Interactions of OspA monoclonal antibody C3.78 with Borrelia burgdorferi within ticks

The Borrelia burgdorferi outer surface protein A (OspA) vaccine induces antibodies that prevent transmission from the tick to the host. Here we describe studies with an OspA monoclonal antibody (C3.78) to understand the mechanism by which antibodies entering the tick block Borrelia transmission. Host complement in the tick's blood meal did not contribute to protection because the antibody was equally effective whether infected ticks fed on normal or complement-deficient mice. Antibody-mediated cross-linking of bacteria or cross-linking of OspA molecules was not required for protection because C3.78 Fab' fragments were as effective as whole antibody molecules. At low C3.78 concentrations, transmission was blocked despite the presence of many live spirochetes within the tick, confirming that clearance of Borrelia organisms was not required to block transmission. We propose that OspA antibody binding to the surface of spirochetes blocks transmission by a mechanism that does not require bacterial killing.