Adenosine A_2A Receptor Agonist Mediated Prevention of Graft-Versus Host Disease In the Mouse Is Associated with An Increase of T-Regulatory Cells In Target Organs.

Abstract 3744

Graft versus host disease (GVHD) is a serious complication of allogenic hematopoietic stem cell transplantation (HSCT) affecting primarily the liver, intestine, skin and lymphoid organs. T-regulatory cells (Tregs) are a subset of T cells that are characterized by CD4⁺CD25^high and express the Foxp3 transcription factor. There is increasing recognition that Tregs likely are important in preventing development, reducing the severity and/or mediating resolution of GVHD. It has been reported that giving donor Tregs will suppress development of GVHD in a mouse model; however it is not clear whether the immunologic suppression by Tregs occurs in the target organs. Agonists of the Gs-coupled Adenosine A_2A receptor (A_2AR) agonists have been demonstrated to terminate inflammation and improve survival in solid organ transplant ischemia models. We previously have reported that the A_2AR specific agonist, ATL146e, also can decrease the incidence and severity of GVHD as well as improve survival of mice in a GVHD transplant mouse model. In this same GVHD mouse model, ATL146e treatment increases the number of donor derived CD4⁺CD25^highFoxp3⁺ Tregs. In order to further understand the role of the agonist in GVHD abrogation we performed studies looking at A_2AR agonist mediated appearance of Tregs in the target organs affected by GVHD in this model. Using a parental into irradiated F1 offspring transplant model (C57BL/6J [B6, H-2^b] -> B6D2F1/J [BDF1, H-2^b/d]) we can induce GVHD as manifested by weight loss and mortality in 100% of mice by infusing an additional 10 million donor T cells into mice previously engrafted with 10 million bone marrow donor cells using 850cGy conditioning. We administered the A_2AR specific agonists, ATL1223 or ATL370, or a PBS control by osmotic mini pumps resulting in continuous subcutaneous infusion for 14 days starting one day before the donor T cell infusion. Mice that received only the donor bone marrow transplant and not the additional donor T cells did not develop GVHD and served as an additional control. We collected ear and colon biopsies from the transplanted mice to assess these target organs (skin and gut) usually affected by GVHD in this model. Tissue was obtained, placed in 10% formalin and paraffin embedded and stained by immunohistochemistry (IHC) for intracellular FoxP3 by incubating overnight with 1:200 mouse anti-mouse/human/rat—FoxP3 IgG. The slides were washed and then incubated with anti-mouse IgG conjugated to Tetramethylrhodamine (TRITC). We then counted the number of Foxp3⁺ cells per high power field. We also examined whether specific agonist activation of A_2AR can increase the number of Tregs in vitro by culturing CD4⁺ CD25⁻ cells in the presence of either TGF-beta and/or the A_2AR specific agonists. As in our previous studies with the specific A_2AR agonist ATL146e, the agonists ATL1223 and ATL370 also were effective at preventing the acute GVHD syndrome of weight loss and mortality seen in the PBS treated controls. Treatment with these agonists resulted in an increase in CD4⁺CD25⁺FoxP3⁺ Tregs in the spleen compared to the PBS treated group at days 14 to 20 after HSCT. In addition we were able to document significant increases in the number of Foxp3 positive T lymphocytes infiltrating skin and colon compared to the PBS treated group. The number of Foxp3⁺ T cells per high power field was 14 ± 6 in the control and 64 ± 14 in the A2_AR agonist treated mice (p = 0.0281). Finally from our in vitro data we also observed a 4.2 to 5.4 fold increase in the number of Foxp3⁺ Tregs in vitro after A_2AR activation compared to TGF beta treatment only. Thus we have confirmed that the specific activation of A_2AR inhibits acute GVHD through the increase of donor-derived CD4⁺ CD25⁺ FoxP3⁺ immunosuppressive T regulatory cells in vivo and in vitro. Furthermore, our histological observation suggests the possibility that increased Tregs by treatment with A_2AR specific agonist is responsible for suppressing the immune response in GVHD target tissues. Therefore, our observation provides additional mechanistic basis for the anti-inflammatory capacity of A_2AR agonist in acute GVHD. Additional studies are ongoing to further elucidate the mechanism by which A2_AR specific agonist increases Tregs.