Antibodies Elicited by Multiple Envelope Glycoprotein Immunogens in Primates Neutralize Primary Human Immunodeficiency Viruses (HIV-1) Sensitized by CD4-Mimetic Compounds

Safety, pharmacokinetics, and biological activity of CD4-mimetic BNM-III-170 in SHIV-infected rhesus macaques

Anti-HIV-1 antibodies capable of mediating ADCC are elicited by the majority of people with HIV-1 and preferentially target the "open," CD4-bound conformation of HIV-1 envelope glycoproteins (Env). However, due to the "closed" conformation sampled by unliganded HIV-1-Envs, these antibodies are ineffective at eliminating infected cells. BNM-III-170 is a small-molecule CD4-mimetic compound that binds the Phe43 cavity of the gp120 subunit of Env, forcing Env to "open up," thus exposing epitopes targeted by CD4-induced (CD4i), ADCC-mediating antibodies. Here, we assessed the safety, pharmacokinetics, and biological activity of BNM-III-170 in uninfected and SHIV-AD8-EO-infected rhesus macaques (RMs). In uninfected RMs, single subcutaneous administrations of 3-36 mg/kg BNM-III-170 were well-tolerated, with serum half-lives ranging from 3 to 6 h. In SHIV-infected RMs, four different regimens were evaluated: 2 × 36 mg/kg daily, 1 × 24 mg/kg, 3 × 36 mg/kg every 7 days, and 3 × 36 mg/kg every 3 days. While toxicity was observed with daily doses, all other regimens demonstrated reasonable safety profiles. No changes in plasma viral loads were observed in SHIV-infected RMs following any of the evaluated BNM-III-170 dosing regimens. However, plasma collected following BNM-III-170 administration was shown to have increased binding to infected cells and to sensitize SHIV AD8-EO virions to neutralization by otherwise non-neutralizing antibodies. In addition, the plasma of treated animals mediated ADCC in the presence of BNM-III-170. These results establish a well-tolerated BNM-III-170 dosing regimen in SHIV-infected RMs and serve as proof of concept for its biological activity in promoting the targeting of infected cells by CD4i ADCC-mediating antibodies. Thus, they inform future studies evaluating CD4mc treatment in ART-treated animals.IMPORTANCEA therapeutic regimen able to eradicate or functionally cure HIV-1 remains elusive and may require a "shock-and-kill" approach to reactivate and then purge the latent HIV-1 reservoir. The small-molecule CD4-mimetic compound BNM-III-170 has previously been shown to (i) sensitize HIV-1-infected cells to ADCC mediated by plasma from people with HIV-1 (PWH) in vitro and (ii) significantly delay the time to viral rebound following ART interruption when combined with anti-CoRBS + anti-cluster A Abs or plasma from PWH in humanized mice. To evaluate the use of BNM-III-170 as part of a kill approach, we characterized the safety, pharmacokinetics, and biological activity of BNM-III-170 in uninfected and SHIV-infected RMs. Our study identifies a tolerable BNM-III-170 dosing regimen in SHIV-infected RMs and provides insights into its antiviral activities; as such, it informs future studies evaluating the efficacy of BNM-III-170 in reducing the viral reservoir.

Membrane HIV-1 envelope glycoproteins stabilized more strongly in a pretriggered conformation than natural virus Envs

The pretriggered conformation of the human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer ((gp120/gp41)3) is targeted by virus entry inhibitors and broadly neutralizing antibodies (bNAbs). The lability of pretriggered Env has hindered its characterization. Here, we produce membrane Env variants progressively stabilized in pretriggered conformations, in some cases to a degree beyond that found in natural HIV-1 strains. Pretriggered Env stability correlated with stronger trimer subunit association, increased virus sensitivity to bNAb neutralization, and decreased capacity to mediate cell-cell fusion and virus entry. For some highly stabilized Env mutants, after virus-host cell engagement, the normally inaccessible gp120 V3 region on an Env intermediate became targetable by otherwise poorly neutralizing antibodies. Thus, evolutionary pressure on HIV-1 Env to maintain trimer integrity, responsiveness to the CD4 receptor, and resistance to antibodies modulates pretriggered Env stability. The strongly stabilized pretriggered membrane Envs reported here will facilitate further characterization of this functionally important conformation.

Inhibition of human immunodeficiency virus (HIV-1) infectivity by expression of poorly or broadly neutralizing antibodies against Env in virus-producing cells

The human immunodeficiency virus (HIV-1) envelope (Env) glycoprotein precursor (gp160) trimerizes, is modified by high-mannose glycans in the endoplasmic reticulum, and is transported via Golgi and non-Golgi secretory pathways to the infected cell surface. In the Golgi, gp160 is partially modified by complex carbohydrates and proteolytically cleaved to produce the mature functional Env trimer, which is preferentially incorporated into virions. Broadly neutralizing antibodies (bNAbs) generally recognize the cleaved Env trimer, whereas poorly neutralizing antibodies (pNAbs) bind the conformationally flexible gp160. We found that expression of bNAbs, pNAbs, or soluble/membrane forms of the receptor, CD4, in cells producing HIV-1 all decreased viral infectivity. Four patterns of co-expressed ligand:Env were observed: (i) ligands (CD4, soluble CD4-Ig, and some pNAbs) that specifically recognize the CD4-bound Env conformation resulted in uncleaved Envs lacking complex glycans that were not incorporated into virions; (ii) other pNAbs produced Envs with some complex carbohydrates and severe defects in cleavage, which were relieved by brefeldin A treatment; (iii) bNAbs that recognize gp160 as well as mature Envs resulted in Envs with some complex carbohydrates and moderate decreases in virion Env cleavage; and (iv) bNAbs that preferentially recognize mature Envs produced cleaved Envs with complex glycans in cells and on virions. The low infectivity observed upon co-expression of pNAbs or CD4 could be explained by disruption of Env trafficking, reducing the level of Env and/or increasing the fraction of uncleaved Env on virions. In addition to bNAb effects on virion Env cleavage, the secreted bNAbs neutralized the co-expressed viruses.IMPORTANCEThe Env trimers on the HIV-1 mediate virus entry into host cells. Env is synthesized in infected cells, modified by complex sugars, and cleaved to form a mature, functional Env, which is incorporated into virus particles. Env elicits antibodies in infected individuals, some of which can neutralize the virus. We found that antibodies co-expressed in the virus-producing cell can disrupt Env transit to the proper compartment for cleavage and sugar modification and, in some cases, block incorporation into viruses. These studies provide insights into the processes by which Env becomes functional in the virus-producing cell and may assist attempts to interfere with these events to inhibit HIV-1 infection.

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.

Characterization of the Human Immunodeficiency Virus (HIV-1) Envelope Glycoprotein Conformational States on Infectious Virus Particles

Human immunodeficiency virus (HIV-1) entry into cells involves triggering of the viral envelope glycoprotein (Env) trimer ([gp120/gp41]3) by the primary receptor, CD4, and coreceptors, CCR5 or CXCR4. The pretriggered (State-1) conformation of the mature (cleaved) Env is targeted by broadly neutralizing antibodies (bNAbs), which are inefficiently elicited compared with poorly neutralizing antibodies (pNAbs). Here, we characterize variants of the moderately triggerable HIV-1AD8 Env on virions produced by an infectious molecular proviral clone; such virions contain more cleaved Env than pseudotyped viruses. We identified three types of cleaved wild-type AD8 Env trimers on virions: (i) State-1-like trimers preferentially recognized by bNAbs and exhibiting strong subunit association; (ii) trimers recognized by pNAbs directed against the gp120 coreceptor-binding region and exhibiting weak, detergent-sensitive subunit association; and (iii) a minor gp41-only population. The first Env population was enriched and the other Env populations reduced by introducing State-1-stabilizing changes in the AD8 Env or by treatment of the virions with crosslinker or the State-1-preferring entry inhibitor, BMS-806. These stabilized AD8 Envs were also more resistant to gp120 shedding induced by a CD4-mimetic compound or by incubation on ice. Conversely, a State-1-destabilized, CD4-independent AD8 Env variant exhibited weaker bNAb recognition and stronger pNAb recognition. Similar relationships between Env triggerability and antigenicity/shedding propensity on virions were observed for other HIV-1 strains. State-1 Envs on virions can be significantly enriched by minimizing the adventitious incorporation of uncleaved Env; stabilizing the pretriggered conformation by Env modification, crosslinking or BMS-806 treatment; strengthening Env subunit interactions; and using CD4-negative producer cells. IMPORTANCE Efforts to develop an effective HIV-1 vaccine have been frustrated by the inability to elicit broad neutralizing antibodies that recognize multiple virus strains. Such antibodies can bind a particular shape of the HIV-1 envelope glycoprotein trimer, as it exists on a viral membrane but before engaging receptors on the host cell. Here, we establish simple yet powerful assays to characterize the envelope glycoproteins in a natural context on virus particles. We find that, depending on the HIV-1 strain, some envelope glycoproteins change shape and fall apart, creating decoys that can potentially divert the host immune response. We identify requirements to keep the relevant envelope glycoprotein target for broad neutralizing antibodies intact on virus-like particles. These studies suggest strategies that should facilitate efforts to produce and use virus-like particles as vaccine immunogens.

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.

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.

SERINC5 Restricts HIV-1 Infectivity by Promoting Conformational Changes and Accelerating Functional Inactivation of Env

SERINC5 incorporates into HIV-1 particles and inhibits the ability of Env glycoprotein to mediate virus-cell fusion. SERINC5-resistance maps to Env, with primary isolates generally showing greater resistance than laboratory-adapted strains. Here, we examined a relationship between the inhibition of HIV-1 infectivity and the rate of Env inactivation using a panel of SERINC5-resistant and -sensitive HIV-1 Envs. SERINC5 incorporation into pseudoviruses resulted in a faster inactivation of sensitive compared to resistant Env strains. A correlation between fold reduction in infectivity and the rate of inactivation was also observed for multiple Env mutants known to stabilize and destabilize the closed Env structure. Unexpectedly, most mutations disfavoring the closed Env conformation rendered HIV-1 less sensitive to SERINC5. In contrast, functional inactivation of SERINC5-containing viruses was significantly accelerated in the presence of a CD4-mimetic compound, suggesting that CD4 binding sensitizes Env to SERINC5. Using a small molecule inhibitor that selectively targets the closed Env structure, we found that, surprisingly, SERINC5 increases the potency of this compound against a laboratory-adapted Env which prefers a partially open conformation, indicating that SERINC5 may stabilize the closed trimeric Env structure. Our results reveal a complex effect of SERINC5 on Env conformational dynamics that promotes Env inactivation and is likely responsible for the observed restriction phenotype.

The HIV Env Glycoprotein Conformational States on Cells and Viruses

The HIV Env glycoprotein is the surface glycoprotein responsible for viral entry into CD4+ immune cells. During infection, Env also serves as a primary target for antibody responses, which are robust but unable to control virus replication. Immune evasion by HIV-1 Env appears to employ complex mechanisms to regulate what antigenic states are presented to the immune system. Immunodominant features appear to be distinct from epitopes that interfere with Env functions in mediating infection. Further, cell-cell transmission studies indicate that vulnerable conformational states are additionally hidden from recognition on infected cells, even though the presence of Env at the cell surface is required for viral infection through the virological synapse. Cell-cell infection studies support that Env on infected cells is presented in distinct conformations from that on virus particles. Here we review data regarding the regulation of conformational states of Env and assess how regulated sorting of Env within the infected cell may underlie mechanisms to distinguish Env on the surface of virus particles versus Env on the surface of infected cells. These mechanisms may allow infected cells to avoid opsonization, providing cell-to-cell infection by HIV with a selective advantage during evolution within an infected individual. Understanding how distinct Env conformations are presented on cells versus viruses may be essential to designing effective vaccine approaches and therapeutic strategies to clear infected cell reservoirs.

Functional and Highly Cross-Linkable HIV-1 Envelope Glycoproteins Enriched in a Pretriggered Conformation

Binding to the receptor, CD4, drives the pretriggered, "closed" (state-1) conformation of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer into more "open" conformations (states 2 and 3). Broadly neutralizing antibodies, which are elicited inefficiently, mostly recognize the state-1 Env conformation, whereas the more commonly elicited poorly neutralizing antibodies recognize states 2/3. HIV-1 Env metastability has created challenges for defining the state-1 structure and developing immunogens mimicking this labile conformation. The availability of functional state-1 Envs that can be efficiently cross-linked at lysine and/or acidic amino acid residues might assist these endeavors. To that end, we modified HIV-1_AD8 Env, which exhibits an intermediate level of triggerability by CD4. We introduced lysine/acidic residues at positions that exhibit such polymorphisms in natural HIV-1 strains. Env changes that were tolerated with respect to gp120-gp41 processing, subunit association, and virus entry were further combined. Two common polymorphisms, Q114E and Q567K, as well as a known variant, A582T, additively rendered pseudoviruses resistant to cold, soluble CD4, and a CD4-mimetic compound, phenotypes indicative of stabilization of the pretriggered state-1 Env conformation. Combining these changes resulted in two lysine-rich HIV-1_AD8 Env variants (E.2 and AE.2) with neutralization- and cold-resistant phenotypes comparable to those of natural, less triggerable tier 2/3 HIV-1 isolates. Compared with these and the parental Envs, the E.2 and AE.2 Envs were cleaved more efficiently and exhibited stronger gp120-trimer association in detergent lysates. These highly cross-linkable Envs enriched in a pretriggered conformation should assist characterization of the structure and immunogenicity of this labile state. IMPORTANCE The development of an efficient vaccine is critical for combating HIV-1 infection worldwide. However, the instability of the pretriggered shape (state 1) of the viral envelope glycoprotein (Env) makes it difficult to raise neutralizing antibodies against HIV-1. Here, by introducing multiple changes in Env, we derived two HIV-1 Env variants that are enriched in state 1 and can be efficiently cross-linked to maintain this shape. These Env complexes are more stable in detergent, assisting their purification. Thus, our study provides a path to a better characterization of the native pretriggered Env, which should assist vaccine development.

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.

Opening the HIV envelope: potential of CD4 mimics as multifunctional HIV entry inhibitors

PURPOSE OF REVIEW

Close to 2 million individuals globally become infected with HIV-1 each year and just over two-thirds will have access to life-prolonging antivirals. However, the rapid development of drug resistance creates challenges, such that generation of more effective therapies is not only warranted but a necessary endeavour. This review discusses a group of HIV-1 entry inhibitors known as CD4 mimics which exploit the highly conserved relationship between the HIV-1 envelope glycoprotein and the receptor, CD4.

RECENT FINDINGS

We review the structure/function guided evolution of these inhibitors, vital mechanistic insights that underpin broad and potent functional antagonism, recent evidence of utility demonstrated in animal and physiologically relevant in-vitro models, and current progress towards effective new-generation inhibitors.

SUMMARY

The current review highlights the promising potential of CD4 mimetics as multifunctional therapeutics.

The HIV-1 Env gp120 Inner Domain Shapes the Phe43 Cavity and the CD4 Binding Site

The HIV-1 envelope glycoproteins (Env) undergo conformational changes upon interaction of the gp120 exterior glycoprotein with the CD4 receptor. The gp120 inner domain topological layers facilitate the transition of Env to the CD4-bound conformation. CD4 engages gp120 by introducing its phenylalanine 43 (Phe43) in a cavity ("the Phe43 cavity") located at the interface between the inner and outer gp120 domains. Small CD4-mimetic compounds (CD4mc) can bind within the Phe43 cavity and trigger conformational changes similar to those induced by CD4. Crystal structures of CD4mc in complex with a modified CRF01_AE gp120 core revealed the importance of these gp120 inner domain layers in stabilizing the Phe43 cavity and shaping the CD4 binding site. Our studies reveal a complex interplay between the gp120 inner domain and the Phe43 cavity and generate useful information for the development of more-potent CD4mc.IMPORTANCE The Phe43 cavity of HIV-1 envelope glycoproteins (Env) is an attractive druggable target. New promising compounds, including small CD4 mimetics (CD4mc), were shown to insert deeply into this cavity. Here, we identify a new network of residues that helps to shape this highly conserved CD4 binding pocket and characterize the structural determinants responsible for Env sensitivity to small CD4 mimetics.

Long-Acting BMS-378806 Analogues Stabilize the State-1 Conformation of the Human Immunodeficiency Virus Type 1 Envelope Glycoproteins

During human immunodeficiency virus type 1 (HIV-1) entry into cells, the viral envelope glycoprotein (Env) trimer [(gp120/gp41)₃] binds the receptors CD4 and CCR5 and fuses the viral and cell membranes. CD4 binding changes Env from a pretriggered (state-1) conformation to more open downstream conformations. BMS-378806 (here called BMS-806) blocks CD4-induced conformational changes in Env important for entry and is hypothesized to stabilize a state-1-like Env conformation, a key vaccine target. Here, we evaluated the effects of BMS-806 on the conformation of Env on the surface of cells and virus-like particles. BMS-806 strengthened the labile, noncovalent interaction of gp120 with the Env trimer, enhanced or maintained the binding of most broadly neutralizing antibodies, and decreased the binding of poorly neutralizing antibodies. Thus, in the presence of BMS-806, the cleaved Env on the surface of cells and virus-like particles exhibits an antigenic profile consistent with a state-1 conformation. We designed novel BMS-806 analogues that stabilized the Env conformation for several weeks after a single application. These long-acting BMS-806 analogues may facilitate enrichment of the metastable state-1 Env conformation for structural characterization and presentation to the immune system.IMPORTANCE The envelope glycoprotein (Env) spike on the surface of human immunodeficiency virus type 1 (HIV-1) mediates the entry of the virus into host cells and is also the target for antibodies. During virus entry, Env needs to change shape. Env flexibility also contributes to the ability of HIV-1 to evade the host immune response; many shapes of Env raise antibodies that cannot recognize the functional Env and therefore do not block virus infection. We found that an HIV-1 entry inhibitor, BMS-806, stabilizes the functional shape of Env. We developed new variants of BMS-806 that stabilize Env in its natural state for long periods of time. The availability of such long-acting stabilizers of Env shape will allow the natural Env conformation to be characterized and tested for efficacy as a vaccine.

A New Family of Small-Molecule CD4-Mimetic Compounds Contacts Highly Conserved Aspartic Acid 368 of HIV-1 gp120 and Mediates Antibody-Dependent Cellular Cytotoxicity

The HIV-1 envelope glycoprotein (Env) trimer mediates virus entry into cells. The "closed" conformation of Env is resistant to nonneutralizing antibodies (nnAbs). These antibodies mostly recognize occluded epitopes that can be exposed upon binding of CD4 or small-molecule CD4 mimetics (CD4mc). Here, we describe a new family of small molecules that expose Env to nnAbs and sensitize infected cells to antibody-dependent cellular cytotoxicity (ADCC). These compounds have a limited capacity to inhibit virus infection directly but are able to sensitize viral particles to neutralization by otherwise nonneutralizing antibodies. Structural analysis shows that some analogs of this family of CD4mc engage the gp120 Phe43 cavity by contacting the highly conserved D368 residue, making them attractive scaffolds for drug development.IMPORTANCE HIV-1 has evolved multiple strategies to avoid humoral responses. One efficient mechanism is to keep its envelope glycoprotein (Env) in its "closed" conformation. Here, we report on a new family of small molecules that are able to "open up" Env, thus exposing vulnerable epitopes. This new family of molecules binds in the Phe43 cavity and contacts the highly conserved D368 residue. The structural and biological attributes of molecules of this family make them good candidates for drug development.

CD4 Incorporation into HIV-1 Viral Particles Exposes Envelope Epitopes Recognized by CD4-Induced Antibodies

CD4 downregulation on infected cells is a highly conserved function of primate lentiviruses. It has been shown to positively impact viral replication by a variety of mechanisms, including enhanced viral release and infectivity, decrease of cell reinfection, and protection from antibody-dependent cellular cytotoxicity (ADCC), which is often mediated by antibodies that require CD4 to change envelope (Env) conformation. Here, we report that incorporation of CD4 into HIV-1 viral particles affects Env conformation resulting in the exposure of occluded epitopes recognized by CD4-induced antibodies. This translates into enhanced neutralization susceptibility by these otherwise nonneutralizing antibodies but is prevented by the HIV-1 Nef accessory protein. Altogether, these findings suggest that another functional consequence of Nef-mediated CD4 downregulation is the protection of viral particles from neutralization by commonly elicited CD4-induced antibodies.IMPORTANCE It has been well established that Env-CD4 complexes expose epitopes recognized by commonly elicited CD4-induced antibodies at the surface of HIV-1-infected cells, rendering them vulnerable to ADCC responses. Here, we show that CD4 incorporation has a profound impact on Env conformation at the surface of viral particles. Incorporated CD4 exposes CD4-induced epitopes on Env, rendering HIV-1 susceptible to neutralization by otherwise nonneutralizing antibodies.

Strain-Dependent Activation and Inhibition of Human Immunodeficiency Virus Entry by a Specific PF-68742 Stereoisomer

Human immunodeficiency virus (HIV-1) entry into cells is mediated by the viral envelope glycoprotein (Env) trimer, which consists of three gp120 exterior glycoproteins and three gp41 transmembrane glycoproteins. When gp120 binds sequentially to the receptors CD4 and CCR5 on the target cell, the metastable Env trimer is triggered to undergo entry-related conformational changes. PF-68742 is a small molecule that inhibits the infection of a subset of HIV-1 strains by interfering with an Env function other than receptor binding. Determinants of HIV-1 resistance to PF-68742 map to the disulfide loop and fusion peptide of gp41. Of the four possible PF-68742 stereoisomers, only one, MF275, inhibited the infection of CD4-positive CCR5-positive cells by some HIV-1 strains. MF275 inhibition of these HIV-1 strains occurred after CD4 binding but before the formation of the gp41 six-helix bundle. Unexpectedly, MF275 activated the infection of CD4-negative CCR5-positive cells by several HIV-1 strains resistant to the inhibitory effects of the compound in CD4-positive target cells. In contrast to CD4 complementation by CD4-mimetic compounds, activation of CD4-independent infection by MF275 did not depend upon the availability of the gp120 Phe 43 cavity. Sensitivity to inhibitors indicates that MF275-activated virus entry requires formation/exposure of the gp41 heptad repeat (HR1) as well as CCR5 binding. MF275 apparently activates a virus entry pathway parallel to that triggered by CD4 and CD4-mimetic compounds. Strain-dependent divergence in Env conformational transitions allows different outcomes, inhibition or activation, in response to MF275. Understanding the mechanisms of MF275 activity should assist efforts to optimize its utility.IMPORTANCE Envelope glycoprotein (Env) spikes on the surface of human immunodeficiency virus (HIV-1) bind target cell receptors, triggering changes in the shape of Env. We studied a small molecule, MF275, that also induced shape changes in Env. The consequences of MF275 interaction with Env depended on the HIV-1 strain, with infection by some viruses inhibited and infection by other viruses enhanced. These studies reveal the strain-dependent diversity of HIV-1 Envs as they undergo shape changes in proceeding down the entry pathway. Appreciation of this diversity will assist attempts to develop broadly active inhibitors of HIV-1 entry.

Effects of the SOS (A501C/T605C) and DS (I201C/A433C) Disulfide Bonds on HIV-1 Membrane Envelope Glycoprotein Conformation and Function

Most broadly neutralizing antibodies and many entry inhibitors target the pretriggered (state 1) conformation of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env). Here we examine two previously reported Env mutants designed to be stabilized in this conformation by the introduction of artificial disulfide bonds: A501C/T605C (called SOS) and I201C/A433C (called DS). SOS Env supported virus entry and cell-cell fusion only after exposure to a reducing agent, dithiothreitol (DTT). Deletion of the Env cytoplasmic tail improved the efficiency with which the SOS Env supported virus infection in a reducing environment. The antigenicity of the SOS Env was similar to that of the unmodified Env, except for greater sensitivity to some state 1-preferring ligands. In contrast, viruses with the DS Env were not infectious, even after DTT treatment. The proteolytic maturation of the DS Env on both cell surfaces and virions was severely compromised compared with that of the unmodified Env. The DS Env exhibited detectable cell-fusing activity when DTT was present. However, the profiles of cell-surface Env recognition and cell-cell fusion inhibition by antibodies differed for the DS Env and the unmodified Env. Thus, the DS Env appears to be stabilized in an off-pathway conformation that is nonfunctional on the virus. The SOS change exerted more subtle, context-dependent effects on Env conformation and function.IMPORTANCE The human immunodeficiency virus type 1 (HIV-1) envelope proteins (Envs) bind receptors on the host cell and change shape to allow the virus to enter the cell. Most virus-inhibiting antibodies and drugs recognize a particular shape of Env called state 1. Disulfide bonds formed by cysteine residues have been introduced into soluble forms of the flexible envelope proteins in an attempt to lock them into state 1 for use in vaccines and as research tools. We evaluated the effect of these cysteine substitutions on the ability of the membrane Env to support virus entry and on susceptibility to inhibition by antibodies and small molecules. We found that the conformation of the envelope proteins with the cysteine substitutions differed from that of the unmodified membrane envelope proteins. Awareness of these effects can assist efforts to create stable HIV-1 Env complexes that more closely resemble the state 1 conformation.

Highlights from the Fourth Biennial Strategies for an HIV Cure Meeting, 10-12 October 2018, Bethesda, MD, USA

The National Institute of Allergy and Infectious Diseases (NIAID) organised the Strategies for an HIV Cure 2018 meeting focused on research to develop innovative strategies for eradicating or achieving long-term remission of HIV infection. The purpose was to bring together researchers studying HIV persistence and cure strategies, including the six National Institutes of Health (NIH)-funded Martin Delaney Collaboratories for HIV Cure Research (MDCs), as well as industry and community partners, to share scientific results and stimulate active discussion among all stakeholders about the merits of various approaches under investigation. These discussions were intended to stimulate new collaborations and ideas for future research. The meeting covered a comprehensive range of topics spanning basic and translational research, drug discovery and development, and clinical research. Aside from the oral presentations described here, the meeting also included 130 poster presentations. Each of the three days of presentations is available for viewing via the NIH VideoCast website at: https://videocast.nih.gov/PastEvents.asp.

3-Hydroxyphthalic Anhydride- Modified Rabbit Anti-PAP IgG as a Potential Bifunctional HIV-1 Entry Inhibitor

Several studies have reported that amyloid fibrils in human semen formed from a naturally occurring peptide fragment of prostatic acidic phosphatase (PAP248-286), known as semen-derived enhancer of viral infection (SEVI), could dramatically enhance human immunodeficiency virus type 1 (HIV-1) infection. Accordingly, SEVI might serve as a novel target for new antiviral drugs or microbicide candidates for the prevention of sexually transmitted HIV. Theoretically, a special anti-PAP or anti-SEVI antibody could reduce the enhancement of viral infection by blocking the binding of HIV and SEVI fibrils. Here, 3-hydroxyphthalic anhydride modified anti-PAP248-286 antibody, named HP-API, exhibited broad-spectrum and highly effective anti-HIV-1 activities on different subtypes and tropism. By using time-of-addition, cell–cell fusion and a single-cycle HIV-1 infection assays, we demonstrated that HP-API is an HIV-1 entry/fusion inhibitor. Mechanism studies suggest that HP-API inhibited HIV-1 entry/fusion by targeting both HIV-1 gp120 envelop and CD4 receptor on the host cell specifically. It is noteworthy that HP-API abrogated the formation of SEVI fibrils and partially interfered with SEVI-mediated enhancement of HIV-1 infection. Based on these findings, HP-API could be considered a bifunctional HIV-1 entry/fusion inhibitor with high potential.

A CD4-mimetic compound enhances vaccine efficacy against stringent immunodeficiency virus challenge

The envelope glycoprotein (Env) trimer ((gp120/gp41)₃) mediates human immunodeficiency virus (HIV-1) entry into cells. The “closed,” antibody-resistant Env trimer is driven to more open conformations by binding the host receptor, CD4. Broadly neutralizing antibodies that recognize conserved elements of the closed Env are potentially protective, but are elicited inefficiently. HIV-1 has evolved multiple mechanisms to evade readily elicited antibodies against more open Env conformations. Small-molecule CD4-mimetic compounds (CD4mc) bind the HIV-1 gp120 Env and promote conformational changes similar to those induced by CD4, exposing conserved Env elements to antibodies. Here, we show that a CD4mc synergizes with antibodies elicited by monomeric HIV-1 gp120 to protect monkeys from multiple high-dose intrarectal challenges with a heterologous simian-human immunodeficiency virus (SHIV). The protective immune response persists for at least six months after vaccination. CD4mc should increase the protective efficacy of any HIV-1 Env vaccine that elicits antibodies against CD4-induced conformations of Env.

The HIV Env trimer exhibits a closed confirmation and restricts access to known antibody binding sites. Here the authors show that a small-molecule CD4-mimetic compound binds the HIV Env trimer and enhances antibody-mediated protection in a non-human primate model of infection.

Trilobatin as an HIV-1 entry inhibitor targeting the HIV-1 Gp41 envelope

HIV-1 transmembrane protein gp41 plays a crucial role by forming a stable six-helix bundle during HIV entry. Due to highly conserved sequence of gp41, the development of an effective and safe small-molecule compound targeting gp41 is a good choice. Currently, natural polyanionic ingredients with anti-HIV activities have aroused concern. Here, we first discovered that a glycosylated dihydrochalcone, trilobatin, exhibited broad anti-HIV-1 activity and low cytotoxicity in vitro. Site-directed mutagenesis analysis suggested that the hydrophobic residue (I564) located in gp41 pocket-forming site is pivotal for anti-HIV activity of trilobatin. Furthermore, trilobatin displayed synergistic anti-HIV activities combined with other antiretroviral agents. Trilobatin has a good potential to be developed as a small-molecule HIV-1 entry inhibitor for clinical combination therapy.

Comparison of Uncleaved and Mature Human Immunodeficiency Virus Membrane Envelope Glycoprotein Trimers

The mature envelope glycoprotein (Env) spike on the surfaces of human immunodeficiency virus type 1 (HIV-1)-infected cells and virions is derived from proteolytic cleavage of a trimeric gp160 glycoprotein precursor. In these studies, we compared the conformations of cleaved and uncleaved membrane Envs with truncated cytoplasmic tails to those of stabilized soluble gp140 SOSIP.664 Env trimers. Deletion of the gp41 cytoplasmic tail did not significantly affect the sensitivity of viruses with the HIV-1_AD8 Env to inhibition by antibodies or a CD4-mimetic compound. After glutaraldehyde fixation and purification from membranes, a cleaved Env exhibited a hydrodynamic radius of ∼10 nm and an antibody-binding profile largely consistent with that expected based on virus neutralization sensitivity. The purified cleaved Env trimers exhibited a hollow architecture with a central void near the trimer axis. Uncleaved Env, cross-linked and purified in parallel, exhibited a hydrodynamic radius similar to that of the cleaved Env. However, the uncleaved Env was recognized by poorly neutralizing antibodies and appeared by negative-stain electron microscopy to sample multiple conformations. Compared with membrane Envs, stabilized soluble gp140 SOSIP.664 Env trimers appear to be more compact, as reflected in their smaller hydrodynamic radii and negative-stain electron microscopy structures. The antigenic features of the soluble gp140 SOSIP.664 Env trimers differed from those of the cleaved membrane Env, particularly in gp120 V3 and some CD4-binding-site epitopes. Thus, proteolytic maturation allows the membrane-anchored Env to achieve a conformation that retains functional metastability but masks epitopes for poorly neutralizing antibodies.IMPORTANCE The entry of human immunodeficiency virus type 1 (HIV-1) into host cells is mediated by the envelope glycoprotein (Env) spike on the surface of the virus. Host antibodies elicited during natural HIV-1 infection or by vaccination can potentially recognize the Env spike and block HIV-1 infection. However, the changing shape of the HIV-1 Env spike protects the virus from antibody binding. Understanding the shapes of natural and man-made preparations of HIV-1 Envs will assist the development of effective vaccines against the virus. Here, we evaluate the effects of several Env modifications commonly used to produce Env preparations for vaccine studies and the determination of structure. We found that the cleavage of the HIV-1 Env precursor helps Env to assume its natural shape, which resists the binding of many commonly elicited antibodies. Stabilized soluble Envs exhibit more compact shapes but expose some Env elements differently than the natural Env.

Antibody Functional Assays as Measures of Fc Receptor-Mediated Immunity to HIV - New Technologies and their Impact on the HIV Vaccine Field

Background:

There is now intense interest in the role of HIV-specific antibodies and the engagement of FcγR functions in the control and prevention of HIV infection. The analyses of the RV144 vaccine trial, natural progression cohorts, and macaque models all point to a role for Fc-dependent effector functions, such as cytotoxicity (ADCC) or phagocytosis (ADCP), in the control of HIV. However, reliable assays that can be reproducibly used across different laboratories to measure Fc-dependent functions, such as antibody dependent cellular cytotoxicity (ADCC) are limited.

Method:

This brief review highlights the importance of Fc properties for immunity to HIV, particular-ly via FcγR diversity and function. We discuss assays used to study FcR mediated functions of HIV-specific Ab, including our recently developed novel cell-free ELISA using homo-dimeric FcγR ecto-domains to detect functionally relevant viral antigen-specific antibodies.

Results:

The binding of these dimeric FcγR ectodomains, to closely spaced pairs of IgG Fc, mimics the engagement and cross-linking of Fc receptors by IgG opsonized virions or infected cells as the es-sential prerequisite to the induction of Ab-dependent effector functions. The dimeric FcγR ELISA reli-ably correlates with ADCC in patient responses to influenza. The assay is amenable to high throughput and could be standardized across laboratories.

Conclusion:

We propose the assay has broader implications for the evaluation of the quality of anti-body responses in viral infections and for the rapid evaluation of responses in vaccine development campaigns for HIV and other viral infections.

Envelope glycoproteins sampling states 2/3 are susceptible to ADCC by sera from HIV-1-infected individuals

Recent analysis of HIV-1 envelope glycoproteins (Env) dynamics showed that the unliganded Env trimer can potentially sample three conformations: a metastable "closed" conformation (State 1), an "open" CD4-bound conformation (State 3), and an intermediate "partially open" conformation (State 2). HIV-1 evolved several mechanisms to avoid "opening" its Env in order to evade immune responses such as antibody-dependent cellular cytotoxicity (ADCC), which preferentially targets Envs in the CD4-bound conformation on the surface of infected cells. Here we took advantage of a well-characterized single-residue change in the gp120 trimer association domain to modify Env conformation and evaluate its impact on ADCC responses. We found that cells infected with viruses expressing Env stabilized in States 2/3 become highly susceptible to ADCC responses by sera from HIV-1-infected individuals. Our results indicate that the conformations spontaneously sampled by the Env trimer at the surface of infected cells has a significant impact on ADCC responses.

Impact of HIV-1 Envelope Conformation on ADCC Responses

HIV-1 envelope glycoproteins (Env) represent the only virus-specific antigen exposed at the surface of infected cells. In its unliganded form, Env from primary viruses samples a 'closed' conformation (State 1), which is preferentially recognized by broadly neutralizing antibodies (bNAbs). CD4 engagement drives Env into an intermediate 'partially open' (State 2) and then into the 'open' CD4-bound conformation (State 3). Emerging evidence suggests a link between Env conformation and Ab-dependent cellular cytotoxicity (ADCC). HIV-1-infected cells exposing Env in the CD4-bound conformation are susceptible to ADCC mediated by CD4-induced Abs and HIV+sera. Cells exposing State 1 Env are susceptible to ADCC mediated by bNAbs. Here, we discuss how Env conformation affects ADCC responses and in vitro measurements.

Unlocking HIV-1 Env: implications for antibody attack

Collective evidence supporting a role of Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC) in controlling HIV-1 transmission and disease progression emerged in the last few years. Non-neutralizing antibodies (nnAbs) recognizing conserved CD4-induced epitopes on Env and able to mediate potent ADCC against HIV-1-infected cells exposing Env in its CD4-bound conformation have been shown to be present in some RV144 vaccinees and most HIV-1-infected individuals. HIV-1 evolved sophisticated strategies to decrease exposure of this Env conformation by downregulating CD4 and by limiting the overall amount of cell-surface Env. In this review, we will summarize our contribution to this rapidly evolving field, discuss how structural properties of HIV-1 Env might have contributed to the modest efficacy of the RV144 trial and how we recently used this knowledge to develop new strategies aimed at sensitizing HIV-1-infected cells to ADCC mediated by easy to elicit nnAbs.

Short Communication: Small-Molecule CD4 Mimetics Sensitize HIV-1-Infected Cells to Antibody-Dependent Cellular Cytotoxicity by Antibodies Elicited by Multiple Envelope Glycoprotein Immunogens in Nonhuman Primates

Recent studies have linked antibody Fc-mediated effector functions with control of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus infections. Interestingly, the presence of antibodies with potent antibody-dependent cellular cytotoxicity (ADCC) activity in RV144 vaccine trial participants correlated inversely with HIV-1 acquisition risk. These antibodies were recently found to recognize epitopes on the HIV-1 envelope (Env) glycoprotein exposed upon Env-CD4 binding. Accordingly, small-molecule CD4 mimetics (CD4mc) that induce Env to sample the CD4-bound conformation were shown to sensitize HIV-1-infected cells to ADCC mediated by sera from HIV-1-infected individuals. However, it remains unknown whether antibodies elicited through immunization can also mediate CD4mc-induced ADCC. In this study, we tested the capacity of CD4mc to sensitize HIV-1-infected cells to ADCC by sera from Env-vaccinated nonhuman primates using a FACS-based ADCC assay. In parallel, we evaluated the ability of CD4mc to sensitize HIV-1 viral particles to neutralization by sera from these immunized animals. We found that the vaccine-induced antibodies were able to mediate ADCC and viral neutralization in the presence, but not the absence, of CD4mc. Thus, CD4mc are capable of sensitizing HIV-1-infected cells to ADCC and infectious viral particles to neutralization by easy-to-elicit antibodies that are otherwise unable to mediate these activities.

Driving HIV-1 into a Vulnerable Corner by Taking Advantage of Viral Adaptation and Evolution

Anti-retroviral therapy (ART) is crucial for controlling human immunodeficiency virus type-1 (HIV-1) infection. Recently, progress in identifying and characterizing highly potent broadly neutralizing antibodies has provided valuable templates for HIV-1 therapy and vaccine design. Nevertheless, HIV-1, like many RNA viruses, exhibits genetically diverse populations known as quasispecies. Evolution of quasispecies can occur rapidly in response to selective pressures, such as that exerted by ART and the immune system. Hence, rapid viral evolution leading to drug resistance and/or immune evasion is a significant barrier to the development of effective HIV-1 treatments and vaccines. Here, we describe our recent investigations into evolutionary pressure exerted by anti-retroviral drugs and monoclonal neutralizing antibodies (NAbs) on HIV-1 envelope sequences. We also discuss sensitivities of HIV-1 escape mutants to maraviroc, a CCR5 inhibitor, and HIV-1 sensitized to NAbs by small-molecule CD4-mimetic compounds. These studies help to develop an understanding of viral evolution and escape from both anti-retroviral drugs and the immune system, and also provide fundamental insights into the combined use of NAbs and entry inhibitors. These findings of the adaptation and evolution of HIV in response to drug and immune pressure will inform the development of more effective antiviral therapeutic strategies.

Evaluation of the contribution of the transmembrane region to the ectodomain conformation of the human immunodeficiency virus (HIV-1) envelope glycoprotein

Background

The human immunodeficiency virus (HIV-1) envelope glycoprotein (Env), a Type 1 transmembrane protein, assembles into a trimeric spike complex that mediates virus entry into host cells. The high potential energy of the metastable, unliganded Env trimer is maintained by multiple non-covalent contacts among the gp120 exterior and gp41 transmembrane Env subunits. Structural studies suggest that the gp41 transmembrane region forms a left-handed coiled coil that contributes to the Env trimer interprotomer contacts. Here we evaluate the contribution of the gp41 transmembrane region to the folding and stability of Env trimers.

Methods

Multiple polar/charged amino acid residues, which hypothetically disrupt the stop-transfer signal, were introduced in the proposed lipid-interactive face of the transmembrane coiled coil, allowing release of soluble cleavage-negative Envs containing the modified transmembrane region (TM_mod). We also examined effects of cleavage, the cytoplasmic tail and a C-terminal fibritin trimerization (FT) motif on oligomerization, antigenicity and functionality of soluble and membrane-bound Envs.

Results

The introduction of polar/charged amino acids into the transmembrane region resulted in the secretion of soluble Envs from the cell. However, these TM_mod Envs primarily formed dimers. By contrast, control cleavage-negative sgp140 Envs lacking the transmembrane region formed soluble trimers, dimers and monomers. TM_mod and sgp140 trimers were stabilized by the addition of a C-terminal FT sequence, but still exhibited carbohydrate and antigenic signatures of a flexible ectodomain structure. On the other hand, detergent-solubilized cleaved and uncleaved Envs isolated from the membranes of expressing cells exhibited "tighter” ectodomain structures, based on carbohydrate modifications. These trimers were found to be unstable in detergent solutions, but could be stabilized by the addition of a C-terminal FT moiety. The C-terminal FT domain decreased Env cleavage and syncytium-forming ability by approximately three-fold; alteration of the FT trimerization interface restored Env cleavage and syncytium formation to near-wild-type levels.

Conclusion

The modified transmembrane region was not conducive to trimerization of soluble Envs. However, for HIV-1 Env ectodomains that are minimally modified, membrane-anchored Envs exhibit the most native structures and can be stabilized by appropriately positioned FT domains.

Activation and Inactivation of Primary Human Immunodeficiency Virus Envelope Glycoprotein Trimers by CD4-Mimetic Compounds

Human immunodeficiency virus type 1 (HIV-1) entry into cells is mediated by the viral envelope glycoproteins (Env), a trimer of three gp120 exterior glycoproteins, and three gp41 transmembrane glycoproteins. The metastable Env is triggered to undergo entry-related conformational changes when gp120 binds sequentially to the receptors, CD4 and CCR5, on the target cell. Small-molecule CD4-mimetic compounds (CD4mc) bind gp120 and act as competitive inhibitors of gp120-CD4 engagement. Some CD4mc have been shown to trigger Env prematurely, initially activating Env function, followed by rapid and irreversible inactivation. Here, we study CD4mc with a wide range of anti-HIV-1 potencies and demonstrate that all tested CD4mc are capable of activating as well as inactivating Env function. Biphasic dose-response curves indicated that the occupancy of the protomers in the Env trimer governs viral activation versus inactivation. One CD4mc bound per Env trimer activated HIV-1 infection. Envs with two CD4mc bound were activated for infection of CD4-negative, CCR5-positive cells, but the infection of CD4-positive, CCR5-positive cells was inhibited. Virus was inactivated when all three Env protomers were occupied by the CD4mc, and gp120 shedding from the Env trimer was increased in the presence of some CD4mc. Env reactivity and the on rates of CD4mc binding to the Env trimer were found to be important determinants of the potency of activation and entry inhibition. Cross-sensitization of Env protomers that do not bind the CD4mc to neutralization by an anti-V3 antibody was not evident. These insights into the mechanism of antiviral activity of CD4mc should assist efforts to optimize their potency and utility.

IMPORTANCE

The trimeric envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) mediate virus entry into host cells. Binding to the host cell receptors, CD4 and CCR5, triggers changes in the conformation of the HIV-1 envelope glycoprotein trimer important for virus entry. Small-molecule CD4-mimetic compounds inhibit HIV-1 infection by multiple mechanisms: (i) direct blockade of the interaction between the gp120 exterior envelope glycoprotein and CD4; (ii) premature triggering of conformational changes in the envelope glycoproteins, leading to irreversible inactivation; and (iii) exposure of cryptic epitopes to antibodies, allowing virus neutralization. The consequences of the binding of the CD4-mimetic compound to the HIV-1 envelope glycoproteins depends upon how many of the three subunits of the trimer are bound and upon the propensity of the envelope glycoproteins to undergo conformational changes. Understanding the mechanistic factors that influence the activity of CD4-mimetic compounds can help to improve their potency and coverage of diverse HIV-1 strains.

Humoral Antibody Responses to HIV Viral Proteins and to CD4 Among HIV Controllers, Rapid and Typical Progressors in an HIV-Positive Patient Cohort

The purpose of this study was to assess humoral antibody responses as a function of disease progression (DP) in a well-defined HIV⁺ cohort. We quantified antibodies to HIV-1 gp120, Gag, and CD4 receptor by enzyme-linked immunosorbent assay in sera from a cohort of 97 HIV⁺ subjects at defined stages of DP. We also measured antibody-dependent cellular cytotoxicity (ADCC) as a function of the clinical status of the patients. We purified antibodies to CD4 and gp120 and assessed them for specificity, ability to block gp120 binding to target cells, ability to block virus infection, and ability to facilitate ADCC. All of the HIV⁺ patient samples were positive for antibodies to HIV gp120 and p24 and 80% showed evidence of hypergammaglobulinemia. Approximately 10% of cohort members were positive for antibodies to CD4, but we noted no significant correlation relevant to DP. There were statistically significant differences between the groups concerning the level of humoral response to gp120 and Gag. However, we observed no distinction in ability of anti-gp120 antibodies purified from each group to neutralize infection. In addition, there was a statistically significant difference in ADCC, with elite controllers exhibiting significantly lower levels of ADCC than the other five groups. We detected IgA anti-gp120 antibodies, but did not correlate their presence with either DP or ADCC levels. The results are consistent with the interpretation that the humoral antibody response to the antigens assessed here represents a signature of the level of viremia but does not correlate with clinical status of HIV infection.

Patterns of HIV/SIV Prevention and Control by Passive Antibody Immunization

Neutralizing antibody (NAb) responses are promising immune effectors for control of human immunodeficiency virus (HIV) infection. Protective activity and mechanisms of immunodeficiency virus-specific NAbs have been increasingly scrutinized in animals infected with simian immunodeficiency virus (SIV), chimeric simian/human immunodeficiency virus (SHIV) and related viruses. Studies on such models have unraveled a previously underscored protective potential against in vivo immunodeficiency virus replication. Pre-challenge NAb titers feasibly provide sterile protection from SIV/SHIV infection by purging the earliest onset of viral replication and likely modulate innate immune cell responses. Sufficient sub-sterile NAb titers after established infection also confer dose-dependent reduction of viremia, and in certain earlier time frames augment adaptive immune cell responses and even provide rebound-free viral control. Here, we provide an overview of the obtained patterns of SIV/SHIV protection and viral control by various types of NAb passive immunizations and discuss how these notions may be extrapolated to NAb-based clinical control of HIV infection.