HIV-specific Fc effector function early in infection predicts the development of broadly neutralizing antibodies

Polyfunctionality and breadth of HIV-1 antibodies are associated with delayed disease progression

HIV-1 infection leads to chronic disease requiring life-long treatment and therefore alternative therapeutics, a cure and/or a protective vaccine are needed. Antibody-mediated effector functions could have a role in the fight against HIV-1. However, the properties underlying the potential beneficial effects of antibodies during HIV-1 infection are poorly understood. To identify a specific profile of antibody features associated with delayed disease progression, we studied antibody polyfunctionality during untreated HIV-1 infection in the well-documented Amsterdam Cohort Studies. Serum samples were analyzed from untreated individuals with HIV-1 at approximately 6 months (n = 166) and 3 years (n = 382) post-seroconversion (post-SC). A Luminex antibody Fc array was used to profile 15 different Fc features for serum antibodies against 20 different HIV-1 envelope glycoprotein antigens and the resulting data was also compared with data on neutralization breadth. We found that high HIV-1 specific IgG1 levels and low IgG2 and IgG4 levels at 3 years post-SC were associated with delayed disease progression. Moreover, delayed disease progression was associated with a broad and polyfunctional antibody response. Specifically, the capacity to interact with all Fc γ receptors (FcγRs) and C1q, and in particular with FcγRIIa, correlated positively with delayed disease progression. There were strong correlations between antibody Fc features and neutralization breadth and several antibody features that were associated with delayed disease progression were also associated with the development of broad and potent antibody neutralization. In summary, we identified a strong association between broad, polyfunctional antibodies and delayed disease progression. These findings contribute new information for the fight against HIV-1, especially for new antibody-based therapy and cure strategies.

IgM N-glycosylation correlates with COVID-19 severity and rate of complement deposition

The glycosylation of IgG plays a critical role during human severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, activating immune cells and inducing cytokine production. However, the role of IgM N-glycosylation has not been studied during human acute viral infection. The analysis of IgM N-glycosylation from healthy controls and hospitalized coronavirus disease 2019 (COVID-19) patients reveals increased high-mannose and sialylation that correlates with COVID-19 severity. These trends are confirmed within SARS-CoV-2-specific immunoglobulin N-glycan profiles. Moreover, the degree of total IgM mannosylation and sialylation correlate significantly with markers of disease severity. We link the changes of IgM N-glycosylation with the expression of Golgi glycosyltransferases. Lastly, we observe antigen-specific IgM antibody-dependent complement deposition is elevated in severe COVID-19 patients and modulated by exoglycosidase digestion. Taken together, this work links the IgM N-glycosylation with COVID-19 severity and highlights the need to understand IgM glycosylation and downstream immune function during human disease.

Characterization of antibody-dependent cellular phagocytosis in patients infected with hepatitis C virus with different clinical outcomes

Early neutralizing antibodies against hepatitis C virus (HCV) and CD8 + T cell effector responses can lead to viral clearance. However, these functions alone are not sufficient to protect patients against HCV infection, thus undefined additional antiviral immune mechanisms are required. In recent years, Fc-receptor-dependent antibody effector functions, particularly, antibody-dependent cellular phagocytosis (ADCP) were shown to offer immune protection against several RNA viruses. However, its development and clinical role in patients with HCV infection remain unknown. In this study, we found that patients with chronic GT1a or GT3a HCV infection had significantly higher concentrations of anti-envelope 2 (E2) antibodies, predominantly IgG1 subclass, than patients that cleared the viruses while the latter had antibodies with higher affinities. 97% of the patients with HCV had measurable ADCP of whom patients with chronic disease showed significantly higher ADCP than those who naturally cleared the virus. Epitope mapping studies showed that patients with antibodies that target antigenic domains on the HCV E2 protein that are known to associate with neutralization function are also strongly associated with ADCP, suggesting antibodies with overlapping/dual functions. Correlation studies showed that ADCP significantly correlated with plasma anti-E2 antibody levels and neutralization function regardless of clinical outcome and genotype of infecting virus, while a significant correlation between ADCP and affinity was only evident in patients that cleared the virus. These results suggest ADCP was mostly driven by antibody titer in patients with chronic disease while maintained in clearers due to the quality (affinity) of their anti-E2 antibodies despite having lower antibody titers.

Serum immunoglobulin and the threshold of Fc receptor-mediated immune activation

Antibodies can mediate immune recruitment or clearance of immune complexes through the interaction of their Fc domain with cellular Fc receptors. Clustering of antibodies is a key step in generating sufficient avidity for efficacious receptor recognition. However, Fc receptors may be saturated with prevailing, endogenous serum immunoglobulin and this raises the threshold by which cellular receptors can be productively engaged. Here, we review the factors controlling serum IgG levels in both healthy and disease states, and discuss how the presence of endogenous IgG is encoded into the functional activation thresholds for low- and high-affinity Fc receptors. We discuss the circumstances where antibody engineering can help overcome these physiological limitations of therapeutic antibodies. Finally, we discuss how the pharmacological control of Fc receptor saturation by endogenous IgG is emerging as a feasible mechanism for the enhancement of antibody therapeutics.

Heavy-chain CDR3-engineered B cells facilitate in vivo evaluation of HIV-1 vaccine candidates

V2-glycan/apex broadly neutralizing antibodies (bnAbs) recognize a closed quaternary epitope of the HIV-1 envelope glycoprotein (Env). This closed structure is necessary to elicit apex antibodies and useful to guide the maturation of other bnAb classes. To compare antigens designed to maintain this conformation, we evaluated apex-specific responses in mice engrafted with a diverse repertoire of B cells expressing the HCDR3 of the apex bnAb VRC26.25. Engineered B cells affinity matured, guiding the improvement of VRC26.25 itself. We found that soluble Env (SOSIP) variants differed significantly in their ability to raise anti-apex responses. A transmembrane SOSIP (SOSIP-TM) delivered as an mRNA-lipid nanoparticle elicited more potent neutralizing responses than multimerized SOSIP proteins. Importantly, SOSIP-TM elicited neutralizing sera from B cells engineered with the predicted VRC26.25-HCDR3 progenitor, which also affinity matured. Our data show that HCDR3-edited B cells facilitate efficient in vivo comparisons of Env antigens and highlight the potential of an HCDR3-focused vaccine approach.

ADCC-activating antibodies correlate with decreased risk of congenital human cytomegalovirus transmission

Human cytomegalovirus (HCMV) is the most common vertically transmitted infection worldwide, yet there are no vaccines or therapeutics to prevent congenital HCMV (cCMV) infection. Emerging evidence indicates that antibody Fc effector functions may be a previously underappreciated component of maternal immunity against HCMV. We recently reported that antibody-dependent cellular phagocytosis (ADCP) and IgG activation of FcγRI/FcγRII were associated with protection against cCMV transmission, leading us to hypothesize that additional Fc-mediated antibody functions may be important. In this same cohort of HCMV-transmitting (n = 41) and nontransmitting (n = 40) mother-infant dyads, we report that higher maternal sera antibody-dependent cellular cytotoxicity (ADCC) activation is also associated with lower risk of cCMV transmission. We investigated the relationship between ADCC and IgG responses against 9 viral antigens and found that ADCC activation correlated most strongly with sera IgG binding to the HCMV immunoevasin protein UL16. Moreover, we determined that higher UL16-specific IgG binding and FcγRIII/CD16 engagement were associated with the greatest risk reduction in cCMV transmission. Our findings indicate that ADCC-activating antibodies against targets such as UL16 may represent an important protective maternal immune response against cCMV infection that can guide future HCMV correlates studies and vaccine or antibody-based therapeutic development.

Engineering strategies of Anti-HIV antibody therapeutics in clinical development

PURPOSE OF REVIEW

Anti-human immunodeficiency virus (HIV) antibody-based therapeutics offer an alternative treatment option to current antiretroviral drugs. This review aims to provide an overview of the Fc- and Fab-engineering strategies that have been developed to optimize broadly neutralizing antibodies and discuss recent findings from preclinical and clinical studies.

RECENT FINDINGS

Multispecific antibodies, including bispecific and trispecific antibodies, DART molecules, and BiTEs, as well as Fc-optimized antibodies, have emerged as promising therapeutic candidates for the treatment of HIV. These engineered antibodies engage multiple epitopes on the HIV envelope protein and human receptors, resulting in increased potency and breadth of activity. Additionally, Fc-enhanced antibodies have demonstrated extended half-life and improved effector function.

SUMMARY

The development of Fc and Fab-engineered antibodies for the treatment of HIV continues to show promising progress. These novel therapies have the potential to overcome the limitations of current antiretroviral pharmacologic agents by more effectively suppressing viral load and targeting latent reservoirs in individuals living with HIV. Further studies are needed to fully understand the safety and efficacy of these therapies, but the growing body of evidence supports their potential as a new class of therapeutics for the treatment of HIV.

Polyfunctional antibodies: a path towards precision vaccines for vulnerable populations

Vaccine efficacy determined within the controlled environment of a clinical trial is usually substantially greater than real-world vaccine effectiveness. Typically, this results from reduced protection of immunologically vulnerable populations, such as children, elderly individuals and people with chronic comorbidities. Consequently, these high-risk groups are frequently recommended tailored immunisation schedules to boost responses. In addition, diverse groups of healthy adults may also be variably protected by the same vaccine regimen. Current population-based vaccination strategies that consider basic clinical parameters offer a glimpse into what may be achievable if more nuanced aspects of the immune response are considered in vaccine design. To date, vaccine development has been largely empirical. However, next-generation approaches require more rational strategies. We foresee a generation of precision vaccines that consider the mechanistic basis of vaccine response variations associated with both immunogenetic and baseline health differences. Recent efforts have highlighted the importance of balanced and diverse extra-neutralising antibody functions for vaccine-induced protection. However, in immunologically vulnerable populations, significant modulation of polyfunctional antibody responses that mediate both neutralisation and effector functions has been observed. Here, we review the current understanding of key genetic and inflammatory modulators of antibody polyfunctionality that affect vaccination outcomes and consider how this knowledge may be harnessed to tailor vaccine design for improved public health.

Differing natural killer cell, T cell and antibody profiles in antiretroviral-naive HIV-1 viraemic controllers with and without protective HLA alleles

Previous work suggests that HIV controllers with protective human leukocyte antigen class I alleles (VC+) possess a high breadth of Gag-specific CD8+ T cell responses, while controllers without protective alleles (VC-) have a different unknown mechanism of control. We aimed to gain further insight into potential mechanisms of control in VC+ and VC-. We studied 15 VC+, 12 VC- and 4 healthy uninfected individuals (UI). CD8+ T cell responses were measured by ELISpot. Flow cytometry was performed to analyse surface markers for activation, maturation, and exhaustion on natural killer (NK) cell and T cells, as well as cytokine secretion from stimulated NK cells. We measured plasma neutralization activity against a panel of 18 Env-pseudotyped viruses using the TZM-bl neutralization assay. We found no significant differences in the magnitude and breadth of CD8+ T cell responses between VC+ and VC-. However, NK cells from VC- had higher levels of activation markers (HLA-DR and CD38) (p = 0.03), and lower cytokine expression (MIP-1β and TNF-α) (p = 0.05 and p = 0.04, respectively) than NK cells from VC+. T cells from VC- had higher levels of activation (CD38 and HLA-DR co-expression) (p = 0.05), as well as a trend towards higher expression of the terminal differentiation marker CD57 (p = 0.09) when compared to VC+. There was no difference in overall neutralization breadth between VC+ and VC- groups, although there was a trend for higher neutralization potency in the VC- group (p = 0.09). Altogether, these results suggest that VC- have a more activated NK cell profile with lower cytokine expression, and a more terminally differentiated and activated T cell profile than VC+. VC- also showed a trend of more potent neutralizing antibody responses that may enhance viral clearance. Further studies are required to understand how these NK, T cell and antibody profiles may contribute to differing mechanisms of control in VC+ and VC-.

IgM N-glycosylation correlates with COVID-19 severity and rate of complement deposition

The glycosylation of IgG plays a critical role during human SARS-CoV-2, activating immune cells and inducing cytokine production. However, the role of IgM N-glycosylation has not been studied during acute viral infection in humans. In vitro evidence suggests that the glycosylation of IgM inhibits T cell proliferation and alters complement activation rates. The analysis of IgM N-glycosylation from healthy controls and hospitalized COVID-19 patients reveals that mannosylation and sialyation levels associate with COVID-19 severity. Specifically, we find increased di- and tri-sialylated glycans and altered mannose glycans in total serum IgM in severe COVID-19 patients when compared to moderate COVID-19 patients. This is in direct contrast with the decrease of sialic acid found on the serum IgG from the same cohorts. Moreover, the degree of mannosylation and sialylation correlated significantly with markers of disease severity: D-dimer, BUN, creatinine, potassium, and early anti-COVID-19 amounts of IgG, IgA, and IgM. Further, IL-16 and IL-18 cytokines showed similar trends with the amount of mannose and sialic acid present on IgM, implicating these cytokines' potential to impact glycosyltransferase expression during IgM production. When examining PBMC mRNA transcripts, we observe a decrease in the expression of Golgi mannosidases that correlates with the overall reduction in mannose processing we detect in the IgM N-glycosylation profile. Importantly, we found that IgM contains alpha-2,3 linked sialic acids in addition to the previously reported alpha-2,6 linkage. We also report that antigen-specific IgM antibody-dependent complement deposition is elevated in severe COVID-19 patients. Taken together, this work links the immunoglobulin M N-glycosylation with COVID-19 severity and highlights the need to understand the connection between IgM glycosylation and downstream immune function during human disease.

Complement contributes to antibody-mediated protection against repeated SHIV challenge

The first clinical efficacy trials of a broadly neutralizing antibody (bNAb) resulted in less benefit than expected and suggested that improvements are needed to prevent HIV infection. While considerable effort has focused on optimizing neutralization breadth and potency, it remains unclear whether augmenting the effector functions elicited by broadly neutralizing antibodies (bNAbs) may also improve their clinical potential. Among these effector functions, complement-mediated activities, which can culminate in the lysis of virions or infected cells, have been the least well studied. Here, functionally modified variants of the second-generation bNAb 10-1074 with ablated and enhanced complement activation profiles were used to examine the role of complement-associated effector functions. When administered prophylactically against simian-HIV challenge in rhesus macaques, more bNAb was required to prevent plasma viremia when complement activity was eliminated. Conversely, less bNAb was required to protect animals from plasma viremia when complement activity was enhanced. These results suggest that complement-mediated effector functions contribute to in vivo antiviral activity, and that their engineering may contribute to the further improvements in the efficacy of antibody-mediated prevention strategies.

ADCC-activating antibodies correlate with protection against congenital human cytomegalovirus infection

Human cytomegalovirus (HCMV) is the most common vertically transmitted infection worldwide, yet there are no licensed vaccines or therapeutics to prevent congenital HCMV (cCMV) infection. Emerging evidence from studies of natural infection and HCMV vaccine trials indicates that antibody Fc effector functions may defend against HCMV infection. We previously reported that antibody-dependent cellular phagocytosis (ADCP) and IgG activation of FcγRI/FcγRII were associated with reduced risk of cCMV transmission, leading us to hypothesize that other Fc-mediated antibody functions may also contribute to protection. In this same cohort of HCMV transmitting (n = 41) and non-transmitting (n = 40) mother-infant dyads, we found that higher maternal sera antibody-dependent cellular cytotoxicity (ADCC) activation was also associated with decreased risk of cCMV infection. We determined that NK cell-mediated ADCC responses correlated strongly with anti-HCMV IgG FcγRIII/CD16 activation and IgG binding to the HCMV immunoevasin protein UL16. Notably, anti-UL16 IgG binding and engagement of FcγRIII/CD16 were higher in non-transmitting versus transmitting dyads and interacted significantly with ADCC responses. These findings indicate that ADCC-activating antibodies against novel targets such as UL16 may represent an important protective maternal immune response against cCMV infection, which can guide future HCMV correlates studies and vaccine development.

Antibody class-switching as a strategy to improve HIV-1 neutralization

Broadly neutralizing antibodies (bNAbs), when administered through passive immunization, are protective against HIV-1 infection. Current HIV-1 vaccine strategies are aimed at guiding the immune system to make bNAbs by mimicking their development during infection. Somatic hypermutation of the variable region is known to be crucial for the development of bNAbs. More recently, however, studies have shown how class-switch recombination (CSR) resulting in the generation of different antibody isotypes may serve as an additional mechanism through which antibodies can gain neutralization breadth and potency. In this review, we discuss the importance of different antibody isotypes for HIV-1 neutralization breadth and potency and how this information can be leveraged to improve passive and active immunization against HIV-1.

Enhanced neutralization potency of an identical HIV neutralizing antibody expressed as different isotypes is achieved through genetically distinct mechanisms

Antibodies with the same variable region can exist as multiple isotypes with varying neutralization potencies, though the mechanism for this is not fully defined. We previously isolated an HIV-directed IgA1 monoclonal antibody (mAb), CAP88-CH06, and showed that IgA1 and IgG3 isotypes of this antibody demonstrated enhanced neutralization compared to IgG1. To explore the mechanism behind this, hinge region and constant heavy chain (CH1) chimeras were constructed between the IgA1, IgG3 and IgG1 mAbs and assessed for neutralization activity, antibody-dependent cellular phagocytosis (ADCP) and antibody-dependent cellular cytotoxicity (ADCC). Hinge chimeras revealed that the increased neutralization potency and phagocytosis of the IgG3 isotype was attributed to its longer hinge region. In contrast, for IgA1, CH1 chimeras showed that this region was responsible both for enhanced neutralization potency and decreased ADCP, though ADCC was not affected. Overall, these data show that the enhanced neutralization potency of CAP88-CH06 IgG3 and IgA1, compared to IgG1, is achieved through distinct mechanisms. Understanding the influence of the hinge and CH1 regions on Fab domain function may provide insights into the engineering of therapeutic antibodies with increased neutralization potency.

Leveraging South African HIV research to define SARS-CoV-2 immunity triggered by sequential variants of concern

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes coronavirus disease 2019 (COVID-19), has shifted our paradigms about B cell immunity and the goals of vaccination for respiratory viruses. The development of population immunity, through responses directed to highly immunogenic regions of this virus, has been a strong driving force in the emergence of progressively mutated variants. This review highlights how the strength of the existing global virology and immunology networks built for HIV vaccine research enabled rapid adaptation of techniques, assays, and skill sets, to expeditiously respond to the SARS-CoV-2 pandemic. Allying real-time genomic surveillance to immunological platforms enabled the characterization of immune responses elicited by infection with distinct variants, in sequential epidemic waves, as well as studies of vaccination and hybrid immunity (combination of infection- and vaccination-induced immunity). These studies have shown that consecutive variants of concern have steadily diminished the ability of vaccines to prevent infection, but that increasing levels of hybrid immunity result in higher frequencies of cross-reactive responses. Ultimately, this rapid pivot from HIV to SARS-CoV-2 enabled a depth of understanding of the SARS-CoV-2 antigenic vulnerabilities as population immunity expanded and diversified, providing key insights for future responses to the SARS-CoV-2 pandemic.

Influenza Vaccination Results in Differential Hemagglutinin Stalk-Specific Fc-Mediated Functions in Individuals Living With or Without HIV

Influenza virus hemagglutinin (HA) stalk-specific antibodies have been shown to potently induce Fc-mediated effector functions which are important in protection from disease. In placebo-controlled maternal influenza (MatFlu) vaccination trials of pregnant women living with or without HIV, reduced risk of influenza illness was associated with high HA stalk antibody titers following trivalent inactivated vaccination (TIV). However, the mechanisms of immunity conferred by the HA stalk antibodies were not well understood. Here, we investigated HA stalk-specific Fc effector functions including antibody-dependent cellular phagocytosis (ADCP), antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent complement deposition (ADCD), and FcγRIIa and FcγRIIIa binding in response to seasonal influenza vaccination. These were measured pre- and 1-month post-vaccination in 141 HIV-uninfected women (67 TIV and 74 placebo recipients) and 119 women living with HIV (WLWH; 66 TIV and 53 placebo recipients). In contrast to HIV-uninfected women, where HA stalk-specific ADCP and FcγRIIa binding were significantly boosted, WLWH showed no increase in response to vaccination. HA stalk-specific ADCC potential and FcγRIIIa binding were not boosted regardless of HIV status but were higher in WLWH compared with HIV-uninfected women prior to vaccination. HA stalk-specific ADCD was significantly increased by vaccination in all women, but was significantly lower in the WLWH both pre- and post- vaccination. Co-ordination between HA stalk-specific ADCP and ADCD in WLWH was improved by vaccination. Fc polyfunctionality was enhanced by vaccination in HIV-uninfected women and driven by the HA stalk antibody titers. However, in the WLWH, higher pre-vaccination Fc polyfunctionality was maintained post-vaccination but was decoupled from titer. Overall, we showed differential regulation of Fc effector HA stalk responses, suggesting that HIV infection results in unique humoral immunity in response to influenza vaccination, with relevance for future strategies that aim to target the HA stalk in this population.

Potent anti-viral activity of a trispecific HIV neutralizing antibody in SHIV-infected monkeys

Broadly neutralizing antibodies (bNAbs) represent an alternative to drug therapy for the treatment of HIV-1 infection. Immunotherapy with single bNAbs often leads to emergence of escape variants, suggesting a potential benefit of combination bNAb therapy. Here, a trispecific bNAb reduces viremia 100- to 1000-fold in viremic SHIV-infected macaques. After treatment discontinuation, viremia rebounds transiently and returns to low levels, through CD8-mediated immune control. These viruses remain sensitive to the trispecific antibody, despite loss of sensitivity to one of the parental bNAbs. Similarly, the trispecific bNAb suppresses the emergence of resistance in viruses derived from HIV-1-infected subjects, in contrast to parental bNAbs. Trispecific HIV-1 neutralizing antibodies, therefore, mediate potent antiviral activity in vivo and may minimize the potential for immune escape.

Frequent Development of Broadly Neutralizing Antibodies in Early Life in a Large Cohort of Children With Human Immunodeficiency Virus

BACKGROUND

Recent studies have indicated that broadly neutralizing antibodies (bnAbs) in children may develop earlier after human immunodeficiency virus (HIV) infection compared to adults.

METHODS

We evaluated plasma from 212 antiretroviral therapy-naive children with HIV (1-3 years old). Neutralization breadth and potency was assessed using a panel of 10 viruses and compared to adults with chronic HIV. The magnitude, epitope specificity, and immunoglobulin (Ig)G subclass distribution of Env-specific antibodies were assessed using a binding antibody multiplex assay.

RESULTS

One-year-old children demonstrated neutralization breadth comparable to chronically infected adults, whereas 2- and 3-year-olds exhibited significantly greater neutralization breadth (P = .014). Likewise, binding antibody responses increased with age, with levels in 2- and 3-year-old children comparable to adults. Overall, there was no significant difference in antibody specificities or IgG subclass distribution between the pediatric and adult cohorts. It is interesting to note that the neutralization activity was mapped to a single epitope (CD4 binding site, V2 or V3 glycans) in only 5 of 38 pediatric broadly neutralizing samples, which suggests that most children may develop a polyclonal neutralization response.

CONCLUSIONS

These results contribute to a growing body of evidence suggesting that initiating HIV immunization early in life may present advantages for the development of broadly neutralizing antibody responses.

Polyclonal Broadly Neutralizing Antibody Activity Characterized by CD4 Binding Site and V3-Glycan Antibodies in a Subset of HIV-1 Virus Controllers

Broadly neutralizing antibodies (bNAbs), known to mediate immune control of HIV-1 infection, only develop in a small subset of HIV-1 infected individuals. Despite being traditionally associated with patients with high viral loads, bNAbs have also been observed in therapy naïve HIV-1+ patients naturally controlling virus replication [Virus Controllers (VCs)]. Thus, dissecting the bNAb response in VCs will provide key information about what constitutes an effective humoral response to natural HIV-1 infection. In this study, we identified a polyclonal bNAb response to natural HIV-1 infection targeting CD4 binding site (CD4bs), V3-glycan, gp120-gp41 interface and membrane-proximal external region (MPER) epitopes on the HIV-1 envelope (Env). The polyclonal antiviral antibody (Ab) response also included antibody-dependent cellular phagocytosis of clade AE, B and C viruses, consistent with both the Fv and Fc domain contributing to function. Sequence analysis of envs from one of the VCs revealed features consistent with potential immune pressure and virus escape from V3-glycan targeting bNAbs. Epitope mapping of the polyclonal bNAb response in VCs with bNAb activity highlighted the presence of gp120-gp41 interface and CD4bs antibody classes with similar binding profiles to known potent bNAbs. Thus, these findings reveal the induction of a broad and polyfunctional humoral response in VCs in response to natural HIV-1 infection.

Selection of HIV Envelope strains for standardized assessments of vaccine-elicited antibody-dependent cellular cytotoxicity (ADCC)-mediating antibodies

Antibody-dependent cellular cytotoxicity (ADCC) has been correlated with reduced risk of HIV-1 infection in several preclinical vaccine trials and the RV144 clinical trial, indicating this is a relevant antibody function to study. Given the diversity of HIV-1, the breadth of vaccine-induced antibody responses is a critical parameter to understand if a universal vaccine is to be realised. Moreover, breadth of ADCC responses can be influenced by different vaccine strategies and regimens, including adjuvants. Therefore, to accurately evaluate ADCC and to compare vaccine regimens, it is important to understand the range of HIV Envelope susceptibility to these responses. These evaluations have been limited because of the complexity of the assay and the lack of a comprehensive panel of viruses for the assessment of these humoral responses. Here, we used twenty-nine HIV-1 infectious molecular clones (IMCs) representing different Envelope subtypes and circulating recombinant forms to characterise susceptibility to ADCC from antibodies in plasma from infected individuals, including thirteen viraemic individuals, ten controllers and six with broadly neutralizing antibody responses. We found in our panel that ADCC susceptibility of the IMCs in our panel did not cluster by subtype, infectivity, level of CD4 downregulation, level of shedding, or neutralization sensitivity. Using partition-around-medoids (PAM) clustering to distinguish smaller groups of IMCs with similar ADCC susceptibility, we identified nested panels of four to eight IMCs that broadly represent the ADCC susceptibility of the entire 29 IMC panel. These panels, together with reagents developed to specifically accommodate circulating viruses at the geographical sites of vaccine trials, will provide a powerful tool to harmonise ADCC data generated across different studies, and detect common themes of ADCC responses elicited by various vaccines. IMPORTANCE Antibody-dependent cellular cytotoxicity (ADCC) responses were found to correlate with reduced risk of infection in the RV144 trial, the only human HIV-1 vaccine to show any efficacy to date. However, reagents to understand the breadth and magnitude of these responses across preclinical and clinical vaccine trials remain underdeveloped. In this study, we characterise HIV-1 infectious molecular clones encoding 29 distinct envelope strains (Env-IMCs) to understand factors which impact virus susceptibility to ADCC and use statistical methods to identify smaller nested panels of four to eight Env-IMCs which accurately represent the full set. These reagents can be used as standardized reagents across studies to fully understand how ADCC may affect efficacy of future vaccine studies, and how studies differed in the breadth of responses developed.

Building a better antibody through the Fc: advances and challenges in harnessing antibody Fc effector functions for antiviral protection

Antibodies can provide antiviral protection through neutralization and recruitment of innate effector functions through the Fc domain. While neutralization has long been appreciated for its role in antibody-mediated protection, a growing body of work indicates that the antibody Fc domain also significantly contributes to antiviral protection. Recruitment of innate immune cells such as natural killer cells, neutrophils, monocytes, macrophages, dendritic cells and the complement system by antibodies can lead to direct restriction of viral infection as well as promoting long-term antiviral immunity. Monoclonal antibody therapeutics against viruses are increasingly incorporating Fc-enhancing features to take advantage of the Fc domain, uncovering a surprising breadth of mechanisms through which antibodies can control viral infection. Here, we review the recent advances in our understanding of antibody-mediated innate immune effector functions in protection from viral infection and review the current approaches and challenges to effectively leverage innate immune cells via antibodies.

ADCC-mediating non-neutralizing antibodies can exert immune pressure in early HIV-1 infection

Despite antibody-dependent cellular cytotoxicity (ADCC) responses being implicated in protection from HIV-1 infection, there is limited evidence that they control virus replication. The high mutability of HIV-1 enables the virus to rapidly adapt, and thus evidence of viral escape is a very sensitive approach to demonstrate the importance of this response. To enable us to deconvolute ADCC escape from neutralizing antibody (nAb) escape, we identified individuals soon after infection with detectable ADCC responses, but no nAb responses. We evaluated the kinetics of ADCC and nAb responses, and viral escape, in five recently HIV-1-infected individuals. In one individual we detected viruses that escaped from ADCC responses but were sensitive to nAbs. In the remaining four participants, we did not find evidence of viral evolution exclusively associated with ADCC-mediating non-neutralizing Abs (nnAbs). However, in all individuals escape from nAbs was rapid, occurred at very low titers, and in three of five cases we found evidence of viral escape before detectable nAb responses. These data show that ADCC-mediating nnAbs can drive immune escape in early infection, but that nAbs were far more effective. This suggests that if ADCC responses have a protective role, their impact is limited after systemic virus dissemination.

Revisiting an IgG Fc Loss-of-Function Experiment: the Role of Complement in HIV Broadly Neutralizing Antibody b12 Activity

The role of the complement system in HIV-1 immunity and pathogenesis is multifaceted, and an improved understanding of complement activities mediated by HIV-1-specific antibodies has the potential to inform and advance clinical development efforts. A seminal nonhuman primate challenge experiment suggested that complement was dispensable for the protective effect of the early broadly neutralizing antibody (bnAb) b12, but recent experiments have raised questions about the breadth of circumstances under which this conclusion may hold. Here, we reassess the original observation using Fc variants of IgG1 b12 that enhance complement activity and report that complement fixation on recombinant antigen, virions, and cells and complement-dependent viral and cellular lysis in vitro vary among bnAbs. Specifically, while the clinically significant V3 glycan-specific bnAb 10-1074 demonstrates activity, we found that b12 does not meaningfully activate the classical complement cascade. Consistent with avid engagement by C1q and its complex system of regulatory factors, these results suggest that complement-mediated antibody activities demonstrate a high degree of context dependence and motivate revisiting the role of complement in antibody-mediated prevention of HIV-1 infection by next-generation bnAbs in new translational studies in animal models.

CD4+ T Cells Are Dispensable for Induction of Broad Heterologous HIV Neutralizing Antibodies in Rhesus Macaques

Induction of broadly neutralizing antibodies (bNAbs) is a major goal for HIV vaccine development. HIV envelope glycoprotein (Env)-specific bNAbs isolated from HIV-infected individuals exhibit substantial somatic hypermutation and correlate with T follicular helper (Tfh) responses. Using the VC10014 DNA-protein co-immunization vaccine platform consisting of gp160 plasmids and gp140 trimeric proteins derived from an HIV-1 infected subject that developed bNAbs, we determined the characteristics of the Env-specific humoral response in vaccinated rhesus macaques in the context of CD4+ T cell depletion. Unexpectedly, both CD4+ depleted and non-depleted animals developed comparable Tier 1 and 2 heterologous HIV-1 neutralizing plasma antibody titers. There was no deficit in protection from SHIV challenge, no diminution of titers of HIV Env-specific cross-clade binding antibodies, antibody dependent cellular phagocytosis, or antibody-dependent complement deposition in the CD4+ depleted animals. These collective results suggest that in the presence of diminished CD4+ T cell help, HIV neutralizing antibodies were still generated, which may have implications for developing effective HIV vaccine strategies.

TLR-Agonist Mediated Enhancement of Antibody-Dependent Effector Functions as Strategy For an HIV-1 Cure

Background

The current treatment for HIV-1 is based on blocking various stages in the viral replication cycle using combination antiretroviral therapy (ART). Even though ART effectively controls the infection, it is not curative, and patients must therefore continue treatment life-long.

Aim

Here we review recent literature investigating the single or combined effect of toll-like receptor (TLR) agonists and broadly neutralizing antibodies (bNAbs) with the objective to evaluate the evidence for this combination as a means towards an HIV-1 cure.

Results

Multiple preclinical studies found significantly enhanced killing of HIV-1 infected cells by TLR agonist-induced innate immune activation or by Fc-mediated effector functions following bNAb administration. However, monotherapy with either agent did not lead to sustained HIV-1 remission in clinical trials among individuals on long-term ART. Notably, findings in non-human primates suggest that a combination of TLR agonists and bNAbs may be able to induce long-term remission after ART cessation and this approach is currently being further investigated in clinical trials.

Conclusion

Preclinical findings show beneficial effects of either TLR agonist or bNAb administration for enhancing the elimination of HIV-1 infected cells. Further, TLR agonist-mediated stimulation of innate effector functions in combination with bNAbs may enhance antibody-dependent cellular cytotoxicity and non-human primate studies have shown promising results for this combination strategy. Factors such as immune exhaustion, proviral bNAb sensitivity and time of intervention might impact the clinical success.

Coordinated Fc-effector and neutralization functions in HIV-infected children define a window of opportunity for HIV vaccination

OBJECTIVES

Antibody function has been extensively studied in HIV-infected adults but is relatively understudied in children. Emerging data suggests enhanced development of broadly neutralizing antibodies (bNAbs) in children but Fc effector functions in this group are less well defined. Here, we profiled overall antibody function in HIV-infected children.

DESIGN

Plasma samples from a cross-sectional study of 50 antiretroviral therapy-naive children (aged 1-11 years) vertically infected with HIV-1 clade A were screened for HIV-specific binding antibody levels and neutralizing and Fc-mediated functions.

METHODS

Neutralization breadth was determined against a globally representative panel of 12 viruses. HIV-specific antibody levels were determined using a multiplex assay. Fc-mediated antibody functions measured were antibody-dependent: cellular phagocytosis (ADCP); neutrophil phagocytosis (ADNP); complement deposition (ADCD) and natural killer function (ADNK).

RESULTS

All children had HIV gp120-specific antibodies, largely of the IgG1 subtype. Fifty-four percent of the children exhibited more than 50% neutralization breadth, with older children showing significantly broader neutralization activity. Apart from ADCC, observed only in 16% children, other Fc-mediated functions were common (>58% children). Neutralization breadth correlated with Fc-mediated functions suggesting shared determinants of enhanced antibody function exist.

CONCLUSIONS

These results are consistent with previous observations that children may develop high levels of neutralization breadth. Furthermore, the striking association between neutralization breadth and Fc effector function suggests that HIV vaccination in children could yield multifunctional antibodies. Paediatric populations may therefore provide an ideal window of opportunity for HIV vaccination strategies.

Evolution of antibodies to native trimeric envelope and their Fc dependent functions in untreated and treated primary HIV infection

People living with HIV (PLWH) develop both anti-Envelope-specific antibodies, which bind the closed trimeric HIV Envelope present on infected cells and anti-gp120-specific antibodies, which bind gp120 monomers shed by infected cells and taken up by CD4 on uninfected bystander cells. Both antibodies have an Fc portion that binds to Fc Receptors on several types of innate immune cells and stimulates them to develop anti-viral functions. Among these Fc dependent functions (FcDFs) are antibody dependent (AD) cellular cytotoxicity (ADCC), AD cellular trogocytosis (ADCT) and AD phagocytosis (ADCP). Here, we assessed the evolution of total immunoglobulin G (IgG), anti-gp120 and anti-Envelope IgG antibodies and their FcDFs in plasma samples from anti-retroviral therapy (ART) naïve subjects during early HIV infection (28-194 days post infection [DPI]). We found that both the concentrations and FcDFs of anti-gp120 and anti-Envelope antibodies increased with time in ART-naïve PLWH. Although generated concurrently, anti-gp120-specific antibodies were 20.7-fold more abundant than anti-Envelpe-specific antibodies, both specificities being strongly correlated with each other and FcDFs. Among the FcDFs, only ADCP activity was inversely correlated with concurrent viral load. PLWH who started ART >90 DPI showed higher anti-Envelope-specific antibody levels, ADCT and ADCP activities than those starting ART <90 DPI. However, in longitudinally collected samples, ART initiation at >90 DPI was accompanied by a faster decline in anti-Envelope-specific antibody levels, which did not translate to a faster decline in FcDFs compared to those starting ART <90 DPI. IMPORTANCE Closed conformation Envelope is expressed on the surface of HIV-infected cells. Antibodies targeting this conformation and that support FcDFs have the potential to control HIV. This study tracks the timing of the appearance and evolution of antibodies to closed conformation Envelope, whose concentration increases over the first 6 mos of infection. Antiretroviral therapy (ART) initiation blunts further increases in the concentration of these antibodies and their and FcDFs. However, antibodies to open conformation Envelope also increase with DPI until ART initiation. These antibodies target uninfected bystander cells, which may contribute to loss of uninfected CD4 cells and pathogenicity. This manuscript presents, for the first time, the evolution of antibodies to closed conformation Envelope and their fate on-ART. This information may be useful in making decisions on the timing of ART initiation in early HIV infection.

HIV Broadly Neutralizing Antibodies Expressed as IgG3 Preserve Neutralization Potency and Show Improved Fc Effector Function

The ability of several broadly neutralizing antibodies (bNAbs) to protect against HIV infection is enhanced through Fc receptor binding. Antibody isotype modulates this effect, with IgG3 associated with improved HIV control and vaccine efficacy. We recently showed that an IgG3 variant of bNAb CAP256-VRC26.25 exhibited more potent neutralization and phagocytosis than its IgG1 counterpart. Here, we expanded this analysis to include additional bNAbs targeting all major epitopes. A total of 15 bNAbs were expressed as IgG1 or IgG3, and pairs were assessed for neutralization potency against the multi-subtype global panel of 11 HIV strains. Binding to the neonatal Fc receptor (FcRn) and Fcγ receptors were measured using ELISA and antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis were measured using infectious viruses and global panel Env SOSIP trimers, respectively. IgG3 bNAbs generally showed similar or increased (up to 60 fold) neutralization potency than IgG1 versions, though the effect was virus-specific. This improvement was statistically significant for CAP256-VRC26.25, 35022, PGT135 and CAP255.G3. IgG3 bNAbs also showed significantly improved binding to FcγRIIa which correlated with enhanced phagocytosis of all trimeric Env antigens. Differences in ADCC were epitope-specific, with IgG3 bNAbs to the MPER, CD4 binding site and gp120-gp41 interface showing increased ADCC. We also explored the pH dependence of IgG1 and IgG3 variants for FcRn binding, as this determines the half-life of antibodies. We observed reduced pH dependence, associated with shorter half-lives for IgG3 bNAbs, with κ-light chains. However, IgG3 bNAbs that use λ-light chains showed similar pH dependence to their IgG1 counterparts. This study supports the manipulation of the constant region to improve both the neutralizing and Fc effector activity of bNAbs, and suggests that IgG3 versions of bNAbs may be preferable for passive immunity given their polyfunctionality.

Antibodies for Human Immunodeficiency Virus-1 Cure Strategies

Human immunodeficiency virus (HIV) infection leads to the establishment of a long-lived latent cellular reservoir. One strategy to eliminate quiescent reservoir cells is to reactivate virus replication to induce HIV envelope glycoprotein (Env) expression on the cell surface exposing them to subsequent antibody targeting. Via the interactions between the antibody Fc domain and Fc-γ receptors (FcγRs) that are expressed on innate effector cells, such as natural killer cells, monocytes, and neutrophils, antibodies can mediate the elimination of infected cells. Over the last decade, a multitude of human monoclonal antibodies that are broadly neutralizing across many HIV-1 subtypes have been identified and are currently being explored for HIV eradication strategies. Antibody development also includes novel Fc engineering approaches to increase engagement of effector cells and optimize antireservoir efficacy. In this review, we discuss the usefulness of antibodies for HIV eradication approaches specifically focusing on antibody-mediated strategies to target latently infected cells and options to increase antibody efficacy.

Recent insights into Fc-mediated effector responses to HIV-1

PURPOSE OF REVIEW

Recent work defining Fc-mediated effector functions for both viral control and protection against infection is summarized and considered along with new strategies to drive robust Fc-mediated responses.

RECENT FINDINGS

In new human and nonhuman primate (NHP) vaccine trials as well as studies of natural infection, Fc-mediated effector responses have sometimes been observed to correlate with decreased risk of infection or with better clinical outcomes, suggesting a potential role for these responses in HIV-1 prevention and therapy. Recent highlights include use of antibody-dependent cellular cytotoxicity-sensitizing CD4-induced mimetic compounds, novel V1V2 immunogens, passive transfer studies, and vaccine regimens that successfully elicited Fc-mediated responses and were reported to decrease risk of infection in challenge studies in NHPs. Lastly, detailed studies of IgG3 forms of HIV-specific antibodies have reported that both neutralizing and Fc-mediated responses can be increased relative to the more prevalent IgG1 subclass.

SUMMARY

Successful harmonization of neutralizing and Fc-mediated responses may make key contributions to the goal of reducing HIV-1 infection via active and passive vaccination. New studies continue to highlight the importance of Fc-mediated antibody responses as correlates of decreased risk of infection and suggest enhanced phagocytosis is a potential mechanism of reduced risk of infection associated with human IgG3 responses. Results from recent studies may help guide the rational design of therapies and vaccines that aim to specifically leverage antibody effector function.

C1q binding to surface-bound IgG is stabilized by C1r2s2 proteases

Antibody-dependent complement activation plays a major role in various pathophysiological processes in our body, including infection, inflammation, autoimmunity, and transplant rejection. In order to activate complement, antibodies should bind to target cells and recruit complement component C1. C1 is a large, multimolecular complex that consists of the antibody recognition protein C1q and a heterotetramer of proteases (C1r₂s₂). Although it is believed that interactions between C1 and IgGs are solely mediated by C1q, we here show that C1r₂s₂ proteases affect the capacity of C1q to form an avid complex with surface-bound IgG molecules. Furthermore, we demonstrate that C1q–IgG stability is influenced by IgG oligomerization and that promoting IgG oligomerization improves phagocytosis of the pathogenic bacterium Staphylococcus aureus.

Complement is an important effector mechanism for antibody-mediated clearance of infections and tumor cells. Upon binding to target cells, the antibody’s constant (Fc) domain recruits complement component C1 to initiate a proteolytic cascade that generates lytic pores and stimulates phagocytosis. The C1 complex (C1qr₂s₂) consists of the large recognition protein C1q and a heterotetramer of proteases C1r and C1s (C1r₂s₂). While interactions between C1 and IgG-Fc are believed to be mediated by the globular heads of C1q, we here find that C1r₂s₂ proteases affect the capacity of C1q to form an avid complex with surface-bound IgG molecules (on various 2,4-dinitrophenol [DNP]-coated surfaces and pathogenic Staphylococcus aureus). The extent to which C1r₂s₂ contributes to C1q–IgG stability strongly differs between human IgG subclasses. Using antibody engineering of monoclonal IgG, we reveal that hexamer-enhancing mutations improve C1q–IgG stability, both in the absence and presence of C1r₂s₂. In addition, hexamer-enhanced IgGs targeting S. aureus mediate improved complement-dependent phagocytosis by human neutrophils. Altogether, these molecular insights into complement binding to surface-bound IgGs could be important for optimal design of antibody therapies.

Optimisation and validation of a new method for antibody dependent cellular phagocytosis in hepatitis C virus infection

Lack of a simple, high throughput antibody-dependent cellular phagocytosis (ADCP) assay has limited our understanding of its potential role of in hepatitis C (HCV) infection. Here, we optimised a flow-cytometry based ADCP assay using HCV envelope (E2)-protein coated microbeads that were opsonised with anti-E2 monoclonal IgG antibody (αE2 mAb) and the THP-1 monocyte cell line as effector cells. We found 1.5 × 10⁹/ml microbeads opsonised with 5 μg/ml αE2 mAb and 1.6 × 10⁶/ml THP-1 cells were optimal conditions to distinguish between healthy controls and patients with HCV. This optimised assay was then used to investigate ADCP in plasma obtained from 72 patients with chronic HCV infection and 15 healthy controls. We found that 75% of patients with genotype 1 and 87% of patients with genotype 3 HCV infection had significantly higher levels of ADCP compared to healthy controls. In patients, there was a significant correlation between increase in ADCP and higher concentrations of anti-E2 IgG antibodies in the plasma. Taken together, we established a simple, quick and high throughput ADCP assay for HCV infection that can readily be used for screening of large cohorts of patients and investigation of the role of ADCP in the pathogenesis or protection from this disease.

Are Fc Gamma Receptor Polymorphisms Important in HIV-1 Infection Outcomes and Latent Reservoir Size?

Fc gamma receptors (FcγR) are cell surface glycoproteins which trigger specific effector-cell responses when cross-linked with the Fc portions of immunoglobulin (IgG) antibodies. During HIV-1 infection, the course of disease progression, ART response, and viral reservoir size vary in different individuals. Several factors may account for these differences; however, Fc gamma receptor gene polymorphisms, which influence receptor binding to IgG antibodies, are likely to play a key role. FcγRIIa (CD32) was recently reported as a potential marker for latent HIV reservoir, however, this assertion is still inconclusive. Whether FcγR polymorphisms influence the size of the viral reservoir, remains an important question in HIV cure studies. In addition, potential cure or viral suppression methods such as broadly neutralizing antibody (bNAbs) may depend on FcγRs to control the virus. Here, we discuss the current evidence on the potential role played by FcγR polymorphisms in HIV-1 infection, treatment and vaccine trial outcomes. Importantly, we highlight contrasting findings that may be due to multiple factors and the relatively limited data from African populations. We recommend further studies especially in sub-Saharan Africa to confirm the role of FcγRIIa in the establishment of latent reservoir and to determine their influence in therapies involving bNAbs.

Targeting Fc effector function in vaccine design

INTRODUCTION

Antibodies mediate pathogen neutralization in addition to several cytotoxic Fc functions through engaging cellular receptors and recruiting effector cells. Fc effector functions have been well described in disease control and protection against infectious diseases including HIV, Ebola, malaria, influenza and tuberculosis, making them attractive targets for vaccine design.

AREAS COVERED

We briefly summarize the role of Fc effector functions in disease control and protection in viral, bacterial and parasitic infectious diseases. We review Fc effector function in passive immunization and vaccination, and primarily focus on strategies to elicit and modulate these functions as part of a robust vaccine strategy.

EXPERT OPINION

Despite their known correlation with vaccine efficacy for several diseases, only recently have seminal studies addressed how these Fc effector functions can be elicited and modulated in vaccination. However, gaps remain in assay standardization and the precise mechanisms of diverse functional assays. Furthermore, there are inherent difficulties in the translation of findings from animal models to humans, given the difference in sequence, expression and function of Fc receptors and Fc portions of antibodies. However, overall it is clear that vaccine development to elicit Fc effector function is an important goal for optimal prevention against infectious disease.

B cell engagement with HIV-1 founder virus envelope predicts development of broadly neutralizing antibodies

Determining which immunological mechanisms contribute to the development of broad neutralizing antibodies (bNAbs) during HIV-1 infection is a major goal to inform vaccine design. Using samples from a longitudinal HIV-1 acute infection cohort, we found key B cell determinants within the first 14-43 days of viremia that predict the development of bNAbs years later. Individuals who develop neutralization breadth had significantly higher B cell engagement with the autologous founder HIV envelope (Env) within 1 month of initial viremia. A higher frequency of founder-Env-specific naive B cells was associated with increased B cell activation and differentiation and predictive of bNAb development. These data demonstrate that the initial B cell interaction with the founder HIV Env is important for the development of broadly neutralizing antibodies and provide evidence that events within HIV acute infection lead to downstream functional outcomes.

The Role of CXCL13 in Antibody Responses to HIV-1 Infection and Vaccination

CXCL13 signals through the G protein-coupled chemokine receptor CXCR5 to drive development of secondary lymphoid tissue as well as B cell and Tfh cell trafficking to germinal centers (GC), which leads to the differentiation of B cells to plasma cells and memory B cells. CXCL13 has been proposed as a general plasma biomarker for GC activities. In HIV-1 infected individuals, plasma CXCL13 levels have been associated with the rate of disease progression to AIDS. Moreover, CXCL13 production has been reported to be increased in HIV-1-infected lymph nodes, which may drive increased downregulation of CXCR5. In this review, we address the role of CXCL13 in HIV-1 infected individuals with regard to GC formation, generation of broadly neutralizing antibodies after infection and vaccination, and AIDS-related B cell lymphoma.

Distinct Features of Germinal Center Reactions in Macaques Infected by SIV or Vaccinated with a T-Dependent Model Antigen

B-cell follicles constitute large reservoirs of infectious HIV/SIV associated to follicular dendritic cells and infecting follicular helper (TFH) and regulatory (TFR) T-cells in germinal centers (GCs). Thus, follicular and GC B-cells are persistently exposed to viral antigens. Despite recent development of potent HIV immunogens, numerous questions are still open regarding GC reaction during early HIV/SIV infection. Here, we dissect the dynamics of B- and T-cells in GCs of macaques acutely infected by SIV (Group SIV+) or vaccinated with Tetanus Toxoid (Group TT), a T-dependent model antigen. Systemic inflammation and mobilization of antigen-presenting cells in inguinal lymph nodes and spleen are lower in Group TT than in Group SIV+. Despite spleen GC reaction of higher magnitude in Group SIV+, the development of protective immunity could be limited by abnormal helper functions of TFH massively polarized into TFH1-like cells, by inflammation-induced recruitment of fCD8 (either regulatory or cytotoxic) and by low numbers of TFR limiting TFH/TFR competition for high affinity B-cells. Increased GC B-cells apoptosis and accumulation of CD21lo memory B-cells, unable to further participate to GC reaction, likely contribute to eliminate SIV-specific B-cells and decrease antibody affinity maturation. Surprisingly, functional GCs and potent TT-specific antibodies develop despite low levels of CXCL13.

Antibody Isotype Switching as a Mechanism to Counter HIV Neutralization Escape

Neutralizing antibodies (nAbs) to highly variable viral pathogens show remarkable diversification during infection, resulting in an “arms race” between virus and host. Studies of nAb lineages have shown how somatic hypermutation (SHM) in immunoglobulin (Ig)-variable regions enables maturing antibodies to neutralize emerging viral escape variants. However, the Ig-constant region (which determines isotype) can also influence epitope recognition. Here, we use longitudinal deep sequencing of an HIV-directed nAb lineage, CAP88-CH06, and identify several co-circulating isotypes (IgG3, IgG1, IgA1, IgG2, and IgA2), some of which share identical variable regions. First, we show that IgG3 and IgA1 isotypes are better able to neutralize longitudinal autologous viruses and epitope mutants than can IgG1. Second, detrimental class-switch recombination (CSR) events that resulted in reduced neutralization can be rescued by further CSR, which we term “switch redemption.” Thus, CSR represents an additional immunological mechanism to counter viral escape from HIV-specific antibody responses.

Scheepers et al. show within an HIV-specific antibody lineage that isotypes confer variable ability to neutralize emerging viral escape variants. This suggests that class switching, in addition to somatic hypermutation of immunoglobulin-variable regions, contributes to antibody maturation during infection.

Mining for humoral correlates of HIV control and latent reservoir size

While antiretroviral therapy (ART) has effectively revolutionized HIV care, the virus is never fully eliminated. Instead, immune dysfunction, driven by persistent non-specific immune activation, ensues and progressively leads to premature immunologic aging. Current biomarkers monitoring immunologic changes encompass generic inflammatory biomarkers, that may also change with other infections or disease states, precluding the antigen-specific monitoring of HIV-infection associated changes in disease. Given our growing appreciation of the significant changes in qualitative and quantitative properties of disease-specific antibodies in HIV infection, we used a systems approach to explore humoral profiles associated with HIV control. We found that HIV-specific antibody profiles diverge by spontaneous control of HIV, treatment status, viral load and reservoir size. Specifically, HIV-specific antibody profiles representative of changes in viral load were largely quantitative, reflected by differential HIV-specific antibody levels and Fc-receptor binding. Conversely, HIV-specific antibody features that tracked with reservoir size exhibited a combination of quantitative and qualitative changes marked by more distinct subclass selection profiles and unique HIV-specific Fc-glycans. Our analyses suggest that HIV-specific antibody Fc-profiles provide antigen-specific resolution on both cell free and cell-associated viral loads, pointing to potentially novel biomarkers to monitor reservoir activity.

Current combination antiretroviral therapy (ART) regimens have reversed the death sentence once associated with an HIV diagnosis. However, the virus is never fully eliminated. Rather, latently infected cells with integrated virus (latent reservoir) persist and the virus rebounds rapidly upon discontinuation of therapy. Further, even for those on ART, immune dysfunction, driven by persistent non-specific immune activation, ensues and progressively leads to premature immunologic aging. Current biomarkers monitoring these changes are non-specific–they focus on generic inflammatory changes that may also track with other infections or disease states. In this manuscript, we used an unbiased analytical systems approach to identify antigen-specific biomarkers of HIV disease state/treatment status, active viremia and the latent reservoir. By virtue of them being antigen-specific, these are robust context-specific biomarkers of HIV disease progression, viremia and reservoir size. Our framework highlights the strength of using systems approaches in identifying humoral biomarkers, and can be used in other contexts to identify antigen-specific correlates of infectious disease outcome.

Systems serology for decoding infection and vaccine-induced antibody responses to HIV-1

PURPOSE OF REVIEW

Experimental and analytical advances have enabled systematic, high-resolution studies of humoral immune responses, and are beginning to define mechanisms of immunity to HIV.

RECENT FINDINGS

High-throughput, information-rich experimental and analytical methods, whether genomic, proteomic, or transcriptomic, have firmly established their value across a diversity of fields. Consideration of these tools as trawlers in 'fishing expeditions' has faded as 'data-driven discovery' has come to be valued as an irreplaceable means to develop fundamental understanding of biological systems. Collectively, studies of HIV-1 infection and vaccination including functional, biophysical, and biochemical humoral profiling approaches have provided insights into the phenotypic characteristics of individual and pools of antibodies. Relating these measures to clinical status, protection/efficacy outcomes, and cellular profiling data using machine learning has offered the possibility of identifying unanticipated mechanisms of action and gaining insights into fundamental immunological processes that might otherwise be difficult to decipher.

SUMMARY

Recent evidence establishes that systematic data collection and application of machine learning approaches can identify humoral immune correlates that are generalizable across distinct HIV-1 immunogens and vaccine regimens and translatable between model organisms and the clinic. These outcomes provide a strong rationale supporting the utility and further expansion of these approaches both in support of vaccine development and more broadly in defining mechanisms of immunity.

The antibody response in HIV-1-infected donors

PURPOSE OF REVIEW

Although the goal of preventive HIV vaccine design is primarily the induction of broadly neutralizing antibodies (bNAbs), recent evidence suggests that a protective response will also benefit from Fc effector functions. Here, we provide an update on the antibody response to HIV infection, including both Fab and Fc-mediated antibody responses. We also highlight recent studies showing the interplay between these functions, focusing primarily on studies published in the last year.

RECENT FINDINGS

Identification and characterization of bNAb donors continues to provide insights into viral factors that are potentially translatable to vaccine design. Improved and more diverse measures of Fc effector function, and modulators thereof, are enabling a deeper understanding of their role in infection. New data providing mechanistic links between the innate and adaptive humoral immune responses are creating exciting opportunities for vaccine strategies, with the aim of eliciting a polyfunctional protective response.

SUMMARY

New insights into the overall humoral response to HIV infection are defining diverse and synergistic mechanisms required for antibody protection from HIV through vaccination.

Correlates of broadly neutralizing antibody development

PURPOSE OF REVIEW

Broadly neutralizing antibodies (bnAbs) are considered a key component of an effective HIV-1 vaccine, but despite intensive efforts, induction of bnAbs by vaccination has thus far not been possible. Potent bnAb activity is rare in natural infection and a deeper understanding of factors that promote or limit bnAb evolution is critical to guide bnAb vaccine development. This review reflects on recent key discoveries on correlates of bnAb development and discusses what further insights are needed to move forward.

RECENT FINDINGS

An increasing number of parameters have been implicated to influence bnAb development in natural infection. Most recent findings highlight a range of immune factors linked with bnAb evolution. Novel approaches have brought exciting progress in defining signatures of the viral envelope associated with bnAb activity.

SUMMARY

Focused efforts of recent years have unraveled a multiply layered process of HIV-1 bnAb development. As it is understood today, bnAb evolution can be triggered and influenced by a range of factors and several different pathways may exist how bnAb induction and maturation can occur. To capitalize on the gained knowledge, future research needs to validate factors to identify independent drivers of bnAb induction to advance vaccine design.

Vaccinal effect of HIV-1 antibody therapy

PURPOSE OF REVIEW

The review recalls recent findings regarding the induction of vaccinal effects by HIV-1 broadly neutralizing antibodies (bNAbs) and highlights potential therapeutic strategies to exploit such immunomodulatory properties.

RECENT FINDINGS

Studies in different animal models have shown that mAbs can generate long-lasting protective immunity. Induction of this vaccinal effect by HIV-1 bNAbs has also been more recently reported in animal models of HIV-1 infection. Notably, bNAbs treatment of macaques infected with the chimeric simian-human immunodeficiency virus (SHIV) improved both humoral and cellular adaptive immune responses that contributed to disease control. Importantly, this concept has been extended to HIV-1-infected patients as enhancement of humoral responses was recently reported in HIV-1 patients treated with bNAbs. Studies aiming at elucidating the mechanisms underlying these immunomodulatory properties of bNAbs have identified a role for immune complexes in shaping immune responses against HIV-1. They also highlight different Fc (fragment crystallizable) region effector functions that might be required for the enhancement of HIV-1 immune responses upon bNAbs treatment.

SUMMARY

HIV-1 bNAbs can elicit protective adaptive immune responses through mechanisms involving multiple cellular and molecular actors of the immune system. Harnessing these mechanisms will be crucial to achieve protective immunity against HIV-1 infection by bNAbs.

Characterization and Immunogenicity of HIV Envelope gp140 Zera® Tagged Antigens

HIV-1 envelope glycoprotein (Env) remains the most relevant target for the elicitation of functional antibodies to HIV by vaccination. However, soluble Env antigens often do not elicit the desired immune responses. Delivering subunit antigens on particulate nanoparticles is an established approach to improve their immunogenicity. In this study the sequence encoding Zera^®, a proline-rich domain derived from the γ-zein storage protein, was fused to either the C- or N-terminus of the superinfecting HIV-1 CAP256 gp140 envelope: Zera^® generally induces the formation of protein bodies (PBs), which can significantly improve both the immunogenicity and yields of the partner protein. The expression of gp140-Zera^® and Zera^®-gp140 (N- and C-terminal fusions respectively) in mammalian cells was confirmed by western blot analysis and immunostaining. However, isopycnic ultracentrifugation showed that neither gp140-Zera^® nor Zera^®-gp140 accumulated in characteristic electron-dense PBs. gp140-Zera^® elicited higher binding antibody titers in rabbits to autologous gp140 and V1V2 scaffold than Zera^®-gp140. Rabbit anti-gp140-Zera^® sera also had significantly higher Tier 1A neutralizing antibody titers than anti-Zera^®-gp140 sera. Neither gp140-Zera^® nor Zera^®-gp140-specific sera neutralized Tier 1B or autologous Tier 2 viruses. These results showed that HIV-1 gp140 tagged with Zera^® at either the N- or C-termini elicited high titers of gp140 and V1V2 binding antibodies, and low levels of Tier 1 neutralizing antibodies in rabbits.

Envelope characteristics in individuals who developed neutralizing antibodies targeting different epitopes in HIV-1 subtype C infection

Broadly neutralizing antibodies (bNAbs) may constitute an essential component of a protective vaccine against HIV-1, yet no immunogen has been able to elicit them. To characterize the development of bNAbs in HIV-1 subtype C infected individuals, a panel of 18 Env-pseudotyped viruses was used to screen 18 study participants. The specificity of plasma neutralization was mapped against Env mutants and MPER chimeras. Envelope (env) gene sequence evolution was characterized by single genome amplification and sequencing. Three out of eighteen individuals developed broad plasma neutralizing activity (>60% breadth). Two of the three participants may target epitopes comprising glycans at position 276 of the D loop in the CD4 binding site and 332 glycan supersite, respectively. Deletion of these glycans was associated with neutralization resistance. Our study describes the kinetics of the development of plasma neutralizing activity and identified amino acid residue changes suggestive of immune pressure on putative epitopes. The study enhances our understanding of how neutralization breadth develops in the course of HIV-1 subtype C infection.

Anti-HIV-1 antibodies trigger non-lytic complement deposition on infected cells

The effect of anti-HIV-1 antibodies on complement activation at the surface of infected cells remains partly understood. Here, we show that a subset of anti-Envelope (Env) broadly neutralizing antibodies (bNAbs), targeting the CD4 binding site and the V3 loop, triggers C3 deposition and complement-dependent cytotoxicity (CDC) on Raji cells engineered to express high surface levels of HIV-1 Env. Primary CD4 T cells infected with laboratory-adapted or primary HIV-1 strains and treated with bNAbs are susceptible to C3 deposition but not to rapid CDC. The cellular protein CD59 and viral proteins Vpu and Nef protect infected cells from CDC mediated by bNAbs or by polyclonal IgGs from HIV-positive individuals. However, complement deposition accelerates the disappearance of infected cells within a few days of culture. Altogether, our results uncover the contribution of complement to the antiviral activity of anti-HIV-1 bNAbs.

Update on Fc-Mediated Antibody Functions Against HIV-1 Beyond Neutralization

Antibodies (Abs) are the major component of the humoral immune response and a key player in vaccination. The precise Ab-mediated inhibitory mechanisms leading to in vivo protection against HIV have not been elucidated. In addition to the desired viral capture and neutralizing Ab functions, complex Ab-dependent mechanisms that involve engaging immune effector cells to clear infected host cells, immune complexes, and opsonized virus have been proposed as being relevant. These inhibitory mechanisms involve Fc-mediated effector functions leading to Ab-dependent cellular cytotoxicity, phagocytosis, cell-mediated virus inhibition, aggregation, and complement inhibition. Indeed, the decreased risk of infection observed in the RV144 HIV-1 vaccine trial was correlated with the production of non-neutralizing inhibitory Abs, highlighting the role of Ab inhibitory functions besides neutralization. Moreover, Ab isotypes and subclasses recognizing specific HIV envelope epitopes as well as pecular Fc-receptor polymorphisms have been associated with disease progression. These findings further support the need to define which Fc-mediated Ab inhibitory functions leading to protection are critical for HIV vaccine design. Herein, based on our previous review Su & Moog Front Immunol 2014, we update the different inhibitory properties of HIV-specific Abs that may potentially contribute to HIV protection.

An Inflammatory Story: Antibodies in Tuberculosis Comorbidities

Mycobacterium tuberculosis (Mtb) resides in a quarter of the world's population and is the causative agent for tuberculosis (TB), the most common infectious reason of death in humans today. Although cellular immunity has been firmly established in the control of Mtb, there is growing evidence that antibodies may also modulate the infection. More specifically, certain antibody features are associated with inflammation and are divergent in different states of human infection and disease. Importantly, TB impacts not just the healthy but also those with chronic conditions. While HIV represents the quintessential comorbid condition for TB, recent epidemiological evidence shows that additional chronic conditions such as diabetes and kidney disease are rising. In fact, the prevalence of diabetes as a comorbid TB condition is now higher than that of HIV. These chronic diseases are themselves independently associated with pro-inflammatory immune states that encompass antibody profiles. This review discusses isotypes, subclasses, post-translational modifications and Fc-mediated functions of antibodies in TB infection and in the comorbid chronic conditions of HIV, diabetes, and kidney diseases. We propose that inflammatory antibody profiles, which are a marker of active TB, may be an important biomarker for detection of TB disease progression within comorbid individuals. We highlight the need for future studies to determine which inflammatory antibody profiles are the consequences of comorbidities and which may potentially contribute to TB reactivation.