Reduced Frequency of FOXP3+ CD4+CD25+ Regulatory T Cells in Patients with Chronic Graft-Versus-Host Disease Short Title: Regulatory T Cells in cGVHD

and HL70149. E. Zorn is a special fellow of the Leukemia and Lymphoma Society. Abstract Chronic graft-versus-host disease (cGVHD) is a major complication of allogeneic hematopoietic stem cell transplantation but the immune mechanisms leading to the diverse clinical manifestations of cGVHD remain unknown. In this study, we examined regulatory T cells (T reg) in 57 transplant recipients (30 with cGVHD and 27 without active cGVHD) and 26 healthy donors. Phenotypic studies demonstrated decreased frequency of CD4+CD25+ T cells in patients with cGVHD compared to patients without cGVHD (p<0.0001) and healthy individuals (p<0.0001). Gene expression of T reg transcription factor FOXP3 was reduced in cGVHD patients compared to patients without cGVHD (p=0.009) or healthy donors (p=0.01). T-cell-receptor-excision-circle (TREC) assays for the evaluation of thymus activity revealed fewer TREC in both transplant groups compared to healthy donors (p<0.001 and p=0.02, respectively) although no difference was observed between patients with or without cGVHD (p=0.13). When tested in functional assays, T reg from both patient cohorts and normal individuals mediated equivalent levels of suppression. Collectively, these studies indicate that patients with active cGVHD have reduced frequencies of T reg but the function of these cells remains normal. These findings support the development of new strategies to increase the number of T reg following allogeneic HSCT to prevent or correct cGVHD.