Distribution of feline leukemia virus DNA sequences in tissues of normal and leukemic domestic cats.

Seroepidemiological studies indicate that leukemia and lymphoma in cats are horizontally transmitted and that feline leukemia virus (FeLV) is the agent responsible for the disease. Yet, in a significant proportion of leukemic cats, FeLV has not been identified, despite the presence of the feline oncornavirus-associated cell membrane antigen (FOCMA) and in some instances, epidemiological evidence indicating exposure to FeLV. Prompted by these so called virus negative cases of cat leukemia, we surveyed a total of 176 tissues from 50 cats for the presence of FeLV proviral DNA sequences. The animals studied included viremic and nonviremic cats that were either healthy or leukemic. We found: (1) DNA from virus-positive tissues from both healthy and leukemic cats hybridized 60–100% of ¹²⁵I-FeLV (strain Rickard) RNA. (These values are normalized to hybrid yield obtained with DNA of cat cells infected with FeLV in culture, which is 55%.) (2) A few virus-negative tissues also hybridized 60–100% of FeLV RNA. These tissues were derived from lymphoma-bearing cats as well as healthy cats. (3) The majority of virus-negative tissues hybridized 30–60% of FeLV RNA, and in this group, tissues from leukemic cats did not show significantly higher hybridization. (4) The results did not define a particular tissue target site since comparable hybridization results were obtained with many different tissues. (5) A few virus-negative tissues from healthy cats hybridized only 15–30% FeLV RNA. This is considerably lower than the level found with the majority of virus-negative tissues (normal or leukemic) and may reflect the true level of endogenous FeLV related sequences in cats. If so, the higher hybridization levels (30–60% of FeLV RNA) observed with most virus-negative tissues indicates more widespread infection in cats than previously believed, and suggests integration of only partial provirus and/or infection of only a fraction of the cells in the tissue. If exogenous FeLV is involved in the leukemogenesis of “virus-negative” cats, the apparent lack of detectable proviral sequences in tissues of many of these cats over and above those of healthy cats could be interpreted in different ways, e.g., partial loss of provirus, integration of a small fragment, or presence of proviral sequences in a limited population of cells. These “virus-negative” cats may serve as a model (S. M. Cotter and M. Essex, 1977, Amer. J. Pathol.87, 265–268) for other animals, including man, where leukemia may sometimes be associated with certain type-C related subviral markers, but detection of provirus is unusual.