PROJECT SUMMARY The objective of this fellowship application is to develop a minimally invasive injectable biomaterial-based drug delivery depot to enhance regulatory T cells (Treg) as a means to promote durable disease control in allogeneic hematopoietic stem cell transplantation (HSCT)-associated acute graft-versus-host disease (aGVHD). Donor Treg in HSCT grafts are widely recognized as the primary suppressors of recipient-reactive pathogenic effector donor T cells. However, Treg are often insufficiently functional and/or display abnormal levels of inflammation- induced instability. For example, when exposed to the inflammatory milieu in aGvHD, Treg may lose forkhead box P3 (FoxP3) expression, essential for immunoregulatory function, and transform into pathogenic T cells which can lead to enhanced inflammation in aGVHD. Here, the project will demonstrate the expansion and stabilization of donor Treg via an injectable biomaterial that releases, in a sustained and localized fashion, a Treg immunomodulatory agent. The project will develop and validate the Treg-modulating biomaterial as a therapeutic option for aGVHD. Aim 1 will develop the biomaterial and characterize the efficacy of the Treg modulating agent in post-HSCT aGVHD-affected mice. Aim 2, will assess durability and validate the mechanism of action of Treg in mitigated aGVHD. The immediate results of this project will assess feasibility of promoting Treg stability to improve outcomes in a well-established aGVHD mouse model. If successful, in the longer term, the project will pave the way for a new approach to treat aGVHD and advance NHLBI's mission of developing new therapeutics for blood disorders. The proposed training, guided by an expert mentorship team, will enrich the applicant's knowledge and skills in biomaterials and drug delivery, flow cytometry and cell sorting and histomorphometry and imaging techniques. Gaining expertise and research competency in the subject matter through a quantitative methods-based training approach will support the development of the applicant’s scientific knowledge towards a career as an independent researcher.