Complement and Tumor Antibody Levels in Cats, and Changes Associated with Natural Feline Leukemia Virus Infection and Malignant Disease¹

Complement-dependent antibody (CDA) and complement levels were measured in sera from 314 cats. Many of the cats (65%) had received prior natural exposure to horizontally transmitted feline leukemia virus (FeLV), and some of the cats (13%) had lymphoid tumors. In CDA assays, sera were tested against ⁵¹Cr-labeled cat lymphoma cells, and normal cat serum was used as a complement source. In assays for total lytic complement activity, the lymphoma cells were presensitized with alloantiserum. CDA was frequently detected in sera from persistently viremic cats, suggesting no direct CDA-FeLV interaction, and lack of such antibody-virus interaction in sera was confirmed by contingency table analysis. CDA was infrequently detected in cats with tumors, and data analysis provided strong support for CDA-tumor interaction (p = 0.002), thereby supporting the concept that CDA has specific antitumor activity. Most individual cat complement values were distributed normally, but a reduction in mean activity was found in FeLV-infected cats with detectable CDA. Wide variations in complement activity occurred from week to week only in FeLV-infected animals, and severely depleted complement levels were sometimes associated with FeLV-related anemia or lymphoid cancers. The possibility is discussed that some lymphoid tumors of cats develop and progress in the face of a CDA-mediated (anti-feline oncornavirus-associated cell membrane antigen) tumor immune response at times when complement levels are depleted.