Endogenous C-type viruses: double agents in natural life processes

C-type RNA viruses have been described in about 20 different vertebrate species. Their presence in man is suggested by electron microscopic, biochemical and serologic studies, although a definite re;icating human virus has not yet been isolated. These viruses are inherited through the germ cell and their production is regulated by genetic information carried in the host cell (i.e. endogenous virus). Two classes of endogenous C-type viruses have been recognized in certain animals particularly the mouse: ecotropic and xenotropic. They may have their counterparts in man. Ecotropic viruses spread through the host and can be easily transmitted to cells of the same species; they can produce malignancy. Xenotropic viruses cannot infect cells from their host species but are infectious for cells from heterologous species. The interaction between xenotropic and ecotropic viruses could lead to the transfer among species of genetic information relating to normal life processes and malignancy. These C-type viruses may play a role in evolution, normal development and differentiation as well as autoimmune disease and cancer.

Interactions between endogenous baboon type-C virus and oncogenic viruses. I. Syncytium induction and development of infectivity assay

Cells releasing the endogenous baboon virus (BV) can interact with human KC cells containing the Rous sarcoma virus (RSV) genome, resulting in cell fusion and syncytium formation. This interaction has been utilized in the development of a sensitive infectivity assay for BV. The titration pattern is of a one-hit type, demonstrating a linear relationship between virus concentration and number of syncytial plaques obtained in the KC co-cultivation assay. Endpoint titration comparisons indicate that the KC test is as sensitive as the immunofluorescence or the RNA-directed DNA-polymerase assays. Attempts to develop an XC test for BV failed, indicating that while BV can interact with the RSV genome it will do so in the human KC cells and not in the rat XC cells. Syncytia are also induced when KC cells are directly exposed to cell-free BV; however, a linear dose relationship is not obtained. When syncytium-positive KC cultures are passaged, the syncytia disappear and a chronic BV infection is established. These KC-BV cells then lose the ability to interact with either the endogenous cat RD-114 virus or the Mason-Pfizer virus which are known to form syncytia with KC cells.

In vitro infection of lymphoid cells by thymotropic radiation leukemia virus grown in vitro

Murine lymphoid cells were infected in vitro with purified leukemogenic radiation leukemia virus (RadLV) produced by virus-induced lymphoblast cell lines. Thymocytes were shown to be highly susceptible to infection by the virus, whereas murine or other fibroblasts were refractory to it. Murine bone marrow and spleen cells were shown to be much less sensitive to infection by this thymotropic RadLV. By comparison, a B-tropic RadLV isolate (RadLV), propagated on a mouse fibroblast cell line, was noninfectious for lymphoid cells but infected fibroblasts. A correlation was shown to exist between in vitro infection of thymocytes, as assayed by immunofluorescence, and in vivo leukemogenicity of the thymotropic RadLV. This constitutes a rapid in vitro test for in vivo leukemogenicity of a natural lymphatic leukemia virus.

Studies on the role of C-type viruses in the development of epithelial tumors induced with DMBA

Epithelial tumors were induced using 0.5 per cent solution of DMBA in two strains of mice--one infected with leukoviruses (Swiss mice), and the other is free of these viruses (hairless mice). Tumors from 15 mice of each strain were examined light- and electron-microscopically. Depending on the period of administration of the carcinogen, benign growths of the type of papilloma or keratoacanthoma were obtained, or malignant tumors. In the tumors in Swiss mice electron microscopy revealed a distinct increase in the numbers of viruses in comparison with surrounding skin and intact skin of healthy mice of this strain. In spite of certain histologic differences between tumors produced in Swiss and hairless mice, the results argue against a role of leukoviruses in the pathogenesis of experimentally induced epithelial tumors in mice.

A possible non-oncogenic effect of C type bone cell virus on serum calcium levels of potentially leukemic mice

In bone of C3H/Fg mice, particles structurally identical to C-type leukemia virus arise from membranes of osteocytes and osteoblasts. Although these virus apparently do not induce morphologic or neoplastic change in bone they may have other, more subtle, effects. Thus, comparison of sera from male C3H/Fg mice, a high leukemia-prone strain, with C57BL and C3H/HeJ mice, low leukemia strains which do not contain C-type virus in bone, reveals that serum calcium levels are significantly lower in the former than in the latter. Further, when C3H/Fg mice develop frank leukemia there is a corresponding increase in virus particles while the serum calcium concentration levels fall to even lower values. The presence of leukemia itself appears not to be the cause as indicated by the failure of implanted lymphocytic leukemic cells in C3H/Fg mice to significantly affect serum calcium concentration. It is postulated that the effects of the virus could be due either to increased osteo blastic activity or to inhibition of osteocytic osteolytic activity or to both.

Genetic control of endogenous C-type virus production in pancreatic acinar cells of C57BL/He and C57BL/6J mice

Electron microscopy revealed very active production of C-type virus particles in the pancreatic acinar cells of untreated normal adult mice of the C57BL/He strain. In C57BL/6J mice, a similar picture was observed after a single intraperitoneal injection of dexamethasone. No viruses were observed in the pancreas of untreated or dexamethasone-treated BALB/c and C3Hf mice. F1 hybrids of both C57BL strains with C3Hf mice produced viruses in the same manner and quantity as the C57BL parents, whereas hybrids with BALB/c mice were entirely negative. Approximately 50% of mice of the first backcross generation of (BALB/c times C57BL/He)F1 hybrids with C57BL/He mice were active producers of C-type particles, while the other 50% were negative. It is suggested that a regulator gene that controls C-type virus production does not function in the pancreatic cells of either C57BL strain, and that BALB/c mice can provide hybrids with an active regulator.

Oncogenicity of an endogenous C-type virus chemically activated from mouse cells in culture

RNA C-type viruses are known to exist in an unexpressed form in all mouse cells. These endogenous viruses can be activated from cells in tissue culture under certain experimental conditions. The biologic activity in vivo of one class of chemically-activated C-type virus has been studied. This virus is shown to induce a specific tumor, lymphatic leukemia, in mice of a low leukemic incidence strain.

The influence of the partner cell on the production of L virus and the expression of viral surface antigen in hybrid cells

The C-type particles produced by the A9 and A9HT sublines of mouse L cells were shown to infect C3H (N type), but not C57BL (B type), mouse embryo fibroblasts. Infection was indicated by distinct single giant cell formation in the XC monolayer used to overlay the mouse embryo fibroblasts. On the basis of these results it was concluded that the L cell virus is N tropic. A9 and A9HT cells were fused to various mouse cells derived from tumors and normal tissues. The ability to produce the Moloney-type surface antigen and to release infectious virus was introduced by the A9 component into the hybrid cell. Virus production, measured by antigen induction on JLS-V9 cells, was suppressed in those hybrids in which the partner cell had a genotype determining low infectibility with that particular virus (B-type cell). It thus appears that the major genetic locus affecting resistance to infection with leukemia viruses, the Fv-1 locus, regulates infectious virus production in somatic cell hybrids also. The same genetic locus did not seem to govern the expression of all virus-related functions, for the virus-determined membrane antigen was demonstrated in many of the N x B-type hybrids in which production of infectious virus was suppressed.