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The influence of in vivo infection with the polycythemic substrain of Friend leukemia virus on noninducible ('natural') resistance against allogeneic normal or malignant grafts was studied in lethally irradiated mice. Parallel studies were performed on the NK system in the same experimental conditions. The results indicate that FLV-P infection of mice with full (DBA/2) vs partial (BALB/c and CD2F1) susceptibility did not suppress their in vivo natural resistance against bone marrow or El-4 leukemia cells. On the other hand, a decline in NK activity paralleled the progression of leukemic disease in the more susceptible DBA/2 hosts.

Spontaneous reticulum cell sarcoma (RCS) tumor induction occurs in 90% of SJL/J mice of 8-13 months of age. Tumor induction and growth has been shown to be under the influence of both H-2 and non-H-2 genes as well as the presence of an intact host T-cell system. We postulated that cellular oncogenes may play a role in the induction, growth, and characteristics of RCS. DNA-mediated gene transfer protocols were adopted to investigate the presence of transforming genes in DNA from RCS of SJL/J mice. High molecular weight DNA was isolated from these tumors as well as from brains and livers of control tumor-free SJL/J mice and transfected into NIH-3T3 mouse and F2408 rat fibroblast cell lines. Foci of transformed cells with a peculiar round morphology were scored in both rat and mouse cultures given tumor DNA, but not in those receiving DNA from normal tissues. DNA from first-cycle transformants was transfected in further cycles of transfection, giving rise to foci with similar morphological appearances and growth properties. These experiments suggest that a transforming gene, present in RCS spontaneous tumors, is involved in the malignant conversion of the transfected normal fibroblasts. The implication of these results with respect to the induction and growth properties of RCS is discussed.

A number of Friend leukemia cell variants with a interferon-gamma (IFN-gamma)-resistant phenotype have been isolated. They appear resistant to the antiproliferative action of IFN-gamma and to the induction of the antiviral state assessed by Friend leukemia virus release and vesicular stomatitis virus yield. Selection was performed via a prolonged exposure to increasing amounts of highly purified recombinant IFN-gamma of wild-type Friend cells or of variant clones thereof already resistant to IFN-alpha/beta (Affabris et al., 1982, Virology 120, 441-452). Only the clones derived from IFN-alpha/beta-resistant variants showed a phenotype fully resistant to IFN-gamma treatment while keeping their previously acquired resistance to IFN-alpha/beta. These cells are not deficient in high-affinity receptors for IFN-gamma so that their resistant phenotype appears to be mediated by events distal to binding of IFN-gamma to its receptors. Furthermore, analysis of IFN-induced dsRNA-dependent 2-5A synthetase and 67K protein kinase enzymatic activities, biochemical markers for cellular responses to IFN, showed that both these activities were not induced in IFN-alpha/beta and IFN-gamma-resistant clones when treated with either type of IFN. Accordingly, no increased expression of 2-5A synthetase mRNA(s) could be detected by probing poly(A)+-enriched RNA from cells exposed to IFN-alpha/beta or IFN-gamma treatment with murine or human specific cDNAs. On the other hand, no major changes in restriction patterns of 2-5A synthetase gene(s) were observed in these variant cells by restriction endonuclease digestion and Southern blotting. In addition, analysis of 2-5A synthetase mRNA induction, performed on wild-type cells, showed that the kinetic of induction due to IFN-gamma treatment is slower than that obtained with IFN-alpha/beta.

Two African green monkey kidney (AGMK) cell lines, 37RC (interferon-producing) and Vero (non-interferon-producing), were infected by egg-grown Sendai virus passaged in eggs at high and low m.o.i. The appearance of haem-adsorption, and cytopathic effect (c.p.e.) as well as the presence of haemagglutinating virions in the supernates were much more pronounced with a virus seed obtained with 10(-3) diluted passages than with a seed obtained with undiluted inoculum. They were also independent of interferon production (data obtained in 37RC and Vero cells were almost superimposable). In studies carried out with the virus seed prepared at high dilution the establishment of infection was maximal when monolayers were infected as soon as 5 h after trypsinization and cell seeding, regardless of cell density. Virus in supernates obtained from cultures infected 5, 20 or 50 h after seeding exhibited a greatly reduced infectivity for monkey cells, but not apparently for chick embryos. Trypsin treatment of the virus supernates restored their infectivity for AGMK cells more efficiently for virus released from cells infected 5 h after seeding than for virus released from cells infected later after seeding. In keeping with these observations, virus in supernates from cultures infected 5, 20 or 50 h after seeding contained increasing amounts of auto-interfering virions. The decreased infectivity obtained in cell supernates was not accounted for by major differences in virus RNA synthesis. Similarly, the optimum infection established in cells seeded only for 5 h was not due to increased virus adsorption. Several lines of cells surviving first infection with egg-grown Sendai virus have been obtained and kept in cultures for 3 to 18 months. Virus release and c.p.e. apparently become reduced in the cells surviving the first infection until the newly repopulated monolayers must undergo trypsinization. Weekly protease treatments of the cells reactivate all parameters of virus infection which again will tend to disappear slowly and then reappear following each trypsinization ('intermittent' carrier state). The establishment and the 'intermittent' reactivation of these lines were not prevented by the inclusion in the medium of anti-Sendai virus serum. Temperature-sensitive ts functions do not seem to play an important role in this virus-host relationship.

The administration of appropriate doses of interferon to cultures of Friend leukemia cells causes a pronounced inhibition of cell growth. Several lines of evidence indicate that this effect is due to interferon itself, rather than to unknown contaminants of interferon preparations. Autoradiograph analysis of growth parameters of Friend leukemia cells during treatment with interferon demonstrates that the rate of entry into the S phase, the percent decline of unlabeled mitoses, and the mitotic indexes are significantly lower in interferon-treated cell cultures than in control untreated cultures when tritiated thymidine was added 12 h after the administration of interferon. These data indicate that fractions of interferon-treated cell population are delayed in both G1 and in G2 phases of the cell cycle. This was confirmed by exact measurements of the length of the various phases of the cycle. The interferon-induced inhibition of growth of Friend leukemia cells is reversible after removal of the compound. Autoradiograph data obtained from control cultures and from cultures previously treated with interferon that had been washed free of interferon and reseeded in interferon-free medium, demonstrate that during the first 12 h after removal of interferon, a large majority of the cells previously treated with interferon had a deranged flow into the S phase, a high number of unlabeled mitoses, and a low mitotic index. These data provide further evidence for the above-mentioned prolongations of G1 and G2 phases of the cell cycle. All growth parameters tested reverted to normal values within 12 h after washing out interferon.

A mild graft-versus-host reaction induced in (BALB/cJ x DBA/2J) F(1) mice by the administration of parental spleen cells that differ at several weak histocompatibility loci did not influence the development of lymphomas in these animals. Rous sarcoma virus also failed to induce tumors in the runt and control animals. Breast carcinomas, presumably due to contamination of the inoculums with mammary tumor virus, occurred in those experimental groups given parental cells, whether or not they were viable or immunologically competent. We found no evidence that the immunologic process-as represented by the graft-versus-host reaction-is causally related to the induction of neoplasia.

Proliferation and erythroid differentiation of transplanted DBA/2 marrow cells and Friend virus-induced leukemic cells were assessed in syngeneic, allogeneic (H-2 compatible), and (BALB/c x DBA/2)F(1) hybrid mice (CDF(1)). Measurements were made 5 days after transplantation of donor cells into nonirradiated or X-irradiated mice by the spleen colony or the (125)IUdR-(59)Fe uptake methods. Growth of DBA/2J (Jackson subline) marrow grafts was poor in irradiated CDF(1)J hybrids as compared with growth in syngeneic and allogeneic hosts. The DBA/2J transplants proliferated, however, without impairment in irradiated CDF(1) hybrids which were the progeny of DBA/2 male parents of other sublines, e.g. DBA/2Ha, DBA/2Cr, and DBA/2Cum. In contrast, tissue-cultured Friend leukemic cells of DBA/2J origin grew deficiently in all CDF(1) hybrids tested, regardless of irradiation and of the DBA/2 parent's subline. The growth pattern of transplanted DBA/2J cells was a manifestation of hybrid resistance. The results with DBA/2J and other DBA/2 subline grafts suggested that hybrid histocompatibility alleles were expressed to a greater extent in leukemic than in normal marrow cells, for the former were consistently recognized as "nonself" by CDF(1) mice, but not the latter cells. The property of deficient growth in irradiated CDF(1)Ha hybrids was acquired by DBA/2J hemopoietic cells within 6 hr from infection in vivo with Friend leukemia virus, and persisted during the following 8 days. It was ascribed to enhanced expression of hybrid histocompatibility gene(s) (Hh) induced by the virus. Autonomous growth potential of hemopoietic cells, manifested by proliferation in nonirradiated recipients, was first detected 24 hr from infection, and likewise persisted at the later intervals. At the same time, the infected cells grew deficiently also in nonirradiated CDF(1)Ha mice. The two irreversible cellular changes were regarded as the earliest signals of virus-induced transformation.