Modulating HIV-1 envelope glycoprotein conformation to decrease the HIV-1 reservoir

Untangling the role of the microbiome across the stages of HIV disease

PURPOSE OF REVIEW

The primate microbiome consists of bacteria, eukaryotes, and viruses that dynamically shape and respond to host health and disease. Understanding how the symbiotic relationship between the host and microbiome responds to HIV has implications for therapeutic design.

RECENT FINDINGS

Advances in microbiome identification technologies have expanded our ability to identify constituents of the microbiome and to infer their functional capacity. The dual use of these technologies and animal models has allowed interrogation into the role of the microbiome in lentiviral acquisition, vaccine efficacy, and the response to antiretrovirals. Lessons learned from such studies are now being harnessed to design microbiome-based interventions.

SUMMARY

Previous studies considering the role of the microbiome in people living with HIV largely described viral acquisition as an intrusion on the host:microbiome interface. Re-framing this view to consider HIV as a novel, albeit unwelcome, component of the microbiome may better inform the research and development of pre and postexposure prophylaxes.

The combination of three CD4-induced antibodies targeting highly conserved Env regions with a small CD4-mimetic achieves potent ADCC activity

The majority of naturally-elicited antibodies against the HIV-1 envelope glycoproteins (Env) are non-neutralizing (nnAbs), because they are unable to recognize the Env timer in its native "closed" conformation. Nevertheless, it has been shown that nnAbs have the potential to eliminate HIV-1-infected cells by Antibody-Dependent Cellular Cytotoxicity (ADCC) provided that Env is present on the cell surface in its "open" conformation. This is because most nnAbs recognize epitopes that become accessible only after Env interaction with CD4 and the exposure of epitopes that are normally occluded in the closed trimer. HIV-1 limits this vulnerability by downregulating CD4 from the surface of infected cells, thus preventing a premature encounter of Env with CD4. Small CD4-mimetics (CD4mc) sensitize HIV-1-infected cells to ADCC by opening the Env glycoprotein and exposing CD4-induced (CD4i) epitopes. There are two families of CD4i nnAbs, termed anti-cluster A and anti-CoRBS Abs, which are known to mediate ADCC in the presence of CD4mc. Here, we performed Fab competition experiments and found that anti-gp41 cluster I antibodies comprise a major fraction of the plasma ADCC activity in people living with HIV (PLWH). Moreover, addition of gp41 cluster I antibodies to cluster A and CoRBS antibodies greatly enhanced ADCC mediated cell killing in the presence of a potent indoline CD4mc, CJF-III-288. This cocktail outperformed broadly-neutralizing antibodies and even showed activity against HIV-1 infected monocyte-derived macrophages. Thus, combining CD4i antibodies with different specificities achieves maximal ADCC activity, which may be of utility in HIV cure strategies.

Three families of CD4-induced antibodies are associated with the capacity of plasma from people living with HIV to mediate ADCC in presence of CD4-mimetics

CD4-mimetics (CD4mcs) are small molecule compounds that mimic the interaction of the CD4 receptor with HIV-1 envelope glycoproteins (Env). Env from primary viruses normally samples a "closed" conformation which occludes epitopes recognized by CD4-induced (CD4i) non-neutralizing antibodies (nnAbs). CD4mcs induce conformational changes on Env resulting in the exposure of these otherwise inaccessible epitopes. Here we evaluated the capacity of plasma from a cohort of 50 people living with HIV to recognize HIV-1-infected cells and eliminate them by antibody-dependent cellular cytotoxicity (ADCC) in the presence of a potent indoline CD4mc. We observed a marked heterogeneity among plasma samples. By measuring the levels of different families of CD4i Abs, we found that the levels of anti-cluster A, anti-coreceptor binding site and anti-gp41 cluster I antibodies are responsible for plasma-mediated ADCC in presence of CD4mc.

Humanized mice for studying HIV latency and potentially its eradication

PURPOSE OF THE REVIEW

The quest for an HIV cure faces a formidable challenge: the persistent presence of latent viral infections within the cells and tissues of infected individuals. This review provides a thorough examination of discussions surrounding HIV latency, the use of humanized mouse models, and strategies aimed at eliminating the latent HIV reservoir. It explores the hurdles and advancements in understanding HIV pathogenesis, mainly focusing on establishing latent reservoirs in CD4 + T cells and macrophages. Introducing the concepts of functional and sterile cures, the review underscores the indispensable role of humanized mouse models in HIV research, offering crucial insights into the efficacy of cART and the ongoing pursuit of an HIV cure.

RECENT FINDINGS

Here, we highlight studies investigating molecular mechanisms and pathogenesis related to HIV latency in humanized mice and discuss novel strategies for eradicating latent HIV. Emphasizing the importance of analytical cART interruption in humanized mouse studies to gauge its impact on the latent reservoir accurately, the review underlines the ongoing progress and challenges in harnessing humanized mouse models for HIV research.

SUMMARY

This review suggests that humanized mice models provide valuable insights into HIV latency and potential eradication strategies, contributing significantly to the quest for an HIV cure.

Idiotopes of antibodies against HIV-1 CD4-induced epitope shared with those against microorganisms

Induction of antibodies (Abs) against the conformational CD4-induced (CD4i) epitope is frequent in HIV-1 infection. However, the mechanism of development of anti-CD4i Abs is unclear. We used anti-idiotypic (aID) monoclonal Abs (mAbs) of anti-CD4i mAbs to isolate anti-CD4i mAbs from infected subjects and track the causative antigens. One anti-aID mAb sorted from infected subjects by aID mAbs had the characteristics of anti-CD4i Abs, including IGHV1-69 usage and ability to bind to HIV-1 Env enhanced by sCD4. Critical amino acid sequences for the binding of six anti-aID mAbs, with shared idiotope to anti-CD4i mAbs, were analysed by phage display. The identified amino acid sequences showed similarity to proteins from human microbiota and infectious agents. Peptides synthesized from Caudoviricetes sp and Vibrio vulnificus based on the identified sequences were reactive to most anti-aID and some anti-CD4i mAbs. These results suggest that anti-CD4i Abs may evolve from B cells primed by microorganisms.

V3 tip determinants of susceptibility to inhibition by CD4-mimetic compounds in natural clade A human immunodeficiency virus (HIV-1) envelope glycoproteins

IMPORTANCE

CD4-mimetic compounds (CD4mcs) are small-molecule inhibitors of human immunodeficiency virus (HIV-1) entry into host cells. CD4mcs target a pocket on the viral envelope glycoprotein (Env) spike that is used for binding to the receptor, CD4, and is highly conserved among HIV-1 strains. Nonetheless, naturally occurring HIV-1 strains exhibit a wide range of sensitivities to CD4mcs. Our study identifies changes distant from the binding pocket that can influence the susceptibility of natural HIV-1 strains to the antiviral effects of multiple CD4mcs. We relate the antiviral potency of the CD4mc against this panel of HIV-1 variants to the ability of the CD4mc to activate entry-related changes in Env conformation prematurely. These findings will guide efforts to improve the potency and breadth of CD4mcs against natural HIV-1 variants.

Soluble CD4 and low molecular weight CD4-mimetic compounds sensitize cells to be killed by anti-HIV cytotoxic immunoconjugates

IMPORTANCE

HIV infection can be effectively treated to prevent the development of AIDS, but it cannot be cured. We have attached poisons to anti-HIV antibodies to kill the infected cells that persist even after years of effective antiviral therapy. Here we show that the killing of infected cells can be markedly enhanced by the addition of soluble forms of the HIV receptor CD4 or by mimics of CD4.

Impact of HIV-1 Vpu-mediated downregulation of CD48 on NK-cell-mediated antibody-dependent cellular cytotoxicity

HIV-1 evades antibody-dependent cellular cytotoxicity (ADCC) responses not only by controlling Env conformation and quantity at the cell surface but also by altering NK cell activation via the downmodulation of several ligands of activating and co-activating NK cell receptors. The signaling lymphocyte activation molecule (SLAM) family of receptors, which includes NTB-A and 2B4, act as co-activating receptors to sustain NK cell activation and cytotoxic responses. These receptors cooperate with CD16 (FcγRIII) and other activating receptors to trigger NK cell effector functions. In that context, Vpu-mediated downregulation of NTB-A on HIV-1-infected CD4 T cells was shown to prevent NK cell degranulation via an homophilic interaction, thus contributing to ADCC evasion. However, less is known on the capacity of HIV-1 to evade 2B4-mediated NK cell activation and ADCC. Here, we show that HIV-1 downregulates the ligand of 2B4, CD48, from the surface of infected cells in a Vpu-dependent manner. This activity is conserved among Vpu proteins from the HIV-1/SIVcpz lineage and depends on conserved residues located in its transmembrane domain and dual phosphoserine motif. We show that NTB-A and 2B4 stimulate CD16-mediated NK cell degranulation and contribute to ADCC responses directed to HIV-1-infected cells to the same extent. Our results suggest that HIV-1 has evolved to downmodulate the ligands of both SLAM receptors to evade ADCC. IMPORTANCE Antibody-dependent cellular cytotoxicity (ADCC) can contribute to the elimination of HIV-1-infected cells and HIV-1 reservoirs. An in-depth understanding of the mechanisms used by HIV-1 to evade ADCC might help develop novel approaches to reduce the viral reservoirs. Members of the signaling lymphocyte activation molecule (SLAM) family of receptors, such as NTB-A and 2B4, play a key role in stimulating NK cell effector functions, including ADCC. Here, we show that Vpu downmodulates CD48, the ligand of 2B4, and this contributes to protect HIV-1-infected cells from ADCC. Our results highlight the importance of the virus to prevent the triggering of the SLAM receptors to evade ADCC.

Current Advances in Humanized Mouse Models for Studying NK Cells and HIV Infection

Human immunodeficiency virus (HIV) has infected millions of people worldwide and continues to be a major global health problem. Scientists required a small animal model to study HIV pathogenesis and immune responses. To this end, humanized mice were created by transplanting human cells and/or tissues into immunodeficient mice to reconstitute a human immune system. Thus, humanized mice have become a critical animal model for HIV researchers, but with some limitations. Current conventional humanized mice are prone to death by graft versus host disease induced by the mouse signal regulatory protein α and CD47 signaling pathway. In addition, commonly used humanized mice generate low levels of human cytokines required for robust myeloid and natural killer cell development and function. Here, we describe recent advances in humanization procedures and transgenic and knock-in immunodeficient mice to address these limitations.

Piperidine CD4-Mimetic Compounds Expose Vulnerable Env Epitopes Sensitizing HIV-1-Infected Cells to ADCC

The ability of the HIV-1 accessory proteins Nef and Vpu to decrease CD4 levels contributes to the protection of infected cells from antibody-dependent cellular cytotoxicity (ADCC) by preventing the exposure of Env vulnerable epitopes. Small-molecule CD4 mimetics (CD4mc) based on the indane and piperidine scaffolds such as (+)-BNM-III-170 and (S)-MCG-IV-210 sensitize HIV-1-infected cells to ADCC by exposing CD4-induced (CD4i) epitopes recognized by non-neutralizing antibodies that are abundantly present in plasma from people living with HIV. Here, we characterize a new family of CD4mc, (S)-MCG-IV-210 derivatives, based on the piperidine scaffold which engages the gp120 within the Phe43 cavity by targeting the highly conserved Asp368 Env residue. We utilized structure-based approaches and developed a series of piperidine analogs with improved activity to inhibit the infection of difficult-to-neutralize tier-2 viruses and sensitize infected cells to ADCC mediated by HIV+ plasma. Moreover, the new analogs formed an H-bond with the α-carboxylic acid group of Asp368, opening a new avenue to enlarge the breadth of this family of anti-Env small molecules. Overall, the new structural and biological attributes of these molecules make them good candidates for strategies aimed at the elimination of HIV-1-infected cells.

The Humanized Mouse Model: What Added Value Does It Offer for HIV Research?

In the early 2000s, novel humanized mouse models based on the transplantation of human hematopoietic stem and progenitor cells (HSPCs) into immunocompromised mice were introduced (hu mice). The human HSPCs gave rise to a lymphoid system of human origin. The HIV research community has greatly benefitted from these hu mice. Since human immunodeficiency virus (HIV) type 1 infection results in a high-titer disseminated HIV infection, hu mice have been of great value for all types of HIV research from pathogenesis to novel therapies. Since the first description of this new generation of hu mice, great efforts have been expended to improve humanization by creating other immunodeficient mouse models or supplementing mice with human transgenes to improve human engraftment. Many labs have their own customized hu mouse models, making comparisons quite difficult. Here, we discuss the different hu mouse models in the context of specific research questions in order to define which characteristics should be considered when determining which hu mouse model is appropriate for the question posed. We strongly believe that researchers must first define their research question and then determine whether a hu mouse model exists, allowing the research question to be studied.

Indoline CD4-mimetic compounds mediate potent and broad HIV-1 inhibition and sensitization to antibody-dependent cellular cytotoxicity

Binding to the host cell receptors, CD4 and CCR5/CXCR4, triggers large-scale conformational changes in the HIV-1 envelope glycoprotein (Env) trimer [(gp120/gp41)3] that promote virus entry into the cell. CD4-mimetic compounds (CD4mcs) comprise small organic molecules that bind in the highly conserved CD4-binding site of gp120 and prematurely induce inactivating Env conformational changes, including shedding of gp120 from the Env trimer. By inducing more "open," antibody-susceptible Env conformations, CD4mcs also sensitize HIV-1 virions to neutralization by antibodies and infected cells to antibody-dependent cellular cytotoxicity (ADCC). Here, we report the design, synthesis, and evaluation of novel CD4mcs based on an indoline scaffold. Compared with our current lead indane scaffold CD4mc, BNM-III-170, several indoline CD4mcs exhibit increased potency and breadth against HIV-1 variants from different geographic clades. Viruses that were selected for resistance to the lead indane CD4mc, BNM-III-170, are susceptible to inhibition by the indoline CD4mcs. The indoline CD4mcs also potently sensitize HIV-1-infected cells to ADCC mediated by plasma from HIV-1-infected individuals. Crystal structures indicate that the indoline CD4mcs gain potency compared to the indane CD4mcs through more favorable π-π overlap from the indoline pose and by making favorable contacts with the vestibule of the CD4-binding pocket on gp120. The rational design of indoline CD4mcs thus holds promise for further improvements in antiviral activity, potentially contributing to efforts to treat and prevent HIV-1 infection.

Piperidine CD4-mimetic compounds expose vulnerable Env epitopes sensitizing HIV-1-infected cells to ADCC

The ability of HIV-1 accessory proteins Nef and Vpu to decrease CD4 levels contributes to the protection of infected cells from antibody-dependent cellular cytotoxicity (ADCC) by preventing the exposure of Env vulnerable epitopes. Small-molecule CD4 mimetics (CD4mc) based on the indane and piperidine scaffolds such as (+)-BNM-III-170 and ( S )-MCG-IV-210 sensitize HIV-1 infected cells to ADCC by exposing CD4-induced (CD4i) epitopes recognized by non-neutralizing antibodies abundantly present in plasma from people living with HIV. Here, we characterize a new family of CD4mc, ( S )-MCG-IV-210 derivatives, based on the piperidine scaffold which engage the gp120 within the Phe43 cavity by targeting the highly-conserved Asp ³⁶⁸ Env residue. We utilized structure-based approaches and developed a series of piperidine analogs with improved activity to inhibit infection of difficult-to-neutralize tier-2 viruses and sensitize infected cells to ADCC mediated by HIV+ plasma. Moreover, the new analogs formed an H-bond with the α-carboxylic acid group of Asp ³⁶⁸ , opening a new avenue to enlarge the breadth of this family of anti-Env small molecules. Overall, the new structural and biological attributes of these molecules make them good candidates for strategies aimed at the elimination HIV-1-infected cells.

Small CD4 mimetics sensitize HIV-1-infected macrophages to antibody-dependent cellular cytotoxicity

HIV-1 envelope (Env) conformation determines the susceptibility of infected CD4+ T cells to antibody-dependent cellular cytotoxicity (ADCC). Upon interaction with CD4, Env adopts more "open" conformations, exposing ADCC epitopes. HIV-1 limits Env-CD4 interaction and protects infected cells against ADCC by downregulating CD4 via Nef, Vpu, and Env. Limited data exist, however, of the role of these proteins in downmodulating CD4 on infected macrophages and how this impacts Env conformation. While Nef, Vpu, and Env are all required to efficiently downregulate CD4 on infected CD4+ T cells, we show here that any one of these proteins is sufficient to downmodulate most CD4 from the surface of infected macrophages. Consistent with this finding, Nef and Vpu have a lesser impact on Env conformation and ADCC sensitivity in infected macrophages compared with CD4+ T cells. However, treatment of infected macrophages with small CD4 mimetics exposes vulnerable CD4-induced Env epitopes and sensitizes them to ADCC.

Characterization of a Novel CD4 Mimetic Compound YIR-821 against HIV-1 Clinical Isolates

Small CD4-mimetic compound (CD4mc), which inhibits the interaction between gp120 with CD4, acts as an entry inhibitor and induces structural changes in the HIV-1 envelope glycoprotein trimer (Env) through its insertion within the Phe43 cavity of gp120. We recently developed YIR-821, a novel CD4mc, that has potent antiviral activity and lower toxicity than the prototype NBD-556. To assess the possibility of clinical application of YIR-821, we tested its antiviral activity using a panel of HIV-1 pseudoviruses from different subtypes. YIR-821 displayed entry inhibitor activity against 53.5% (21/40) of the pseudoviruses tested and enhanced neutralization mediated by coreceptor binding site (CoRBS) antibodies in 50% (16/32) of these. Furthermore, when we assessed the antiviral effects using a panel of pseudoviruses and autologous plasma IgG, enhancement of antibody-mediated neutralization activity was observed for 48% (15/31) of subtype B strains and 51% (28/55) of non-B strains. The direct antiviral activity of YIR-821 as an entry inhibitor was observed in 53% of both subtype B (27/51) and non-B subtype (40/75) pseudoviruses. Enhancement of antibody-dependent cellular cytotoxicity was also observed with YIR-821 for all six selected clinical isolates, as well as for the transmitted/founder (T/F) CH58 virus-infected cells. The sequence diversity in the CD4 binding site as well as other regions, such as the gp120 inner domain layers or gp41, may be involved in the multiple mechanisms related to the sensitive/resistant phenotype of the virus to YIR-821. Our findings may facilitate the clinical application of YIR-821. IMPORTANCE Small CD4-mimetic compound (CD4mc) interacts with the Phe43 cavity and triggers conformational changes, enhancing antibody-mediated neutralization and antibody-dependent cellular cytotoxicity (ADCC). Here, we evaluated the effect of YIR-821, a novel CD4mc, against clinical isolates, including both subtype B and non-B subtype viruses. Our results confirm the desirable properties of YIR-821, which include entry inhibition, enhancement of IgG-neutralization, binding, and ADCC, in addition to low toxicity and long half-life in a rhesus macaque model, that might facilitate the clinical application of this novel CD4mc. Our observation of primary viruses that are resistant to YIR-821 suggests that further development of CD4mcs with different structural properties is required.

Human Hematopoietic Stem Cell Engrafted IL-15 Transgenic NSG Mice Support Robust NK Cell Responses and Sustained HIV-1 Infection

Mice reconstituted with human immune systems are instrumental in the investigation of HIV-1 pathogenesis and therapeutics. Natural killer (NK) cells have long been recognized as a key mediator of innate anti-HIV responses. However, established humanized mouse models do not support robust human NK cell development from engrafted human hematopoietic stem cells (HSCs). A major obstacle to human NK cell reconstitution is the lack of human interleukin-15 (IL-15) signaling, as murine IL-15 is a poor stimulator of the human IL-15 receptor. Here, we demonstrate that immunodeficient NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice expressing a transgene encoding human IL-15 (NSG-Tg(IL-15)) have physiological levels of human IL-15 and support long-term engraftment of human NK cells when transplanted with human umbilical-cord-blood-derived HSCs. These Hu-NSG-Tg(IL-15) mice demonstrate robust and long-term reconstitution with human immune cells, but do not develop graft-versus-host disease (GVHD), allowing for long-term studies of human NK cells. Finally, we show that these HSC engrafted mice can sustain HIV-1 infection, resulting in human NK cell responses in HIV-infected mice. We conclude that Hu-NSG-Tg(IL-15) mice are a robust novel model to study NK cell responses to HIV-1.

Subtle Longitudinal Alterations in Env Sequence Potentiate Differences in Sensitivity to Broadly Neutralizing Antibodies following Acute HIV-1 Subtype C Infection

Broadly neutralizing antibodies (bNAbs) for HIV-1 prevention or cure strategies must inhibit transmitted/founder and reservoir viruses. Establishing sensitivity of circulating viruses to bNAbs and genetic patterns affecting neutralization variability may guide rational bNAbs selection for clinical development. We analyzed 326 single env genomes from nine individuals followed longitudinally following acute HIV-1 infection, with samples collected at ~1 week after the first detection of plasma viremia; 300 to 1,709 days postinfection but prior to initiating antiretroviral therapy (ART) (median = 724 days); and ~1 year post ART initiation. Sequences were assessed for phylogenetic relatedness, potential N- and O-linked glycosylation, and variable loop lengths (V1 to V5). A total of 43 env amplicons (median = 3 per patient per time point) were cloned into an expression vector and the TZM-bl assay was used to assess the neutralization profiles of 15 bNAbs targeting the CD4 binding site, V1/V2 region, V3 supersite, MPER, gp120/gp41 interface, and fusion peptide. At 1 μg/mL, the neutralization breadths were as follows: VRC07-LS and N6.LS (100%), VRC01 (86%), PGT151 (81%), 10-1074 and PGT121 (80%), and less than 70% for 10E8, 3BNC117, CAP256.VRC26, 4E10, PGDM1400, and N123-VRC34.01. Features associated with low sensitivity to V1/V2 and V3 bNAbs were higher potential glycosylation sites and/or relatively longer V1 and V4 domains, including known "signature" mutations. The study shows significant variability in the breadth and potency of bNAbs against circulating HIV-1 subtype C envelopes. VRC07-LS, N6.LS, VRC01, PGT151, 10-1074, and PGT121 display broad activity against subtype C variants, and major determinants of sensitivity to most bNAbs were within the V1/V4 domains. IMPORTANCE Broadly neutralizing antibodies (bNAbs) have potential clinical utility in HIV-1 prevention and cure strategies. However, bNAbs target diverse epitopes on the HIV-1 envelope and the virus may evolve to evade immune responses. It is therefore important to identify antibodies with broad activity in high prevalence settings, as well as the genetic patterns that may lead to neutralization escape. We investigated 15 bNAbs with diverse biophysical properties that target six epitopes of the HIV-1 Env glycoprotein for their ability to inhibit viruses that initiated infection, viruses circulating in plasma at chronic infection before antiretroviral treatment (ART), or viruses that were archived in the reservoir during ART in subtype C infected individuals in South Africa, a high burden country. We identify the antibodies most likely to be effective for clinical use in this setting and describe mutational patterns associated with neutralization escape from these antibodies.

A humanized mouse model to study NK cell biology during HIV infection

NK cells are an important subset of innate immune effectors with antiviral activity. However, NK cell development and immune responses in different tissues during acute and chronic HIV infection in vivo have been difficult to study due to the impaired development and function of NK cells in conventional humanized mouse models. In this issue of the JCI, Sangur et al. report on a transgenic MISTRG-6-15 mouse model with human IL-6 and IL-15 knocked into the previously constructed MISTRG mice. The predecessor model was deficient in Rag2 and γ chain (γc) with knock-in expression of human M-CSF, IL-3, GM-CSF, and TPO, and transgenic expression of human SIRPα. The researchers studied tissue-specific NK cell immune responses during HIV infection and clearly show that the endogenous human NK cells in the humanized mouse model suppressed HIV-1 replication in vivo. These findings provide insight into harnessing the innate immune response for clinical antiviral therapies.

Associations between NK Cells in Different Immune Organs and Cellular SIV DNA and RNA in Regional HLADR- CD4+ T Cells in Chronically SIVmac239-Infected, Treatment-Naïve Rhesus Macaques

With the development of NK cell-directed therapeutic strategies, the actual effect of NK cells on the cellular SIV DNA levels of the virus in SIV-infected macaques in vivo remains unclear. In this study, five chronically SIV_mac239-infected, treatment-naïve rhesus macaques were euthanized, and the blood, spleen, pararectal/paracolonic lymph nodes (PaLNs), and axillary lymph nodes (ALNs) were collected. The distributional, phenotypic, and functional profiles of NK cells were detected by flow cytometry. The highest frequency of NK cells was found in PBMC, followed by the spleen, while only 0~0.5% were found in LNs. Peripheral NK cells also exhibited higher cytotoxic potential (CD56^- CD16⁺ NK subsets) and IFN-γ-producing capacity but low PD-1 and Tim-3 levels than those in the spleen and LNs. Our results demonstrated a significant positive correlation between the frequency of NK cells and the ratios of cellular SIV DNA/RNA in HLADR^- CD4⁺ T cells (r = 0.6806, p < 0.001) in SIV-infected macaques, despite no discrepancies in the cellular SIV DNA or RNA levels that were found among the blood, spleen, and LNs. These findings showed a profile of NK cell frequencies and NK cytotoxicity levels in different immune organs from chronically SIV_mac239-infected, treatment-naïve rhesus macaques. It was suggested that NK cell frequencies could be closely related to SIV DNA/RNA levels, which could affect the transcriptional activity of SIV proviruses. However, the cytotoxicity effect of NK cells on the latent SIV viral load in LNs could be limited due to the sparse abundance of NK cells in LNs. The development of NK cell-directed treatment approaches aiming for HIV clearance remains challenging.

HIV-1 Vpu restricts Fc-mediated effector functions in vivo

Non-neutralizing antibodies (nnAbs) can eliminate HIV-1-infected cells via antibody-dependent cellular cytotoxicity (ADCC) and were identified as a correlate of protection in the RV144 vaccine trial. Fc-mediated effector functions of nnAbs were recently shown to alter the course of HIV-1 infection in vivo using a vpu-defective virus. Since Vpu is known to downregulate cell-surface CD4, which triggers conformational changes in the viral envelope glycoprotein (Env), we ask whether the lack of Vpu expression was linked to the observed nnAbs activity. We find that restoring Vpu expression greatly reduces nnAb recognition of infected cells, rendering them resistant to ADCC. Moreover, administration of nnAbs in humanized mice reduces viral loads only in animals infected with a vpu-defective but not with a wild-type virus. CD4-mimetics administration, known to "open" Env and expose nnAb epitopes, renders wild-type viruses sensitive to nnAbs Fc-effector functions. This work highlights the importance of Vpu-mediated evasion of humoral responses.

Humanized mouse models for preclinical evaluation of HIV cure strategies

Although the world is currently focused on the COVID-19 pandemic, HIV/AIDS remains a significant threat to public health. To date, the HIV/AIDS pandemic has claimed the lives of over 36 million people, while nearly 38 million people are currently living with the virus. Despite the undeniable success of antiretroviral therapy (ART) in controlling HIV, the medications are not curative. Soon after initial infection, HIV integrates into the genome of infected cells as a provirus, primarily, within CD4+ T lymphocytes and tissue macrophages. When not actively transcribed, the provirus is referred to as a latent reservoir because it is hidden to the immune system and ART. Following ART discontinuation, HIV may emerge from the replication-competent proviruses and resumes the infection of healthy cells. Thus, these latent reservoirs are a major obstacle to an HIV cure, and their removal remains a priority. A vital aspect in the development of curative therapies is the demonstration of efficacy in an animal model, such as the humanized mouse model. Therefore, optimization, standardization, and validation of the humanized mouse model are a priority. The purpose of this review article is to provide an update on existing humanized mouse models, highlighting the advantages and disadvantages of each as they pertain to HIV cure studies and to review the approaches to curative therapies that are under investigation.

Characterization of Human Immunodeficiency Virus (HIV-1) Envelope Glycoprotein Variants Selected for Resistance to a CD4-Mimetic Compound

Binding to the host cell receptors CD4 and CCR5/CXCR4 triggers conformational changes in the human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer that promote virus entry. CD4 binding allows the gp120 exterior Env to bind CCR5/CXCR4 and induces a short-lived prehairpin intermediate conformation in the gp41 transmembrane Env. Small-molecule CD4-mimetic compounds (CD4mcs) bind within the conserved Phe-43 cavity of gp120, near the binding site for CD4. CD4mcs like BNM-III-170 inhibit HIV-1 infection by competing with CD4 and by prematurely activating Env, leading to irreversible inactivation. In cell culture, we selected and analyzed variants of the primary HIV-1_AD8 strain resistant to BNM-III-170. Two changes (S375N and I424T) in gp120 residues that flank the Phe-43 cavity each conferred an ~5-fold resistance to BNM-III-170 with minimal fitness cost. A third change (E64G) in layer 1 of the gp120 inner domain resulted in ~100-fold resistance to BNM-III-170, ~2- to 3-fold resistance to soluble CD4-Ig, and a moderate decrease in viral fitness. The gp120 changes additively or synergistically contributed to BNM-III-170 resistance. The sensitivity of the Env variants to BNM-III-170 inhibition of virus entry correlated with their sensitivity to BNM-III-170-induced Env activation and shedding of gp120. Together, the S375N and I424T changes, but not the E64G change, conferred >100-fold and 33-fold resistance to BMS-806 and BMS-529 (temsavir), respectively, potent HIV-1 entry inhibitors that block Env conformational transitions. These studies identify pathways whereby HIV-1 can develop resistance to CD4mcs and conformational blockers, two classes of entry inhibitors that target the conserved gp120 Phe-43 cavity. IMPORTANCE CD4-mimetic compounds (CD4mcs) and conformational blockers like BMS-806 and BMS-529 (temsavir) are small-molecule inhibitors of human immunodeficiency virus (HIV-1) entry into host cells. Although CD4mcs and conformational blockers inhibit HIV-1 entry by different mechanisms, they both target a pocket on the viral envelope glycoprotein (Env) spike that is used for binding to the receptor CD4 and is highly conserved among HIV-1 strains. Our study identifies changes near this pocket that can confer various levels of resistance to the antiviral effects of a CD4mc and conformational blockers. We relate the antiviral potency of a CD4mc against this panel of HIV-1 variants to the ability of the CD4mc to activate changes in Env conformation and to induce the shedding of the gp120 exterior Env from the spike. These findings will guide efforts to improve the potency and breadth of small-molecule HIV-1 entry inhibitors.

Enhanced development of functional human NK cells in NOD-scid-IL2rgnull mice expressing human IL15

Human innate immunity plays a critical role in tumor surveillance and in immunoregulation within the tumor microenvironment. Natural killer (NK) cells are innate lymphoid cells that have opposing roles in the tumor microenvironment, including NK cell subsets that mediate tumor cell cytotoxicity and subsets with regulatory function that contribute to the tumor immune suppressive environment. The balance between effector and regulatory NK cell subsets has been studied extensively in murine models of cancer, but there is a paucity of models to study human NK cell function in tumorigenesis. Humanized mice are a powerful alternative to syngeneic mouse tumor models for the study of human immuno-oncology and have proven effective tools to test immunotherapies targeting T cells. However, human NK cell development and survival in humanized NOD-scid-IL2rg^null (NSG) mice are severely limited. To enhance NK cell development, we have developed NSG mice that constitutively expresses human Interleukin 15 (IL15), NSG-Tg(Hu-IL15). Following hematopoietic stem cell engraftment of NSG-Tg(Hu-IL15) mice, significantly higher levels of functional human CD56+ NK cells are detectable in blood and spleen, as compared to NSG mice. Hematopoietic stem cell (HSC)-engrafted NSG-Tg(Hu-IL15) mice also supported the development of human CD3+ T cells, CD20+ B cells, and CD33+ myeloid cells. Moreover, the growth kinetics of a patient-derived xenograft (PDX) melanoma were significantly delayed in HSC-engrafted NSG-Tg(Hu-IL15) mice as compared to HSC-engrafted NSG mice demonstrating that human NK cells have a key role in limiting the tumor growth. Together, these data demonstrate that HSC-engrafted NSG-Tg(Hu-IL15) mice support enhanced development of functional human NK cells, which limit the growth of PDX tumors.

Engaging innate immunity in HIV-1 cure strategies

Combination antiretroviral therapy (ART) can block multiple stages of the HIV-1 life cycle to prevent progression to AIDS in people living with HIV-1. However, owing to the persistence of a reservoir of latently infected CD4⁺ T cells, life-long ART is necessary to prevent viral rebound. One strategy currently under consideration for curing HIV-1 infection is known as 'shock and kill'. This strategy uses latency-reversing agents to induce expression of HIV-1 genes, allowing for infected cells to be cleared by cytolytic immune cells. The role of innate immunity in HIV-1 pathogenesis is best understood in the context of acute infection. Here, we suggest that innate immunity can also be used to improve the efficacy of HIV-1 cure strategies, with a particular focus on dendritic cells (DCs) and natural killer cells. We discuss novel latency-reversing agents targeting DCs as well as DC-based strategies to enhance the clearance of infected cells by CD8⁺ T cells and strategies to improve the killing activity of natural killer cells.

Identification of gp120 Residue His105 as a Novel Target for HIV-1 Neutralization by Small-Molecule CD4-Mimics

The design and synthesis of butyl chain derivatives at the indane ring 3-position of our lead CD4-mimetic compound BNM-III-170 that inhibits human immunodeficiency virus (HIV-1) infection are reported. Optimization efforts were guided by crystallographic and computational analysis of the small-molecule ligands of the Phe43 cavity of the envelope glycoprotein gp120. Biological evaluation of 11-21 revealed that members of this series of CD4-mimetic compounds are able to inhibit HIV-1 viral entry into target cells more potently and with greater breadth compared to BNM-III-170. Crystallographic analysis of the binding pocket of 14, 16, and 17 revealed a novel hydrogen bonding interaction between His105 and a primary hydroxyl group on the butyl side chain. Further optimization of this interaction with the His105 residue holds the promise of more potent CD4-mimetic compounds.

HIV-1 Envelope Glycoproteins Proteolytic Cleavage Protects Infected Cells from ADCC Mediated by Plasma from Infected Individuals

The HIV-1 envelope glycoprotein (Env) is synthesized in the endoplasmic reticulum as a trimeric gp160 precursor, which requires proteolytic cleavage by a cellular furin protease to mediate virus-cell fusion. Env is conformationally flexible but controls its transition from the unbound "closed" conformation (State 1) to downstream CD4-bound conformations (States 2/3), which are required for fusion. In particular, HIV-1 has evolved several mechanisms that reduce the premature "opening" of Env which exposes highly conserved epitopes recognized by non-neutralizing antibodies (nnAbs) capable of mediating antibody-dependent cellular cytotoxicity (ADCC). Env cleavage decreases its conformational transitions favoring the adoption of the "closed" conformation. Here we altered the gp160 furin cleavage site to impair Env cleavage and to examine its impact on ADCC responses mediated by plasma from HIV-1-infected individuals. We found that infected primary CD4+ T cells expressing uncleaved, but not wildtype, Env are efficiently recognized by nnAbs and become highly susceptible to ADCC responses mediated by plasma from HIV-1-infected individuals. Thus, HIV-1 limits the exposure of uncleaved Env at the surface of HIV-1-infected cells at least in part to escape ADCC responses.

Unleashing natural antibodies against HIV-1

One barrier to HIV-1 eradication is the viral Env protein that is invisible to most antibodies. In this issue of Cell Host & Microbe, Rajashekar et al. (2021) remove the "invisibility cloak" from Env, make it accessible to antibodies, and demonstrate NK-mediated in vivo killing of infected cells by human plasma antibodies.