Persistence of viral RNA in mouse brains after recovery from acute alphavirus encephalitis

Little is known about the relationship between recovery from acute viral encephalitis and the clearance of viral genetic material from the central nervous system. In a mouse model of Sindbis virus encephalitis, we have previously shown that clearance of infectious virus is mediated by antibody-induced restriction of viral gene expression rather than by cytotoxic destruction of virally infected cells. To explore whether Sindbis virus genomes persist in mouse brain after the clearance of infectious virus, we used reverse transcriptase-polymerase chain reaction amplification methods to detect Sindbis virus RNA in brain samples from immunocompetent BALB/c and antibody-treated immunodeficient scid/CB17 mice. RNA sequences from both the nonstructural region (NSP1 gene) and structural regions (E2 gene) of Sindbis virus were detected in the brains of all BALB/c and antibody-treated scid mice examined at 1, 2, and 3 months after infection. Additional BALB/c mouse brains were also positive at 8, 12, and 17 months after infection. To determine whether persistent RNA was capable of resuming unrestricted replication in the absence of the continuous presence of antiviral antibodies, viral titers were measured in the brains of scid mice at 1, 2, 3, and 6 months after antibody treatment. Viral reactivation was seen in scid mice treated with hyperimmune serum or a low dose of monoclonal antibody to the E2 envelope glycoprotein, but not in mice treated with a high dose of monoclonal antibody to E2. Replication of infectious virus isolated from scid mouse brain could be restricted by repeat treatment with immune serum, indicating that viral reactivation is not due to antibody-escape mutations. These results demonstrate that Sindbis virus can persist long term in a nonproductive form in mouse brain and suggest that the humoral immune response plays an important role in preventing viral reactivation.