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Broadly neutralizing antibodies targeting the V2 apex of the HIV-1 envelope trimer are among the most common specificities elicited in HIV-1-infected humans and simian-human immunodeficiency virus (SHIV)-infected macaques. To gain insight into the prevalent induction of these antibodies, we isolated and characterized 11 V2 apex-directed neutralizing antibody lineages from SHIV-infected rhesus macaques. Remarkably, all SHIV-induced V2 apex lineages were derived from reading frame two of the rhesus DH3-15*01 gene. Cryo-EM structures of envelope trimers in complex with antibodies from nine rhesus lineages revealed modes of recognition that mimicked three canonical human V2 apex-recognition modes. Notably, amino acids encoded by DH3-15*01 played divergent structural roles, inserting into a hole at the trimer apex, H-bonding to an exposed strand, or forming part of a loop scaffold. Overall, we identify a DH3-15*01-signature for rhesus V2 apex broadly neutralizing antibodies and show that highly selected genetic elements can play multiple roles in antigen recognition., Highlights: Isolated 11 V2 apex-targeted HIV-neutralizing lineages from 10 SHIV-infected Indian-origin rhesus macaquesCryo-EM structures of Fab-Env complexes for nine rhesus lineages reveal modes of recognition that mimic three modes of human V2 apex antibody recognitionAll SHIV-elicited V2 apex lineages, including two others previously published, derive from the same DH3-15*01 gene utilizing reading frame twoThe DH3-15*01 gene in reading frame two provides a necessary, but not sufficient, signature for V2 apex-directed broadly neutralizing antibodiesStructural roles played by DH3-15*01-encoded amino acids differed substantially in different lineages, even for those with the same recognition modePropose that the anionic, aromatic, and extended character of DH3-15*01 in reading frame two provides a selective advantage for V2 apex recognition compared to B cells derived from other D genes in the naïve rhesus repertoireDemonstrate that highly selected genetic elements can play multiple roles in antigen recognition, providing a structural means to enhance recognition diversity.

Host restriction factors play key roles in innate antiviral defense, but it remains poorly understood which of them restricts HIV-1 in vivo. Here, we used single-cell transcriptomic analysis to identify host factors associated with HIV-1 control during acute infection by correlating host gene expression with viral RNA abundance within individual cells. Wide sequencing of cells from one participant with the highest plasma viral load revealed that intracellular viral RNA transcription correlates inversely with expression of the gene PTMA , which encodes prothymosin α. This association was genome-wide significant ( P adjusted < 0.05) and was validated in 28 additional participants from Thailand and the Americas with HIV-1 CRF01&#95;AE and subtype B infections, respectively. Overexpression of prothymosin α in vitro confirmed that this cellular factor inhibits HIV-1 transcription and infectious virus production. Our results identify prothymosin α as a host factor that restricts HIV-1 infection in vivo, which has implications for viral transmission and cure strategies.

HIV-1 and its SIV precursors share a broadly neutralizing antibody (bNAb) epitope in variable loop 2 (V2) at the envelope glycoprotein (Env) trimer apex. Here, we tested the immunogenicity of germ line-targeting versions of a chimpanzee SIV (SIVcpz) Env in human V2-apex bNAb heavy-chain precursor-expressing knock-in mice and as chimeric simian-chimpanzee immunodeficiency viruses (SCIVs) in rhesus macaques (RMs). Trimer immunization of knock-in mice induced V2-directed NAbs, indicating activation of V2-apex bNAb precursor-expressing mouse B cells. SCIV infection of RMs elicited high-titer viremia, potent autologous tier 2 neutralizing antibodies, and rapid sequence escape in the canonical V2-apex epitope. Six of seven animals also developed low-titer heterologous plasma breadth that mapped to the V2-apex. Antibody cloning from two of these animals identified multiple expanded lineages with long heavy chain third complementarity determining regions that cross-neutralized as many as 7 of 19 primary HIV-1 strains, but with low potency. Negative stain electron microscopy (NSEM) of members of the two most cross-reactive lineages confirmed V2 targeting but identified an angle of approach distinct from prototypical V2-apex bNAbs, with antibody binding either requiring or inducing an occluded-open trimer. Probing with conformation-sensitive, nonneutralizing antibodies revealed that SCIV-expressed, but not wild-type SIVcpz Envs, as well as a subset of primary HIV-1 Envs, preferentially adopted a more open trimeric state. These results reveal the existence of a cryptic V2 epitope that is exposed in occluded-open SIVcpz and HIV-1 Env trimers and elicits cross-neutralizing responses of limited breadth and potency. IMPORTANCE An effective HIV-1 vaccination strategy will need to stimulate rare precursor B cells of multiple bNAb lineages and affinity mature them along desired pathways. Here, we searched for V2-apex germ line-targeting Envs among a large set of diverse primate lentiviruses and identified minimally modified versions of one chimpanzee SIV Env that bound several human V2-apex bNAb precursors and stimulated one of these in a V2-apex bNAb precursor-expressing knock-in mouse. We also generated chimeric simian-chimpanzee immunodeficiency viruses and showed that they elicit low-titer V2-directed heterologous plasma breadth in six of seven infected rhesus macaques. Characterization of this antibody response identified a new class of weakly cross-reactive neutralizing antibodies that target the V2-apex, but only in occluded-open Env trimers. The existence of this cryptic epitope, which in some Env backgrounds is immunodominant, needs to be considered in immunogen design.

Of the 13 known independent zoonoses of simian immunodeficiency viruses to humans, only one, leading to human immunodeficiency virus (HIV) type 1(M) has become pandemic, causing over 80 million human infections. To understand the specific features associated with pandemic human-to-human HIV spread, we compared replication of HIV-1(M) with non-pandemic HIV-(O) and HIV-2 strains in myeloid cell models. We found that non-pandemic HIV lineages replicate less well than HIV-1(M) owing to activation of cGAS and TRIM5-mediated antiviral responses. We applied phylogenetic and X-ray crystallography structural analyses to identify differences between pandemic and non-pandemic HIV capsids. We found that genetic reversal of two specific amino acid adaptations in HIV-1(M) enables activation of TRIM5, cGAS and innate immune responses. We propose a model in which the parental lineage of pandemic HIV-1(M) evolved a capsid that prevents cGAS and TRIM5 triggering, thereby allowing silent replication in myeloid cells. We hypothesize that this capsid adaptation promotes human-to-human spread through avoidance of innate immune response activation., (© 2022. The Author(s).)

Not all persons recovering from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection develop SARS-CoV-2-specific antibodies. We show that nonseroconversion is associated with younger age and higher reverse transcription PCR cycle threshold values and identify SARS-CoV-2 viral loads in the nasopharynx as a major correlate of the systemic antibody response.

T-cell immunity is likely to play a role in protection against SARS-CoV-2 by helping generate neutralizing antibodies. We longitudinally studied CD4 T-cell responses to the M, N, and S structural proteins of SARS-CoV-2 in 26 convalescent individuals. Within the first two months following symptom onset, a majority of individuals (81%) mounted at least one CD4 T-cell response, and 48% of individuals mounted detectable SARS-CoV-2-specific circulating T follicular helper cells (cTfh, defined as CXCR5+PD1+ CD4 T cells). SARS-CoV-2-specific cTfh responses across all three protein specificities correlated with antibody neutralization with the strongest correlation observed for S protein-specific responses. When examined over time, cTfh responses, particularly to the M protein, increased in convalescence, and robust cTfh responses with magnitudes greater than 5% were detected at the second convalescent visit, a median of 38 days post-symptom onset. CD4 T-cell responses declined but persisted at low magnitudes three months and six months after symptom onset. These data deepen our understanding of antigen-specific cTfh responses in SARS-CoV-2 infection, suggesting that in addition to S protein, M and N protein-specific cTfh may also assist in the development of neutralizing antibodies and that cTfh response formation may be delayed in SARS-CoV-2 infection., Competing Interests: The authors have declared that no competing interests exist.

Small CD4-mimetic compounds (CD4mc) sensitize HIV-1-infected cells to antibody-dependent cellular cytotoxicity (ADCC) by facilitating antibody recognition of epitopes that are otherwise occluded on the unliganded viral envelope (Env). Combining CD4mc with two families of CD4-induced (CD4i) antibodies, which are frequently found in plasma of HIV-1-infected individuals, stabilizes Env in a conformation that is vulnerable to ADCC. We employed new-generation SRG-15 humanized mice, supporting natural killer (NK) cell and Fc-effector functions to demonstrate that brief treatment with CD4mc and CD4i-Abs significantly decreases HIV-1 replication, the virus reservoir and viral rebound after ART interruption. These effects required Fc-effector functions and NK cells, highlighting the importance of ADCC. Viral rebound was also suppressed in HIV-1+-donor cell-derived humanized mice supplemented with autologous HIV-1+-donor-derived plasma and CD4mc. These results indicate that CD4mc could have therapeutic utility in infected individuals for decreasing the size of the HIV-1 reservoir and/or achieving a functional cure., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Infection with human and simian immunodeficiency viruses (HIV/SIV) requires binding of the viral envelope glycoprotein (Env) to the host protein CD4 on the surface of immune cells. Although invariant in humans, the Env binding domain of the chimpanzee CD4 is highly polymorphic, with nine coding variants circulating in wild populations. Here, we show that within-species CD4 diversity is not unique to chimpanzees but found in many African primate species. Characterizing the outermost (D1) domain of the CD4 protein in over 500 monkeys and apes, we found polymorphic residues in 24 of 29 primate species, with as many as 11 different coding variants identified within a single species. D1 domain amino acid replacements affected SIV Env-mediated cell entry in a single-round infection assay, restricting infection in a strain- and allele-specific fashion. Several identical CD4 polymorphisms, including the addition of N -linked glycosylation sites, were found in primate species from different genera, providing striking examples of parallel evolution. Moreover, seven different guenons ( Cercopithecus spp.) shared multiple distinct D1 domain variants, pointing to long-term trans-specific polymorphism. These data indicate that the HIV/SIV Env binding region of the primate CD4 protein is highly variable, both within and between species, and suggest that this diversity has been maintained by balancing selection for millions of years, at least in part to confer protection against primate lentiviruses. Although long-term SIV-infected species have evolved specific mechanisms to avoid disease progression, primate lentiviruses are intrinsically pathogenic and have left their mark on the host genome., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

Type 1 interferons (IFN-I) are potent innate antiviral effectors that constrain HIV-1 transmission. However, harnessing these cytokines for HIV-1 cure strategies has been hampered by an incomplete understanding of their antiviral activities at later stages of infection. Here, we characterized the IFN-I sensitivity of 500 clonally derived HIV-1 isolates from the plasma and CD4 + T cells of 26 individuals sampled longitudinally after transmission or after antiretroviral therapy (ART) and analytical treatment interruption. We determined the concentration of IFNα2 and IFNβ that reduced viral replication in vitro by 50% (IC 50 ) and found consistent changes in the sensitivity of HIV-1 to IFN-I inhibition both across individuals and over time. Resistance of HIV-1 isolates to IFN-I was uniformly high during acute infection, decreased in all individuals in the first year after infection, was reacquired concomitant with CD4 + T cell loss, and remained elevated in individuals with accelerated disease. HIV-1 isolates obtained by viral outgrowth during suppressive ART were relatively IFN-I sensitive, resembling viruses circulating just before ART initiation. However, viruses that rebounded after treatment interruption displayed the highest degree of IFNα2 and IFNβ resistance observed at any time during the infection course. These findings indicate a dynamic interplay between host innate responses and the evolving HIV-1 quasispecies, with the relative contribution of IFN-I to HIV-1 control affected by both ART and analytical treatment interruption. Although elevated at transmission, host innate pressures are the highest during viral rebound, limiting the viruses that successfully become reactivated from latency to those that are IFN-I resistant., (Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Neutralizing antibodies elicited by HIV-1 coevolve with viral envelope proteins (Env) in distinctive patterns, in some cases acquiring substantial breadth. We report that primary HIV-1 envelope proteins-when expressed by simian-human immunodeficiency viruses in rhesus macaques-elicited patterns of Env-antibody coevolution very similar to those in humans, including conserved immunogenetic, structural, and chemical solutions to epitope recognition and precise Env-amino acid substitutions, insertions, and deletions leading to virus persistence. The structure of one rhesus antibody, capable of neutralizing 49% of a 208-strain panel, revealed a V2 apex mode of recognition like that of human broadly neutralizing antibodies (bNAbs) PGT145 and PCT64-35S. Another rhesus antibody bound the CD4 binding site by CD4 mimicry, mirroring human bNAbs 8ANC131, CH235, and VRC01. Virus-antibody coevolution in macaques can thus recapitulate developmental features of human bNAbs, thereby guiding HIV-1 immunogen design., (Copyright © 2021, American Association for the Advancement of Science.)

HLA-C-mediated antigen presentation induces the killing of human immunodeficiency virus (HIV)-infected CD4 + T cells by cytotoxic T lymphocytes (CTLs). To evade killing, many HIV-1 group M strains decrease HLA-C surface levels using their accessory protein Vpu. However, some HIV-1 group M isolates lack this activity, possibly to prevent the activation of natural killer (NK) cells. Analyzing diverse primate lentiviruses, we found that Vpu-mediated HLA-C downregulation is not limited to pandemic group M but is also found in HIV-1 groups O and P as well as several simian immunodeficiency viruses (SIVs). We show that Vpu targets HLA-C primarily at the protein level, independently of its ability to suppress NF-κB-driven gene expression, and that in some viral lineages, HLA-C downregulation may come at the cost of efficient counteraction of the restriction factor tetherin. Remarkably, HIV-2, which does not carry a vpu gene, uses its accessory protein Vif to decrease HLA-C surface expression. This Vif activity requires intact binding sites for the Cullin5/Elongin ubiquitin ligase complex but is separable from its ability to counteract APOBEC3G. Similar to HIV-1 Vpu, the degree of HIV-2 Vif-mediated HLA-C downregulation varies considerably among different virus isolates. In agreement with opposing selection pressures in vivo , we show that the reduction of HLA-C surface levels by HIV-2 Vif is accompanied by increased NK cell-mediated killing. In summary, our results highlight the complex role of HLA-C in lentiviral infections and demonstrate that HIV-1 and HIV-2 have evolved at least two independent mechanisms to decrease HLA-C levels on infected cells. IMPORTANCE Genome-wide association studies suggest that HLA-C expression is a major determinant of viral load set points and CD4 + T cell counts in HIV-infected individuals. On the one hand, efficient HLA-C expression enables the killing of infected cells by cytotoxic T lymphocytes (CTLs). On the other hand, HLA-C sends inhibitory signals to natural killer (NK) cells and enhances the infectivity of newly produced HIV particles. HIV-1 group M viruses modulate HLA-C expression using the accessory protein Vpu, possibly to balance CTL- and NK cell-mediated immune responses. Here, we show that the second human immunodeficiency virus, HIV-2, can use its accessory protein Vif to evade HLA-C-mediated restriction. Furthermore, our mutational analyses provide insights into the underlying molecular mechanisms. In summary, our results reveal how the two human AIDS viruses modulate HLA-C, a key component of the antiviral immune response., (Copyright © 2020 Hopfensperger et al.)

The HIV-1 accessory protein Vpu enhances viral release by counteracting the restriction factor BST-2. Furthermore, Vpu promotes NK cell evasion by downmodulating cell surface NTB-A and PVR, known ligands of the NK cell receptors NTB-A and DNAM-1, respectively. While it has been established that Vpu's transmembrane domain (TMD) is required for the interaction and intracellular sequestration of BST-2, NTB-A, and PVR, it remains unclear how Vpu manages to target these proteins simultaneously. In this study, we show that upon upregulation, BST-2 is preferentially downregulated by Vpu over its other TMD substrates. We found that type I interferon (IFN)-mediated BST-2 upregulation greatly impairs the ability of Vpu to downregulate NTB-A and PVR. Our results suggest that occupation of Vpu by BST-2 affects its ability to downregulate other TMD substrates. Accordingly, knockdown of BST-2 increases Vpu's potency to downmodulate NTB-A and PVR in the presence of type I IFN treatment. Moreover, we show that expression of human BST-2, but not that of the macaque orthologue, decreases Vpu's capacity to downregulate NTB-A. Importantly, we show that type I IFNs efficiently sensitize HIV-1-infected cells to NTB-A- and DNAM-1-mediated direct and antibody-dependent NK cell responses. Altogether, our results reveal that type I IFNs decrease Vpu's polyfunctionality, thus reducing its capacity to protect HIV-1-infected cells from NK cell responses. IMPORTANCE The restriction factor BST-2 and the NK cell ligands NTB-A and PVR are among a growing list of membrane proteins found to be downregulated by HIV-1 Vpu. BST-2 antagonism enhances viral release, while NTB-A and PVR downmodulation contributes to NK cell evasion. However, it remains unclear how Vpu can target multiple cellular factors simultaneously. Here we provide evidence that under physiological conditions, BST-2 is preferentially targeted by Vpu over NTB-A and PVR. Specifically, we show that type I IFNs decrease Vpu's polyfunctionality by upregulating BST-2, thus reducing its capacity to protect HIV-1-infected cells from NK cell responses. This indicates that there is a hierarchy of Vpu substrates upon IFN treatment, revealing that for the virus, targeting BST-2 as part of its resistance to IFN takes precedence over evading NK cell responses. This reveals a potential weakness in HIV-1's immunoevasion mechanisms that may be exploited therapeutically to harness NK cell responses against HIV-1., (Copyright © 2019 Prévost et al.)

Human and simian immunodeficiency viruses (HIV/SIVs) use CD4 as the primary receptor to enter target cells. Here, we show that the chimpanzee CD4 is highly polymorphic, with nine coding variants present in wild populations, and that this diversity interferes with SIV envelope (Env)-CD4 interactions. Testing the replication fitness of SIVcpz strains in CD4 + T cells from captive chimpanzees, we found that certain viruses were unable to infect cells from certain hosts. These differences were recapitulated in CD4 transfection assays, which revealed a strong association between CD4 genotypes and SIVcpz infection phenotypes. The most striking differences were observed for three substitutions (Q25R, Q40R, and P68T), with P68T generating a second N-linked glycosylation site (N66) in addition to an invariant N32 encoded by all chimpanzee CD4 alleles. In silico modeling and site-directed mutagenesis identified charged residues at the CD4-Env interface and clashes between CD4- and Env-encoded glycans as mechanisms of inhibition. CD4 polymorphisms also reduced Env-mediated cell entry of monkey SIVs, which was dependent on at least one D1 domain glycan. CD4 allele frequencies varied among wild chimpanzees, with high diversity in all but the western subspecies, which appeared to have undergone a selective sweep. One allele was associated with lower SIVcpz prevalence rates in the wild. These results indicate that substitutions in the D1 domain of the chimpanzee CD4 can prevent SIV cell entry. Although some SIVcpz strains have adapted to utilize these variants, CD4 diversity is maintained, protecting chimpanzees against infection with SIVcpz and other SIVs to which they are exposed., Competing Interests: The authors declare no conflict of interest., (Copyright © 2019 the Author(s). Published by PNAS.)

Short tandem repeats (STRs), also known as microsatellites, are commonly used to noninvasively genotype wild-living endangered species, including African apes. Until recently, capillary electrophoresis has been the method of choice to determine the length of polymorphic STR loci. However, this technique is labor intensive, difficult to compare across platforms, and notoriously imprecise. Here we developed a MiSeq-based approach and tested its performance using previously genotyped fecal samples from long-term studied chimpanzees in Gombe National Park, Tanzania. Using data from eight microsatellite loci as a reference, we designed a bioinformatics platform that converts raw MiSeq reads into locus-specific files and automatically calls alleles after filtering stutter sequences and other PCR artifacts. Applying this method to the entire Gombe population, we confirmed previously reported genotypes, but also identified 31 new alleles that had been missed due to sequence differences and size homoplasy. The new genotypes, which increased the allelic diversity and heterozygosity in Gombe by 61% and 8%, respectively, were validated by replicate amplification and pedigree analyses. This demonstrated inheritance and resolved one case of an ambiguous paternity. Using both singleplex and multiplex locus amplification, we also genotyped fecal samples from chimpanzees in the Greater Mahale Ecosystem in Tanzania, demonstrating the utility of the MiSeq-based approach for genotyping nonhabituated populations and performing comparative analyses across field sites. The new automated high-throughput analysis platform (available at https://github.com/ShawHahnLab/chiimp) will allow biologists to more accurately and effectively determine wildlife population size and structure, and thus obtain information critical for conservation efforts.

Pandemic HIV-1 originated from the cross-species transmission of SIVcpz, which infects chimpanzees, while SIVcpz itself emerged following the cross-species transmission and recombination of monkey SIVs, with env contributed by the SIVgsn/mus/mon lineage that infects greater spot-nosed, mustached and mona monkeys. SIVcpz and HIV-1 are pathogenic in their respective hosts, while the phenotype of their SIVgsn/mus/mon ancestors is unknown. However, two well-studied SIV infected natural hosts, sooty mangabeys (SMs) and African green monkeys (AGMs), typically remain healthy despite high viral loads; these species express low levels of the canonical coreceptor CCR5, and recent work shows that CXCR6 is a major coreceptor for SIV in these hosts. It is not known what coreceptors were used by the precursors of SIVcpz, whether coreceptor use changed during emergence of the SIVcpz/HIV-1 lineage, and what T cell subsets express CXCR6 in natural hosts. Using species-matched coreceptors and CD4, we show here that SIVcpz uses only CCR5 for entry and, like HIV-1, cannot use CXCR6. In contrast, SIVmus efficiently uses both CXCR6 and CCR5. Coreceptor selectivity was determined by Env, with CXCR6 use abrogated by Pro326 in the V3 crown, which is absent in monkey SIVs but highly conserved in SIVcpz/HIV-1. To characterize which cells express CXCR6, we generated a novel antibody that recognizes CXCR6 of multiple primate species. Testing lymphocytes from SM, the best-studied natural host, we found that CXCR6 is restricted to CD4+ effector memory cells, and is expressed by a sub-population distinct from those expressing CCR5. Thus, efficient CXCR6 use, previously identified in SM and AGM infection, also characterizes a member of the SIV lineage that gave rise to SIVcpz/HIV-1. Loss of CXCR6 usage by SIVcpz may have altered its cell tropism, shifting virus from CXCR6-expressing cells that may support replication without disrupting immune function or homeostasis, towards CCR5-expressing cells with pathogenic consequences.

Plasmodium falciparum , the major cause of malaria morbidity and mortality in humans, has been shown to have emerged after cross-species transmission of one of six host-specific parasites (subgenus Laverania ) infecting wild chimpanzees ( Pan troglodytes ) and western gorillas ( Gorilla gorilla ). Binding of the parasite-encoded ligand RH5 to the host protein basigin is essential for erythrocyte invasion and has been implicated in host specificity. A recent study claimed to have found two amino acid changes in RH5 that "drove the host shift leading to the emergence of P. falciparum as a human pathogen." However, the ape Laverania data available at that time, which included only a single distantly related chimpanzee parasite sequence, were inadequate to justify any such conclusion. Here, we have investigated Laverania Rh5 gene evolution using sequences from all six ape parasite species. Searching for gene-wide episodic selection across the entire Laverania phylogeny, we found eight codons to be under positive selection, including three that correspond to contact residues known to form hydrogen bonds between P. falciparum RH5 and human basigin. One of these sites (residue 197) has changed subsequent to the transmission from apes to humans that gave rise to P. falciparum , suggesting a possible role in the adaptation of the gorilla parasite to the human host. We also found evidence that the patterns of nucleotide polymorphisms in P. falciparum are not typical of Laverania species and likely reflect the recent demographic history of the human parasite. IMPORTANCE A number of closely related, host-specific malaria parasites infecting wild chimpanzees and gorillas have recently been described. The most important cause of human malaria, Plasmodium falciparum , is now known to have resulted from a cross-species transmission of one of the gorilla parasites. Overcoming species-specific interactions between a parasite ligand, RH5, and its receptor on host cells, basigin, was likely an important step in the origin of the human parasite. We have investigated the evolution of the Rh5 gene and found evidence of adaptive changes during the diversification of the ape parasite species at sites that are known to form bonds with human basigin. One of these changes occurred at the origin of P. falciparum , implicating it as an important adaptation to the human host., (Copyright © 2018 Plenderleith et al.)

Background: Simian immunodeficiency virus of chimpanzees (SIVcpz), the progenitor of human immunodeficiency virus type 1 (HIV-1), is associated with increased mortality and AIDS-like immunopathology in wild-living chimpanzees (Pan troglodytes). Surprisingly, however, similar findings have not been reported for chimpanzees experimentally infected with SIVcpz in captivity, raising questions about the intrinsic pathogenicity of this lentivirus., Findings: Here, we report progressive immunodeficiency and clinical disease in a captive western chimpanzee (P. t. verus) infected twenty years ago by intrarectal inoculation with an SIVcpz strain (ANT) from a wild-caught eastern chimpanzee (P. t. schweinfurthii). With sustained plasma viral loads of 10 5 to 10 6 RNA copies/ml for the past 15 years, this chimpanzee developed CD4+ T cell depletion (220 cells/μl), thrombocytopenia (90,000 platelets/μl), and persistent soft tissue infections refractory to antibacterial therapy. Combination antiretroviral therapy consisting of emtricitabine (FTC), tenofovir disoproxil fumarate (TDF), and dolutegravir (DTG) decreased plasma viremia to undetectable levels (<200 copies/ml), improved CD4+ T cell counts (509 cell/μl), and resulted in the rapid resolution of all soft tissue infections. However, initial lack of adherence and/or differences in pharmacokinetics led to low plasma drug concentrations, which resulted in transient rebound viremia and the emergence of FTC resistance mutations (M184V/I) identical to those observed in HIV-1 infected humans., Conclusions: These data demonstrate that SIVcpz can cause immunodeficiency and other hallmarks of AIDS in captive chimpanzees, including P. t. verus apes that are not naturally infected with this virus. Moreover, SIVcpz-associated immunodeficiency can be effectively treated with antiretroviral therapy, although sufficiently high plasma concentrations must be maintained to prevent the emergence of drug resistance. These findings extend a growing body of evidence documenting the immunopathogenicity of SIVcpz and suggest that experimentally infected chimpanzees may benefit from clinical monitoring and therapeutic intervention.

Antibodies recognizing conserved CD4-induced (CD4i) epitopes on human immunodeficiency virus type 1 (HIV-1) Env and able to mediate antibody-dependent cellular cytotoxicity (ADCC) have been shown to be present in sera from most HIV-1-infected individuals. These antibodies preferentially recognize Env in its CD4-bound conformation. CD4 downregulation by Nef and Vpu dramatically reduces exposure of CD4i HIV-1 Env epitopes and therefore reduce the susceptibility of HIV-1-infected cells to ADCC mediated by HIV-positive (HIV+) sera. Importantly, this mechanism of immune evasion can be circumvented with small-molecule CD4 mimetics (CD4mc) that are able to transition Env into the CD4-bound conformation and sensitize HIV-1-infected cells to ADCC mediated by HIV+ sera. However, HIV-1 developed additional mechanisms to avoid ADCC, including Vpu-mediated BST-2 antagonism, which decreases the overall amount of Env present at the cell surface. Accordingly, BST-2 upregulation in response to alpha interferon (IFN-α) was shown to increase the susceptibility of HIV-1-infected cells to ADCC despite the activity of Vpu. Here we show that BST-2 upregulation by IFN-β and interleukin-27 (IL-27) also increases the surface expression of Env and thus boosts the ability of CD4mc to sensitize HIV-1-infected cells to ADCC by sera from HIV-1-infected individuals. IMPORTANCE HIV-1 evolved sophisticated strategies to conceal Env epitopes from ADCC-mediating antibodies present in HIV+ sera. Vpu-mediated BST-2 downregulation was shown to decrease ADCC responses by limiting the amount of Env present at the cell surface. This effect of Vpu was shown to be attenuated by IFN-α treatment. Here we show that in addition to IFN-α, IFN-β and IL-27 also affect Vpu-mediated BST-2 downregulation and greatly enhance ADCC responses against HIV-1-infected cells in the presence of CD4mc. These findings may inform strategies aimed at HIV prevention and eradication., (Copyright © 2017 American Society for Microbiology.)

Simian immunodeficiency viruses (SIVs) use their Nef proteins to counteract the restriction factor tetherin. However, a deletion in human tetherin prevents antagonism by the Nef proteins of SIVcpz and SIVgor, which represent the ape precursors of human immunodeficiency virus type 1 (HIV-1). To promote virus release from infected cells, pandemic HIV-1 group M strains evolved Vpu as a tetherin antagonist, while the Nef protein of less widespread HIV-1 group O strains acquired the ability to target a region adjacent to this deletion. In this study, we identified an unusual HIV-1 group O strain (RBF206) that evolved Vpu as an effective antagonist of human tetherin. While both RBF206 Vpu and Nef exert anti-tetherin activity in transient-transfection assays, mainly Vpu promotes RBF206 release in infected CD4 + T cells. Although mutations distinct from the adaptive changes observed in group M Vpus (M-Vpus) were critical for the acquisition of its anti-tetherin activity, RBF206 O-Vpu potently suppresses NF-κB activation and reduces CD4 cell surface expression. Interestingly, RBF206 Vpu counteracts tetherin in a largely species-independent manner, degrading both the long and short isoforms of human tetherin. Downmodulation of CD4, but not counteraction of tetherin, by RBF206 Vpu was dependent on the cellular ubiquitin ligase machinery. Our data present the first example of an HIV-1 group O Vpu that efficiently antagonizes human tetherin and suggest that counteraction by O-Nefs may be suboptimal. IMPORTANCE Previous studies showed that HIV-1 groups M and O evolved two alternative strategies to counteract the human ortholog of the restriction factor tetherin. While HIV-1 group M switched from Nef to Vpu due to a deletion in the cytoplasmic domain of human tetherin, HIV-1 group O, which lacks Vpu-mediated anti-tetherin activity, acquired a Nef protein that is able to target a region adjacent to the deletion. Here we report an unusual exception, identifying a strain of HIV-1 group O (RBF206) whose Vpu protein evolved an effective antagonism of human tetherin. Interestingly, the adaptive changes in RBF206 Vpu are distinct from those found in M-Vpus and mediate efficient counteraction of both the long and short isoforms of this restriction factor. Our results further illustrate the enormous flexibility of HIV-1 in counteracting human defense mechanisms., (Copyright © 2017 American Society for Microbiology.)

Sexual transmission of HIV-1 is an inefficient process, with only one or few variants of the donor quasispecies establishing the new infection. A critical, and as yet unresolved, question is whether the mucosal bottleneck selects for viruses with increased transmission fitness. Here, we characterized 300 limiting dilution-derived virus isolates from the plasma, and in some instances genital secretions, of eight HIV-1 donor and recipient pairs. Although there were no differences in the amount of virion-associated envelope glycoprotein, recipient isolates were on average threefold more infectious (P = 0.0001), replicated to 1.4-fold higher titers (P = 0.004), were released from infected cells 4.2-fold more efficiently (P < 0.00001), and were significantly more resistant to type I IFNs than the corresponding donor isolates. Remarkably, transmitted viruses exhibited 7.8-fold higher IFNα2 (P < 0.00001) and 39-fold higher IFNβ (P < 0.00001) half-maximal inhibitory concentrations (IC 50 ) than did donor isolates, and their odds of replicating in CD4 + T cells at the highest IFNα2 and IFNβ doses were 35-fold (P < 0.00001) and 250-fold (P < 0.00001) greater, respectively. Interestingly, pretreatment of CD4 + T cells with IFNβ, but not IFNα2, selected donor plasma isolates that exhibited a transmitted virus-like phenotype, and such viruses were also detected in the donor genital tract. These data indicate that transmitted viruses are phenotypically distinct, and that increased IFN resistance represents their most distinguishing property. Thus, the mucosal bottleneck selects for viruses that are able to replicate and spread efficiently in the face of a potent innate immune response., Competing Interests: The authors declare no conflict of interest.

Humans encode seven APOBEC3 proteins (A-H), with A3G, 3F and 3H as the major factors restricting HIV-1 replication. HIV-1, however, encodes Vif, which counteracts A3 proteins by chaperoning them to the proteasome where they are degraded. Vif polymorphisms found in HIV-1s isolated from infected patients have varying anti-A3G potency when assayed in vitro, but there are few studies demonstrating this in in vivo models. Here, we created Friend murine leukemia viruses encoding vif alleles that were previously shown to differentially neutralize A3G in cell culture or that were originally found in primary HIV-1 isolates. Infection of transgenic mice expressing different levels of human A3G showed that these naturally occurring Vif variants differed in their ability to counteract A3G during in vivo infection, although the effects on viral replication were not identical to those seen in cultured cells. We also found that the polymorphic Vifs that attenuated viral replication reverted to wild type only in A3G transgenic mice. Finally, we found that the level of A3G-mediated deamination was inversely correlated with the level of viral replication. This animal model should be useful for studying the functional significance of naturally occurring vif polymorphisms, as well as viral evolution in the presence of A3G.

Cross-species transmissions of viruses from animals to humans are at the origin of major human pathogenic viruses. While the role of ecological and epidemiological factors in the emergence of new pathogens is well documented, the importance of host factors is often unknown. Chimpanzees are the closest relatives of humans and the animal reservoir at the origin of the human AIDS pandemic. However, despite being regularly exposed to monkey lentiviruses through hunting, chimpanzees are naturally infected by only a single simian immunodeficiency virus, SIVcpz. Here, we asked why chimpanzees appear to be protected against the successful emergence of other SIVs. In particular, we investigated the role of the chimpanzee APOBEC3 genes in providing a barrier to infection by most monkey lentiviruses. We found that most SIV Vifs, including Vif from SIVwrc infecting western-red colobus, the chimpanzee's main monkey prey in West Africa, could not antagonize chimpanzee APOBEC3G. Moreover, chimpanzee APOBEC3D, as well as APOBEC3F and APOBEC3H, provided additional protection against SIV Vif antagonism. Consequently, lentiviral replication in primary chimpanzee CD4(+) T cells was dependent on the presence of a lentiviral vif gene that could antagonize chimpanzee APOBEC3s. Finally, by identifying and functionally characterizing several APOBEC3 gene polymorphisms in both common chimpanzees and bonobos, we found that these ape populations encode APOBEC3 proteins that are uniformly resistant to antagonism by monkey lentiviruses.

Unlabelled: Broadly cross-reactive neutralizing antibodies (bNabs) represent powerful tools to combat human immunodeficiency virus type 1 (HIV-1) infection. Here, we examined whether HIV-1-specific bNabs are capable of cross-neutralizing distantly related simian immunodeficiency viruses (SIVs) infecting central (Pan troglodytes troglodytes) (SIVcpzPtt) and eastern (Pan troglodytes schweinfurthii) (SIVcpzPts) chimpanzees (n = 11) as well as western gorillas (Gorilla gorilla gorilla) (SIVgor) (n = 1). We found that bNabs directed against the CD4 binding site (n = 10), peptidoglycans at the base of variable loop 3 (V3) (n = 5), and epitopes at the interface of surface (gp120) and membrane-bound (gp41) envelope glycoproteins (n = 5) failed to neutralize SIVcpz and SIVgor strains. In addition, apex V2-directed bNabs (n = 3) as well as llama-derived (heavy chain only) antibodies (n = 6) recognizing both the CD4 binding site and gp41 epitopes were either completely inactive or neutralized only a fraction of SIVcpzPtt strains. In contrast, one antibody targeting the membrane-proximal external region (MPER) of gp41 (10E8), functional CD4 and CCR5 receptor mimetics (eCD4-Ig, eCD4-Ig(mim2), CD4-218.3-E51, and CD4-218.3-E51-mim2), as well as mono- and bispecific anti-human CD4 (iMab and LM52) and CCR5 (PRO140, PRO140-10E8) receptor antibodies neutralized >90% of SIVcpz and SIVgor strains with low-nanomolar (0.13 to 8.4 nM) potency. Importantly, the latter antibodies blocked virus entry not only in TZM-bl cells but also in Cf2Th cells expressing chimpanzee CD4 and CCR5 and neutralized SIVcpz in chimpanzee CD4(+) T cells, with 50% inhibitory concentrations (IC50s) ranging from 3.6 to 40.5 nM. These findings provide new insight into the protective capacity of anti-HIV-1 bNabs and identify candidates for further development to combat SIVcpz infection., Importance: SIVcpz is widespread in wild-living chimpanzees and can cause AIDS-like immunopathology and clinical disease. HIV-1 infection of humans can be controlled by antiretroviral therapy; however, treatment of wild-living African apes with current drug regimens is not feasible. Nonetheless, it may be possible to curb the spread of SIVcpz in select ape communities using vectored immunoprophylaxis and/or therapy. Here, we show that antibodies and antibody-like inhibitors developed to combat HIV-1 infection in humans are capable of neutralizing genetically diverse SIVcpz and SIVgor strains with considerable breadth and potency, including in primary chimpanzee CD4(+) T cells. These reagents provide an important first step toward translating intervention strategies currently developed to treat and prevent AIDS in humans to SIV-infected apes., (Copyright © 2015 Barbian et al.)

Most simian immunodeficiency viruses use their Nef protein to antagonize the host restriction factor tetherin. A deletion in human tetherin confers Nef resistance, representing a hurdle to successful zoonotic transmission. HIV-1 group M evolved to utilize the viral protein U (Vpu) to counteract tetherin. Although HIV-1 group O has spread epidemically in humans, it has not evolved a Vpu-based tetherin antagonism. Here we show that HIV-1 group O Nef targets a region adjacent to this deletion to inhibit transport of human tetherin to the cell surface, enhances virion release, and increases viral resistance to inhibition by interferon-&#945;. The Nef protein of the inferred common ancestor of group O viruses is also active against human tetherin. Thus, Nef-mediated antagonism of human tetherin evolved prior to the spread of HIV-1 group O and likely facilitated secondary virus transmission. Our results may explain the epidemic spread of HIV-1 group O., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Plasmodium vivax is the leading cause of human malaria in Asia and Latin America but is absent from most of central Africa due to the near fixation of a mutation that inhibits the expression of its receptor, the Duffy antigen, on human erythrocytes. The emergence of this protective allele is not understood because P. vivax is believed to have originated in Asia. Here we show, using a non-invasive approach, that wild chimpanzees and gorillas throughout central Africa are endemically infected with parasites that are closely related to human P. vivax. Sequence analyses reveal that ape parasites lack host specificity and are much more diverse than human parasites, which form a monophyletic lineage within the ape parasite radiation. These findings indicate that human P. vivax is of African origin and likely selected for the Duffy-negative mutation. All extant human P. vivax parasites are derived from a single ancestor that escaped out of Africa.

APOBEC3G (A3G) is a cytidine deaminase that restricts human immunodeficiency virus type 1 (HIV-1) and other lentiviruses. Most of these viruses encode a Vif protein that directly binds A3G and leads to its proteasomal degradation. Both Vif proteins of HIV-1 and African green monkey simian immunodeficiency virus (SIVagm) bind residue 128 of A3G. However, this position does not control the A3G degradation by Vif variants derived from HIV-2 and SIVmac, which both originated from SIV of sooty mangabey monkeys (SIVsmm), suggesting that the A3G binding site for Vif proteins of the SIVsmm/HIV-2 lineage differs from that of HIV-1. To map the SIVsmm Vif binding site of A3G, we performed immunoprecipitations of individual A3G domains, Vif/A3G degradation assays and a detailed mutational analysis of human A3G. We show that A3G residue 129, but not the adjacent position 128, confers susceptibility to degradation by SIVsmm Vif. An artificial A3G mutant, the P129D mutant, was resistant to degradation by diverse Vifs from HIV-1, HIV-2, SIVagm, and chimpanzee SIV (SIVcpz), suggesting a conserved lentiviral Vif binding site. Gorilla A3G naturally contains a glutamine (Q) at position 129, which makes its A3G resistant to Vifs from diverse lineages. We speculate that gorilla A3G serves as a barrier against SIVcpz strains. In summary, we show that Vif proteins from distinct lineages bind to the same A3G loop, which includes positions 128 and 129. The multiple adaptations within this loop among diverse primates underscore the importance of counteracting A3G in lentiviral evolution.

Background: The acquisition of effective Vpu-mediated anti-tetherin activity to promote virion release following transmission of SIVcpzPtt from central chimpanzees (Pan troglodytes troglodytes) to humans distinguishes pandemic HIV-1 group M strains from non-pandemic group N, O and P viruses and may have been a prerequisite for their global spread. Some functional motifs in the cytoplasmic region of HIV-1 M Vpus proposed to be important for anti-tetherin activity are more frequently found in the Vpu proteins of SIVcpzPtt than in those of SIVcpzPts infecting eastern chimpanzees (P. t. schweinfurthii), that have not been detected in humans, and SIVgor from gorillas, which is closely related to HIV-1 O and P. Thus, SIVcpzPtt strains may require fewer adaptive changes in Vpu than SIVcpzPts or SIVgor strains to counteract human tetherin., Results: To examine whether SIVcpzPtt may only need changes in the transmembrane domain (TMD) of Vpu to acquire anti-tetherin activity, whereas SIVcpzPts and SIVgor may also require changes in the cytoplasmic region, we analyzed chimeras between the TMD of an HIV-1 M Vpu and the cytoplasmic domains of SIVcpzPtt (n = 2), SIVcpzPts (n = 2) and SIVgor (n = 2) Vpu proteins. Unexpectedly, all of these chimeras were capable of counteracting human tetherin to enhance virion release, irrespective of the presence or absence of the putative adaptor protein binding sites and the DSGxxS β-TrCP binding motif reported to be critical for effective anti-tetherin activity of M Vpus. It was also surprising that in three of the six chimeras the gain of anti-tetherin function was associated with a loss of the CD4 degradation activity since this function was conserved among all parental HIV-1, SIVcpz and SIVgor Vpu proteins., Conclusions: Our results show that changes in the TMD of SIVcpzPtt, SIVcpzPts and SIVgor Vpus are sufficient to render them active against human tetherin. Thus, several previously described domains in the extracellular region of Vpu are not absolutely essential for tetherin antagonism but may be required for other Vpu functions.

HIV-1 groups M and N emerged within the last century following two independent cross-species transmissions of SIVcpz from chimpanzees to humans. In contrast to pandemic group M strains, HIV-1 group N viruses are exceedingly rare, with only about a dozen infections identified, all but one in individuals from Cameroon. Poor adaptation to the human host may be responsible for this limited spread of HIV-1 group N in the human population. Here, we analyzed the function of Vpu proteins from seven group N strains from Cameroon, the place where this zoonosis originally emerged. We found that these N-Vpus acquired four amino acid substitutions (E15A, V19A and IV25/26LL) in their transmembrane domain (TMD) that allow efficient interaction with human tetherin. However, despite these adaptive changes, most N-Vpus still antagonize human tetherin only poorly and fail to down-modulate CD4, the natural killer (NK) cell ligand NTB-A as well as the lipid-antigen presenting protein CD1d. These functional deficiencies were mapped to amino acid changes in the cytoplasmic domain that disrupt putative adaptor protein binding sites and an otherwise highly conserved ßTrCP-binding DSGxxS motif. As a consequence, N-Vpus exhibited aberrant intracellular localization and/or failed to recruit the ubiquitin-ligase complex to induce tetherin degradation. The only exception was the Vpu of a group N strain recently discovered in France, but originally acquired in Togo, which contained intact cytoplasmic motifs and counteracted tetherin as effectively as the Vpus of pandemic HIV-1 M strains. These results indicate that HIV-1 group N Vpu is under strong host-specific selection pressure and that the acquisition of effective tetherin antagonism may lead to the emergence of viral variants with increased transmission fitness.

Recent studies have shown that natural infection by HIV-2 leads to the elicitation of high titers of broadly neutralizing antibodies (NAbs) against primary HIV-2 strains (T. I. de Silva, et al., J. Virol. 86:930-946, 2012; R. Kong, et al., J. Virol. 86:947-960, 2012; G. Ozkaya Sahin, et al., J. Virol. 86:961-971, 2012). Here, we describe the envelope (Env) binding and neutralization properties of 15 anti-HIV-2 human monoclonal antibodies (MAbs), 14 of which were newly generated from 9 chronically infected subjects. All 15 MAbs bound specifically to HIV-2 gp120 monomers and neutralized heterologous primary virus strains HIV-2(7312A) and HIV-2(ST). Ten of 15 MAbs neutralized a third heterologous primary virus strain, HIV-2(UC1). The median 50% inhibitory concentrations (IC(50)s) for these MAbs were surprisingly low, ranging from 0.007 to 0.028 μg/ml. Competitive Env binding studies revealed three MAb competition groups: CG-I, CG-II, and CG-III. Using peptide scanning, site-directed mutagenesis, chimeric Env constructions, and single-cycle virus neutralization assays, we mapped the epitope of CG-I antibodies to a linear region in variable loop 3 (V3), the epitope of CG-II antibodies to a conformational region centered on the carboxy terminus of V4, and the epitope(s) of CG-III antibodies to conformational regions associated with CD4- and coreceptor-binding sites. HIV-2 Env is thus highly immunogenic in vivo and elicits antibodies having diverse epitope specificities, high potency, and wide breadth. In contrast to the HIV-1 Env trimer, which is generally well shielded from antibody binding and neutralization, HIV-2 is surprisingly vulnerable to broadly reactive NAbs. The availability of 15 human MAbs targeting diverse HIV-2 Env epitopes can facilitate comparative studies of HIV/SIV Env structure, function, antigenicity, and immunogenicity.

Chimpanzees in west central Africa (Pan troglodytes troglodytes) are endemically infected with simian immunodeficiency viruses (SIVcpzPtt) that have crossed the species barrier to humans and gorillas on at least five occasions, generating pandemic and nonpandemic forms of human immunodeficiency virus type 1 (HIV-1) as well as gorilla SIV (SIVgor). Chimpanzees in east Africa (Pan troglodytes schweinfurthii) are also infected with SIVcpz; however, their viruses (SIVcpzPts) have never been found in humans. To examine whether this is due to a paucity of natural infections, we used noninvasive methods to screen wild-living eastern chimpanzees in the Democratic Republic of the Congo (DRC), Uganda, and Rwanda. We also screened bonobos (Pan paniscus) in the DRC, a species not previously tested for SIV in the wild. Fecal samples (n = 3,108) were collected at 50 field sites, tested for species and subspecies origin, and screened for SIVcpz antibodies and nucleic acids. Of 2,565 samples from eastern chimpanzees, 323 were antibody positive and 92 contained viral RNA. The antibody-positive samples represented 76 individuals from 19 field sites, all sampled north of the Congo River in an area spanning 250,000 km(2). In this region, SIVcpzPts was common and widespread, with seven field sites exhibiting infection rates of 30% or greater. The overall prevalence of SIVcpzPts infection was 13.4% (95% confidence interval, 10.7% to 16.5%). In contrast, none of the 543 bonobo samples from six sites was antibody positive. All newly identified SIVcpzPts strains clustered in strict accordance to their subspecies origin; however, they exhibited considerable genetic diversity, especially in protein domains known to be under strong host selection pressure. Thus, the absence of SIVcpzPts zoonoses cannot be explained by an insufficient primate reservoir. Instead, greater adaptive hurdles may have prevented the successful colonization of humans by P. t. schweinfurthii viruses.

The interferon-induced host restriction factor tetherin poses a barrier for SIV transmission from primates to humans. After cross-species transmission, the chimpanzee precursor of pandemic HIV-1 switched from the accessory protein Nef to Vpu to effectively counteract human tetherin. As we report here, the experimental reintroduction of HIV-1 into its original chimpanzee host resulted in a virus that can use both Vpu and Nef to antagonize chimpanzee tetherin. Functional analyses demonstrated that alterations in and near the highly conserved ExxxLL motif in the C-terminal loop of Nef were critical for the reacquisition of antitetherin activity. Strikingly, just two amino acid changes allowed HIV-1 Nef to counteract chimpanzee tetherin and promote virus release. Our data demonstrate that primate lentiviruses can reacquire lost accessory gene functions during a single in vivo passage and suggest that other functional constraints keep Nef ready to regain antitetherin activity., (Copyright © 2012 Elsevier Inc. All rights reserved.)

SIVs infecting wild-living apes in west central Africa have crossed the species barrier to humans on at least four different occasions, one of which spawned the AIDS pandemic. Although the chimpanzee precursor of pandemic HIV-1 strains must have been able to infect humans, the capacity of SIVcpz strains to replicate in human lymphoid tissues (HLTs) is not known. Here, we show that SIVcpz strains from two chimpanzee subspecies are capable of replicating in human tonsillary explant cultures, albeit only at low titers. However, SIVcpz replication in HLT was significantly improved after introduction of a previously identified human-specific adaptation at position 30 in the viral Gag matrix protein. An Arg or Lys at this position significantly increased SIVcpz replication in HLT, while the same mutation reduced viral replication in chimpanzee-derived CD4(+) T cells. Thus, naturally occurring SIVcpz strains are capable of infecting HLTs, the major site of HIV-1 replication in vivo. However, efficient replication requires the acquisition of a host-specific adaptation in the viral matrix protein. These results identify Gag matrix as a major determinant of SIVcpz replication fitness in humans and suggest a critical role in the emergence of HIV/AIDS.

We recently reported that human immunodeficiency virus type 1 (HIV-1) carrying PTAP and LYPX(n)L L domains ceased budding when the nucleocapsid (NC) domain was mutated, suggesting a role for NC in HIV-1 release. Here we investigated whether NC involvement in virus release is a property specific to HIV-1 or a general requirement of retroviruses. Specifically, we examined a possible role for NC in the budding of retroviruses relying on divergent L domains and structurally homologous NC domains that harbor diverse protein sequences. We found that NC is critical for the release of viruses utilizing the PTAP motif whether it functions within its native Gag in simian immunodeficiency virus cpzGAB2 (SIVcpzGAB2) or SIVsmmE543 or when it is transplanted into the heterologous Gag protein of equine infectious anemia virus (EIAV). In both cases, virus release was severely diminished even though NC mutant Gag proteins retained the ability to assemble spherical particles. Moreover, budding-defective NC mutants, which displayed particles tethered to the plasma membrane, were triggered to release virus when access to the cell endocytic sorting complex required for transport pathway was restored (i.e., in trans expression of Nedd4.2s). We also examined the role of NC in the budding of EIAV, a retrovirus relying exclusively on the (L)YPX(n)L-type L domain. We found that EIAV late budding defects were rescued by overexpression of the isolated Alix Bro1 domain (Bro1). Bro1-mediated rescue of EIAV release required the wild-type NC. EIAV NC mutants lost interactions with Bro1 and failed to produce viruses despite retaining the ability to self-assemble. Together, our studies establish a role for NC in the budding of retroviruses harboring divergent L domains and evolutionarily diverse NC sequences, suggesting the utilization of a common conserved mechanism and/or cellular factor rather than a specific motif.

Compared with human immunodeficiency virus type 1 (HIV-1), little is known about the susceptibility of HIV-2 to antibody neutralization. We characterized the potency and breadth of neutralizing antibody (NAb) responses in 64 subjects chronically infected with HIV-2 against three primary HIV-2 strains: HIV-2(7312A), HIV-2(ST), and HIV-2(UC1). Surprisingly, we observed in a single-cycle JC53bl-13/TZM-bl virus entry assay median reciprocal 50% inhibitory concentration (IC(50)) NAb titers of 1.7 × 10(5), 2.8 × 10(4), and 3.3 × 10(4), respectively. A subset of 5 patient plasma samples tested against a larger panel of 17 HIV-2 strains where the extracellular gp160 domain was substituted into the HIV-2(7312A) proviral backbone showed potent neutralization of all but 4 viruses. The specificity of antibody neutralization was confirmed using IgG purified from patient plasma, HIV-2 Envs cloned by single-genome amplification, viruses grown in human CD4(+) T cells and tested for neutralization sensitivity on human CD4(+) T target cells, and, as negative controls, env-minus viruses pseudotyped with HIV-1, vesicular stomatitis virus, or murine leukemia virus Env glycoproteins. Human monoclonal antibodies (MAbs) specific for HIV-2 V3 (6.10F), V4 (1.7A), CD4 binding site (CD4bs; 6.10B), CD4 induced (CD4i; 1.4H), and membrane-proximal external region (MPER; 4E10) epitopes potently neutralized the majority of 32 HIV-2 strains bearing Envs from 13 subjects. Patient antibodies competed with V3, V4, and CD4bs MAbs for binding to monomeric HIV-2 gp120 at titers that correlated significantly with NAb titers. HIV-2 MPER antibodies did not contribute to neutralization breadth or potency. These findings indicate that HIV-2 Env is highly immunogenic in natural infection, that high-titer broadly neutralizing antibodies are commonly elicited, and that unlike HIV-1, native HIV-2 Env trimers expose multiple broadly cross-reactive epitopes readily accessible to NAbs.

Few studies have explored the role of neutralizing antibody (NAb) responses in controlling HIV-2 viremia and disease progression. Using a TZM-bl neutralization assay, we assessed heterologous and autologous NAb responses from a community cohort of HIV-2-infected individuals with a broad range of disease outcomes in rural Guinea-Bissau. All subjects (n = 40) displayed exceptionally high heterologous NAb titers (50% inhibitory plasma dilution or 50% inhibitory concentration [IC(50)], 1:7,000 to 1:1,000,000) against 5 novel primary HIV-2 envelopes and HIV-2 7312A, whereas ROD A and 3 primary envelopes were relatively resistant to neutralization. Most individuals also showed high autologous NAb against contemporaneous envelopes (78% of plasma-envelope combinations in 69 envelopes from 21 subjects), with IC(50)s above 1:10,000. No association between heterologous or autologous NAb titer and greater control of HIV-2 was found. A subset of envelopes was found to be more resistant to neutralization (by plasma and HIV-2 monoclonal antibodies). These envelopes were isolated from individuals with greater intrapatient sequence diversity and were associated with changes in potential N-linked glycosylation sites but not CD4 independence or CXCR4 use. Plasma collected from up to 15 years previously was able to potently neutralize recent autologous envelopes, suggesting a lack of escape from NAb and the persistence of neutralization-sensitive variants over time, despite significant NAb pressure. We conclude that despite the presence of broad and potent NAb responses in HIV-2-infected individuals, these are not the primary forces behind the dichotomous outcomes observed but reveal a limited capacity for adaptive selection and escape from host immunity in HIV-2 infection.

Nef is an accessory protein critical for the ability of human and simian immunodeficiency viruses (HIV and SIV) to replicate efficiently in their respective hosts. Previous analyses of members of 15 different primate lentivirus lineages revealed a link between Nef function and the presence of a vpu gene. In particular, Nef proteins of all vpu-containing viruses had lost their ability to downmodulate the T cell (TCR-CD3) receptor. Here we examined Nef proteins from eight additional SIV lineages, including SIVgor, SIVwrc, SIVolc, SIVgri, SIVdrl, SIVlho, SIVden, and SIVasc, from western lowland gorillas, western red colobus monkeys, olive colobus monkeys, grivet monkeys, drills, L'Hoest's monkeys, Dent's mona monkeys, and red-tailed monkeys, respectively. We found that except for the nef gene of SIVdrl, all of them were efficiently expressed and modulated CD4, major histocompatibility complex class I (MHC-I), CD28, CXCR4, and Ii cell surface expression and/or enhanced viral infectivity and replication. Furthermore, the Nef proteins of SIVgri, SIVlho, SIVwrc, SIVolc, and SIVgor antagonized tetherin. As expected, the Nef protein of SIVgor, which carries vpu, failed to downmodulate CD3, whereas those of SIVwrc, SIVgri, SIVlho, and SIVasc, which lack vpu, were capable of performing this function. Surprisingly, however, the Nef protein of the vpu-containing SIVden strain retained the ability to downmodulate TCR-CD3, whereas that of SIVolc, which does not contain vpu, was unable to perform this function. Although the SIVden Vpu is about 20 amino acids shorter than other Vpu proteins, it degrades CD4 and antagonizes tetherin. Our data show that there are exceptions to the link between the presence of a vpu gene and nef alleles deficient in CD3 modulation, indicating that host properties also affect the selective pressure for Nef-mediated disruption of TCR-CD3 signaling. Our results are also further evidence that tetherin antagonism is a common function of primate lentivirus Nef proteins and that the resistance of human tetherin to Nef represents a relevant barrier to cross-species transmission of SIVs to humans.

Human antibody 4E10 targets the highly conserved membrane-proximal external region (MPER) of the HIV-1 transmembrane glycoprotein, gp41, and has extraordinarily broad neutralizing activity. It is considered by many to be a prototype for vaccine development. In this study, we describe four subjects infected with viruses carrying rare MPER polymorphisms associated with resistance to 4E10 neutralization. In one case resistant virus carrying a W680G substitution was transmitted from mother to infant. We used site-directed mutagenesis to demonstrate that the W680G substitution is necessary for conferring the 4E10-resistant phenotype, but that it is not sufficient to transfer the phenotype to a 4E10-sensitive Env. Our third subject carried Envs with a W680R substitution causing variable resistance to 4E10, indicating that residues outside the MPER are required to confer the phenotype. A fourth subject possessed a F673L substitution previously associated with 4E10 resistance. For all three subjects with W680 polymorphisms, we observed additional residues in the MPER that co-varied with position 680 and preserved charged distributions across this region. Our data provide important caveats for vaccine development targeting the MPER. Naturally occurring Env variants described in our study also represent unique tools for probing the structure-function of HIV-1 envelope.

Vpu proteins of pandemic HIV-1 M strains degrade the viral receptor CD4 and antagonize human tetherin to promote viral release and replication. We show that Vpus from SIVgsn, SIVmus, and SIVmon infecting Cercopithecus primate species also degrade CD4 and antagonize tetherin. In contrast, SIVcpz, the immediate precursor of HIV-1, whose Vpu shares a common ancestry with SIVgsn/mus/mon Vpu, uses Nef rather than Vpu to counteract chimpanzee tetherin. Human tetherin, however, is resistant to Nef and thus poses a significant barrier to zoonotic transmission of SIVcpz to humans. Remarkably, Vpus from nonpandemic HIV-1 O strains are poor tetherin antagonists, whereas those from the rare group N viruses do not degrade CD4. Thus, only HIV-1 M evolved a fully functional Vpu following the three independent cross-species transmissions that resulted in HIV-1 groups M, N, and O. This may explain why group M viruses are almost entirely responsible for the global HIV/AIDS pandemic.

Human immunodeficiency virus type 1 (HIV-1) originated in chimpanzees; yet, several previous studies have shown that primary HIV-1 isolates replicate poorly in chimpanzee CD4+ T lymphocytes in vitro and in vivo. The reasons for this apparent restriction are not understood. Here, we describe a new activation protocol that led to a reproducible expansion and activation of chimpanzee CD4+ T lymphocytes in vitro. Using this protocol, we uncovered species-specific differences in the activation profiles of human and chimpanzee CD4+ T-cells, including HLA-DR and CD62L. Moreover, we found that improved activation facilitated the replication of both CXCR4 and CCR5-tropic HIV-1 in CD4+ T-cell cultures from over 30 different chimpanzees. Thus, the previously reported "replication block" of CCR5-tropic HIV-1 in chimpanzee lymphocytes appears to be due, at least in large part, to suboptimal T-cell activation.