Monoclonal IgM antibodies mediate potent complement neutralization by targeting cell-derived epitopes on enveloped viruses.

When studying virus neutralization by specific antibodies of the adaptive immune response, serum heat treatment is a standard procedure done to eliminate any non-specific effects by the complement system. Although non-specific, innate immune responses such as these may be relevant in determining infection outcome. We observed extensive variation between negative control donors in their ability to neutralize Ebola virus and other enveloped viruses in the presence of complement. The effect was mediated by the classical pathway of complement activation, with serum IgM the main initiator in a manner that depended on the cell line that produced the virus. To identify the epitope, three monoclonal IgM antibodies were isolated after a neutralization screen and probed against arrayed glycans. The antibodies were found to bind ganglioside GM2, and the test virus was rendered refractory to anti-GM2 neutralization when propagated in the presence of an inhibitor of ganglioside synthesis. Gangliosides and other host membrane moieties are known to be carried by viruses and to aid entry to cells. The findings here suggest that under specific circumstances, such moieties incorporated in lipid envelopes of viruses can be recognized by potently neutralizing IgM antibodies present in the serum of immunologically naïve individuals.IMPORTANCENeutralization, that is, the ability to block infection in cell culture, remains the primary functional attribute of therapeutic antibodies and humoral immune responses against viruses. Beyond direct effects-such as preventing virus attachment to the cell-virus inactivation by antibody-mediated complement deposition or lysis is often considered a type of neutralization. Viral envelope glycoproteins are the main targets of neutralizing antibodies. However, because viruses obtain their membranes from cells in which they replicate, the membranes are known to contain cell-derived markers in addition to virally encoded proteins. Having observed marked individual variance in naïve serum donors' ability to neutralize Ebola virus and other enveloped viruses, we sought to understand the mechanism. Three IgM monoclonals targeting a cell-derived epitope were isolated. The findings expand on the importance of cellular factors exposed on the virion membrane and suggest that they may be targets of potent antibody-mediated immune monitoring.