Mechanisms of tolerance induction: splenocytes from total lymphoid irradiated mice inhibit the IL-2 pathway in TCR-activated CD4 cells.

ABSTRACT

Mice given total lymphoid irradiation (TLI) can be tolerized by an incompletely understood mechanism to foreign Ags given during a brief window of time immediately after completing radiation. We previously showed that splenocytes taken from mice immediately after completing TLI treatment (TLI cells) block the ability of control cells to proliferate or to produce IL-2 in response to alloantigen, suggesting that TLI cells interfere with the IL-2 pathway in responder lymphocytes. To further characterize the mechanism by which TLI cells affect IL-2, we utilized an in vitro system of T cell activation. Adding TLI cells into cultures decreased the ability of TCR-stimulated T cells to proliferate and to secrete IL-2 by 40 to 60%. Exogenous IL-2 did not restore the proliferative response. The inhibition of IL-2 secretion resulted both from a decrease in the number of IL-2-producing cells and a decrease in IL-2 mRNA transcription. TLI cells directly inhibited IL-2 secretion by TCR-stimulated CD4 cells, as determined by ELISPOT. Competitive PCR demonstrated that TLI cells decreased IL-2 transcription of TCR-stimulated CD4 cells by 75%. In situ hybridization confirmed that unfractionated spleen and sorted CD4 cells from TCR-stimulated cultures transcribed less IL-2 RNA in the presence of TLI cells. The results show that TLI cells directly inhibit IL-2 transcription and protein secretion of normal TCR-stimulated CD4 cells.