Upfront Denileukin Diftitox as in vivo regulatory T-cell depletion in order to enhance vaccination effects in a canine allogeneic hematopoietic stem cell transplantation model

Abstract Denileukin Diftitox (ONTAK ® , DAB 389 IL-2) is a recombinant DNA-derived fusion protein depleting cells that express high-affinity IL-2 receptor. Important cell targets are CD4 + CD25 + Foxp3 + regulatory T cells (T reg ). Elimination of immunosuppressive T reg by Denileukin Diftitox may provide a way to modulate immune tolerance following stem cell transplantation. Here, we combined T reg depletion with a vaccination approach to induce donor-specific immune reactions. To investigate this approach we chose the mixed chimerism canine stem cell transplantation model which represents a high state of tolerance between two hematopoietic systems. The aim was therefore to induce a graft versus hematopoiesis effect thereby converting mixed to full donor chimerism. Dog leukocyte antigen identical siblings that had developed a stable mixed chimerism after non-myeloablative stem cell transplantation received a single dose of Denileukin Diftitox (18 μg/kg, i.v.) followed by several cell-lysate vaccinations. Host peripheral blood mononuclear cell lysates combined with CpG-ODN, and Montanide ® ISA 51 were locally applied. In vitro studies demonstrated that canine T reg are a target of Denileukin Diftitox. The suppression of T-cell proliferation by T reg was abolished by addition of Denileukin Diftitox (10 nM). An increase of proliferation of median 300% (range: 200%–425%) was observed. No change in donor chimerism was observed after administration of Denileukin Diftitox and vaccination. This study highlights that application of Denileukin Diftitox resulted in a depletion of T reg followed by an increase of immune response in vitro . This effect could not be confirmed in vivo even if the immune system was stimulated by vaccinations.