Effect of interleukin-2 on the inhibition of the human mixed lymphocyte reaction (MLR) by azathioprine.

The effect of interleukin-2 on the response of the human MLR to azathioprine was studied. Azathioprine (36 microM) inhibited the MLR by 86% and interleukin-2 (25%) stimulated the MLR by 99%. Only partial relief (16%) of azathioprine (36 microM) inhibition was found in the interleukin-2 treated MLR compared with azathioprine treatment alone. However, [3H]-thymidine incorporation by azathioprine (36 microM) + interleukin-2 treated MLR was 4.3 times greater than that exposed to azathioprine alone. As the residual number of immunocompetent cells after drug exposure is pathologically more important than the fraction suppressed, this result may be of clinical relevance. These results show that the major mode of action of azathioprine is not the inhibition of interleukin-2 production.