Horizontally and vertically transmitted oncornaviruses of cats

Leukemia specific antigens: FOCMA and immune surveillance

In cats, horizontally transmitted viruses cause leukemia and lymphoma under natural conditions. As with other retroviruses, feline leukemia virus (FeLV) contains products of 3 major genes; the virus core gag gene products, the polymerase, and the virus envelope glycoprotein. When cells are transformed in vitro by the related feline sarcoma virus (FeSV), an additional protein, FOCMA is expressed at the cell membrane. FOCMA, which is FeSV-coded, is transformation and/or tumor specific and expressed regardless of whether or not the cells make virus or contain virus structural antigens. Lymphoid leukemia cells also express FOCMA, both when FeLV is used to induce the disease in laboratory cats and when the tumors occur under natural conditions. FOCMA is expressed on both T and B lymphoid leukemia cells, but not expressed on non-malignant lymphoid cells, even when they are infected with FeLV. About one-third of the naturally occurring lymphoid tumors of cats lack detectable FeLV proteins and varying portions of the FeLV provirus. Despite this, they regularly express FOCMA, which is the target of an immuno-surveillance response that functions effectively under most conditions. FOCMA thus provides a useful model for antigens that might be expressed in "virus-negative" leukemias of man.

Induction of C-type virus in cell lines derived from calf form bovine lymphosarcoma

For attempt to detect an etiological agent, cultures from bovine lymphosarcoma cases (adult form (ALS), calf form (CLS), and thymic form (TLS) were maintained in vitro for over a 18 month period. In two cultures from ALS, bovine leukemia virus (BLV) antigen was constantly detected. On the other hand, BLV antigen remained negative in cultures from two CLS and one TLS cases up to 40 passages. The RNA dependent DNA polymerase activities in these cultures were also negative. Treatment of a culture from CLS (3178) originated from liver tumor with 5'-iodo-2'-deoxyuridine (IdU) and dexamethasone (DXM) resulted in production of an agent serologically and morphologically similar to BLV and in alteration of cell morphology. No virus was detected in culture from TLS after treatment with IdU and DXM.

Genetic variation in antibody response and natural killer cell activity against a Moloney virus-induced lymphoma (YAC)

Antibody formation against the Moloney virus-determined surface antigen (MCSA) was found to be under genetic control. In the (A X C57BL)F1 cross one dominant gene played a major role, resulting in bimodal distribution of the antibody response. This gene showed no linkage to H-2, IgG heavy chain immunoglobulin allotype, the coat color markers B and C, and five different isozyme markers representing chromosome numbers 1, 4, 7, 8 and 9. Antibody response to MCSA was not correlated with antibody titers against the virion proteins, confirming that MCSA was an independent entity. There was no relationship between the segregation of natural killer cell activity and antibody response in a [(A X C57BL) X A] backcross population.

The viral etiology of cancer: a realistic approach

The etiology of cancer resembles that of many other diseases in that multiple factors may be required. Because of this, the role or viruses in the etiology of human cancers is especially difficult to assess. When animal tumor systems were used as models, the roles of various predisposing characteristics in virus oncogenesis were elucidated. Extrapolation of these findings to the human diseases suggests the importance of genetics, age, hormones, immune competence, and stress in determining susceptibility to tumor development in individuals infected with an oncogenic virus. The importance of cofactors in induction of those human tumors most strongly associated with virus infection, including Burkitt's lymphoma, nasopharyngeal carcinoma, cerviccal carcinoma, acute myelogenous leukemia, and breast cancer, is reviewed. Understanding of the role of these cofactors in virus carcinogenesis may lead to disease prevention through elimination of one or more of the cofactors.

Autoimmunity, oncornaviruses, and lymphomagenesis

The interactions between aberrant immune responses and C-type oncornaviruses in mice are complex. These viruses may be activated during certain immune responses, e.g., histocompatibility reactions, in the face of chronic immunosuppression. Oncornaviruses, themselves, may induce autoaggressive cell-mediated responses in certain lymphoid subpopulations, and these autoimmune reactions may be important in subsequent lymphomagenesis. Parallel events may occur in other animal models, e.g., the NZB mouse, and in certain human disorders, e.g., Sjögren's syndrome, systemic lupus erythematosus, and myasthenia gravis.

Horizontal transmission of feline leukemia virus under natural conditions in a feline leukemia cluster household

Ten post-weanling 4-month-old cats, designated "tracers", were placed in a feline leukemia cluster household to determine the efficiency of horizontal transmission of feline leukemia virus (FeLV). The tracer cats were confirmed as negative for prior exposure to FeLV. Following the placement in the leukemia cluster environment, the tracer cats were serologically monitored at intervals of 3-6 weeks for a total period of 1 year. The tests employed included the detection of FeLV using fixed-cell immunofluorescence and the detection and titration of antibody to : (1) the feline oncornavirus-associated cell membrane antigen (FOCMA), as detected by membrane immunofluorescence; (2) viable FeLV, using serum neutralization; (3) virion core protein p30, using radioimmunoprecipitation; and (4) virion glycoprotein gp70, using radioimmunoprecipitation. All of the tracers had evidence of horizontal infection by FeLV, by several criteria. Seven of the 10 had virus that could be isolated from plasma. All of these 7 developed a terminal illness within 18 months; 3 developed aplastic anemia, 3 infectious peritonitis, and 1 lymphoma. The remaining 3 were negative for FeLV by both virus isolation and fixed-cell immunofluorescence. These 3 did, however, develop high antibody titers by all four criteria and they remained healthy throughout the examination period. These results clearly indicate that unprotected pros-weanling cats brought into a leukemia exposure household environment have a high risk of becoming infected with FeLV. Furthermore, a large proportion of the cats are at risk for development of persistent viremia and FeLV-related diseases.

Germ line integration and Mendelian transmission of the exogenous Moloney leukemia virus

Mice were infected with the exogenous Moloney leukemia virus (M-MuLV) at two different stages of development. Either newborn mice (which can be considered as essentially fully differentiated animals) or preimplantation mouse embryos (at the 4-8 cell stage) were infected with M-MuLV. In both cases, animals that had developed an M-MuLV-induced leukemia were obtained. Two lines of evidence indicate that infection of preimplantation embryos, in contrast to infection of newborns, can lead to integration of the virus into the germ line. 1. Viremic males of the first backcross generation (N-1 generation) transmitted the virus to 50% of their offspring (N-2 generation) when mated with uninfected females. Likewise, a 50% transmission was observed from viremic N-2 and N-3 males to the next generations. 2. Molecular hybridization experiments revealed that viremic N-1 and N-2 animals carried one copy of M-MuLV per diploid mouse genome equivalent in all "non-target" organs tested. Together, both experiments indicate that the exogenous M-MuLV can be converted to an endogenous virus after infection of preimplantation embryos. The available evidence suggests that M-MuLV integrated into the germ line at one out of two possible integration sites. Thus, viremic backcross animals are heterozygous for a single Mendelian locus carrying the M-MuLV gene. During leukemogenesis an amplification of the M-MuLV from one copy to a maximum of four copies per diploid mouse genome equivalent takes place in the tumor tissues.

Humoral immune responses of cats to mammalian type-C virus p30s

Natural and experimental cat sera were tested in radioimmune precipitation assays vs purified p30s from FeLV, RD114 and MuLV. Antibodies with specificity for FeLV p30 comparable to hyperimmune sera from heterologous species but of low titer were found in a high percentage of normal cats from households with a high incidence of FeLV and neoplasia. Sera from cats with neoplasms were generally negative. Cats immunized with FeLV gave low-level immune response, also of the same general specificity as heterologous hyperimmune sera. Cat sera do not normally show antibody to RD114 p30 although two immunized weanling cats produced low titered but highly specific p30 antibody. Thus, for both classes of feline type-C virus p30s, there is an evident capability of the cat to mount an immune response to natural or experimental exposure to the respective proteins. The magnitude of the response is between 100 and 1,000 fold below that seen in heterologous species. In contrast, cats immunized with MuLV p30 gave immune responses comparable to those seen in guinea-pigs, rabbits and goats. Several very old cats with carcinoma had antibody which preferentially precipitated MuVL p30. A competition assay using one such serum and labelled MuVL p30 was inhibited by FeVL, RD114, and MuLV p30s. This indicates that the assay is "interspecies" in nature. Among the possible explanations of this reaction category is that it represents antibody to the p30 of an as yet undefined class of feline type-C virus.

Immunosurveillance of naturally occurring feline leukemia

When compared to their housemates that subsequently developed leukemia, cats that remained healthy had five-to tenfold higher (geometric mean) humoral antibody titers to the feline oncornavirus-associated cell membrane antigen. This is compatible with the application of the immunosurveillance hypothesis to the natural development of leukemia in an outbred mammalian species.