The inhibitory effect of cyclosporine on the nuclear proliferative response to a variety of T cell activators.

Previous work has demonstrated that cyclosporine inhibits generation of the cytoplasmic activation signal. The present study shows that the drug also diminishes the proliferative response of isolated nuclei upon phytohemagglutinin, OKT3, or mixed lymphocyte culture stimulation. Following in vitro action of normal human peripheral blood lymphocytes, nuclei isolated by homogenization and differential centrifugation were tested for 3H-thymidine incorporation upon stimulation with cytoplasmic fractions derived from homologous or heterologous activators. Nuclei isolated from CsA-treated cells showed significantly reduced responses to activation by cytoplasmic fractions from cells stimulated by all three agents with inhibition of OKT3 greater than MLC greater than PHA activation. This inhibition was less than the effect of CsA to disrupt generation of the activation signal by cytoplasmic fractions. Exogenous addition of Interleukin 2 to PHA-treated cells almost completely overcame the inhibitory effect of CsA on both nuclei and cytoplasm. A direct effect was documented by incubation of isolated nuclei with CsA prior to addition of a cytoplasmic activator and assessment of 3H-thymidine incorporation. The degree of direct nuclear inhibition by CsA was proportionate to the intranuclear drug concentration: cells displaying greater than 25% inhibition had higher intranuclear CsA concentrations (126 +/- 49 ng/ml) than unaffected cells (43 +/- 3 ng/ml) (P less than 0.005). These data suggested direct effects of CsA on nuclear proliferative responses, probably related to intranuclear drug binding and independent of its action to inhibit cytoplasmic transduction of the activation signal.