Enucleation as a technique in the study of virus-host interactions

Enucleated L929 cells support invasion, differentiation, and multiplication of Trypanosoma cruzi parasites

Cell infection with Trypanosoma cruzi, the agent of Chagas' disease, begins with the uptake of infective trypomastigotes within phagosomes and their release into the cytosol, where they transform into replicating amastigotes; the latter, in turn, differentiate into cytolytically released and infective trypomastigotes. We ask here if the T. cruzi infection program can develop in enucleated host cells. Monolayers of L929 cells, enucleated by centrifugation in the presence of cytochalasin B and kept at 34 degrees C to extend the survival of cytoplasts, were infected with parasites of the CL strain. Percent infection, morphology, stage-specific markers, and numbers of parasites per cell were evaluated in nucleated and enucleated cells, both of which were present in the same preparations. Parasite uptake, differentiation and multiplication of amastigotes, development of epimastigote- and trypomastigote-like forms, and initial cytolytic release of parasites were all documented for cytoplasts and nucleated cells. Although the doubling times were similar, parasite loads at 48 and 72 h were significantly lower in the cytoplasts than in nucleated cells. Similar results were obtained with the highly virulent strain Y as well as with strains CL-14 and G, which exhibit low virulence for mice. Cytoplasts could also be infected with the CL strain 24 or 48 h after enucleation. Thus, infection of cells by T. cruzi can take place in enucleated host cells, i.e., in the absence of modulation of chromosomal and nucleolar gene transcription and of RNA modification and processing in the nucleus.

Association of adenovirus with the microtubule organizing center

Adenoviruses (Ad) must deliver their genomes to the nucleus of the target cell to initiate an infection. Following entry into the cell and escape from the endosome, Ad traffics along the microtubule cytoskeleton toward the nucleus. In the final step in Ad trafficking, Ad must leave the microtubule and establish an association with the nuclear envelope. We hypothesized that in cells lacking a nucleus, the capsid moves to and associates with the microtubule organizing center (MTOC). To test this hypothesis, we established an experimental system to examine Ad trafficking in enucleated cells compared to Ad trafficking in intact, mock-enucleated cells. Enucleation of a monolayer of A549 human lung epithelial cells was accomplished by depolymerization of the actin cytoskeleton followed by centrifugation. Upon infection of enucleated cells with Cy3-labeled Ad, the majority of Ad capsid trafficked to a discrete, centrally located site which colocalized with pericentrin, a component of the MTOC. MTOC-associated Ad had escaped from endosomes and thus had direct access to MTOC components. Ad localization at this site was sensitive to the microtubule-depolymerizing agent nocodazole, but not to the microfilament-depolymerizing agent cytochalasin B, indicating that intact microtubules were required to maintain the localization with the MTOC. Ad localization to the MTOC in the enucleated cells was stable, as demonstrated by continuing Ad localization with pericentrin for more than 5 h after infection, a strong preference for Ad arrival at rather than Ad departure from the MTOC, and minimal redistribution of Ad between MTOCs within a single cell. In summary, the data demonstrate that the Ad capsid establishes a stable interaction with the MTOC when a nucleus is not present, suggesting that dissociation of Ad from microtubules likely requires nuclear factors.

A survey of respiratory syncytial virus and parainfluenza virus type 3 neutralising and immunoprecipitating antibodies in relation to Paget disease

The aetiology of Paget disease of bone has not been established but certain features have suggested involvement of a parainfluenzalike virus. To seek further evidence of the possible role of paramyxoviruses in Paget disease we have surveyed the presence of neutralising and immunoprecipitating antibodies to both respiratory syncytial virus and parainfluenza virus type 3 in the sera of patients attending a bone disease clinic. These two viruses were implicated by the sporadic observation of viral antigen in individual nuclei of osteoclasts in Paget disease bone lesions. A total of 315 samples were obtained from 177 patients attending the clinic during 1 year. Thirty-six of the patients had confirmed Paget disease and the remainder other conditions. All sera possessed neutralising activity to both viruses. The mean titres for each virus were similar in patients with Paget disease and those with other conditions whether matched or not. In the case of respiratory syncytial virus the neutralising titres were distributed closer to the mean in the Paget group and showed little variation in repeat samples taken over periods of up to 1 year in contrast to the greater variability of the control group. The antigenic specificity of 20 age- and sex-matched sera from each group was examined by immunoprecipitation. No significant differences were observed between Paget and non-Paget patients. These results do not provide confirmation of involvement of either virus in Paget disease, but the serological data suggest that persistent infection with respiratory syncytial virus can occur.

Japanese encephalitis virus replication: studies on host cell nuclear involvement

Replication, as measured by virus production, of both the flavivirus Japanese encephalitis virus (JEV) and the alphavirus Venezuelan encephalitis virus (VEV) was unaffected by short pulses of actinomycin D (act D) at early times postinfection (PI). Replication of JEV was found to be partially inhibited by continuous exposure to act D under conditions where VEV replication was equally sensitive to the drug. JEV replication proceeded normally in the presence of mitomycin C, a DNA synthesis inhibitor. Autoradiographic analysis revealed that virus-specific RNA was present only in the cytoplasm at both early and late times PI. When infected cell membranes were separated on a discontinuous sucrose gradient, most of the virus-specific RNA was associated with the endoplasmic reticulum fraction.

Initiation and maintenance of persistent infection by respiratory syncytial virus

Propagation of cells infected with temperature-sensitive (ts) mutants of respiratory syncytial (RS) virus at nonpermissive temperature (39 degrees C) resulted in cytolytic, abortive, or persistent infection, depending on the mutant used to initiate infection. Five mutants from complementation group B produced cytolytic or abortive infections, whereas a single mutant (ts1) from group D and a noncomplbmenting mutant produced persistent infections. The persistently infected culture initiated by mutant ts1 (RS ts1/BS-C-1) has been maintained in serial culture for greater than 100 transfers, and infectious-center assays and immunofluorescent staining indicated that all cells harbored the RS virus genome. RS ts1/BS-C-1 cultures were resistant to superinfection by homologous and some heterologous viruses, and interferon-like activity against some heterologous viruses was present in the culture medium. Small amounts (0.002 to 0.2 PFU/cell) of infectious virus were present in the culture fluid, but autointerfering defective particles were not detected. This released virus formed small plaques and produced persistent infection of BS-C-1 cells at 37 degrees C. The RS ts1/BS-C-1 cells contained abundant RS virus antigen internally, but little at the surface, although the cells showed enhanced agglutinability by concanavalin A. Nucleocapsids and the 41,000-molecular-weight nucleoprotein were present in extracts of both nucleated and enucleated cells. No infectious RS virus was obtained by transfection of DNA from RS tsl/BS-C-1 cells to susceptible BS-C-1 or feline embryo cells under conditions allowing efficient transfection of a foamy virus proviral DNA. It was concluded that persistent infection was maintained in part by a non-ts variant of RS virus partially defective in maturation. The karyotype of the RS ts1/BS-C-1 culture differed from that of unifected cells.