Circulating immune complexes associated with naturally occurring lymphosarcoma in pet cats.

Cats were classified into 4 categories by immunofluorescence antibody assay for the presence of feline leukemia virus (FeLV) and histologically as a) normal, FeLV-, b) normal, FeLV+, c) lymphosarcoma (LSA), FeLV+, and d) LSA, FeLV-. Determinations by Raji cell radioimmunoassay modified for cat serum revealed circulating immune complex (CIC) levels of healthy cats to be less than or equal to 50 micrograms equivalent aggregated cat immunoglobulin/ml (microgram/ml). In contrast, sera of cats in groups b, c, and d all contained significantly higher CIC levels (up to 12,000 micrograms/ml) associated with marked hypocomplementemia and C activation occurring via the classical pathway. Sera from FeLV+, LSA+ cats with high levels of CIC and sera of healthy cats were fractionated according to size and bouyant density by centrifugation through 10 to 40% sucrose gradients. Analysis of fractions from LSA+, FeLV+ sera revealed that both immune complexes (ICs), FeLV reverse transcriptase (RT), and IgG were present in fractions corresponding to a bouyant density of 1.15 to 1.18 g/ml. The CIC containing fractions activated C by the classical pathway. Sera from normal cats did not have CIC or RT and none of the fractions activated the classical pathway. These data suggest that vital antigen-antibody complexes are present in sera of viremic cats with LSA and these complexes activate the C system.