Monoclonal antibody cure and prophylaxis of lethal Sindbis virus encephalitis in mice

Neuroadapted Sindbis virus (NSV) causes acute encephalitis and paralyzes and kills adult mice unless they are treated with primary immune serum after infection. To study the nature and specificity of curative antibodies, we gave mice 30 different monoclonal antibodies (MAbs) against Sindbis virus (SV) 24 h after lethal intracerebral inoculation of NSV. By the time of MAb treatment, NSV replication in the brain had been well established (7.5 X 10(7) PFU/g). Seventeen MAbs directed against multiple biological domains on the NSV E1 and E2 envelope glycoproteins prevented paralysis and death. Anticapsid MAbs failed to protect. Altogether, 15 of 17 curative MAbs either neutralized NSV infectivity or lysed NSV-infected cells with complement, but neither ability was necessary or sufficient to guarantee recovery. All 5 protective anti-E2 MAbs neutralized NSV infectivity; 6 of 10 protective anti-E1 MAbs neutralized NSV; 4 did not. Plaque assay or immunohistochemical staining showed that neutralizing and nonneutralizing curative MAbs decreased NSV in the brain, brainstem, and spinal cord. Despite high neutralization titers, hyperimmune anti-SV and anti-NSV mouse sera prevented only 6 and 30% of deaths, respectively, while primary immune sera prevented 50 (SV) and 90% (NSV) of deaths. Secondary intravenous immunization with a live virus apparently diminished, obscured, or failed to boost a class of protective antibodies. When separate mouse groups were given these 30 MAbs 24 h before lethal intracerebral inoculation of NSV, a slightly different set of 17 neutralizing or nonneutralizing anti-E1 and anti-E2 antibodies protected. Two nonneutralizing MAbs and hyperimmune anti-SV serum, which had failed to promote recovery, prophylactically protected 100% of the mice. The antibody requirements or mechanisms of prophylaxis and recovery may differ.