No abstract

Mouse embryo fibroblasts (MEFs) are a widely used cell culture system in life sciences, including virology. Here, we show that although primary MEFs are nonpermissive to myxoma virus replication, the corresponding immortalized MEFs support a highly productive myxoma virus infection. We further demonstrate that this permissive phenotype for myxoma virus in immortalized MEFs is due to the immortalization-associated selective block to the cellular alpha/beta interferon induction machinery involved in responding to myxoma virus challenge. Thus, our report presents a clear example, illustrating that a drastic phenotypic alteration can occur with respect to virus infection between primary cells and their immortalized counterparts.Myxoma virus is a member of the poxvirus family and causes highly lethal infections in rabbits but not in any other species, such as mice (7,17). During our efforts to elucidate the molecular basis for the myxoma virus species barrier, we demonstrated that primary mouse embryo fibroblasts (pMEFs) are resistant to productive myxoma virus infection, a phenotype congruent with the noninfectivity of the virus in vivo (32). In striking contrast, however, when nonpermissive pMEFs become spontaneously immortalized, the resultant immortalized MEFs (iMEFs) developed a permissive state that supported a highly productive myxoma virus infection. Until now, little if anything was known about the underlying molecular mechanisms responsible for this phenotypic alteration of myxoma virus infectivity in iMEFs. Here, we show that the immortalization of pMEFs causes a selective block to the induction machinery of cellular alpha/beta interferon (IFN-␣/␤), rendering the resultant iMEFs unable to mount an antiviral response to the infecting myxoma virus.Previously, we reported that pMEFs (C57BL/6 or 129Sv/Ev background) do not support permissive myxoma virus infection (32). As demonstrated here in Fig. 1A, left, infection of pMEFs with a recombinant myxoma virus expressing ␤-galactosidase under the control of a late viral promoter (22) produced only isolated blue cells with X-Gal (5-bromo-4-chloro-3-indolyl-␤-D-galactopyranoside) staining. In contrast, when pMEFs were cultured for multiple passages until they became spontaneously immortalized through the standard 3T3 protocol (30), classical myxoma virus-induced blue foci (Fig. 1A, right) formed in the resulting iMEFs, indicating that a full-fledged permissive state had evolved following pMEF immortalization. Quantitatively, myxoma virus infection of iMEFs resulted in a 3-log amplification of progeny virus in comparison to that of