Clearance of retroviremia and regression of malignancy in cats with leukemia-lymphoma during treatment with staphylococcal protein a
Engelman, R.W.; Good, R.A.; Day, N.K.
Cancer Detection and Prevention 10(5-6): 435-444
1987
ISSN/ISBN: 0361-090X PMID: 3032440 Document Number: 296367
Cats persistently infected with a type C retrovirus (feline leukemia virus; FeLV) were treated either by the extracorporeal perfusion of their plasma through filter columns containing staphylococcal protein A (SPA) or by the intraperitoneal injection of SPA for up to 32 treatments. The clearance of evidence of viral infection occurred during treatment with SPA in 67% of the treated cats with chronic viral infection that had FeLV-related abnormalities other than overt malignancy. This rate of viral clearance is more than 30 times greater than the incidence of spontaneous clearance of persistent FeLV viremia in the untreated cat. Actual clearance of viremia occurred in 28% of all cats treated. Regression of malignancy, which occurred in four of 12 (33%) treated cats, was demonstrated by reductions in tumor size and bone marrow neoplastic cell populations. Immediately preceding or concurrent with these remissions from viral infection and malignant disease, a marked though transient elevation in the plasma level of circulating gamma-like interferon occurred in some responding cats and was followed by the appearance and rising titer of a complement-dependent cytotoxic antibody that reacted with FeLV-infected cells. This IgG antibody was shown by several analyses to be specific for the major viral envelope glycoprotein gp70. Further investigations demonstrated that high levels of FeLV at the cellular level might be responsible for the reduced ability of normal feline lymphocytes to secrete gamma-interferon in response to mitogenic stimuli in vitro. These findings may provide some clue to the variation in the responsiveness of individual cats to immunotherapy with staphylococcal protein A.