Rejection of solid organ allograft involves alloreactive T-cell expansion. The importance of NF-jB and NFAT in this process is underscored by the therapeutic efficacy of immunosuppressive agents, which target the two transcription factors. Since calpains, calciumactivated proteases, are involved in the activation of NF-jB and NFAT, we investigated the role of calpains in allograft rejection. In human transplant kidneys undergoing acute or chronic rejection, we show an increased expression of CAPN 1 gene encoding l-calpain, associated with a marked expression of l-calpain, mainly in infiltrating T cells. To address the role of calpain in rejection, we used a skin transplant model in transgenic mice expressing high levels of calpastatin, a calpain-specific inhibitor. We show that calpain inhibition extended skin allograft survival, from 11 to 20 days. This delay was associated with a limitation in allograft infiltration by T cells. In vitro, calpain inhibition by calpastatin transgene expression limited dramatically T-cell migration but, unexpectedly, increased slightly T-cell proliferation. Amplification of IL-2 signaling via the stabilization of IL-2R common c-chain provided an explanation for the proliferation response. This is the first study establishing that calpain inhibition delays allograft rejection by slowing down T-cell migration rather than proliferation.Key words: Allograft . Calpain . Calpastatin . T cells IntroductionSolid organ transplantation represents an important means of treating end stage organ failure. However, this strategy is limited by the immune response mounted by the recipient against donor tissue. Acute allograft rejection involves T cells of the adaptive immune system in addition to cells of the innate immune system [1]. T-cell receptor (TCR) engagement in naïve T cells initiates changes in gene expression that are essential for the generation of effector T cells. They require the activation of transcription factors, primarily NF-kB and NFAT [2,3]. TCR/CD28-induced NF-kB activation characteristically elicits the expression of both IL-2 and IL-2 receptor a chain together with the antiapoptotic molecule Bcl-x L [2]. NFAT, which is activated by the calmodulin-dependent phosphatase calcineurin, is also required for the expression of the IL-2 gene through its interaction with proteins of the AP1 family of transcription factors [3]. Given the importance of these two pathways in the generation à These authors contributed equally to this work. Eur. J. Immunol. 2011. 41: 473-484 DOI 10.1002 Leukocyte signaling 473 of effector T cells, a number of different pharmacological inhibitors of NF-kB and/or calcineurin/NFAT are currently used as immunosuppressive agents in transplantation, including steroids, cyclosporin A and tacrolimus (FK-506) [4]. Calpains are calcium-activated neutral cysteine proteases [5]. Two major isoforms, calpain m (or I) and calpain m (or II), which require micromolar and millimolar Ca 21 concentrations for activity, respectively, are ubiquitously expressed, whereas the ...

Calpains are intracellular proteases that play a key role in inflammation/immunity. Rare studies show that they are partially externalized. However, the mechanism of this secretion and the functions of exteriorized calpains remain poorly understood. In this study, we found that mouse and human lymphocytes secreted calpains through an ABCA1-driven process. In turn, extracellular calpains inhibited IL-17A expression. We were able to attribute this function to a cleavage of the TLR2 extracellular domain, which prevented TLR2-induced transcription of molecules essential for IL-17A induction. Calpain exteriorization and TLR2 cleavage were critical for the control of IL-17A expression by low doses of IL-2. By using newly developed transgenic mice in which extracellular calpains are specifically inactivated, we provide evidence for the relevance of calpain externalization in vivo in regulating IL-17A expression and function in experimental sterile peritonitis and autoimmune arthritis, respectively. Thus, this study identifies calpain exteriorization as a potential target for immune modulation.

The cover image of this issue consists of the immunohistochemical staining of μ‐calpain in a healthy human kidney and is taken from Letavernier et al. (pp. 473–484). In this article, the authors analyse the role of calpains in acute allograft rejection. The authors demonstrate that μ‐calpain expression is increased in human transplant kidneys undergoing acute or chronic rejection, mainly in infiltrating T cells. The authors further show that calpain inhibition delays allograft rejection. Interestingly, this delay in allograft rejection is associated with a limitation in allograft infiltration by T cells that is due to the slowing down in T‐cell migration rather than proliferation.