Summary Human antibodies to HIV-1 can neutralize a broad range of viral isolates in vitro and protect non-human primates against infection1,2. Previous work showed that antibodies exert selective pressure on the virus but escape variants emerge within a short period of time3,4. However, these experiments were performed before the recent discovery of more potent anti-HIV-1 antibodies and their improvement by structure-based design5-9. Here we re-examine passive antibody transfer as a therapeutic modality in HIV-1-infected humanized mice (hu-mice). Although HIV-1 can escape from antibody monotherapy, combinations of broadly neutralizing antibodies (bNAbs) can effectively control HIV-1 infection and suppress viral load to levels below detection. Moreover, in contrast to antiretroviral therapy (ART)10-12, the longer half-life of antibodies led to viremic control for an average of 60 days after cessation of therapy. Thus, combinations of potent monoclonal antibodies can effectively control HIV-1 replication in hu-mice, and should be re-examined as a therapeutic modality in HIV-1-infected individuals.

Summary Broadly neutralizing antibodies (bNAbs) against HIV-1 provide both effective pre-exposure prophylaxis and treatment of HIV-1 infection in murine and non-human primate models, suggesting their potential use in humans. While 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 novel strategies for generating bNAbs with improved efficacy.

Antibodies produced in response to a foreign antigen are characterized by polyclonality, not only in the diverse epitopes to which their variable domains bind but also in the various effector molecules to which their constant regions (Fc domains) engage. Thus, the antibody's Fc domain mediates diverse effector activities by engaging two distinct classes of Fc receptors (type I and type II) on the basis of the two dominant conformational states that the Fc domain may adopt. These conformational states are regulated by the differences among antibody subclasses in their amino acid sequence and by the complex, biantennary Fc-associated N-linked glycan. Here we discuss the diverse downstream proinflammatory, anti-inflammatory and immunomodulatory consequences of the engagement of type I and type II Fc receptors in the context of infectious, autoimmune, and neoplastic disorders.

Summary Latent reservoirs of HIV-1 infected cells are refractory to antiretroviral therapies (ART) and remain the major barrier to curing HIV-1. Because latently infected cells are long-lived, immunologically invisible, and may undergo homeostatic proliferation, a “shock and kill” approach has been proposed to eradicate this reservoir by combining ART with inducers of viral transcription. However, all attempts to alter the HIV-1 reservoir in vivo have failed to date. Using humanized mice, we show that broadly neutralizing antibodies (bNAbs) can interfere with establishment of a silent reservoir by Fc-FcR mediated mechanisms. In established infection, bNAbs or bNAbs plus single inducers are ineffective in preventing viral rebound. However, bNAbs plus a combination of inducers that act by independent mechanisms synergize to decrease the reservoir as measured by viral rebound. Thus, combinations of inducers and bNAbs constitute a therapeutic strategy that impacts the establishment and maintenance of the HIV-1 reservoir in humanized mice.

The in vivo biological activities of IgG antibodies result from their bifunctional nature, in which antigen recognition by the Fab is coupled to the effector and immunomodulatory diversity found in the Fc domain. This diversity, resulting from both amino acid and glycan heterogeneity, is translated into cellular responses through Fcγ receptors (FcγRs), a structurally and functionally diverse family of cell surface receptors found throughout the immune system. Although many of the overall features of this system are maintained throughout mammalian evolution, species diversity has precluded direct analysis of human antibodies in animal species, and, thus, detailed investigations into the unique features of the human IgG antibodies and their FcγRs have been limited. We now report the development of a mouse model in which all murine FcγRs have been deleted and human FcγRs, encoded as transgenes, have been inserted into the mouse genome resulting in recapitulation of the unique profile of human FcγR expression. These human FcγRs are shown to function to mediate the immunomodulatory, inflammatory, and cytotoxic activities of human IgG antibodies and Fc engineered variants and provide a platform for the detailed mechanistic analysis of therapeutic and pathogenic IgG antibodies.

Amid the ongoing COVID-19 pandemic, efforts to actively vaccinate the general population against the SARS-CoV-2 virus in the context of poorly neutralizing and waning immunity have renewed interest in the phenomenon of antibody-dependent enhancement (ADE). This property of antibodies attributes enhanced disease pathogenesis in specific instances of viral infection to the presence of sub-neutralizing titres of antiviral host antibodies. In cases of ADE, rather than contributing to antiviral immunity, pre-existing antibodies facilitate viral entry and subsequent infection of host cells, leading to both increased infectivity and virulence. ADE was first clearly described in dengue virus (DENV) infection by Halstead et al. in 1973 (refs 1,2), although earlier epidemiological evidence had identified that two specific Thai patient populations, specifically first-time infected infants born to immune mothers and children suffering from secondary infection, were associated with an increased incidence of dengue haemor rhagic fever and dengue shock syndrometwo patient groups that ostensibly had pre-existing antibodies to DENV 3. It was not until years later that studies proposed models of ADE, identified optimal conditions for in vitro ADE and quantified antibody titres permissive for ADE 4-9. Numerous studies have since identified Fcγ receptors (FcγRs), surface receptors on immune cells that recognize the Fc portion of IgG and trigger a wide array of downstream effector functions, as the key mediators of ADE in dengue pathogenesis, as they allow for the internalization of multimeric virus-bound IgG and subsequent productive infection.

SARS-CoV-2, the causative agent of COVID-19, has been responsible for over 42 million infections and 1 million deaths since its emergence in December 2019. There are few therapeutic options and no approved vaccines. Here, we examine the properties of highly potent human monoclonal antibodies (hu-mAbs) in a Syrian hamster model of SARS-CoV-2 and in a mouse-adapted model of SARS-CoV-2 infection (SARS-CoV-2 MA). Antibody combinations were effective for prevention and in therapy when administered early. However, in vitro antibody neutralization potency did not uniformly correlate with in vivo protection, and some hu-mAbs were more protective in combination in vivo. Analysis of antibody Fc regions revealed that binding to activating Fc receptors contributes to optimal protection against SARS-CoV-2 MA. The data indicate that intact effector function can affect hu-mAb protective activity and that in vivo testing is required to establish optimal hu-mAb combinations for COVID-19 prevention.

Anti-retroviral drugs and antibodies limit HIV-1 infection by interfering with the viral life-cycle. In addition, antibodies also have the potential to guide host immune effector cells to kill HIV-1 infected cells. Examination of the kinetics of HIV-1 suppression in infected individuals by passively administered 3BNC117, a broadly neutralizing antibody (bNAb), suggested that the effects of the antibody are not limited to free viral clearance and blocking new infection, but also include acceleration of infected cell clearance. Consistent with these observations, we find that bNAbs can target CD4+ T cells infected with patient viruses and decrease their in vivo half-lives by a mechanism that requires FcγR engagement in a humanized mouse model. The results indicate that passive immunotherapy can accelerate elimination of HIV-1 infected cells.