Remission of FeLV-associated lymphosarcoma and persistent viral infection after extracorporeal immunoadsorption of plasma using staphylococcal protein A columns: details of immune response.

Sixteen feline leukemia virus (FeLV)-infected cats with lymphosarcoma (LSA) were treated by extracorporeal immunoadsorption using staphylococcal protein A columns in order to remove immunoglobulin G (IgG) and circulating immune complexes (CIC) from plasma. Complete viral clearance and long-lasting tumor regression were achieved in nine of the cats and tumor regression without virus clearance was observed in two other cats. Since LSA cats rarely go into spontaneous remission, and since other forms of therapy are ineffective, these cats offered a unique system for analyzing details of the immune response to LSA and FeLV as they are cleared. Immunological parameters associated with the FeLV and LSA responses were assessed in detail in three responder cats and three nonresponders during the treatment and follow-up periods. Two serological parameters that always correlated with complete clearance of LSA were development of precipitating antibodies against FeLV-C gp70 and development of cytotoxic antibodies that kill cultured FL74 LSA cells in the presence of complement. The precipitating antibodies were detected prior to the clearance of LSA and prior to the detection of free cytotoxic antibodies. One serological parameter that always correlated with complete clearance of. FeLV was development of free antibodies to FeLV-AB gp70. Quantitative levels of FeLV-specific CIC and feline oncornavirus-associated cell membrane antigen (FOCMA)-specific CIC correlated well with fluctuating levels of the corresponding antigens and antibodies. These results suggest that the staphylococcal protein A treatment columns remove CIC "blocking factors" directly or indirectly and thereby stimulate existing antibody responses. These antibodies mediate clearance of FeLV and LSA.