Appearance of mink cell focus-inducing recombinants during in vivo infection by moloney murine leukemia virus (M-MuLV) or the Mo+PyF101 M-MuLV enhancer variant: implications for sites of generation and roles in leukemogenesis.

One hallmark of murine leukemia virus (MuLV) leukemogenesis in mice is the appearance of env gene recombinants known as mink cell focus-inducing (MCF) viruses. The site(s) of MCF recombinant generation in the animal during Moloney MuLV (M-MuLV) infection is unknown, and the exact roles of MCF viruses in disease induction remain unclear. Previous comparative studies between M-MuLV and an enhancer variant, Mo+PyF101 MuLV, suggested that MCF generation or early propagation might take place in the bone marrow under conditions of efficient leukemogenesis. Moreover, M-MuLV induces disease efficiently following both intraperitoneal (i.p.) and subcutaneous (s.c.) inoculation but leukemogenicity by Mo+PyF101 M-MuLV is efficient following i.p. inoculation but attenuated upon s. c. inoculation. Time course studies of MCF recombinant appearance in the bone marrow, spleen, and thymus of wild-type and Mo+PyF101 M-MuLV i.p.- and s.c.-inoculated mice were carried out by performing focal immunofluorescence assays. Both the route of inoculation and the presence of the PyF101 enhancer sequences affected the patterns of MCF generation or early propagation. The bone marrow was a likely site of MCF recombinant generation and/or early propagation following i.p. inoculation of M-MuLV. On the other hand, when the same virus was inoculated s.c., the primary site of MCF generation appeared to be the thymus. Also, when Mo+PyF101 M-MuLV was inoculated i.p., MCF generation appeared to occur primarily in the thymus. The time course studies indicated that MCF recombinants are not involved in preleukemic changes such as splenic hyperplasia. On the other hand, MCFs were detected in tumors from Mo+PyF101 M-MuLV s. c.-inoculated mice even though they were largely undetectable at preleukemic times. These results support a role for MCF recombinants late in disease induction.

Differential behavior of the Mo + PyF101 enhancer variant of Moloney murine leukemia virus in rats and mice.

The Mo + PyF101 enhancer variant of Moloney murine leukemia virus (M-MuLV) has been very useful in investigating M-MuLV leukemogenesis. When inoculated subcutaneously (s.c.) into neonatal mice, Mo + PyF101 M-MuLV is attenuated for development of disease. Previous studies in mice infected with wild-type M-MuLV have revealed several important preleukemic events, including development of splenic hyperplasia, defects in bone marrow hematopoiesis, and in vivo generation of MCF viruses that arise by recombination in the uninfected mouse. Mo + PyF101 M-MuLV is defective in inducing these effects after s.c. inoculation. In the experiments reported here, a study of Mo + PyF101 M-MuLV infection in rats was carried out. Wild-type M-MuLV is leukemogenic in rats, but infected rats do not form MCF recombinants since they lack the necessary endogenous polytropic envelope sequences. Since Mo + PyF101 M-MuLV's leukemogenic defect is correlated with a failure to generate MCF recombinants, it seemed possible that wild-type M-MuLV might not have a leukemogenic advantage over Mo + PyF101 M-MuLV in rats, where MCF recombinants cannot form. Neonatal Fisher F344 rats were inoculated s.c. or intraperitoneally by wild-type and Mo + PyF101 M-MuLVs. Surprisingly, Mo + PyF101 M-MuLV was completely deficient in leukemogenesis in rats when inoculated by either route while wild-type M-MuLV induced lymphoma with the predicted time course. The leukemogenic defect for Mo + PyF101 M-MuLV resulted from a pronounced defect for establishing infection in rats. Further studies of wild-type M-MuLV in rats indicated that infection was confined almost exclusively to the thymus at early times. In mice wild-type M-MuLV establishes substantial infection in other hematopoietic organs such as spleen and bone marrow as well. Thymic infection was also correlated with a decrease in thymic cellularity at early times.

Low-frequency loss of heterozygosity in Moloney murine leukemia virus-induced tumors in BRAKF1/J mice.

To identify potential involvement of tumor suppressor gene inactivation during leukemogenesis by Moloney murine leukemia virus (M-MuLV), a genome-wide scan for loss of heterozygosity (LOH) in tumor DNAs was made. To assess LOH, it is best to study mice that are heterozygous at many loci across the genome. Accordingly, we generated a collection of 52 M-MULV-induced tumor DNAs from C57BR/cdJ x AKR/J F1 (BRAKF1) hybrid mice. By using direct hybridization with oligonucleotides specific for three different classes of endogenous MuLV-related proviruses, 48 markers on 16 of 19 autosomes were simultaneously examined for allelic loss. No allelic losses were detected, with the exception of a common loss of markers on chromosome 4 in two tumors. The three autosomes that lacked informative endogenous proviral markers were also analyzed for LOH by PCR with simple-sequence length polymorphisms (SSLPs); one additional tumor showed LOH on chromosome 15. Further screening with chromosome 4 SSLPs identified one additional tumor with LOH on chromosome 4. Therefore, in total, the average fractional allelic loss was quite low (0.002), but the LOH frequency of 6% on chromosome 4 was highly statistically significant (P < 0.0005). Detailed SSLP mapping of the three tumors with LOH on chromosome 4 localized the region of common LOH to the distal 45 centimorgans, a region syntenic with human chromosomes 1 and 9. Candidate tumor suppressor genes, Mts1 (p16INK4a) and Mts2 (p15INK4b), have been mapped to this region, but by Southern blot analysis, no homozygous deletions were detected in either gene. One of three tumors with LOH on chromosome 4 also showed a proviral insertion near the c-myc proto-oncogene. These results suggested that tumor suppressor inactivation is generally infrequent in M-MuLV-induced tumors but that a subset of these tumors may have lost a tumor suppressor gene on chromosome 4.

Development and in vitro characterization of a GM-CSF secreting human ovarian carcinoma tumor vaccine.

A human ovarian carcinoma cell line (UCI-107) was genetically engineered to secrete the cytokine granulocyte-macrophage colony stimulating factor (GM-CSF), by retroviral medicated gene transduction. This line was transduced with the LXSN retroviral vector containing the human GM-CSF gene and the neomycin resistance selection marker. Numerous GM-CSF secreting clones were randomly isolated and one clone, termed UCI-107M GM-CSF-MPS, extensively characterized. This clone was shown to constitutively secrete high levels of GM-CSF (ie 420-585 pg ml-1 105 cells-1 48 h-1 for over 35 passages and 6 months of study. Like the parental cell line UCI-107, UCI-107M GM-CSF-MPS cells expressed MHC class I and Her2/Neu surface antigens but did not express detectable MHC class II, ICAM-1 or CA-125. No change in the expression of these surface proteins was noted between the parental cells and this GM-CSF secreting clone. The morphology of UCI-107M GM-CSF-MPS did not differ from that of the parental or LXSN vector control cells; however, parental cells had a slightly faster growth rate than the transductants. UCI-107M GM-CSF-MPS was sensitive to gamma irradiation, since as little as 2500 rads killed the cells within 10 days of irradiation. However, even after higher doses of irradiation (ie 10000 rads), GM-CSF secretion continued in vitro until about day 8. Interestingly, irradiation induced up-regulation of the surface antigens previously expressed, and they remained up-regulated for as long as the cells remained viable. The potential use of these GM-CSF secreting ovarian carcinoma cells as vaccines for women with advanced ovarian cancer will be discussed.

Development and characterization of an IL-4-secreting human ovarian carcinoma cell line.

Human ovarian carcinoma cell lines were genetically engineered to secrete the cytokine interleukin-4 (IL-4) by retroviral-mediated gene transduction. These cells were transduced with the LXSN retroviral vector containing the human IL-4 gene and the neomycin resistance selection marker. Numerous IL-4-secreting clones were isolated from different papillary serous carcinoma cell lines, including SKOV-3, UCI-101, and UCI-107, and one clone derived from UCI-107 extensively characterized. This clone, termed UCI 107E IL-4 GS, was shown to constitutively express high levels of IL-4 (i.e., 900 to 1300 pg/ml/10(5) cells/48 hr) for over 35 passages and 6 months of study. Like the parental cell line (UCI-107), UCI 107E IL-4 GS cells expressed MHC class I and Her-2/neu surface antigens but did not express detectable MHC class II, ICAM 1, CA 125, or IL-4 receptors. No increase in expression of surface proteins was noted between parental and UCI 107E IL-4 GS. The morphology of this clone did not differ from that of the parental or LXSN vector control cells; however, parental cells had a faster growth rates than transductants. UCI 107E IL-4 GS was sensitive to gamma irradiation since as little as 2500 rad killed most of the cells within 10 days of irradiation. However, after irradiation, IL-4 secretion continued until about Day 8. The potential use of these IL-4-secreting ovarian carcinoma cells as vaccines for woman with advanced ovarian cancer will be discussed.