Vaccine-Induced Protection from Homologous Tier 2 SHIV Challenge in Nonhuman Primates Depends on Serum-Neutralizing Antibody Titers.

Passive administration of HIV neutralizing antibodies (nAbs) can protect macaques from hard-to-neutralize (tier 2) chimeric simian-human immunodeficiency virus (SHIV) challenge. However, conditions for nAb-mediated protection after vaccination have not been established. Here, we selected groups of 6 rhesus macaques with either high or low serum nAb titers from a total of 78 animals immunized with recombinant native-like (SOSIP) Env trimers. Repeat intrarectal challenge with homologous tier 2 SHIVBG505 led to rapid infection in unimmunized and low-titer animals. High-titer animals, however, demonstrated protection that was gradually lost as nAb titers waned over time. An autologous serum ID50 nAb titer of ∼1:500 afforded more than 90% protection from medium-dose SHIV infection. In contrast, antibody-dependent cellular cytotoxicity and T cell activity did not correlate with protection. Therefore, Env protein-based vaccination strategies can protect against hard-to-neutralize SHIV challenge in rhesus macaques by inducing tier 2 nAbs, provided appropriate neutralizing titers can be reached and maintained.

Potent broadly neutralizing antibodies mediate efficient antibody-dependent phagocytosis of HIV-infected cells.

Antibody-dependent cellular phagocytosis (ADCP) has been implicated in protection against HIV-1. However, methods for measuring ADCP currently rely on the phagocytosis of gp120- or gp41-coated beads that do not reflect physiologically relevant conformations of the viral envelope glycoprotein or the size of a virus-infected cell. We therefore developed a novel approach for measuring ADCP of HIV-infected cells expressing natural conformations of Env. A monocytic cell line (THP-1 cells) or primary human monocytes were incubated with a CD4+ T cell line that expresses eGFP upon HIV-1 infection in the presence of antibodies and ADCP was measured as the accumulation of eGFP+ material by flow cytometry. The internalization of HIV-infected cells by monocytes was confirmed visually by image-capture flow cytometry. Cytoskeletal remodeling, pseudopod formation and phagocytosis were also observed by confocal microscopy. We found that potent broadly neutralizing antibodies (bnAbs), but not non-neutralizing antibodies (nnAbs), mediate efficient phagocytosis of cells infected with either primary or lab-adapted HIV-1. A nnAb to a CD4-inducible epitope of gp120 (A32) failed to enable ADCP of HIV-infected cells but mediated efficient phagocytosis of gp120-coated beads. Conversely, a bnAb specific to intact Env trimers (PGT145) mediated potent ADCP of HIV-infected cells but did not facilitate the uptake of gp120-coated beads. These results underscore the importance of measuring ADCP of HIV-infected cells expressing physiologically relevant conformations of Env and show that most antibodies that are capable of binding to Env trimers on virions to neutralize virus infectivity are also capable of binding to Env on the surface of virus-infected cells to mediate ADCP.

Potent antibody-dependent cellular cytotoxicity of a V2-specific antibody is not sufficient for protection of macaques against SIV challenge.

Fc-mediated antibody effector functions, such as antibody-dependent cellular cytotoxicity (ADCC), can contribute to the containment HIV-1 replication but whether such activities are sufficient for protection is unclear. We previously identified an antibody to the variable 2 (V2) apex of the HIV-1 Env trimer (PGT145) that potently directs the lysis of SIV-infected cells by NK cells but poorly neutralizes SIV infectivity. To determine if ADCC is sufficient for protection, separate groups of six rhesus macaques were treated with PGT145 or a control antibody (DEN3) by intravenous infusion followed five days later by intrarectal challenge with SIVmac239. Despite high concentrations of PGT145 and potent ADCC activity in plasma on the day of challenge, all animals became infected and viral loads did not differ between the PGT145- and DEN3-treated animals. To determine if PGT145 can protect against a neutralization-sensitive virus, two additional groups of six macaques were treated with PGT145 and DEN3 and challenged with an SIVmac239 variant with a single amino acid change in Env (K180S) that increases PGT145 binding and renders the virus susceptible to neutralization by this antibody. Although there was no difference in virus acquisition, peak and chronic phase viral loads were significantly lower and time to peak viremia was significantly delayed in the PGT145-treated animals compared to the DEN3-treated control animals. Env changes were also selected in the PGT145-treated animals that confer resistance to both neutralization and ADCC. These results show that ADCC is not sufficient for protection by this V2-specific antibody. However, protection may be achieved by increasing the affinity of antibody binding to Env above the threshold required for neutralization.

Rhesus Macaque Killer Cell Ig-like Receptor Domain 0 Glycans Impact Surface Expression and Ligand Specificity.

Defining the MHC class I ligands of rhesus macaque killer cell Ig-like receptors (KIRs) is fundamental to NK cell biology in this species as a model for infectious diseases and comparative immunogenetics. Several rhesus macaque KIRs belong to a phylogenetically distinct group with a three-amino acid deletion in domain 0 (D0). This deletion results in polymorphic differences in potential N-linked glycosylation (PNG) sites adjacent to a predicted KIR-MHC class I contact site. Whereas most KIRs have two tandem PNG sites in D0 (N36FTN39FT), the KIRs containing the deletion only have a single site in this region (N36FT). To discern the contribution of glycosylation to KIR expression and ligand recognition, we constructed PNG mutants for six lineage II KIR genes that eliminate or create sites for N-glycan addition at these locations. The impact of these mutations on total and surface expression was determined by immunoblotting and flow cytometry. Ligand engagement was assessed by coincubating reporter cell lines bearing chimeric KIR-CD3ζ receptors with target cells expressing individual MHC class I molecules and were corroborated by staining with KIR IgG-Fc fusion proteins. We found that N36FT is glycosylated in KIR with a single site, and at least one site is glycosylated in KIRs with two tandem sites. In general, for rhesus KIRs with a single D0 glycosylation site, that site contributes to surface expression. For KIRs with two tandem sites, the first site can contribute to ligand specificity. This study establishes that D0 glycosylation of rhesus macaque KIRs modulates surface expression and contributes to ligand specificity.

Antibody-dependent cellular cytotoxicity, infected cell binding and neutralization by antibodies to the SIV envelope glycoprotein.

Antibodies specific for diverse epitopes of the simian immunodeficiency virus envelope glycoprotein (SIV Env) have been isolated from rhesus macaques to provide physiologically relevant reagents for investigating antibody-mediated protection in this species as a nonhuman primate model for HIV/AIDS. With increasing interest in the contribution of Fc-mediated effector functions to protective immunity, we selected thirty antibodies representing different classes of SIV Env epitopes for a comparison of antibody-dependent cellular cytotoxicity (ADCC), binding to Env on the surface of infected cells and neutralization of viral infectivity. These activities were measured against cells infected with neutralization-sensitive (SIVmac316 and SIVsmE660-FL14) and neutralization-resistant (SIVmac239 and SIVsmE543-3) viruses representing genetically distinct isolates. Antibodies to the CD4-binding site and CD4-inducible epitopes were identified with especially potent ADCC against all four viruses. ADCC correlated well with antibody binding to virus-infected cells. ADCC also correlated with neutralization. However, several instances of ADCC without detectable neutralization or neutralization without detectable ADCC were observed. The incomplete correspondence between ADCC and neutralization shows that some antibody-Env interactions can uncouple these antiviral activities. Nevertheless, the overall correlation between neutralization and ADCC implies that most antibodies that are capable of binding to Env on the surface of virions to block infectivity are also capable of binding to Env on the surface of virus-infected cells to direct their elimination by ADCC.

MHC Class I Ligands of Rhesus Macaque Killer Cell Ig-like Receptors.

Definition of MHC class I ligands of rhesus macaque killer cell Ig-like receptors (KIRs) is fundamental to NK cell biology in this species as an animal model for infectious diseases, reproductive biology, and transplantation. To provide a more complete foundation for studying NK cell responses, rhesus macaque KIRs representing common allotypes of lineage II KIR genes were tested for interactions with MHC class I molecules representing diverse Macaca mulatta (Mamu)-A, -B, -E, -F, -I, and -AG alleles. KIR-MHC class I interactions were identified by coincubating reporter cell lines bearing chimeric KIR-CD3ζ receptors with target cells expressing individual MHC class I molecules and were corroborated by staining with KIR IgG-Fc fusion proteins. Ligands for 12 KIRs of previously unknown specificity were identified that fell into three general categories: interactions with multiple Mamu-Bw4 molecules, interactions with Mamu-A-related molecules, including allotypes of Mamu-AG and the hybrid Mamu-B*045:03 molecule, or interactions with Mamu-A1*012:01. Whereas most KIRs found to interact with Mamu-Bw4 are inhibitory, most of the KIRs that interact with Mamu-AG are activating. The KIRs that recognize Mamu-A1*012:01 belong to a phylogenetically distinct group of macaque KIRs with a 3-aa deletion in the D0 domain that is also present in human KIR3DL1/S1 and KIR3DL2. This study more than doubles the number of rhesus macaque KIRs with defined MHC class I ligands and identifies interactions with Mamu-AG, -B*045, and -A1*012. These findings support overlapping, but nonredundant, patterns of ligand recognition that reflect extensive functional diversification of these receptors.

Effect of Passive Administration of Monoclonal Antibodies Recognizing Simian Immunodeficiency Virus (SIV) V2 in CH59-Like Coil/Helical or ß-Sheet Conformations on Time of SIVmac251 Acquisition.

The monoclonal antibodies (MAbs) NCI05 and NCI09, isolated from a vaccinated macaque that was protected from multiple simian immunodeficiency virus (SIV) challenges, both target an overlapping, conformationally dynamic epitope in SIV envelope variable region 2 (V2). Here, we show that NCI05 recognizes a CH59-like coil/helical epitope, whereas NCI09 recognizes a ß-hairpin linear epitope. In vitro, NCI05 and, to a lesser extent, NCI09 mediate the killing of SIV-infected cells in a CD4-dependent manner. Compared to NCI05, NCI09 mediates higher titers of antibody-dependent cellular cytotoxicity (ADCC) to gp120-coated cells, as well as higher levels of trogocytosis, a monocyte function that contributes to immune evasion. We also found that passive administration of NCI05 or NCI09 to macaques did not affect the risk of SIVmac251 acquisition compared to controls, demonstrating that these anti-V2 antibodies alone are not protective. However, NCI05 but not NCI09 mucosal levels strongly correlated with delayed SIVmac251 acquisition, and functional and structural data suggest that NCI05 targets a transient state of the viral spike apex that is partially opened, compared to its prefusion-closed conformation. IMPORTANCE Studies suggest that the protection against SIV/simian-human immunodeficiency virus (SHIV) acquisition afforded by the SIV/HIV V1 deletion-containing envelope immunogens, delivered by the DNA/ALVAC vaccine platform, requires multiple innate and adaptive host responses. Anti-inflammatory macrophages and tolerogenic dendritic cells (DC-10), together with CD14+ efferocytes, are consistently found to correlate with a vaccine-induced decrease in the risk of SIV/SHIV acquisition. Similarly, V2-specific antibody responses mediating ADCC, Th1 and Th2 cells expressing no or low levels of CCR5, and envelope-specific NKp44+ cells producing interleukin 17 (IL-17) also are reproducible correlates of decreased risk of virus acquisition. We focused on the function and the antiviral potential of two monoclonal antibodies (NCI05 and NCI09) isolated from vaccinated animals that differ in antiviral function in vitro and recognize V2 in a linear (NCI09) or coil/helical (NCI05) conformation. We demonstrate that NCI05, but not NCI09, delays SIVmac251 acquisition, highlighting the complexity of antibody responses to V2.

Substitutions in Nef That Uncouple Tetherin and SERINC5 Antagonism Impair Simian Immunodeficiency Virus Replication in Primary Rhesus Macaque Lymphocytes.

Most simian immunodeficiency viruses (SIVs) use Nef to counteract restriction by the tetherin proteins of their nonhuman primate hosts. In addition to counteracting tetherin, SIV Nef has a number of other functions, including the downmodulation of CD3, CD4, and major histocompatibility complex class I (MHC I) molecules from the surface of SIV-infected cells and the enhancement of viral infectivity by preventing the incorporation of SERINC5 into virions. Although these activities require different surfaces of Nef, they can be difficult to separate because of their dependence on similar interactions with AP-1 or AP-2 for clathrin-mediated endocytosis. We previously observed extensive overlap of the SIV Nef residues required for counteracting tetherin and SERINC5. Here, we define substitutions in Nef that separate anti-tetherin activity from SERINC5 antagonism and other activities of Nef. This information was used to engineer an infectious molecular clone of SIV (SIVmac239nefSA) that is sensitive to tetherin but retains CD3, CD4, MHC I, and SERINC5 downmodulation. In primary rhesus macaque CD4+ T cells, SIVmac239nefSA exhibits impaired replication compared to wild-type SIVmac239 under conditions of interferon-induced upregulation of tetherin. These results demonstrate that tetherin antagonism can be separated from other Nef functions and that resistance to tetherin is essential for optimal replication in primary CD4+ T cells. IMPORTANCE Tetherin is an interferon-inducible transmembrane protein that prevents the detachment of enveloped viruses from infected cells by physically tethering nascent virions to cellular membranes. SIV Nef downmodulates simian tetherin to overcome this restriction in nonhuman primate hosts. Nef also enhances virus infectivity by preventing the incorporation of SERINC5 into virions and contributes to immune evasion by downmodulating other proteins from the cell surface. To assess the contribution of tetherin antagonism to virus replication, we engineered an infectious molecular clone of SIV with substitutions in Nef that uncouple tetherin antagonism from other Nef functions. These substitutions impaired virus replication in interferon-treated macaque CD4+ T cells, revealing the impact of tetherin on SIV replication under physiological conditions in primary CD4+ lymphocytes.

Selective Disruption of SERINC5 Antagonism by Nef Impairs SIV Replication in Primary CD4+ T Cells.

The Nef proteins of HIV-1 and SIV enhance viral infectivity by preventing the incorporation of the multipass transmembrane protein serine incorporator 5 (SERINC5), and to a lesser extent SERINC3, into virions. In addition to counteracting SERINCs, SIV Nef also downmodulates several transmembrane proteins from the surface of virus-infected cells, including simian tetherin, CD4 and MHC class I (MHC I) molecules. From a systematic analysis of alanine substitutions throughout the SIVmac239 Nef protein, we identified residues that are required to counteract SERINC5. This information was used to engineer an infectious molecular clone of SIV (SIVmac239nef AV), which differs by two amino acids in the N-terminal domain of Nef that make the virus sensitive to SERINC5 while retaining other activities of Nef. SIVmac239nef AV downmodulates CD3, CD4, MHC I and simian tetherin, but cannot counteract SERINC5. In primary rhesus macaque CD4+ T cells, SIVmac239nef AV exhibits impaired infectivity and replication compared to wild-type SIVmac239. These results demonstrate that SERINC5 antagonism can be separated from other Nef functions and reveal the impact of SERINC5 on lentiviral replication.Importance: SERINC5, a multipass transmembrane protein, is incorporated into retroviral particles during assembly. This leads to a reduction of particle infectivity by inhibiting virus fusion with the target cell membrane. The Nef proteins of HIV-1 and SIV enhance viral infectivity by preventing the incorporation of SERINC5 into virions. However, the relevance of this restriction factor in viral replication has not been elucidated. Here we report a systematic mapping of Nef residues required for SERINC5 antagonism. Counter screens for three other functions of Nef helped identify two residues in the N-terminal domain of Nef, which when mutated make Nef selectively susceptible to SERINC5. Since Nef is multi-functional, genetic separation of SERINC5 antagonism from its other functions affords comparison of the replication of isogenic viruses that are or are not sensitive to SERINC5. Such a strategy revealed the impact of SERINC5 on SIV replication in primary rhesus macaque CD4+ T-cells.

Enhanced Ability of Plant-Derived PGT121 Glycovariants To Eliminate HIV-1-Infected Cells.

The activity of broadly neutralizing antibodies (bNAbs) targeting HIV-1 depends on pleiotropic functions, including viral neutralization and the elimination of HIV-1-infected cells. Several in vivo studies have suggested that passive administration of bNAbs represents a valuable strategy for the prevention or treatment of HIV-1. In addition, different strategies are currently being tested to scale up the production of bNAbs to obtain the large quantities of antibodies required for clinical trials. Production of antibodies in plants permits low-cost and large-scale production of valuable therapeutics; furthermore, pertinent to this work, it also includes an advanced glycoengineering platform. In this study, we used Nicotiana benthamiana to produce different Fc-glycovariants of a potent bNAb, PGT121, with near-homogeneous profiles and evaluated their antiviral activities. Structural analyses identified a close similarity in overall structure and glycosylation patterns of Fc regions for these plant-derived Abs and mammalian cell-derived Abs. When tested for Fc-effector activities, afucosylated PGT121 showed significantly enhanced FcγRIIIa interaction and antibody dependent cellular cytotoxicity (ADCC) against primary HIV-1-infected cells, both in vitro and ex vivo. However, the overall galactosylation profiles of plant PGT121 did not affect ADCC activities against infected primary CD4+ T cells. Our results suggest that the abrogation of the Fc N-linked glycan fucosylation of PGT121 is a worthwhile strategy to boost its Fc-effector functionality. IMPORTANCE PGT121 is a highly potent bNAb and its antiviral activities for HIV-1 prevention and therapy are currently being evaluated in clinical trials. The importance of its Fc-effector functions in clearing HIV-1-infected cells is also under investigation. Our results highlight enhanced Fc-effector activities of afucosylated PGT121 MAbs that could be important in a therapeutic context to accelerate infected cell clearance and slow disease progression. Future studies to evaluate the potential of plant-produced afucosylated PGT121 in controlling HIV-1 replication in vivo are warranted.

Loss of tetherin antagonism by Nef impairs SIV replication during acute infection of rhesus macaques.

Most simian immunodeficiency viruses use Nef to counteract the tetherin proteins of their nonhuman primate hosts. Nef also downmodulates cell-surface CD4 and MHC class I (MHC I) molecules and enhances viral infectivity by counteracting SERINC5. We previously demonstrated that tetherin antagonism by SIV Nef is genetically separable from CD4- and MHC I-downmodulation. Here we show that disruption of tetherin antagonism by Nef impairs virus replication during acute SIV infection of rhesus macaques. A combination of mutations was introduced into the SIVmac239 genome resulting in three amino acid substitutions in Nef that impair tetherin antagonism, but not CD3-, CD4- or MHC I-downmodulation. Further characterization of this mutant (SIVmac239AAA) revealed that these changes also result in partial sensitivity to SERINC5. Separate groups of four rhesus macaques were infected with either wild-type SIVmac239 or SIVmac239AAA, and viral RNA loads in plasma and sequence changes in the viral genome were monitored. Viral loads were significantly lower during acute infection in animals infected with SIVmac239AAA than in animals infected with wild-type SIVmac239. Sequence analysis of the virus population in plasma confirmed that the substitutions in Nef were retained during acute infection; however, changes were observed by week 24 post-infection that fully restored anti-tetherin activity and partially restored anti-SERINC5 activity. These observations reveal overlap in the residues of SIV Nef required for counteracting tetherin and SERINC5 and selective pressure to overcome these restriction factors in vivo.

Functional Interactions of Common Allotypes of Rhesus Macaque FcγR2A and FcγR3A with Human and Macaque IgG Subclasses.

The rhesus macaque is an important animal model for AIDS and other infectious diseases. However, the investigation of Fc-mediated Ab responses in macaques is complicated by species-specific differences in FcγRs and IgG subclasses relative to humans. To assess the effects of these differences on FcγR-IgG interactions, reporter cell lines expressing common allotypes of human and rhesus macaque FcγR2A and FcγR3A were established. FcγR-mediated responses to B cells were measured in the presence of serial dilutions of anti-CD20 Abs with Fc domains corresponding to each of the four subclasses of human and rhesus IgG and with Fc variants of IgG1 that enhance binding to FcγR2A or FcγR3A. All of the FcγRs were functional and preferentially recognized either IgG1 or IgG2. Whereas allotypes of rhesus FcγR2A were identified with responses similar to variants of human FcγR2A with higher (H131) and lower (R131) affinity for IgG, all of the rhesus FcγR3A allotypes exhibited responses most similar to the higher affinity V158 variant of human FcγR3A. Unlike responses to human IgGs, there was little variation in FcγR-mediated responses to different subclasses of rhesus IgG. Phylogenetic comparisons suggest that this reflects limited sequence variation of macaque IgGs as a result of their relatively recent diversification from a common IGHG gene since humans and macaques last shared a common ancestor. These findings reveal species-specific differences in FcγR-IgG interactions with important implications for investigating Ab effector functions in macaques.

Vaccine protection against rectal acquisition of SIVmac239 in rhesus macaques.

A prophylactic vaccine against human immunodeficiency virus (HIV) remains a top priority in biomedical research. Given the failure of conventional immunization protocols to confer robust protection against HIV, new and unconventional approaches may be needed to generate protective anti-HIV immunity. Here we vaccinated rhesus macaques (RMs) with a recombinant (r)DNA prime (without any exogenous adjuvant), followed by a booster with rhesus monkey rhadinovirus (RRV)-a herpesvirus that establishes persistent infection in RMs (Group 1). Both the rDNA and rRRV vectors encoded a near-full-length simian immunodeficiency virus (SIVnfl) genome that assembles noninfectious SIV particles and expresses all nine SIV gene products. This rDNA/rRRV-SIVnfl vaccine regimen induced persistent anti-Env antibodies and CD8+ T-cell responses against the entire SIV proteome. Vaccine efficacy was assessed by repeated, marginal-dose, intrarectal challenges with SIVmac239. Encouragingly, vaccinees in Group 1 acquired SIVmac239 infection at a significantly delayed rate compared to unvaccinated controls (Group 3). In an attempt to improve upon this outcome, a separate group of rDNA/rRRV-SIVnfl-vaccinated RMs (Group 2) was treated with a cytotoxic T-lymphocyte antigen-4 (CTLA-4)-blocking monoclonal antibody during the vaccine phase and then challenged in parallel with Groups 1 and 3. Surprisingly, Group 2 was not significantly protected against SIVmac239 infection. In sum, SIVnfl vaccination can protect RMs against rigorous mucosal challenges with SIVmac239, a feat that until now had only been accomplished by live-attenuated strains of SIV. Further work is needed to identify the minimal requirements for this protection and whether SIVnfl vaccine efficacy can be improved by means other than anti-CTLA-4 adjuvant therapy.

Immunophenotyping of Rhesus CMV-Specific CD8 T-Cell Populations.

A vaccine to ameliorate cytomegalovirus (CMV)-related pathogenicity in transplantation patients is considered a top priority. A therapeutic vaccine must include components that elicit both neutralizing antibodies, and highly effective CD8 T-cell responses. The most important translational model of vaccine development is the captive-bred rhesus macaque (Macaca mulatta) of Indian origin. There is a dearth of information on rhesus cytomegalovirus (rhCMV)-specific CD8 T cells due to the absence of well-defined CD8 T-cell epitopes presented by classical MHC-I molecules. In the current study, we defined two CD8 T-cell epitopes restricted by high-frequency Mamu alleles: the Mamu-A1*002:01 restricted VY9 (VTTLGMALY aa291-299) epitope of protein IE-1, and the Mamu-A1*008:01 restricted NP8 (NPTDRPIP aa96-103) epitope of protein phosphoprotein 65-2. We developed tetramers and determined the level, phenotype, and functional capability of the two epitope-specific T-cell populations in circulation and various tissues. We demonstrated the value of these tetramers for in situ tetramer staining. Here, we first provided critical reagents and established a flow cytometric staining strategy to study rhCMV-specific T-cell responses in up to 40% of captive-bred rhesus macaques. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals LLC on behalf of International Society for Advancement of Cytometry.

AAV-expressed eCD4-Ig provides durable protection from multiple SHIV challenges.

Long-term in vivo expression of a broad and potent entry inhibitor could circumvent the need for a conventional vaccine for HIV-1. Adeno-associated virus (AAV) vectors can stably express HIV-1 broadly neutralizing antibodies (bNAbs). However, even the best bNAbs neutralize 10-50% of HIV-1 isolates inefficiently (80% inhibitory concentration (IC80) > 5 µg ml(-1)), suggesting that high concentrations of these antibodies would be necessary to achieve general protection. Here we show that eCD4-Ig, a fusion of CD4-Ig with a small CCR5-mimetic sulfopeptide, binds avidly and cooperatively to the HIV-1 envelope glycoprotein (Env) and is more potent than the best bNAbs (geometric mean half-maximum inhibitory concentration (IC50) < 0.05 µg ml(-1)). Because eCD4-Ig binds only conserved regions of Env, it is also much broader than any bNAb. For example, eCD4-Ig efficiently neutralized 100% of a diverse panel of neutralization-resistant HIV-1, HIV-2 and simian immunodeficiency virus isolates, including a comprehensive set of isolates resistant to the CD4-binding site bNAbs VRC01, NIH45-46 and 3BNC117. Rhesus macaques inoculated with an AAV vector stably expressed 17-77 µg ml(-1) of fully functional rhesus eCD4-Ig for more than 40 weeks, and these macaques were protected from several infectious challenges with SHIV-AD8. Rhesus eCD4-Ig was also markedly less immunogenic than rhesus forms of four well-characterized bNAbs. Our data suggest that AAV-delivered eCD4-Ig can function like an effective HIV-1 vaccine.

Antibody-Induced Internalization of HIV-1 Env Proteins Limits Surface Expression of the Closed Conformation of Env.

To minimize immune responses against infected cells, HIV-1 limits the surface expression of its envelope glycoprotein (Env). Here, we demonstrate that this mechanism is specific for the Env conformation and affects the efficiency of antibody-dependent cellular cytotoxicity (ADCC). Using flow cytometry and confocal microscopy, we show that broadly neutralizing antibodies (bNAbs) targeting the "closed" conformation of Env induce its internalization from the surface. In contrast, non-neutralizing antibodies (nNAbs) are displayed on the cell surface for prolonged period of times. The bNAb-induced Env internalization can be decreased by blocking dynamin function, which translates into higher susceptibilities of infected cells to ADCC. Our results suggest that antibody-mediated Env internalization is a mechanism used by HIV-1 to evade immune responses against the "closed" conformation of Env expressed on HIV-1-infected cells.IMPORTANCE HIV-1 has evolved to acquire several strategies to limit the exposure of its envelope glycoproteins (Env) on the surface of infected cells. In this study, we show that antibody-induced Env internalization is conformation specific and reduces the susceptibility of infected cells to antibody-dependent cellular cytotoxicity (ADCC). Thus, a better understanding of this mechanism might help develop antibodies with improved capacities to mediate ADCC.

The Tat inhibitor didehydro-cortistatin A suppresses SIV replication and reactivation.

The HIV-1 transactivation protein (Tat) binds the HIV mRNA transactivation responsive element (TAR), regulating transcription and reactivation from latency. Drugs against Tat are unfortunately not clinically available. We reported that didehydro-cortistatin A (dCA) inhibits HIV-1 Tat activity. In human CD4+ T cells isolated from aviremic individuals and in the humanized mouse model of latency, combining dCA with antiretroviral therapy accelerates HIV-1 suppression and delays viral rebound upon treatment interruption. This drug class is amenable to block-and-lock functional cure approaches, aimed at a durable state of latency. Simian immunodeficiency virus (SIV) infection of rhesus macaques (RhMs) is the best-characterized model for AIDS research. Here, we demonstrate, using in vitro and cell-based assays, that dCA directly binds to SIV Tat's basic domain. dCA specifically inhibits SIV Tat binding to TAR, but not a Tat-Rev fusion protein, which activates transcription when Rev binds to its cognate RNA binding site replacing the apical region of TAR. Tat-TAR inhibition results in loss of RNA polymerase II recruitment to the SIV promoter. Importantly, dCA potently inhibits SIV reactivation from latently infected Hut78 cells and from primary CD4+ T cells explanted from SIVmac239-infected RhMs. In sum, dCA's remarkable breadth of activity encourages SIV-infected RhM use for dCA preclinical evaluation.-Mediouni, S., Kessing, C. F., Jablonski, J. A., Thenin-Houssier, S., Clementz, M., Kovach, M. D., Mousseau, G., de Vera, I.M.S., Li, C., Kojetin, D. J., Evans, D. T., Valente, S. T. The Tat inhibitor didehydro-cortistatin A suppresses SIV replication and reactivation.

Diversification of Bw4 Specificity and Recognition of a Nonclassical MHC Class I Molecule Implicated in Maternal-Fetal Tolerance by Killer Cell Ig-like Receptors of the Rhesus Macaque.

The rhesus macaque is an important animal model for AIDS and other infectious diseases; however, studies to address NK cell function in this species have been limited by the lack of defined ligands for killer cell Ig-like receptors (KIRs). To identify ligands for rhesus macaque KIRs, we adopted a novel approach based on a pair of stable cell lines. NFAT-responsive luciferase reporter cell lines expressing the extracellular domains of macaque KIRs fused to the transmembrane and cytoplasmic domains of CD28 and CD3ζ were incubated with target cells expressing individual MHC class I molecules, and ligand recognition was detected by the MHC class I-dependent upregulation of luciferase. Using this approach, we found that Mamu-KIR3DL01, -KIR3DL06, -KIR3DL08, and -KIR3DSw08 all recognize Mamu-Bw4 molecules but with differing allotype specificity. In contrast, Mamu-KIR3DL05 recognizes Mamu-A and Mamu-A-related molecules, including Mamu-A1*002 and -A3*13, Mamu-B*036, the product of a recombinant Mamu-B allele with α1 and α2 domain sequences derived from a MHC-A gene, and Mamu-AG*01, a nonclassical molecule expressed on placental trophoblasts that originated from an ancestral duplication of a MHC-A gene. These results reveal an expansion of the lineage II KIRs in macaques that recognize Bw4 ligands and identify a nonclassical molecule implicated in placental development and pregnancy as a ligand for Mamu-KIR3DL05. In addition to offering new insights into KIR-MHC class I coevolution, these findings provide an important foundation for investigating the role of NK cells in the rhesus macaque as an animal model for infectious diseases and reproductive biology.

Polymorphisms in Rhesus Macaque Tetherin Are Associated with Differences in Acute Viremia in Simian Immunodeficiency Virus Δnef-Infected Animals.

Tetherin (BST-2 or CD317) is an interferon-inducible transmembrane protein that inhibits virus release from infected cells. To determine the extent of sequence variation and the impact of polymorphisms in rhesus macaque tetherin on simian immunodeficiency virus (SIV) infection, tetherin alleles were sequenced from 146 rhesus macaques, including 68 animals infected with wild-type SIVmac239 and 47 animals infected with SIVmac239Δnef Since Nef is the viral gene product of SIV that counteracts restriction by tetherin, these groups afford a comparison of the effects of tetherin polymorphisms on SIV strains that are, and are not, resistant to tetherin. We identified 15 alleles of rhesus macaque tetherin with dimorphic residues at 9 positions. The relationship between these alleles and plasma viral loads was compared during acute infection, prior to the onset of adaptive immunity. Acute viremia did not differ significantly among the wild-type SIV-infected animals; however, differences in acute viral loads were associated with polymorphisms in tetherin among the animals infected with SIVΔnef In particular, polymorphisms at positions 43 and 111 (P43 and H111) were associated with lower acute-phase viral loads for SIVΔnef infection. These observations reveal extensive polymorphism in rhesus macaque tetherin, maintained perhaps as a consequence of variability in the selective pressure of diverse viral pathogens, and identify tetherin alleles that may have an inherently greater capacity to restrict SIV replication in the absence of Nef.IMPORTANCE As a consequence of ongoing evolutionary conflict with viral pathogens, tetherin has accumulated numerous species-specific differences that represent important barriers to the transmission of viruses between species. This study reveals extensive polymorphism in rhesus macaque tetherin and identifies specific alleles that are associated with lower viral loads during the first few weeks after infection with nef-deleted SIV. These observations suggest that the variable selective pressure of viral pathogens, in addition to driving the diversification of tetherin among species, also operates within certain species to maintain sequence variation in tetherin.