Clinicopathologic responses in cats with feline leukemia virus-associated leukemia-lymphoma treated with staphylococcal protein A.

Purified protein A from Staphylococcus aureus Cowan I was injected intraperitoneally or was incorporated in filters ex vivo through which plasma from cats with feline leukemia virus (FeLV)-associated leukemia-lymphoma was passed. Before treatment, 65% of the FeLV-infected cats were anemic, and 70% were thrombocytopenic. Concomitant infections, or immune-mediated disease, was common. During treatment 50% of the cats with FeLV-associated disease improved objectively with normal posttreatment hematocrits, thrombocyte and leukocyte counts, disappearance of dysplastic hematologic elements, and correction of marrow dyscrasias. A 33% response to treatment occurred in cats with unequivocal manifestations of malignant disease and was characterized by reductions in tumor size and marrow and peripheral blood neoplastic cell populations. Clearance of FeLV viremia was documented in 28% of the treated cats. The several possible mechanisms by which treatment with staphylococcal protein A causes reduction in the extent of malignant disease are considered.