Infectious virus-antibody complexes of sindbis virus

Infectious virus-antibody complexes were formed when Sindbis virus was reacted with antibodies raised against purified viral envelope glycoproteins E1 and E2 as well as against preparations of intact virus. Results from rate zonal centrifugation in sucrose gradients of the complex formed with anti-E1 sera showed this complex to be about the same size as virions. A test of virus neutralization, based on direct plaque assay, by antibodies raised in rabbits and mice given virus in complete Freund adjuvant indicated the presence of antibodies able to complex but not neutralize virus. Conditions were found in which most of the virus was complexed and protected fron neutralization, suggesting that these sera may contain a mixed population of antiviral antibodies with different specificities and different avidities.