with SARS-CoV-2, surface MHC-I levels were reduced even in the absence of ORF7a, suggesting additional mechanisms of MHC-I down-regulation. ORF7a proteins from a sample of sarbecoviruses varied in their ability to induce MHC-I down-regulation and, unlike SARS-CoV-2, the ORF7a protein from SARS-CoV lacked MHC-I downregulating activity. A single amino acid at position 59 (T/F) that is variable among sarbecovirus ORF7a proteins governed the difference in MHC-I downregulating activity. SARS-CoV-2 ORF7a physically associated with the MHC-I heavy chain and inhibited the presentation of expressed antigen to CD8⁺ T cells. Specifically, ORF7a prevented the assembly of the MHC-I peptide loading complex and caused retention of MHC-I in the endoplasmic reticulum. The differential ability of ORF7a proteins to function in this way might affect sarbecovirus dissemination and persistence in human populations, particularly those with infection- or vaccineelicited immunity.

The encapsidation of HIV-1 gRNA into virions is enabled by the binding of the nucleocapsid (NC) domain of the HIV-1 Gag polyprotein to the structured viral RNA packaging signal (Ψ) at the 5’ end of the viral genome. However, the subcellular location and oligomeric status of Gag during the initial Gag-Ψ encounter remain uncertain. Domains other than NC, such as capsid (CA), may therefore indirectly affect RNA recognition. To investigate the contribution of Gag domains to Ψ recognition in a cellular environment, we performed protein-protein crosslinking and protein-RNA crosslinking immunoprecipitation coupled with sequencing (CLIP-seq) experiments. We demonstrate that NC alone does not bind specifically to Ψ in living cells, whereas full-length Gag and a CANC subdomain bind to Ψ with high specificity. Perturbation of the Ψ RNA structure or NC zinc fingers affected CANC:Ψ binding specificity. Notably, CANC variants with substitutions that disrupt CA:CA dimer, trimer, or hexamer interfaces in the immature Gag lattice also affected RNA binding, and mutants that were unable to assemble a nascent Gag lattice were unable to specifically bind to Ψ. Artificially multimerized NC domains did not specifically bind Ψ. CA variants with substitutions in inositol phosphate coordinating residues that prevent CA hexamerization were also deficient in Ψ binding and second-site revertant mutants that restored CA assembly also restored specific binding to Ψ. Overall, these data indicate that the correct assembly of a nascent immature CA lattice is required for the specific interaction between Gag and Ψ in cells.

Vertebrate genomes contain fewer CG dinucleotides than would be expected by chance, and this pattern is mimicked by many viruses; HIV-1 derivatives mutated to contain more CG dinucleotides are targeted by the human antiviral protein ZAP, suggesting that CG suppression has evolved in viruses to evade recognition. Viruses evade the ZAP Many virus genomes possess lower than expected numbers of 5′-CG-3′ dinucleotides, mimicking the CG suppression of their vertebrate hosts. The reason for this has been unclear, but Paul Bieniasz and colleagues now show that viruses use CG suppression to avoid recognition by the zinc-finger antiviral protein ZAP. They show that CG suppression is essential for HIV-1 replication; otherwise, the virus genome is recognized by ZAP and targeted for degradation. Vertebrate genomes exhibit marked CG suppression—that is, lower than expected numbers of 5′-CG-3′ dinucleotides 1 . This feature is likely to be due to C-to-T mutations that have accumulated over hundreds of millions of years, driven by CG-specific DNA methyl transferases and spontaneous methyl-cytosine deamination. Many RNA viruses of vertebrates that are not substrates for DNA methyl transferases mimic the CG suppression of their hosts 2 , 3 , 4 . This property of viral genomes is unexplained 4 , 5 , 6 . Here we show, using synonymous mutagenesis, that CG suppression is essential for HIV-1 replication. The deleterious effect of CG dinucleotides on HIV-1 replication was cumulative, associated with cytoplasmic RNA depletion, and was exerted by CG dinucleotides in both translated and non-translated exonic RNA sequences. A focused screen using small inhibitory RNAs revealed that zinc-finger antiviral protein (ZAP) 7 inhibited virion production by cells infected with CG-enriched HIV-1. Crucially, HIV-1 mutants containing segments whose CG content mimicked random nucleotide sequence were defective in unmanipulated cells, but replicated normally in ZAP-deficient cells. Crosslinking–immunoprecipitation–sequencing assays demonstrated that ZAP binds directly and selectively to RNA sequences containing CG dinucleotides. These findings suggest that ZAP exploits host CG suppression to identify non-self RNA. The dinucleotide composition of HIV-1, and perhaps other RNA viruses, appears to have adapted to evade this host defence.

Numerous challenges have impeded HIV-1 vaccine development. Among these is the lack of a convenient small animal model in which to study antibody elicitation and efficacy. We describe a chimeric Rhabdo-Immunodeficiency virus (RhIV) murine model that recapitulates key features of HIV-1 entry, tropism and antibody sensitivity. RhIVs are based on vesicular stomatitis viruses (VSV), but viral entry is mediated by HIV-1 Env proteins from diverse HIV-1 strains. RhIV infection of transgenic mice expressing human CD4 and CCR5, exclusively on mouse CD4+ cells, at levels mimicking those on human CD4+ T-cells, resulted in acute, resolving viremia and CD4+ T-cell depletion. RhIV infection elicited protective immunity, and antibodies to HIV-1 Env that were primarily non-neutralizing and had modest protective efficacy following passive transfer. The RhIV model enables the convenient in vivo study of HIV-1 Env-receptor interactions, antiviral activity of antibodies and humoral responses against HIV-1 Env, in a genetically manipulatable host. One of the main obstacles to developing a vaccine against HIV-1 is teaching the immune system to recognize the envelope proteins on the surface of the virus, which are also found on infected cells. Envelope proteins allow HIV-1 to attach to and infect a type of human immune cell known as a T-cell, by interacting with proteins on its membrane called CD4 and CCR5. Antibodies are proteins produced by the immune system that can stop HIV-1 from spreading. They can recognize and attach to envelope proteins, thus tagging infected cells so the immune system can attack them, and ‘neutralizing’ viral particles to prevent them from infecting more cells. To make a vaccine against HIV-1, scientists need to teach the immune system how to make neutralizing antibodies. Unfortunately, HIV-1 only replicates in humans and chimpanzees, making it difficult to study how these antibodies are generated. Now, Liberatore et al. have developed a hybrid virus that recreates key features of HIV-1 infection in mice. The interior of these viruses is made up of components from a rhabdovirus, which replicates well in mice, with envelope proteins from HIV-1 incorporated into the viruses’ exterior. Therefore, despite having different replication machinery, these hybrid viruses – nicknamed ‘RhIV’ – are able to infect the cells of mice using the same attachment mechanism as HIV-1. Next, Liberatore et al. genetically modified mice to produce human CD4 and CCR5 proteins, so RhIV could attach to their T-cells and get inside. The virus rapidly killed the cells it infected, similar to early HIV-1 infection in humans. But, unlike HIV-1 infection in humans, the mice were able to get rid of the virus within a couple of weeks. When the mice were exposed to RhIV a second time, they were partially protected against re-infection. This ‘vaccine effect’ was even stronger if the mice were exposed a third time, making them almost immune to the virus. However, the effect could not be attributed exclusively to antibodies, since mice unable to make antibodies still gained some immune protection after infection with RhIV. The results showed that antibodies produced by the infected mice could recognize HIV-1 envelope proteins, but were unable to neutralize viral particles. Nevertheless, transferring antibodies from infected mice after recovery into healthy mice that had never been exposed to the virus partially protected the healthy mice from infection. This new model system for HIV-1 infection should make it easier to test new types of vaccines in a context where the vaccinated animal can be challenged with RhIV. Additionally, the ability to genetically engineer both the virus and the mouse host – for example by making mice that produce human antibodies – allows further studies into the development of antibodies that recognize the HIV-1 envelope.

The encapsidation of HIV-1 genomic RNA into virions is enabled by the binding of the nucleocapsid (NC) domain of the HIV-1 Gag polyprotein to the structured viral RNA packaging signal (Ψ) at the 5′ end of the viral genome. However, the subcellular location and oligomeric status of Gag during the initial Gag-Ψ encounter remains uncertain. Domains other than NC, such as capsid (CA), may therefore indirectly affect RNA recognition. To investigate the contribution of Gag domains to Ψ recognition in a cellular environment, we performed protein-protein crosslinking and protein-RNA crosslinking immunoprecipitation coupled with sequencing (CLIP-seq) experiments. We demonstrate that NC alone does not bind specifically to Ψ in living cells, whereas full-length Gag and a CANC subdomain bind to Ψ with high specificity. Perturbation of the Ψ RNA structure or NC zinc fingers affected CANC:Ψ binding specificity. Notably, CANC variants with substitutions that disrupt CA:CA dimer, trimer or hexamer interfaces in the immature Gag lattice also affected RNA binding, and mutants that were unable to assemble a nascent Gag lattice completely lost the ability to specific bind to Ψ. Artificially multimerized NC domains did not specifically bind Ψ. CA variants with substitutions in inositol phosphate coordinating residues that prevent CA hexamerization were also deficient in Ψ binding and second-site revertant mutants that restored CA assembly also restored specific binding to Ψ. Overall, these data indicate that the correct assembly of a nascent immature CA lattice is required for the specific interaction between Gag and Ψ in cells. Competing Interest Statement The authors have declared no competing interest.

MX2 is shown to be an interferon-induced inhibitor of HIV-1 infection, and this antiviral activity may involve the inhibition of nuclear import of subviral complexes. Human MX2 protein is an HIV-1 resistance factor Two groups report in this issue of Nature that the human interferon-induced GTP-binding protein MX2 is a potent inhibitor of human immunodeficiency virus type 1 (HIV-1) and a number of other lentiviruses. For some years it had been known that the related protein MX1 can inhibit HIV-1 replication in humans, but MX2 was thought to be devoid of antiviral activity. The anti-HIV-1 action of MX2 is much less dependent on GTPase activity than is the broader antiviral activity of MX1, pointing to possible mechanistic differences between them. HIV-1 replication can be inhibited by type I interferon (IFN), and the expression of a number of gene products with anti-HIV-1 activity is induced by type I IFN 1 , 2 . However, none of the known antiretroviral proteins can account for the ability of type I IFN to inhibit early, preintegration phases of the HIV-1 replication cycle in human cells 3 , 4 . Here, by comparing gene expression profiles in cell lines that differ in their ability to support the inhibitory action of IFN-α at early steps of the HIV-1 replication cycle, we identify myxovirus resistance 2 (MX2) as an interferon-induced inhibitor of HIV-1 infection. Expression of MX2 reduces permissiveness to a variety of lentiviruses, whereas depletion of MX2 using RNA interference reduces the anti-HIV-1 potency of IFN-α. HIV-1 reverse transcription proceeds normally in MX2-expressing cells, but 2-long terminal repeat circular forms of HIV-1 DNA are less abundant, suggesting that MX2 inhibits HIV-1 nuclear import, or destabilizes nuclear HIV-1 DNA. Consistent with this notion, mutations in the HIV-1 capsid protein that are known, or suspected, to alter the nuclear import pathways used by HIV-1 confer resistance to MX2, whereas preventing cell division increases MX2 potency. Overall, these findings indicate that MX2 is an effector of the anti-HIV-1 activity of type-I IFN, and suggest that MX2 inhibits HIV-1 infection by inhibiting capsid-dependent nuclear import of subviral complexes.

Passive immunotherapy with a combination of neutralizing monoclonal antibodies is shown to be effective in suppressing HIV replication in a humanized mouse model. A boost for HIV-1 therapy Broadly neutralizing antibodies to human immunodeficiency virus-1 (HIV-1) are slow to develop and are found in only a fraction of patients, but they can prevent infection and so are of great importance for HIV therapy design. Previous work has shown that the virus can quickly evolve resistance against these antibodies; however, more potent antibodies are now available. Michel Nussenzweig and colleagues therefore re-examined the potential of antibody therapy in 'humanized' mice. They demonstrate that passive immunotherapy with combinations of broadly neutralizing antibodies effectively controls HIV-1 infection. The authors suggest that it is time to re-examine monoclonal antibodies as therapeutics in HIV-1-infected individuals. Human antibodies to human immunodeficiency virus-1 (HIV-1) can neutralize a broad range of viral isolates in vitro and protect non-human primates against infection 1 , 2 . Previous work showed that antibodies exert selective pressure on the virus but escape variants emerge within a short period of time 3 , 4 . However, these experiments were performed before the recent discovery of more potent anti-HIV-1 antibodies and their improvement by structure-based design 5 , 6 , 7 , 8 , 9 . Here we re-examine passive antibody transfer as a therapeutic modality in HIV-1-infected humanized mice. Although HIV-1 can escape from antibody monotherapy, combinations of broadly neutralizing antibodies can effectively control HIV-1 infection and suppress viral load to levels below detection. Moreover, in contrast to antiretroviral therapy 10 , 11 , 12 , the longer half-life of antibodies led to control of viraemia for an average of 60 days after cessation of therapy. Thus, combinations of potent monoclonal antibodies can effectively control HIV-1 replication in humanized mice, and should be re-examined as a therapeutic modality in HIV-1-infected individuals.

A central problem in achieving vaccine-based protection against viral infections is eliciting antibodies that are resilient to viral variation. Successive waves of SARS-CoV-2 infection during the COVID19 pandemic were driven by variants that acquired resistance to neutralizing antibodies elicited by prior SARS-CoV-2 variants. To the extent that serum neutralization breadth occurs in individuals with multiple exposures to SARS-CoV-2 antigens, we and others find that it is largely comprised of antibodies that target the variable receptor binding domain (RBD), rather than more conserved spike protein domains. By designing synthetic dimeric RBD immunogens we show that limiting divergence in heterodimeric components favors the generation of cross-reactive B cells and antibodies. We thus devised a vaccine approach based on a two-dose immunization with a pool of five overlapping heterodimeric synthetic RBD variants. Collectively, the RBD heterodimer pool was designed to cover 10% sequence variation and elicited greater antibody cross-reactivity and neutralization breadth than homodimers or heterodimers with highly divergent components. Using an unconventional 'prospective' challenge model in mice, we demonstrate the effectiveness of the RBD heterodimer pool in inducing antibody responses that attenuate infection by future SARS-CoV-2 variants, as well as protection in a challenge model based on a chimeric vesicular stomatitis virus bearing a spike protein from SARS-CoV-1.