Lysis of Feline Lymphoma Cells by Complement-Dependent Antibodies in Feline Leukemia Virus Contact Cats. Correlation of Lysis and Antibodies to Feline Oncornavirus-Associated Cell Membrane Antigen 2

Abstract

Sera were collected from normal cats, from cats exposed to feline leukemia virus (FeLV) infection, and from sick animals including many with leukemias and lymphomas. In blind studies, the sera (346 samples) were tested 1) for lytic antibodies, with the use of cat complement in a 20-hour 51Cr-release test, and 2) for antibodies to feline oncornavirus-associated cell membrane antigen (FOCMA) by the membrane immunofluorescence test. In both assays the target cells were FL74, a cat lymphoblastoid line which replicates FeLV. Correlation of presence or absence of antibodies detected by both tests was 91% overall and 100% for the 93 samples that contained lytic antibody predominantly at a titer of ≥1:25 and anti-FOCMA antibody at a titer of ≥1:16. Complement-dependent and anti-FOCMA antibodies were detected in sera from viremic cats; by radioimmunoprecipitation, these sera did not contain detectable antibodies to either the major envelope or core proteins of FeLV (gp70 and p30), nor did they contain antibodies to the endogenous cat “oncornavirus” RD114. Immune sera that lysed FL74 target cells, which replicate A-, B-, and C-virus subgroups, also lysed F422 cells, which replicate only A-subgroup virus. Complement-dependent antibodies were detected in sera from laboratory-bred normal cats only after contact exposure to cats infected with FeLV; lytic antibodies appeared between 8 and 32 weeks after exposure commenced. Appearance of lytic antibody coincided with first evidence in blood smears of virus infection, and prolonged high antibody titers were maintained, whether or not the leukemia virus infection persisted. The incidence of detection of complement-dependent antibodies was ≤2% in sera from cats maintained in controlled FeLV-free environments, 25% in sera from a randomized sampling of privately owned cats, 36–45% in sera from virus-infected cats of leukemia-cluster households, and 8% in sera from cats with leukemia or lymphoma. Exposure of cats to horizontal FeLV infection induced antibodies that lysed cat lymphoma cells slowly with cat complement; these antibodies were similar to anti-FOCMA antibodies, which have a proved antitumor and immune surveillance function in vivo.