Project Summary The promise of marrow transplantation has been diminished by the development of relapse, opportunistic infection and GVHD. These three complications interlock in a Gordian knot that precludes independent resolution strategies. Immunologic approaches to relapse result in more GVHD. Conventional pharmacologic approaches to treat GVHD with increasing immunosuppression result in more opportunistic infection. This is particularly a concern in non-malignant disease where relapse is less concerning. Our group has adopted an alternative strategy to treat GVHD, by emphasizing approaches that enhance immunoregulatory networks, rather than brute force immunosuppression. Our laboratory research has identified several exploitable targets to control GVHD without global immunosuppression. In this project we focus on chronic GVHD. First, we specifically expand on our prior work demonstrating that regulatory T cells (Treg) can be selectively expanded in patients with cGVHD using very low doses of interleukin-2 (IL-2). We demonstrated previously that Treg are depressed in patients with cGVHD. While infusion of Treg may transiently increase Treg numbers, infusions are by their nature expensive, time consuming, and logistically complex. Alternatively, Treg can be expanded in vivo with daily subcutaneous injections that maintain Treg levels at 9-fold above baseline, reverse the ratio of Treg to conventional CD4+ T cells (Tcon), and ameliorate the manifestations of steroid resistant cGVHD in a clinically relevant and significant fashion. Moreover, this approach also allows a substantial reduction in immunosuppressive medications. We hypothesize that expanding Treg at the outset of cGVHD therapy will allow more effective control of cGVHD manifestations with lower cumulative steroid utilization. Second, we have demonstrated the critical role of humoral immunity in cGVHD. Basic studies in mice and in humans indicate that germinal center formation, the development of autologous and allogeneic antibodies, and B cell cytokines are all important in the generation of cGVHD. Moreover we and others have demonstrated the benefit of anti-B cell approaches in the therapy (rituximab, ibrutinib) and prophylaxis (rituximab) of cGVHD. We propose a 3-armed randomized trial comparing the use of the combination of prednisone alone with prednisone and low dose IL-2, to prednisone, IL-2, and rituximab as primary therapy for cGVHD. The intermediate goal is to compare the ability of each therapy effectively treat cGVHD. The long-term goal is to allow us to pick-the-winner for a phase 3 trial and develop an effective, non-toxic, and physiologically relevant therapy for cGVHD.