FATAL SINDBIS VIRUS-INFECTION OF NEONATAL MICE IN THE ABSENCE OF ENCEPHALITIS

A comparative pathogenesis study was performed in neonatal mice using a molecularly cloned laboratory variant of Sindbis strain AR339, desig nated TRSB, and a single-site attenuated mutant of TRSB derived by sit e-directed mutagenesis of the E2 glycoprotein from Ser to Arg at resid ue 114 (TRSBr114). TRSB caused 100% mortality with an average survival time of 3.0+/-0.7 days, whereas mice inoculated with TRSBr114 exhibit ed an attenuated disease course with 46% mortality and an extended ave rage survival time of 7.5+/-3.4 days for those animals that died. Redu ced virulence of TRSBr114 was characterized by delayed appearance of d etectable virus, relative to TRSB, and by lower peak virus titers in b oth 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 demons trated replication of TRSB in brain, but only minimal histopathologica l changes and no evidence of encephalitis were observed. However, exte nsive extraneural lesions and viral replication were found in skin, co nnective tissue, and muscle. Moreover, dramatic involution of the thym us and loss of hematopoietic tissues were observed in the absence of v irus replication at these sites, suggesting the involvement of a syste mic physiological stress response in TRSB infection. TRSBr114 infectio n did not cause thymic lesions. Otherwise, the attenuated mutant demon strated a similar pattern of tissue and organ involvement, but lesions and positive in situ hybridization signal were much more limited in s cope and intensity compared to TRSB. TRSBr114-infected mice developed myositis and encephalomyelitis approximately 6 days postinfection. The refore, TRSB-infected animals may succumb to an early syndrome associa ted with the stress response, preventing their survival for a time suf ficient for the development of encephalitis. Alternatively, a systemic stress response, as evidenced by thymic involution, may result in imm unosuppression, thus contributing to the absence of encephalitis. In a ny event, the attenuating mutation in the 52 glycoprotein significantl y altered the course of Sindbis-induced disease by limiting virus repl ication and associated damage early in infection. Mutant-infected anim als survived beyond Day 4 and progressed to a classical encephalomyeli tis from which about half recovered. (C) 1996 Academic Press, Inc.