Spontaneous tumor cell cytotoxicity mediated by peripheral blood mononuclear leukocytes of the dog

Peripheral blood samples from 37 dogs of different breeds were separated into mononuclear leukocytes, strongly plastic adherent cells, plastic nonadherent cells, nylon wool nonadherent lymphocytes (NAL), Percoll density gradient enriched lymphocyte fractions and monoclonal antibody M-T606-negative cells. The inherent ability of each cell fraction to exert spontaneous cell-mediated cytotoxicity (SCMC) was tested in an 18-hour chromium release assay using two allogeneic and one xenogeneic tumor cell line(s) as the targets. Among the lymphocytes enriched by Percoll density gradient centrifugation natural killer cells could be clearly identified due to their morphology (large granular lymphocytes) and increased cytotoxic potency. Even high SCMC was mediated by effector cell populations showing plastic or nylon wool adherence and a high proportion of peroxidase-positive cells. Immunomagnetic selection of monoclonal antibody M-T606-nonreactive canine NAL regularly resulted in a strong increase in cytotoxicity mediated by M-T606- cells. Cytotoxic activity was found in cell-free supernatants from effector:target cell mixtures indicating the release of soluble leukolysins.