Introduction: CD134 (OX40), a TNF receptor family member expressed on activated T cells, is involved in T cell activation and IL-4 production, and in T cell-dependent antibody production. Until now, information on regulation of CD134 expression is limited. We examined expression of CD134 in HgCl 2 -induced Th2-mediated systemic autoimmunity in the BN rat, and the regulation of CD134 expression in vitro.Materials and Methods: Expression of CD134 was studied by tri-colour flowcytometry on T lymphocytes from BN rats at day 14 after the first injection of HgCl 2 . Furthermore, lymphnode cells from untreated BN rats were exposed to HgCl 2 , IL-4 and several other cytokines and T cell activating compounds in vitro; expression of CD134 on CD4 + lymphocytes was measured by flowcytometry, using propidium iodide for identification of dead cells.Results: CD134 expression was strongly increased on CD4 + CD45RC l o T lymphocytes in lymphnodes, spleen and peripheral blood from BN rats suffering from HgCl 2 -induced autoimmunity. Exposure of lymphnode cells to HgCl 2 in vitro induced a rapid increase of the fraction of CD134 + cells within the CD4 + cell population. Experiments using Con A or PMA and ionomycin revealed an early and parallel upregulation of CD134, CD25 and CD71. A dose-dependent increase of CD134 expression was induced by IL-4, but not by IFN-γ or TNF-α. The effect of IL-4, but not the effect of HgCl 2 , could be totally inhibited by an IL-4-blocking monoclonal antibody. Effects of IL-4 and PMA on CD134 expression could be blocked by the protein kinase-inhibitor staurosporin. Combination of these stimuli with ionomycin resulted in a synergistic increase of CD134 expression, which was blocked by cyclosporin A, an inhibitor of calcineurin. HgCl 2 also synergized with PMA in increasing CD134 expression, but its effects were resistent to both cyclosporin A and staurosporin. Flow cytometric analysis of intracellular thiol levels revealed that all stimuli which increased CD134 expression induced a decrease of thiol levels in CD4 + T cells, whereas CD4 + CD134 + cells were relatively protected against thiol depletion induced by HgCl 2 or IL-4.Conclusion: Optimal induction of CD134 expression requires protein kinase and calcineurin activity, and may involve modification of the redox balance. Furthermore, the data indicate that CD134 expression is favoured in a Th2-type cytokine environment. In vivo modulation studies to investigate the relevance of the CD134-CD134L interaction for HgCl 2 -induced systemic autoimmunity are currently underway.