| Summary: | Intro: Antibodies play a critical role in protection and pathogenesis of dengue virus (DENV). In the presence of preexisting antibodies that bind but do not neutralize viral particles, virus-antibody immune complexes are internalized by Fcγ receptor (FcγR)-expressing cells, generating increased viral titers and the liberation of pathogenic mediators. The complement protein C1q can bind to immune complexes and reduce the magnitude of antibody-dependent enhancement (ADE). We aimed to investigate the mechanisms of C1q modulation of ADE, focusing on processes of viral entry. Methods: We established a model of C1q modulation of ADE of DENV-1 infection in human cell lines using monoclonal antibodies (anti-flavivirus envelope protein 4G2 and anti-DENV pre-membrane protein 2H2), and physiological concentrations of C1q. We analyzed the mechanism of this modulation using cell-based adsorption and internalization assays and blockade of specific routes of endocytosis with pharmacological inhibitors. Findings: The modulation of ADE of DENV-1 by C1q is dependent on the FcγR and the specificity of the antibody comprising the immune complex. C1q produced a 250- to 23-fold reduction of ADE of DENV-1 in cells that express the low affinity type II FcγR (K562 cells), but not in cells that express the high affinity type I FcγR (U937 cells). C1q reduced entry of immune complexes composed of DENV-1 and 4G2 or 2H2 during adsorption (99% and 90% reduction in viral titer; p=0.015 and p=0.02, respectively), but in the case of immune complexes formed by 2H2, C1q also reduced ADE during internalization of immune complexes (82% reduction, p=0.02). The mechanism of C1q modulation of ADE corresponded to the participation of different components of endocytic pathways. Conclusion: The study of virus-host interactions that drive protective immunity, and the immunological mechanisms that regulate the balance between protection and pathogenesis mediated by the humoral immune response in DENV infections, are key for effective antiviral and vaccine design.
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