Evaluation of Antibody-Dependent Fc-Mediated Viral Entry, as Compared With Neutralization, in SARS-CoV-2 Infection

The SARS-CoV-2 antibody-dependent enhancement façade

Antibody-dependent enhancement (ADE) is an immunological paradox whereby sensitization following a primary viral infection results in the subsequent enhancement of a similar secondary infection. This idiosyncratic immune response has been established in dengue virus infections, driven by four antigenically related serotypes co-circulating in endemic regions. Several coronaviruses exhibit antibody-mediated mechanisms of viral entry, which has led to speculation of an ADE capacity for SARS-CoV-2, though in vivo and epidemiological evidence do not currently support this phenomenon. Three distinct antibody-dependent mechanisms for SARS-CoV-2 entry have recently been demonstrated: 1. FcR-dependent, 2. ACE2-FcR-interdependent, and 3. FcR-independent. These mechanisms of viral entry may be dependent on SARS-CoV-2 antibody specificity; antibodies targeting the receptor binding domain (RBD) typically result in Fc-dependent and ACE2-FcR-interdependent entry, whereas antibodies targeting the N-terminal domain can induce a conformational change to the RBD that optimizes ACE2-receptor binding domain interactions independent of Fc receptors. Whether these antibody-dependent entry mechanisms of SARS-CoV-2 result in the generation of infectious progenies and enhancement of infection has not been robustly demonstrated. Furthermore, ADE of SARS-CoV-2 mediated by antigenic seniority remains a theoretical concern, as no evidence suggests that SARS-CoV-2 imprinting blunts a subsequent immune response, contributing to severe COVID-19 disease.

Beyond bNAbs: Uses, Risks, and Opportunities for Therapeutic Application of Non-Neutralising Antibodies in Viral Infection

The vast majority of antibodies generated against a virus will be non-neutralising. However, this does not denote an absence of protective capacity. Yet, within the field, there is typically a large focus on antibodies capable of directly blocking infection (neutralising antibodies, NAbs) of either specific viral strains or multiple viral strains (broadly-neutralising antibodies, bNAbs). More recently, a focus on non-neutralising antibodies (nNAbs), or neutralisation-independent effects of NAbs, has emerged. These can have additive effects on protection or, in some cases, be a major correlate of protection. As their name suggests, nNAbs do not directly neutralise infection but instead, through their Fc domains, may mediate interaction with other immune effectors to induce clearance of viral particles or virally infected cells. nNAbs may also interrupt viral replication within infected cells. Developing technologies of antibody modification and functionalisation may lead to innovative biologics that harness the activities of nNAbs for antiviral prophylaxis and therapeutics. In this review, we discuss specific examples of nNAb actions in viral infections where they have known importance. We also discuss the potential detrimental effects of such responses. Finally, we explore new technologies for nNAb functionalisation to increase efficacy or introduce favourable characteristics for their therapeutic applications.

Identification of a highly conserved neutralizing epitope within the RBD region of diverse SARS-CoV-2 variants

The constant emergence of SARS-CoV-2 variants continues to impair the efficacy of existing neutralizing antibodies, especially XBB.1.5 and EG.5, which showed exceptional immune evasion properties. Here, we identify a highly conserved neutralizing epitope targeted by a broad-spectrum neutralizing antibody BA7535, which demonstrates high neutralization potency against not only previous variants, such as Alpha, Beta, Gamma, Delta and Omicron BA.1-BA.5, but also more recently emerged Omicron subvariants, including BF.7, CH.1.1, XBB.1, XBB.1.5, XBB.1.9.1, EG.5. Structural analysis of the Omicron Spike trimer with BA7535-Fab using cryo-EM indicates that BA7535 recognizes a highly conserved cryptic receptor-binding domain (RBD) epitope, avoiding most of the mutational hot spots in RBD. Furthermore, structural simulation based on the interaction of BA7535-Fab/RBD complexes dissects the broadly neutralizing effect of BA7535 against latest variants. Therapeutic and prophylactic treatment with BA7535 alone or in combination with BA7208 protected female mice from the circulating Omicron BA.5 and XBB.1 variant infection, suggesting the highly conserved neutralizing epitope serves as a potential target for developing highly potent therapeutic antibodies and vaccines.

Advax-CpG55.2-adjuvanted monovalent or trivalent SARS-CoV-2 recombinant spike protein vaccine protects hamsters against heterologous infection with Beta or Delta variants

The ongoing evolution of SARS-CoV-2 variants emphasizes the need for vaccines providing broad cross-protective immunity. This study was undertaken to assess the ability of Advax-CpG55.2 adjuvanted monovalent recombinant spike protein (Wuhan, Beta, Gamma) vaccines or a trivalent formulation to protect hamsters againstBeta or Delta virus infection. The ability of vaccines to block virus transmission to naïve co-housed animals was also assessed. In naïve hosts, the Beta variant induced higher virus loads than the Delta variant, and conversely the Delta variant caused more severe disease and was more likely to be associated with virus transmission. The trivalent vaccine formulation provided the best protection against both Beta and Delta infection and also completely prevented virus transmission. The next best performing vaccine was the original monovalent Wuhan-based vaccine. Notably, hamsters that received the monovalent Gamma spike vaccine had the highest viral loads and clinical disease of all the vaccine groups, a potential signal of antibody dependent-enhancement (ADE). These hamsters were also the most likely to transmit Delta virus to naïve recipients. In murine studies, the Gamma spike vaccine induced the highest total spike protein to RBD IgG ratio and the lowest levels of neutralizing antibody, a context that could predispose to ADE. Overall, the study results confirmed that the current SpikoGen® vaccine based on Wuhan spike protein was still able to protect against clinical disease caused by either the Beta or Delta virus variants but suggested additional protection may be obtained by combining it with extra variant spike proteins to make a multivalent formulation. This study highlights the complexity of optimizing vaccine protection against multiple SARS-CoV-2 variants and stresses the need to continue to pursue new and improved COVID-19 vaccines able to provide robust, long-lasting, and broadly cross-protective immunity against constantly evolving SARS-CoV-2 variants.

In Vitro Antibody-Dependent Enhancement of SARS-CoV-2 Infection Could Be Abolished by Adding Human IgG

Evidence of antibody-dependent enhancement (ADE) of other viruses has raised concerns about the safety of SARS-CoV-2 vaccines and antibody therapeutics. In vitro studies have shown ADE of SARS-CoV-2 infection. In this study, we also found that vaccination/convalescent sera and some approved monoclonal antibodies can enhance SARS-CoV-2 infection of FcR-expressing B cells in vitro. However, the enhancement of SARS-CoV-2 infection can be prevented by blocking Fc-FcR interaction through the addition of human serum/IgG or the introduction of mutations in the Fc portion of the antibody. It should be noted that ADE activity observed on FcR-expressing cells in vitro may not necessarily reflect the situation in vivo; therefore, animal and clinical data should be included for ADE evaluation.

SARS-CoV-2 infection of phagocytic immune cells and COVID-19 pathology: Antibody-dependent as well as independent cell entry

Our review summarizes the evidence that COVID-19 can be complicated by SARS-CoV-2 infection of immune cells. This evidence is widespread and accumulating at an increasing rate. Research teams from around the world, studying primary and established cell cultures, animal models, and analyzing autopsy material from COVID-19 deceased patients, are seeing the same thing, namely that some immune cells are infected or capable of being infected with the virus. Human cells most vulnerable to infection include both professional phagocytes, such as monocytes, macrophages, and dendritic cells, as well as nonprofessional phagocytes, such as B-cells. Convincing evidence has accumulated to suggest that the virus can infect monocytes and macrophages, while data on infection of dendritic cells and B-cells are still scarce. Viral infection of immune cells can occur directly through cell receptors, but it can also be mediated or enhanced by antibodies through the Fc gamma receptors of phagocytic cells. Antibody-dependent enhancement (ADE) most likely occurs during the primary encounter with the pathogen through the first COVID-19 infection rather than during the second encounter, which is characteristic of ADE caused by other viruses. Highly fucosylated antibodies of vaccinees seems to be incapable of causing ADE, whereas afucosylated antibodies of persons with acute primary infection or convalescents are capable. SARS-CoV-2 entry into immune cells can lead to an abortive infection followed by host cell pyroptosis, and a massive inflammatory cascade. This scenario has the most experimental evidence. Other scenarios are also possible, for which the evidence base is not yet as extensive, namely productive infection of immune cells or trans-infection of other non-immune permissive cells. The chance of a latent infection cannot be ruled out either.