Elimination of T cells during an immune response is mediated by activation-induced cell death (AICD) and CD95-mediated apoptosis. Chronic graft-vs-host disease and T cell-mediated autoimmune diseases are caused by the persistence of activated T cells that escaped tolerance induction by deletion or silencing. To mimic the in vivo situation of long-term activated T cells, we generated an in vitro system using HLA-A1-specific T cells, weekly restimulated by Ag. While short-term activated T cells (two to five rounds of stimulation) were CD95 sensitive and susceptible to AICD, T cells stimulated more than eight times acquired constitutive CD95 resistance and exhibited reduced AICD. Phenotypically, these long-term activated T cells could be identified as effector/memory T cells. The expression of the proforms of the CD95 receptor initiator caspases, caspase-8 and -10, and the effector caspase-3 was strongly decreased in these cells, and only active caspase fragments were detected. In contrast to short-term activated T cells, constitutive CD95 receptor clustering was observed on the cell surface, and caspase-8 was bound to the CD95 receptor in the absence of receptor triggering. After further cross-linking of CD95, additional formation of the death-inducing signaling complex (DISC) was strongly impaired. Reduced DISC formation in long-term activated T cells was associated with the loss of PTEN expression and the increased phosphorylation of protein kinase B. Inhibitors of phosphoinositol 3-kinase restored CD95 sensitivity and DISC formation in long-term activated T cells. These data suggest that defective CD95 signaling in effector/memory T cells may contribute to the apoptosis resistance toward physiological stimuli in T cells mediating tissue destruction in vivo.

Genetically modified antigen-presenting cells (APC) represent an attractive strategy for in vitro immunomodulation. In the human system, APC expressing HLA-A1 and a membrane-bound form of CD95L (m-CD95L) were used for selective depletion of HLA-A1-specific T cells. In short-term assays, m-CD95L-expressing APC-induced apoptosis in activated T cells and the constitutive presence of m-CD95L and HLA-A1 expressing APC in long-term T cell cultures prevented the expansion of CD4 þ and CD8 þ HLA-A1-specifc T cells and the development of HLA-A1-specific cytotoxicity. However, immunity towards third party, viral and bacterial antigens was maintained and T cells spared from depletion could be induced to develop cytotoxicity towards unrelated antigens. Interestingly, inhibition of HLA-A1-specific T cell response absolutely requires the coexpression of m-CD95L and HLA-A1 antigen on the same APC. Thus, m-CD95L expressing APC might be used in clinical settings to obtain tolerance induction in allogeneic transplantation systems or autoimmune diseases. Apoptosis mediated by the CD95 system plays a pivotal regulatory role in the maintenance of immune homeostasis. Deficiency in CD95 or CD95L expression in T cells mediates lymphoproliferative disorders due to the lack of apoptosis induction. 1 Paradoxically, several molecules of the CD95 signaling pathway are also required for T cell activation and proliferation. 2 In addition to T cells, live and death of antigenpresenting cells, which initiate T cell responses has to be tightly controlled to avoid the establishment of autoimmunity. 3 Activation-induced cell death (AICD) represents an apoptosis mechanism mediated via the interaction of CD95 with its ligand CD95L, to mediate T cell death and initiate the termination of an immune response to maintain tolerance to self antigens and to prevent autoimmunity. 4 Owing to the constitutive expression of CD95L in immune privileged organs such as testis or eye, 5,6 CD95L was considered as an immunoprotective agent able to counterattack activated T cells in vivo. However, the role of CD95L in transplantation tolerance is still controversial. While allogeneic pancreatic islands transplanted together with syngeneic CD95L expressing myoblasts were tolerated, 7 allogeneic CD95L expressing pancreatic b cells were rejected due to increased neutrophil infiltration. 8 Lack of immunogenicity of tumors and resistance towards T cell attacks were also correlated with the expression of CD95L on tumor tissues. Such tumors were shown to kill CD95-positive cells in vitro. 9 However, transfection of CD95L into murine tumor cells did not enhance tumor growth, but instead induced accelerated tumor rejection by neutrophils in vivo. [10][11][12] Systemic immunomodulation by CD95L was successfully achieved by antigen-presenting cells modified to express CD95L. 13 In mice antigen-specific elimination of activated T cells by engagement of CD95L on APC and CD95 on the activated T cells has been demonstrated in several models. 14,15 This approach benefits from the hig...

Immune reactions after trauma are characterized by immediate activation of innate immunity and simultaneously downregulation of adaptive immunity leading to a misbalanced immunohomeostasis and immunosuppression of the injured host. Therefore, the susceptibility to secondary infections is strongly increased after trauma. Immune responses are regulated by a network of immune cells influencing each other and at the same time modifying their functions dependent on the inflammatory environment. Although myeloid-derived suppressor cells (MDSCs) are initially described as T-cell suppressors, their immunomodulatory capacity after trauma is mostly undefined. Therefore, in vitro-generated MDSCs were adoptively transferred into mice after blunt chest trauma (TxT). A single MDSC treatment-induced splenic T-cell expansion decreased apoptosis sensitivity and improved proliferation in the absence of T-cell exhaustion until 2 weeks after trauma. MDSC treatment had a long-lasting effect on the genomic landscape of CD4⁺ T cells by upregulating primarily Th2-associated genes. Remarkably, immune-activating functions of MDSCs supported the ability of TxT mice to respond to post-traumatic secondary antigen challenge. Secondary insults were mimicked by immunizing MDSC-treated TxT mice with ovalbumin (OVA), followed by OVA restimulation in vitro. MDSC treatment significantly increased the frequency of OVA-specific T cells, enhanced their Th1/Th2 cytokine expression, and induced upregulation of cytolytic molecules finally improving OVA-specific cytotoxicity. Overall, we could show that therapeutic MDSC treatment after TxT improves post-traumatic T-cell functions, which might enable the traumatic host to counterbalance trauma-induced immunoparalysis.