Type 1 Cytokines Polarize Thymocytes During T Cell Development in Adult Thymus Organ Cultures

Peripheral T cells can be polarized towards type 1 or type 2 cytokine immune responses during TCR engagement. Because T cell selection by peptide plus self-MHC in the thymus requires TCR engagement, we hypothesized that type 1 cytokines may polarize developing T cells. We cultured thymi from BBDR rats in adult thymus organ cultures (ATOC) under type 1 cytokine conditions in the absence of exogenous antigen. Type 1 cytokine-conditioned ATOC generated cells that spontaneously secreted high levels of IFNgamma, but not IL-4. A second exposure to type 1 cytokines further increased IFNgamma secretion by these cells, most of which were blasts that expressed the activation markers CD25, CD71, CD86, and CD134. Studies using blocking antibodies and pharmacological inhibitors suggested that both IL-18 and cognate TCR-MHC/ligand interactions were important for activation. Blocking anti-MHC class I plus anti-MHC class II antibodies, neutralizing anti-IL-18 antibody, and the p38 MAP-kinase inhibitor SB203580 each reduced IFNgamma production by approximately 75-80%. Cyclosporin A, which prevents TCR signaling, inhibited IFNgamma production by approximately 50%. These data demonstrate that exposure to type 1 cytokines during intrathymic development can polarize differentiating T cells, and suggest a mechanism by which intrathymic exposure to type 1 cytokines may modulate T cell development.