The formation of tumors in adult nude mice from transformed human mammary epithelial cells was drastically inhibited (greater than 80%) both after coinjection of tumoral cells and virus or after a single s.c. injection of parvovirus H-1 at the site of cell implantation prior to tumor formation. Moreover, when injected i.v. in animals bearing preformed tumors, H-1 virus was able to slow down and even in some cases to revert neoplastic growth. Thus, H-1 virus achieved the suppression of implanted tumors of human origin under conditions where the immune antitumor mechanisms of the recipient animals were dramatically impaired. Viral infection was not accompanied by detectable deleterious side effects. Imprints of H-1 virus DNA were found in one residual tumor. Normal human mammary epithelial cells were also compared with homologous transformed cells, either derived from tumors (three lines) or containing simian virus 40 (one line), for their susceptibility to the lytic replication of H-1 virus in vitro. Transformed cell lines were more sensitive to virus-induced killing than secondary cultures of normal cells. Moreover, the former had much greater abilities than the latter to amplify viral DNA and to express the viral nonstructural protein NS-1. Altogether, these results are compatible with the idea that the oncosuppressive activity exerted by H-1 virus may be mediated, at least in part, by virus replication in developing tumors.
Morphologically altered and established human fibroblasts, obtained either by 60Co gamma irradiation, treatment with the carcinogen 4-nitroquinoline 1-oxide, or simian virus 40 (SV40) infection, were compared with their normal finite-life parental strains for susceptibility to the autonomous parvoviruses H-1 virus and the prototype strain of minute virus of mice (MVMp). All transformed cells suffered greater virus-induced killing than their untransformed progenitors. The cytotoxic effect of H-1 virus was more severe than that of MVMp. Moreover, the level of viral DNA replication was much (10- to 85-fold) enhanced in the transformants compared with their untransformed parent cells. Thus, in this system, cell transformation appears to correlate with an increase in both DNA amplification and cytotoxicity of the parvoviruses. However, the accumulation of parvovirus DNA in the transformants was not always accompanied by the production of infectious virus. Like in vitro-transformed fibroblasts, a fibrosarcoma-derived cell line was sensitive to the killing effect of both H-1 virus and MVMp and amplified viral DNA to high extents. The results indicate that oncogenic transformation can be included among cellular states which modulate permissiveness to parvoviruses under defined growth conditions.
Human fibroblasts and epithelial cells differing in their susceptibility to killing by the autonomous parvoviruses H-1 and minute virus of mice were compared for their capacity to express viral mRNAs and proteins. The transition from a parvovirus-resistant to a parvovirus-sensitive phenotype correlated with a proportional increase in the production of the three major viral transcripts and of structural and nonstructural proteins. In contrast, cell sensitization to parvovirus could not be correlated with detectable changes in virus uptake, intracellular localization of gene products, stability of viral mRNAs, or phosphorylation of viral nonstructural polypeptides. Moreover, the H-1 virus-sensitive keratinocyte line studied did not sustain a greater level of viral DNA amplification than its resistant derivative. Therefore, the differential susceptibility of the human cells tested to parvovirus infection appears to be mainly controlled at the level of transcription of the viral genome. Parvoviral gene expression could not be elevated by increasing the input multiplicity of infection in either of the cell systems analyzed. Together, these data suggest that a cellular factor(s) regulating parvoviral transcription may be modulated by oncogenic transformation or by differentiation, as both features have been shown to affect cell susceptibility to parvoviruses.
