Two plasmids containing nonoverlapping deletions of the herpes simplex virus thymidine kinase gene were introduced into thymidine kinase-deficient mouse L cells by DNA-mediated gene transfer. Thymidine kinase-producing transformants were generated by a mixture of the two plasmids at a frequency significantly greater than that generated by either plasmid alone. Southern blot analyses demonstrated that functional thymidine kinase genes were generated by homologous recombination between the two deletion mutants.

JC virus and BK virus are ubiquitous human viruses that share sequence and structural homology with simian virus 40. To characterize tissue-specific expression of these viruses and to establish model systems for the study of human viral-induced disease, transgenic mice containing early regions of each of the viruses were produced. The viral sequences induced tumors in a distinct and tissue-specific manner that was similar to their tissue tropism in humans. Ten JC virus-containing founder mice were produced, of which 5 survived to maturity. Four of them developed adrenal neuroblastomas, which metastasized to several other tissues. JC virus tumor-antigen RNA was detected at high levels in the tumor tissues and at low levels in the normal tissues of these mice. One of the three BK virus-containing mice was abnormally shaped and died at 2 weeks of age. The other two BK virus-containing mice developed primary hepatocellular carcinomas and renal tumors and died at 8-10 months of age. BK virus tumor-antigen RNA was expressed in tumor tissues of both mice. Since each of the viruses retained the general tissue tropism that it exhibits in humans, these data suggest that transgenic mice harboring human viruses will be useful as animal models for viral-induced diseases.

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Biochemical and cellular properties of three immortalized Schwann cell lines expressing different levels of the myelin-associated glycoprotein (MAG) were compared . The S16 line generated by repetitive passaging was described previously and expresses a level of MAG comparable to that in adult sciatic nerve. The S42 line was generated independently by the same procedure, divides more slowly than the S16 line, and expresses an even higher level of MAG . The S1 6Y line arose spontaneously from a passage of the S16 cells, divides much more rapidly, and does not express MAG . The levels of MAG expression in the three lines are inversely related to their rates of proliferation, and MAG mRNA levels parallel the amounts of MAG . The S16 and S42 lines consist mainly of flat cells at low density and develop many processes at high density, whereas most of the S16Y cells are spindle-shaped, resembling primary Schwann cells in appearance . Surface immunostaining with the 04 antibody was positive for the S16 and S42 cells and negative for the S16Y cells, but all three lines were negative for surface staining with the 01 antibody . The overall protein compositions of the three lines are very similar, but the S16 and S42 cells express larger amounts of several glycoproteins than the S16Y cells, including the adhesion proteins, neural cell adhesion molecule, L1, and laminin . S16 and S42 cells (but not S1 6Y cells) also express P o glycoprotein, galactocerebroside, and sulfatide, but, unlike MAG, these other myelin-related components were present at much lower levels than in adult nerve . Myelin basic protein and proteolipid protein were not detected in any of the lines, although all three lines contained proteolipid protein mRNA . 2',3'-Cyclic nucleotide 3'-phosphodiesterase and glial fibrillary acidic protein were present in all three lines . Conditions have not yet been found in which any of the lines will myelinate dorsal root ganglion neurons in vitro, but the S16 and S42 cells differ from the S1 6Y cells by clustering around neurons after 1 week in coculture . In many respects, the S16 and S42 cells biochemically resemble Schwann cells at an early stage in their preparation to myelinate and should be useful for investigating the cell biology of MAG and other myelin-related components .