RECOMBINANT MEASLES VIRUSES WITH MUTATIONS IN THE C-GENE, V-GENE, OR F-GENE HAVE ALTERED GROWTH PHENOTYPES IN-VIVO

An understanding of the determinants of measles virus (MV) virulence h as been hampered by the lack of an experimental model of infection. We have previously demonstrated that virulence phenotypes in human infec tions are faithfully reproduced by infection of human thymus/liver (th y/liv) implants engrafted into SCID mice, where the virus grows primar ily in stromal cells but induces thymocyte apoptosis (P. G. Auwaerter et al., J. Virol. 70:3734-3740, 1996). To begin to elucidate the roles of the C protein, V protein, and the 5' untranslated region of the F gene (F 5'UTR) in MV infection in vivo, the replication of strains bea ring mutations of these genes was compared to that of the parent seque nce-tagged Edmonston strain (EdTag). Growth curves show that mutants f all into two phenotypic classes. One class of mutants demonstrated kin etics of growth similar to that of EdTag, with decreased peak titers. The second class of mutants manifested peak titers similar to that of EdTag but had different replication kinetics. Abrogation of V expressi on led to delayed and markedly prolonged replication. Additionally, th ymocyte survival was prolonged and implant architecture was preserved throughout the course of infection. In contrast, massive bystander thy mocyte death occurred after infection with EdTag and all other mutants . A mutant which overexpressed V in Vero cells (V+) had the opposite p henotype of the A mutant not expressing V (V-). V+ grew more rapidly t han EdTag with 100-fold-greater levels of virus production 3 days afte r infection. These results suggest that C, V, and the F 5'UTR are acce ssory factors required for efficient virus replication in vivo. In add ition, thymocyte survival after V- infection suggests this protein may play multiple roles in pathogenesis of MV infection of thymus. Since these recombinant mutant viruses grew identically to the parent virus in Vero cells, the data show that thy/liv implants are an excellent mo del for investigating the determinants of MV virulence.