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Antibody-dependent enhancement (ADE) of viral entry has been a major concern for epidemiology, vaccine development, and antibody-based drug therapy. However, the molecular mechanism behind ADE is still elusive. Coronavirus spike protein mediates viral entry into cells by first binding to a receptor on the host cell surface and then fusing viral and host membranes. In this study, we investigated how a neutralizing monoclonal antibody (MAb), which targets the receptorbinding domain (RBD) of Middle East respiratory syndrome (MERS) coronavirus spike, mediates viral entry using pseudovirus entry and biochemical assays. Our results showed that MAb binds to the virus surface spike, allowing it to undergo conformational changes and become prone to proteolytic activation. Meanwhile, MAb binds to cell surface IgG Fc receptor, guiding viral entry through canonical viral-receptordependent pathways. Our data suggest that the antibody/Fc-receptor complex functionally mimics viral receptor in mediating viral entry. Moreover, we characterized MAb dosages in viral-receptor-dependent, Fc-receptor-dependent, and both-receptorsdependent viral entry pathways, delineating guidelines on MAb usages in treating viral infections. Our study reveals a novel molecular mechanism for antibody-enhanced viral entry and can guide future vaccination and antiviral strategies. IMPORTANCE Antibody-dependent enhancement (ADE) of viral entry has been observed for many viruses. It was shown that antibodies target one serotype of viruses but only subneutralize another, leading to ADE of the latter viruses. Here we identify a novel mechanism for ADE: a neutralizing antibody binds to the surface spike protein of coronaviruses like a viral receptor, triggers a conformational change of the spike, and mediates viral entry into IgG Fc receptor-expressing cells through canonical viral-receptor-dependent pathways. We further evaluated how antibody dosages impacted viral entry into cells expressing viral receptor, Fc receptor, or both receptors. This study reveals complex roles of antibodies in viral entry and can guide future vaccine design and antibody-based drug therapy.KEYWORDS antibody-dependent enhancement of viral entry, MERS coronavirus, SARS coronavirus, spike protein, neutralizing antibody, viral receptor, IgG Fc receptor, antibody-dependent enhancement of viral entry A ntibody-dependent enhancement (ADE) occurs when antibodies facilitate viral entry into host cells and enhance viral infection in these cells (1, 2). ADE has been observed for a variety of viruses, most notably flaviviruses (e.g., dengue virus) (3-6). It has been shown that when patients are infected by one serotype of dengue virus (i.e., Downloaded fromprimary infection), they produce neutralizing antibodies targeting the same serotype of the virus. However, if they are later infected by another serotype of dengue virus (i.e., secondary infection), the preexisting antibodies cannot fully neutralize the virus. Instead, the antibodies first bind to the virus and then bind to the IgG Fc rec...

Abstract

Antigen presenting cells recognize pathogens via pattern recognition receptors (PRR), which upon ligation transduce intracellular signals that can induce innate immune responses. Because some C-type lectin-like receptors (e.g. dectin-1 and DC-SIGN) were shown to act as PRR for particular microbes, we considered a similar role for dectin-2. Binding assays using soluble dectin-2 receptors showed the extracellular domain to bind preferentially to hyphal (rather than yeast/conidial) components of Candida albicans, Microsporum audouinii, and Trichophyton rubrum. Selective binding for hyphae was also observed using RAW macrophages expressing dectin-2, the ligation of which by hyphae or cross-linking with dectin-2-specific antibody led to protein tyrosine phosphorylation. Because dectin-2 lacks an intracellular signaling motif, we searched for a signal adaptor that permits it to transduce intracellular signals. First, we found that the Fc receptor ␥ (FcR␥) chain can bind to dectin-2. Second, ligation of dectin-2 on RAW cells induced tyrosine phosphorylation of FcR␥, activation of NF-B, internalization of a surrogate ligand, and up-regulated secretion of tumor necrosis factor ␣ and interleukin-1 receptor antagonist. Finally, these dectin-2-induced events were blocked by PP2, an inhibitor of Src kinases that are mediators for FcR␥ chain-dependent signaling. We conclude that dectin-2 is a PRR for fungi that employs signaling through FcR␥ to induce innate immune responses.

Family studies of first-degree relatives and analysis of twins indicate that as much as 75% of the differences in quantitative IL-10 production in man derive from heritable genetic factors. Studies of single nucleotide polymorphisms (SNP) in the proximal 1.0 kb of the IL-10 promoter have yielded inconsistent association with IL-10 production and variable results in promoter-reporter studies. However, in normal donors, an association of quantitative production with certain alleles of the IL-10.R short tandem repeat polymorphism at −4.0 kb suggested that SNPs in the more distal promoter might be informative. We have identified seven novel SNP sites in the genomic sequence of the first 4 kb of the IL-10 promoter region 5′ to the ATG start site from Caucasian individuals with either a high or a low IL-10 production phenotype. We have also identified eight SNP haplotypes in the distal promoter that segregate with significant differences in quantitative IL-10 production in normal donors. These SNPs are significantly associated with systemic lupus erythematosus in African-Americans and may define one component of the genetic susceptibility to systemic lupus erythematosus in this group.