In vivo interactions of ecotropic and polytropic murine leukemia viruses in mixed retrovirus infections

Latent murine leukemia virus infection characterized by the release of non-infectious virions

Clonal cell lines derived from cultures infected with a polytropic MuLV release vastly different levels of infectious virions ranging from undetectable to very high. Low producing clones release an overwhelming proportion of non-infectious virions containing retroviral RNA but deficient in the Env protein. Non-infectious virion production is not due to an inability of the cells to support infectious MuLV production or to an inherent replicative defectiveness of the proviruses. Reinfection of the lowest producing lines with the polytropic or an ecotropic MuLV results in enormous increases in the specific infectivity of the released virions. This indicates a reversible state of retroviral latency characterized by the release of non-infectious virions that is likely the result of insufficient levels of Env protein required for infectivity. The latency state described here may have important roles in in vivo retroviral infections including alterations of the immune response and the production of defective interfering particles.

Endogenous retroviruses mobilized during friend murine leukemia virus infection

We have demonstrated in a mouse model that infection with a retrovirus can lead not only to the generation of recombinants between exogenous and endogenous gammaretrovirus, but also to the mobilization of endogenous proviruses by pseudotyping entire polytropic proviral transcripts and facilitating their infectious spread to new cells. However, the frequency of this occurrence, the kinetics, and the identity of mobilized endogenous proviruses was unclear. Here we find that these mobilized transcripts are detected after only one day of infection. They predominate over recombinant polytropic viruses early in infection, persist throughout the course of disease and are comprised of multiple different polytropic proviruses. Other endogenous retroviral elements such as intracisternal A particles (IAPs) were not detected. The integration of the endogenous transcripts into new cells could result in loss of transcriptional control and elevated expression which may facilitate pathogenesis, perhaps by contributing to the generation of polytropic recombinant viruses.

Inhibition of Borna disease virus replication by an endogenous bornavirus-like element in the ground squirrel genome

Animal genomes contain endogenous viral sequences, such as endogenous retroviruses and retrotransposons. Recently, we and others discovered that nonretroviral viruses also have been endogenized in many vertebrate genomes. Bornaviruses belong to the Mononegavirales and have left endogenous fragments, called "endogenous bornavirus-like elements" (EBLs), in the genomes of many mammals. The striking features of EBLs are that they contain relatively long ORFs which have high sequence homology to the extant bornavirus proteins. Furthermore, some EBLs derived from bornavirus nucleoprotein (EBLNs) have been shown to be transcribed as mRNA and probably are translated into proteins. These features lead us to speculate that EBLs may function as cellular coopted genes. An EBLN element in the genome of the thirteen-lined ground squirrel (Ictidomys tridecemlineatus), itEBLN, encodes an ORF with 77% amino acid sequence identity to the current bornavirus nucleoprotein. In this study, we cloned itEBLN from the ground squirrel genome and investigated its involvement in Borna disease virus (BDV) replication. Interestingly, itEBLN, but not a human EBLN, colocalized with the viral factory in the nucleus and appeared to affect BDV polymerase activity by being incorporated into the viral ribonucleoprotein. Our data show that, as do certain endogenous retroviruses, itEBLN potentially may inhibit infection by related exogenous viruses in vivo.

Profound amplification of pathogenic murine polytropic retrovirus release from coinfected cells

Previous studies indicate that mice infected with mixtures of mouse retroviruses (murine leukemia viruses [MuLVs]) exhibit dramatically altered pathology compared to mice infected with individual viruses of the mixture. Coinoculation of the ecotropic virus Friend MuLV (F-MuLV) with Fr98, a polytropic MuLV, induced a rapidly fatal neurological disease that was not observed in infections with either virus alone. The polytropic virus load in coinoculated mice was markedly enhanced, while the ecotropic F-MuLV load was unchanged. Furthermore, pseudotyping of the polytropic MuLV genome within ecotropic virions was nearly complete in coinoculated mice. In an effort to better understand these phenomena, we examined mixed retrovirus infections by utilizing in vitro cell lines. Similar to in vivo mixed infections, the polytropic MuLV genome was extensively pseudotyped within ecotropic virions; polytropic virus release was profoundly elevated in coinfected cells, and the ecotropic virus release was unchanged. A reduced level of polytropic SU protein on the surfaces of coinfected cells was observed and correlated with a reduced level of nonpseudotyped polytropic virion release. Marked amplification and pseudotyping of the polytropic MuLV were also observed in mixed Fr98-F-MuLV infections of cell lines derived from the central nervous system (CNS), the target for Fr98 pathogenesis. Additional experiments indicated that pseudotyping contributed to the elevated polytropic virus titer by increasing the efficiency of packaging and release of the polytropic genomes within ecotropic virions. Mixed infections are the rule rather than the exception in retroviral infection, and the ability to examine them in vitro should facilitate a more thorough understanding of retroviral interactions in general.

Absence of infection in pigs inoculated with high-titre recombinant PERV-A/C

Porcine endogenous retroviruses (PERVs) represent a risk for xenotransplantation using pig cells or organs since they are integrated in the genome of all pigs and infect human cells in vitro. Recombinants between PERV-A and PERV-C have been described in pigs in vivo and found de novo integrated in the genome of somatic cells, but not in the germ line. To study whether PERV-A/C can infect and have a pathogenic effect in normal pigs, German landrace pigs were inoculated with high-titre PERV-A/C. No provirus integration was found in blood cells or in various tissues, and no antibody production was observed, indicating the absence of infection.

Expression of infectious murine leukemia viruses by RAW264.7 cells, a potential complication for studies with a widely used mouse macrophage cell line

The mouse macrophage-like cell line RAW264.7, the most commonly used mouse macrophage cell line in medical research, was originally reported to be free of replication-competent murine leukemia virus (MuLV) despite its origin in a tumor induced by Abelson MuLV containing Moloney MuLV as helper virus. As currently available, however, we find that it produces significant levels of ecotropic MuLV with the biologic features of the Moloney isolate and also MuLV of the polytropic or MCF class. Newborn mice developed lymphoma following inoculation with the MuLV mixture expressed by these cells. These findings should be considered in interpretation of increasingly widespread use of these cells for propagation of other viruses, studies of biological responses to virus infection and use in RNA interference and cell signalling studies.