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Abstract: Background: Acute rejection (AR), initiated by alloreactive CD4+ T cells, hampers allograft survival. Soluble fibrinogen-like protein 2 (sFGL2) is a novel effector of CD4+ T cells. We previously found that serum sFGL2 significantly increased in renal allograft recipients with AR. In this study, sFGL2 secretion by CD4+ T cells and its mechanism were further explored both in vivo and in vitro. Materials and methods: Forty cases of living-related renal transplant recipients with biopsy-proven AR or stable renal function were collected and detected serum sFGL2, tumor necrosis factor (TNF)-α and interferon (IFN)-γ, and peripheral CD4+ T cells. In vitro, the isolated human CD4+ T cells were stimulated by TNF-α or IFN-γ. sFGL2 in the supernatant and mitogen-activated protein kinase (MAPK) proteins in the CD4+ T cells were investigated. Approval for this study was obtained from the Ethics Committee of Fudan University. Results: sFGL2, TNF-α, IFN-γ, and CD4+ T cells were significantly increased in the peripheral blood of renal allograft recipients with AR. Stimulation with 1000 U/mL TNF-α or 62.5 U/mL IFN-γ for 48 h provided an optimal condition for CD4+ T cells to secrete sFGL2 in vitro. Phosphorylated (p-) c-Jun N-terminal kinase was remarkably upregulated in the activated CD4+ T cells, whereas no significant changes were found in p-p38 MAPK or p-ERK1/2 expression. Furthermore, inhibition of c-Jun N-terminal kinase significantly reduced sFGL2 secretion by CD4+ T cells. Conclusions: sFGL2 secretion by CD4+ T cells can be induced with TNF-α and IFN-γ stimulation through MAPK signaling in renal allograft AR. Our study suggests that sFGL2 is a potential mediator in the pathogenesis of allograft rejection. [Copyright &y& Elsevier]