Interleukin-2 suppression in cats naturally infected with feline leukemia virus

Feline peripheral blood lymphocytes (PBLs) produce interleukin-2 (IL-2) when stimulated with concanavalin A (Con A) or the combination of the Ca2+ ionophore A23187 (A2) plus the phorbol ester phorbol-12-myristate-13-acetate (PMA). To determine the status of T helper cell function in cats naturally infected with feline leukemia virus (FeLV), PBLs from 19 normal cats and 49 FeLV-infected (34 asymptomatic and 15 symptomatic) cats were tested for their ability to produce IL-2 in response to Con A or A2/PMA stimulation. In the case of Con A stimulation, 21, 41, and 67% of normal, FeLV-infected asymptomatic, and FeLV-infected symptomatic cats, respectively, failed to produce detectable levels of IL-2. In contrast, A2/PMA stimulation yielded high titers of IL-2 in 100% of the normal cats, 92% of asymptomatic cats, but only 44% of clinically ill cats. All of the A2/PMA nonresponders in the FeLV-infected symptomatic population were also unresponsive to Con A. T Helper immunosuppression was observed in cats with FeLV-induced lymphoproliferative diseases as well as cats with FeLV-associated acquired immune deficiency syndrome-like diseases. These data suggest that TH immunosuppression in naturally infected viremic cats may precede development of clinical disease. Moreover, the ability to restore TH function with A2/PMA is diminished with development of clinical disease.