A comparative pathogenesis study was performed in neonatal mice using a molecularly cloned laboratory variant of Sindbis strain AR339, designated TRSB, and a single-site attenuated mutant of TRSB derived by site-directed mutagenesis of the E2 glycoprotein from Ser to Arg at residue 114 (TRSBr114). TRSB caused 100% mortality with an average survival time of 3.0 ± 0.7 days, whereas mice inoculated with TRSBr114 exhibited an attenuated disease course with 46% mortality and an extended average survival time of 7.5 ± 3.4 days for those animals that died. Reduced virulence of TRSBr114 was characterized by delayed appearance of detectable virus, relative to TRSB, and by lower peak virus titers in both sera and brains of infected mice. TRSB infection induced very high peak serum titers of interferon alpha/beta (215,000 units/ml compared to 2100 units/ml for TRSBr114). In situ hybridization analysis demonstrated replication of TRSB in brain, but only minimal histopathological changes and no evidence of encephalitis were observed. However, extensive extraneural lesions and viral replication were found in skin, connective tissue, and muscle. Moreover, dramatic involution of the thymus and loss of hematopoietic tissues were observed in the absence of virus replication at these sites, suggesting the involvement of a systemic physiological stress response in TRSB infection. TRSBr114 infection did not cause thymic lesions. Otherwise, the attenuated mutant demonstrated a similar pattern of tissue and organ involvement, but lesions and positive in situ hybridization signal were much more limited in scope and intensity compared to TRSB. TRSBr114-infected mice developed myositis and encephalomyelitis approximately 6 days postinfection. Therefore, TRSB-infected animals may succumb to an early syndrome associated with the stress response, preventing their survival for a time sufficient for the development of encephalitis. Alternatively, a systemic stress response, as evidenced by thymic involution, may result in immunosuppression, thus contributing to the absence of encephalitis. In any event, the attenuating mutation in the E2 glycoprotein significantly altered the course of Sindbis-induced disease by limiting virus replication and associated damage early in infection. Mutant-infected animals survived beyond Day 4 and progressed to a classical encephalomyelitis from which about half recovered.
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