Abstract

The complement system is a critical host defense against infection, playing a protective role that can also enhance disease if misregulated. Although many consequences of complement activation during viral infection are well-established, specific mechanisms that contribute to activation by different human viruses remain elusive. Here, we investigate complement activation by human respiratory syncytial virus (RSV), a respiratory pathogen that causes severe disease in infants, the immunocompromised, and the elderly. Using a strain of RSV harboring tags on the surface glycoproteins F and G, we were able to monitor opsonization of single RSV particles with monoclonal antibodies and complement components using fluorescence microscopy. These experiments revealed an antigenic hierarchy in complement activation, where antibodies that bind towards the apex of F in either the pre- or postfusion conformation are able to activate complement whereas other antibodies are not. Additionally, among antibodies that were able to activate complement, we observed preferential targeting of a subset of particles with globular morphology, in contrast to the more prevalent viral filaments. We found that enhanced complement activation on these particles arises from changes in surface curvature that occur when the viral matrix detaches from the surrounding membrane. This transformation occurs naturally over time under mild conditions, and correlates with the accumulation of postfusion F on the viral surface. Collectively, these results identify antigenic and biophysical characteristics of virus particles that contribute to the formation of immune complexes, and suggest models for how these factors may shape disease severity and adaptive immune responses to RSV.