Antibody-DEPENDENT, FcγRI-mediated neutralization of HIV-1 in TZM-bl cells occurs independently of phagocytosis

HIV-1 prehairpin intermediate inhibitors show efficacy independent of neutralization tier

HIV-1 strains are categorized into one of three neutralization tiers based on the relative ease by which they are neutralized by plasma from HIV-1-infected donors not on antiretroviral therapy; tier-1 strains are particularly sensitive to neutralization while tier-2 and tier-3 strains are increasingly difficult to neutralize. Most broadly neutralizing antibodies (bnAbs) previously described target the native prefusion conformation of HIV-1 Envelope (Env), but the relevance of the tiered categories for inhibitors targeting another Env conformation, the prehairpin intermediate, is not well understood. Here, we show that two inhibitors targeting distinct highly conserved regions of the prehairpin intermediate have strikingly consistent neutralization potencies (within ~100-fold for a given inhibitor) against strains in all three neutralization tiers of HIV-1; in contrast, best-in-class bnAbs targeting diverse Env epitopes vary by more than 10,000-fold in potency against these strains. Our results indicate that antisera-based HIV-1 neutralization tiers are not relevant for inhibitors targeting the prehairpin intermediate and highlight the potential for therapies and vaccine efforts targeting this conformation.

Enhancing HIV-1 Neutralization by Increasing the Local Concentration of Membrane-Proximal External Region-Directed Broadly Neutralizing Antibodies

Broadly neutralizing antibodies (bNAbs) against the membrane-proximal external region (MPER) of the gp41 component of the human immunodeficiency virus type 1 (HIV-1) envelope (Env) are characterized by long, hydrophobic, heavy chain complementarity-determining region 3s (HCDR3s) that interact with the MPER and some viral membrane lipids to achieve increased local concentrations. Here, we show that increasing the local concentration of MPER-directed bNAbs at the cell surface via binding to the high-affinity Fc receptor FcγRI potentiates their ability to prevent viral entry in a manner analogous to the previously reported observation wherein the lipid-binding activity of MPER bNAbs increases their concentration at the viral surface membrane. However, binding of MPER-directed bNAb 10E8 to FcγRI abolishes the neutralization synergy that is seen with the N-heptad repeat (NHR)-targeting antibody D5_AR and NHR-targeting small molecule enfuvirtide (T20), possibly due to decreased accessibility of the NHR in the FcγRI-10E8-MPER complex. Taken together, our results suggest that lipid-binding activity and FcγRI-mediated potentiation function in concert to improve the potency of MPER-directed bNAbs by increasing their local concentration near the site of viral fusion. Therefore, lipid binding may not be a strict requirement for potent neutralization by MPER-targeting bNAbs, as alternative methods can achieve similar increases in local concentrations while avoiding potential liabilities associated with immunologic host tolerance. IMPORTANCE The trimeric glycoprotein Env, the only viral protein expressed on the surface of HIV-1, is the target of broadly neutralizing antibodies and the focus of most vaccine development efforts. Broadly neutralizing antibodies targeting the membrane proximal external region (MPER) of Env show lipid-binding characteristics, and modulating this interaction affects neutralization. In this study, we tested the neutralization potencies of variants of the MPER-targeting antibody 10E8 with different viral-membrane-binding and host FcγRI-binding capabilities. Our results suggest that binding to both lipid and FcγRI improves the neutralization potency of MPER-directed antibodies by concentrating the antibodies at sites of viral fusion. As such, lipid binding may not be uniquely required for MPER-targeting broadly neutralizing antibodies, as alternative methods to increase local concentration can achieve similar improvements in potency.

A Holistic Review of the Preclinical Landscape for Long-Acting Anti-infective Drugs Using HIV as a Paradigm

Lack of predictive preclinical models is a key contributor to the steep attrition rate in drug development. Successful clinical translation may be higher for new chemical entities or existing approved drugs reformulated for long-acting (LA) administration if preclinical studies designed to identify any new uncertainties are predictive of human exposure and response. In this review, we present an overview of standard preclinical assessments deployed for LA formulations and delivery systems, using human immunodeficiency virus LA therapeutics preclinical development as a paradigm. Key progress in the preclinical development of novel LA antiretrovirals formulations and delivery systems are summarized, including bispecific broadly neutralizing monoclonal antibody and small molecule technologies for codelivery of multiple drugs with disparate solubility properties. There are new opportunities to take advantage of recent developments in tissue engineering and 3-dimensional in vitro modeling to advance preclinical modeling of anti-infective activity, developmental and reproductive toxicity assessment, and to apply quantitative modeling and simulation strategies. These developments are likely to drive the progression of more LA anti-infective drugs and multipurpose technologies into clinical development in the coming years.

A Derivative of the D5 Monoclonal Antibody That Targets the gp41 N-Heptad Repeat of HIV-1 with Broad Tier-2-Neutralizing Activity

HIV-1 infection is initiated by the viral glycoprotein Env, which, after interaction with cellular coreceptors, adopts a transient conformation known as the prehairpin intermediate (PHI). The N-heptad repeat (NHR) is a highly conserved region of gp41 exposed in the PHI; it is the target of the FDA-approved drug enfuvirtide and of neutralizing monoclonal antibodies (mAbs). However, to date, these mAbs have only been weakly effective against tier-1 HIV-1 strains, which are most sensitive to neutralizing antibodies. Here, we engineered and tested 11 IgG variants of D5, an anti-NHR mAb, by recombining previously described mutations in four of D5's six antibody complementarity-determining regions. One variant, D5_AR, demonstrated 6-fold enhancement in the 50% inhibitory dose (ID50) against lentivirus pseudotyped with HXB2 Env. D5_AR exhibited weak cross-clade neutralizing activity against a diverse set of tier-2 HIV-1 viruses, which are less sensitive to neutralizing antibodies than tier-1 viruses and are the target of current antibody-based vaccine efforts. In addition, the neutralization potency of D5_AR IgG was greatly enhanced in target cells expressing FcγRI, with ID50 values of <0.1 μg/ml; this immunoglobulin receptor is expressed on macrophages and dendritic cells, which are implicated in the early stages of HIV-1 infection of mucosal surfaces. D5 and D5_AR have equivalent neutralization potency in IgG, Fab, and single-chain variable-fragment (scFv) formats, indicating that neutralization is not impacted by steric hindrance. Taken together, these results provide support for vaccine strategies that target the PHI by eliciting antibodies against the gp41 NHR and support investigation of anti-NHR mAbs in nonhuman primate passive immunization studies. IMPORTANCE Despite advances in antiretroviral therapy, HIV remains a global epidemic and has claimed more than 32 million lives. Accordingly, developing an effective HIV vaccine remains an urgent public health need. The gp41 N-heptad repeat (NHR) of the HIV-1 prehairpin intermediate (PHI) is highly conserved (>90%) and is inhibited by the FDA-approved drug enfuvirtide, making it an attractive vaccine target. However, to date, anti-NHR antibodies have not been potent. Here, we engineered D5_AR, a more potent variant of the anti-NHR antibody D5, and established its ability to inhibit HIV-1 strains that are more difficult to neutralize and are more representative of circulating strains (tier-2 strains). The neutralizing activity of D5_AR was greatly potentiated in cells expressing FcγRI; FcγRI is expressed on cells that are implicated at the earliest stages of sexual HIV-1 transmission. Taken together, these results bolster efforts to target the gp41 NHR and the PHI for vaccine development.

Antibody-drug conjugates with dual payloads for combating breast tumor heterogeneity and drug resistance

Breast tumors generally consist of a diverse population of cells with varying gene expression profiles. Breast tumor heterogeneity is a major factor contributing to drug resistance, recurrence, and metastasis after chemotherapy. Antibody-drug conjugates (ADCs) are emerging chemotherapeutic agents with striking clinical success, including T-DM1 for HER2-positive breast cancer. However, these ADCs often suffer from issues associated with intratumor heterogeneity. Here, we show that homogeneous ADCs containing two distinct payloads are a promising drug class for addressing this clinical challenge. Our conjugates show HER2-specific cell killing potency, desirable pharmacokinetic profiles, minimal inflammatory response, and marginal toxicity at therapeutic doses. Notably, a dual-drug ADC exerts greater treatment effect and survival benefit than does co-administration of two single-drug variants in xenograft mouse models representing intratumor HER2 heterogeneity and elevated drug resistance. Our findings highlight the therapeutic potential of the dual-drug ADC format for treating refractory breast cancer and perhaps other cancers.

The high-affinity immunoglobulin receptor FcγRI potentiates HIV-1 neutralization via antibodies against the gp41 N-heptad repeat

Despite decades of research, an effective HIV-1 vaccine remains elusive. One potential vaccine target is the N-heptad repeat (NHR) region of gp41, which is the target of the FDA-approved drug enfuvirtide. However, monoclonal antibodies and antisera targeting this region have only been modestly neutralizing to date. Here, we show that the neutralization potency of the well-characterized anti-NHR antibody D5 is increased >5,000-fold by expression of FcγRI (CD64) on cells. Since FcγRI is expressed on macrophages and dendritic cells, which are implicated in the early establishment of HIV-1 infection following sexual transmission, these results may be important to HIV-1 vaccine development.

The HIV-1 gp41 N-heptad repeat (NHR) region of the prehairpin intermediate, which is transiently exposed during HIV-1 viral membrane fusion, is a validated clinical target in humans and is inhibited by the Food and Drug Administration (FDA)-approved drug enfuvirtide. However, vaccine candidates targeting the NHR have yielded only modest neutralization activities in animals; this inhibition has been largely restricted to tier-1 viruses, which are most sensitive to neutralization by sera from HIV-1–infected individuals. Here, we show that the neutralization activity of the well-characterized NHR-targeting antibody D5 is potentiated >5,000-fold in TZM-bl cells expressing FcγRI compared with those without, resulting in neutralization of many tier-2 viruses (which are less susceptible to neutralization by sera from HIV-1–infected individuals and are the target of current antibody-based vaccine efforts). Further, antisera from guinea pigs immunized with the NHR-based vaccine candidate (ccIZN36)₃ neutralized tier-2 viruses from multiple clades in an FcγRI-dependent manner. As FcγRI is expressed on macrophages and dendritic cells, which are present at mucosal surfaces and are implicated in the early establishment of HIV-1 infection following sexual transmission, these results may be important in the development of a prophylactic HIV-1 vaccine.

Neutralizing Antibodies Targeting HIV-1 gp41

HIV-1 vaccine research has obtained an enormous boost since the discovery of many broadly neutralizing antibodies (bnAbs) targeting all accessible sites on the HIV-1 envelope glycoprotein (Env). This in turn facilitated high-resolution structures of the Env glycoprotein in complex with bnAbs. Here we focus on gp41, its highly conserved heptad repeat region 1 (HR1), the fusion peptide (FP) and the membrane-proximal external region (MPER). Notably, the broadest neutralizing antibodies target MPER. Both gp41 HR1 and MPER are only fully accessible once receptor-induced conformational changes have taken place, although some studies suggest access to MPER in the close to native Env conformation. We summarize the data on the structure and function of neutralizing antibodies targeting gp41 HR1, FP and MPER and we review their access to Env and their complex formation with gp41 HR1, MPER peptides and FP within native Env. We further discuss MPER bnAb binding to lipids and the role of somatic mutations in recognizing a bipartite epitope composed of the conserved MPER sequence and membrane components. The problematic of gp41 HR1 access and MPER bnAb auto- and polyreactivity is developed in the light of inducing such antibodies by vaccination.

Rare Detection of Antiviral Functions of Polyclonal IgA Isolated from Plasma and Breast Milk Compartments in Women Chronically Infected with HIV-1

The humoral response to invading mucosal pathogens comprises multiple antibody isotypes derived from systemic and mucosal compartments. To understand the contribution of each antibody isotype/source to the mucosal humoral response, parallel investigation of the specificities and functions of antibodies within and across isotypes and compartments is required. The role of IgA against HIV-1 is complex, with studies supporting a protective role as well as a role for serum IgA in blocking effector functions. Thus, we explored the fine specificity and function of IgA in both plasma and mucosal secretions important to infant HIV-1 infection, i.e., breast milk. IgA and IgG were isolated from milk and plasma from 20 HIV-1-infected lactating Malawian women. HIV-1 binding specificities, neutralization potency, inhibition of virus-epithelial cell binding, and antibody-mediated phagocytosis were measured. Fine-specificity mapping showed IgA and IgG responses to multiple HIV-1 Env epitopes, including conformational V1/V2 and linear V2, V3, and constant region 5 (C5). Env IgA was heterogeneous between the milk and systemic compartments (Env IgA, τ = 0.00 to 0.63, P = 0.0046 to 1.00). Furthermore, IgA and IgG appeared compartmentalized as there was a lack of correlation between the specificities of Env-specific IgA and IgG (in milk, τ = -0.07 to 0.26, P = 0.35 to 0.83). IgA and IgG also differed in functions: while neutralization and phagocytosis were consistently mediated by milk and plasma IgG, they were rarely detected in IgA from both milk and plasma. Understanding the ontogeny of the divergent IgG and IgA antigen specificity repertoires and their effects on antibody function will inform vaccination approaches targeted toward mucosal pathogens.IMPORTANCE Antibodies within the mucosa are part of the first line of defense against mucosal pathogens. Evaluating mucosal antibody isotypes, specificities, and antiviral functions in relationship to the systemic antibody profile can provide insights into whether the antibody response is coordinated in response to mucosal pathogens. In a natural immunity cohort of HIV-infected lactating women, we mapped the fine specificity and function of IgA in breast milk and plasma and compared these with the autologous IgG responses. Antigen specificities and functions differed between IgG and IgA, with antiviral functions (neutralization and phagocytosis) predominantly mediated by the IgG fraction in both milk and plasma. Furthermore, the specificity of milk IgA differed from that of systemic IgA. Our data suggest that milk IgA and systemic IgA should be separately examined as potential correlates of risk. Preventive vaccines may need to employ different strategies to elicit functional antiviral immunity by both antibody isotypes in the mucosa.

Neutralizing Antibodies Against a Specific Human Immunodeficiency Virus gp41 Epitope are Associated With Long-term Non-progressor Status

Antibodies (Abs) play a central role in human immunodeficiency virus (HIV) protection due to their multiple functional inhibitory activities. W614A-3S Abs recognize a specific form of a highly conserved motif of the gp41 envelope protein and can elicit viral neutralization to protect CD4+ T cells. Here, we describe in detail the neutralizing profile of W614A-3S Abs in untreated long-term non-progressor (LTNP) HIV-infected patients. W614A-3S Abs were detected in 23.5% (16/68) of untreated LTNP patients compared with <5% (5/104) of HIV-1 progressor patients. The W614A-3S Abs had efficient neutralizing activity that inhibited transmitted founder primary viruses and exhibited Fc-mediated inhibitory functions at low concentrations in primary monocyte-derived macrophages. The neutralizing capacity of W614A-3S Abs was inversely correlated with viral load (r=-0.9013; p<0.0001), viral DNA (r=-0.7696; p=0.0005) and was associated the preservation of high CD4+ T-cell counts and T-cell responses. This study demonstrates that W614A-3S neutralizing Abs may confer a crucial advantage to LTNP patients. These results provide insights for both pathophysiological research and the development of vaccine strategies.

Deletion of Nlrp3 Augments Survival during Polymicrobial Sepsis by Decreasing Autophagy and Enhancing Phagocytosis

NLRP3 inflammasome is a critical player in innate immunity. Neutrophil recruitment to tissues and effective neutrophil function are critical innate immune mechanisms for bacterial clearance. However, the role of NLRP3 in neutrophil-dependent bacterial clearance in polymicrobial sepsis is unclear. In this study, we evaluated the role of NLRP3 in polymicrobial sepsis induced by cecal ligation and puncture (CLP). Our results showed protection from death in NLRP3-deficient (Nlrp3^-/-) and NLRP3 inhibitor-treated wild-type (C57BL/6) mice. Nlrp3^-/- and NLRP3 inhibitor-treated mice displayed lower bacterial load but no impairment in neutrophil recruitment to peritoneum. However, neutrophil depletion abrogated protection from death in Nlrp3^-/- mice in response to CLP. Intriguingly, following CLP, Nlrp3^-/- peritoneal cells (primarily neutrophils) demonstrate decreased autophagy, augmented phagocytosis, and enhanced scavenger receptor (macrophage receptor with collagenous structure) and mannose-binding leptin expression. These findings enhance our understanding of the critical role of NLRP3 in modulating autophagy and phagocytosis in neutrophils and suggest that therapies should be targeted to modulate autophagy and phagocytosis in neutrophils to control bacterial burden in tissues during CLP-induced polymicrobial sepsis.

Broadly Neutralizing Antibodies Display Potential for Prevention of HIV-1 Infection of Mucosal Tissue Superior to That of Nonneutralizing Antibodies

Definition of the key parameters mediating effective antibody blocking of HIV-1 acquisition within mucosal tissue may prove critical to effective vaccine development and the prophylactic use of monoclonal antibodies. Although direct antibody-mediated neutralization is highly effective against cell-free virus, antibodies targeting different sites of envelope vulnerability may display differential activity against mucosal infection. Nonneutralizing antibodies (nnAbs) may also impact mucosal transmission events through Fc-gamma receptor (FcγR)-mediated inhibition. In this study, a panel of broadly neutralizing antibodies (bnAbs) and nnAbs, including those associated with protection in the RV144 vaccine trial, were screened for the ability to block HIV-1 acquisition and replication across a range of cellular and mucosal tissue models. Neutralization potency, as determined by the TZM-bl infection assay, did not fully predict activity in mucosal tissue. CD4-binding site (CD4bs)-specific bnAbs, in particular VRC01, were consistent in blocking HIV-1 infection across all cellular and tissue models. Membrane-proximal external region (MPER) (2F5) and outer domain glycan (2G12) bnAbs were also efficient in preventing infection of mucosal tissues, while the protective efficacy of bnAbs targeting V1-V2 glycans (PG9 and PG16) was more variable. In contrast, nnAbs alone and in combinations, while active in a range of cellular assays, were poorly protective against HIV-1 infection of mucosal tissues. These data suggest that tissue resident effector cell numbers and low FcγR expression may limit the potential of nnAbs to prevent establishment of the initial foci of infection. The solid protection provided by specific bnAbs clearly demonstrates their superior potential over that of nonneutralizing antibodies for preventing HIV-1 infection at the mucosal portals of infection.

IMPORTANCE Key parameters mediating effective antibody blocking of HIV-1 acquisition within mucosal tissue have not been defined. While bnAbs are highly effective against cell-free virus, they are not induced by current vaccine candidates. However, nnAbs, readily induced by vaccines, can trigger antibody-dependent cellular effector functions, through engagement of their Fc-gamma receptors. Fc-mediated antiviral activity has been implicated as a secondary correlate of decreased HIV-1 risk in the RV144 vaccine efficacy trial, suggesting that protection might be mediated in the absence of classical neutralization. To aid vaccine design and selection of antibodies for use in passive protection strategies, we assessed a range of bnAbs and nnAbs for their potential to block ex vivo challenge of mucosal tissues. Our data clearly indicate the superior efficacy of neutralizing antibodies in preventing mucosal acquisition of infection. These results underscore the importance of maintaining the central focus of HIV-1 vaccine research on the induction of potently neutralizing antibodies.

Engineering broadly neutralizing antibodies for HIV prevention and therapy

A combination of advances spanning from isolation to delivery of potent HIV-specific antibodies has begun to revolutionize understandings of antibody-mediated antiviral activity. As a result, the set of broadly neutralizing and highly protective antibodies has grown in number, diversity, potency, and breadth of viral recognition and neutralization. These antibodies are now being further enhanced by rational engineering of their anti-HIV activities and coupled to cutting edge gene delivery and strategies to optimize their pharmacokinetics and biodistribution. As a result, the prospects for clinical use of HIV-specific antibodies to treat, clear, and prevent HIV infection are gaining momentum. Here we discuss the diverse methods whereby antibodies are being optimized for neutralization potency and breadth, biodistribution, pharmacokinetics, and effector function with the aim of revolutionizing HIV treatment and prevention options.

Broadly neutralizing anti-HIV-1 antibodies require Fc effector functions for in vivo activity

Broadly neutralizing antibodies (bNAbs) against HIV-1 provide both effective pre-exposure prophylaxis and treatment of HIV-1 infection in murine and nonhuman primate models, suggesting their potential use in humans. Although much is known about the role of variable domains in the neutralization breadth and potency of these bNAbs, the contribution of Fc domains to their activities is, by contrast, poorly characterized. Assessment of the in vivo activity of several bNAbs revealed that FcγR-mediated effector function contributes substantially to their capacity to block viral entry, suppress viremia, and confer therapeutic activity. Enhanced in vivo potency of anti-HIV-1 bNAbs was associated with preferential engagement of activating, but not inhibitory FcγRs, and Fc domain-engineered bNAb variants with selective binding capacity for activating FcγRs displayed augmented protective activity. These findings reveal key roles for Fc effector function in the in vivo activity of anti-HIV-1 bNAbs and provide strategies for generating bNAbs with improved efficacy.

Antibody to gp41 MPER alters functional properties of HIV-1 Env without complete neutralization

Human antibody 10E8 targets the conserved membrane proximal external region (MPER) of envelope glycoprotein (Env) subunit gp41 and neutralizes HIV-1 with exceptional potency. Remarkably, HIV-1 containing mutations that reportedly knockout 10E8 binding to linear MPER peptides are partially neutralized by 10E8, producing a local plateau in the dose response curve. Here, we found that virus partially neutralized by 10E8 becomes significantly less neutralization sensitive to various MPER antibodies and to soluble CD4 while becoming significantly more sensitive to antibodies and fusion inhibitors against the heptad repeats of gp41. Thus, 10E8 modulates sensitivity of Env to ligands both pre- and post-receptor engagement without complete neutralization. Partial neutralization by 10E8 was influenced at least in part by perturbing Env glycosylation. With unliganded Env, 10E8 bound with lower apparent affinity and lower subunit occupancy to MPER mutant compared to wild type trimers. However, 10E8 decreased functional stability of wild type Env while it had an opposite, stabilizing effect on MPER mutant Envs. Clade C isolates with natural MPER polymorphisms also showed partial neutralization by 10E8 with altered sensitivity to various gp41-targeted ligands. Our findings suggest a novel mechanism of virus neutralization by demonstrating how antibody binding to the base of a trimeric spike cross talks with adjacent subunits to modulate Env structure and function. The ability of an antibody to stabilize, destabilize, partially neutralize as well as alter neutralization sensitivity of a virion spike pre- and post-receptor engagement may have implications for immunotherapy and vaccine design.

Which Antibody Functions are Important for an HIV Vaccine?

HIV antibody (Ab) functions capable of preventing mucosal cell-free or cell-to-cell HIV transmission are critical for the development of effective prophylactic and therapeutic vaccines. In addition to CD4(+) T cells, other potential HIV-target cell types including antigen-presenting cells (APCs) (dendritic cells, macrophages) residing at mucosal sites are infected. Moreover, the interactions between APCs and HIV lead to HIV cell-to-cell transmission. Recently discovered broadly neutralizing antibodies (NAbs) are able to neutralize a broad spectrum of HIV strains, inhibit cell-to-cell transfer, and efficiently protect from infection in the experimentally challenged macaque model. However, the 31% protection observed in the RV144 vaccine trial in the absence of detectable NAbs in blood samples pointed to the possible role of additional Ab inhibitory functions. Increasing evidence suggests that IgG Fcγ receptor (FcγR)-mediated inhibition of Abs present at the mucosal site may play a role in protection against HIV mucosal transmission. Moreover, mucosal IgA Abs may be determinant in protection against HIV sexual transmission. Therefore, defining Ab inhibitory functions that could lead to protection is critical for further HIV vaccine design. Here, we review different inhibitory properties of HIV-specific Abs and discuss their potential role in protection against HIV sexual transmission.

Transglutaminase-based chemo-enzymatic conjugation approach yields homogeneous antibody-drug conjugates

Most chemical techniques used to produce antibody-drug conjugates (ADCs) result in a heterogeneous mixture of species with variable drug-to-antibody ratios (DAR) which will potentially display different pharmacokinetics, stability, and safety profiles. Here we investigated two strategies to obtain homogeneous ADCs based on site-specific modification of deglycosylated antibodies by microbial transglutaminase (MTGase), which forms isopeptidic bonds between Gln and Lys residues. We have previously shown that MTGase solely recognizes Gln295 within the heavy chain of IgGs as a substrate and can therefore be exploited to generate ADCs with an exact DAR of 2. The first strategy included the direct, one-step attachment of the antimitotic toxin monomethyl auristatin E (MMAE) to the antibody via different spacer entities with a primary amine functionality that is recognized as a substrate by MTGase. The second strategy was a chemo-enzymatic, two-step approach whereby a reactive spacer entity comprising a bio-orthogonal thiol or azide function was attached to the antibody by MTGase and subsequently reacted with a suitable MMAE-derivative. To this aim, we investigated two different chemical approaches, namely, thiol-maleimide and strain-promoted azide-alkyne cycloaddition (SPAAC). Direct enzymatic attachment of MMAE-spacer derivatives at an 80 molar excess of drug yielded heterogeneous ADCs with a DAR of between 1.0 to 1.6. In contrast to this, the chemo-enzymatic approach only required a 2.5 molar excess of toxin to yield homogeneous ADCs with a DAR of 2.0 in the case of SPAAC and 1.8 for the thiol-maleimide approach. As a proof-of-concept, trastuzumab (Herceptin) was armed with the MMAE via the chemo-enzymatic approach using SPAAC and tested in vitro. Trastuzumab-MMAE efficiently killed BT-474 and SK-BR-3 cells with an IC50 of 89.0 pM and 21.7 pM, respectively. Thus, the chemo-enzymatic approach using MTGase is an elegant strategy to form ADCs with a defined DAR of 2. Furthermore, the approach is directly applicable to a broad variety of antibodies as it does not require prior genetic modifications of the antibody sequence.

Neutralizing IgG at the portal of infection mediates protection against vaginal simian/human immunodeficiency virus challenge

Neutralizing antibodies may have critical importance in immunity against human immunodeficiency virus type 1 (HIV-1) infection. However, the amount of protective antibody needed at mucosal surfaces has not been fully established. Here, we evaluated systemic and mucosal pharmacokinetics (PK) and pharmacodynamics (PD) of 2F5 IgG and 2F5 Fab fragments with respect to protection against vaginal challenge with simian-human immunodeficiency virus-BaL in macaques. Antibody assessment demonstrated that 2F5 IgG was more potent than polymeric forms (IgM and IgA) across a range of cellular and tissue models. Vaginal challenge studies demonstrated a dose-dependent protection for 2F5 IgG and no protection with 2F5 Fab despite higher vaginal Fab levels at the time of challenge. Animals receiving 50 or 25 mg/kg of body weight 2F5 IgG were completely protected, while 3/5 animals receiving 5 mg/kg were protected. In the control animals, infection was established by a minimum of 1 to 4 transmitted/founder (T/F) variants, similar to natural human infection by this mucosal route; in the two infected animals that had received 5 mg 2F5 IgG, infection was established by a single T/F variant. Serum levels of 2F5 IgG were more predictive of sterilizing protection than measured vaginal levels. Fc-mediated antiviral activity did not appear to influence infection of primary target cells in cervical explants. However, PK studies highlighted the importance of the Fc portion in tissue biodistribution. Data presented in this study may be important in modeling serum levels of neutralizing antibodies that need to be achieved by either vaccination or passive infusion to prevent mucosal acquisition of HIV-1 infection in humans.

Fc receptor-mediated immune responses: new tools but increased complexity in HIV prevention

The modest success of the RV144 HIV vaccine trial in Thailand and the ensuing suggestion that a Fc-receptormediated antibody activity might have played a role in the protection observed have intensified investigations on Fcrelated immune responses. HIV neutralizing antibodies have been and continue to be the focal point of research into humoral immune protection. However, recent knowledge that their protective efficacy can be augmented by Fc-FcR interactions has increased the complexity of identifying immune correlates of protection. If anything, continued studies of both humoral and cellular immune mechanisms point to the lack of a single protective anti-HIV immune response. Here we focus on humoral immunity, analyzing the role played by Fc receptor-related responses and discussing how new knowledge of their interactions requires further investigation, but may also spur novel vaccination approaches. We initially address classical Fc-receptor mediated anti-viral mechanisms including antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cell mediated viral inhibition (ADCVI), and antibody-dependent cellular phagocytosis (ADCP), as well as the effector cells that mediate these functions. Next, we summarize key aspects of FcR-Fc interactions that are important for potential control of HIV/SIV such as FcR polymorphisms and post-transcriptional modifications. Finally we discuss less commonly studied non-mechanistic anti-HIV immune functions: antibody avidity and envelopespecific B cell memory. Overall, a spectrum of immune responses, reflecting the immune system's redundancy, will likely be needed to prevent HIV infection and/or disease progression. Aside from elicitation of critical immune mechanisms, a successful vaccine will need to induce mature B cell responses and long-lasting immune memory.