Inhibition of normal lymphocyte mitogenic reactivity by serum from feline leukemia virus-infected cats.

The effect of serum from 12 cats with lymphosarcoma induced by feline leukemia virus (FeLV) on the response of normal cat peripheral blood lymphocytes to phytomitogen-induced blastogenesis was studied. The majority of FeLV sera, when tested at a concentration of 20% of the incubation medium, caused a 40 to 70% reduction in the mean blastogenic response to concanavalin A compared to the response obtained with a similar concentration of normal feline serum. Results with pokeweed mitogen were similar, but the depression in blastogenesis was less than with concanavalin A. Further studies showed that the blastogenic inhibitory activity of FeLV serum (a) was heat labile at 56 degrees for 30 min, (b) could not be overcome by greater concentrations of mitogens, (c) was proportional to the concentration of FeLV serum in the incubation medium, and (d) could not be demonstrated when lymphocytes were preincubated in FeLV serum followed by washing and reincubating in normal feline serum. The results suggested that a substance present in the serum of FeLV-infected cats contributes to altered immunological reactivity during leukemogenesis in the cat.