CD28 co-stimulation is required for the generation of naturally occurring regulatory T cells (nTregs) in the thymus through Lck-signaling. However, high level of CD28 suppresses the generation of induced Tregs (iTregs) from naïve CD4 T cells, although underlying mechanism(s) has not been defined. Here we investigated the role of CD28-mediated signaling pathways in the suppression of Treg generation. We used a series of transgenic (Tg) mice on CD28-deficient background that bears WT CD28 or mutated CD28 in its cytosolic tail incapable of binding to Lck, PI3K or Itk. Regardless of exogenous IL-2, strong CD28 costimulation suppressed iTreg generation through Lck signaling. Using a GVHD model to test the role of CD28-mediated iTreg suppression in T cell pathogenicity in vivo, we found that CD28-Lck T cells induced significantly less GVHD than T cells from CD28-WT mice. Furthermore, we found that the recipients of T cells from CD28-Lck mice generated significantly more iTregs than those with T cells from CD28-WT, which contribute to reduced graft-versus-host disease (GVHD) development in recipients of CD28-Lck T cells. These results indicate that CD28 costimulation can negatively regulate Treg generation and may provide an avenue for control of T-cell immunity or tolerance by regulating Tregs using the CD28 signal as a target. We went a step forward and investigated the therapeutic potential of antigen-specific iTregs in the prevention of GVHD. Donor hematopoietic stem cells and mature T cells are transplanted into a lymphopenic host to potentially cure many cancers and hematopoietic diseases like leukemia in bone marrow transplantation (BMT) or hematopoietic stem cell transplantation (HCT), but the frequent development of GVHD is the main drawback of this treatment. nTregs suppress the development of GVHD and may spare graft-versus-tumor effect. However, nTregs are a minor (~5%) subpopulation of CD4 helper T cells in healthy individuals, and using in vitro expanded nTregs is a common strategy to test their therapeutic potential in BMT. The concern of in vitro expanded nTregs may include their stability of Foxp3 (master regulatory gene for the development and function of regulatory T cell) expression and suppressive function, survival in vivo, and the non-selective suppression of the pre-activated nTregs. Antigen-specific activation of the regulatory T cells is important for optimal function. In this study, we used an alternative strategy to generate antigen-specific, iTregs and assessed their suppressive potential by comparing their effectiveness in preventing GVHD with polyclonal iTregs. We found that antigen-specific iTregs prevented GVHD lethality in recipients that expressed the target antigen, but were not protective of recipients who did not express the target antigen. Furthermore, antigen-specific iTregs were significantly more efficient than those polyclonal Tregs in the prevention of GVHD. These results reveal the therapeutic potential of antigen-specific iTregs to prevent GVHD efficiently and selectively, and provide the rationale to use antigen-specific iTregs in clinical HCT.

Das Auftreten manifester transplantationsassoziierter thrombotischer Mikroangiopathie sowie das Auftreten leichterer, zumeist subklinischer mikroangiopathischer Veränderungen wurde in einem Kollektiv von Patienten untersucht, die zur Behandlung einer steroidrefraktären oder abhängigen akuten oder chronischen GvHD nach allogener Stammzelltransplantation eine Therapie mit extrakorporalen Photopheresen erhielten. Subklinische mikroangiopathische Schädigung wurde durch ein auf den Messwerten von Fragmentozyten, Thrombozyten sowie der LDH-Aktivität beruhendes Bewertungsschema festgestellt. Das Auftreten von ausgeprägten mikroangiopathischen Veränderungen (Schweregrad 2) ging mit einem erniedrigten Gesamtüberleben einher. Die Reduktion der Steroiddosis und die Verringerung der GvHD-Manifestationen wurden als Parameter für die Wirksamkeit der ECP analysiert. Ihre Effektivität wurde bestätigt. Trotz der nachgewiesenen Effektivität der ECP in der Kontrolle der GvHD zeigte sich eine Tendenz zur Zunahme mikroangiopathischer Veränderungen unter Therapie mit ECP, in Einzelfällen mit schwerwiegenden klinischen Konsequenzen.

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