A Phase IIa single arm open-label study to investigate the safety, tolerability, and PK of F-652 in combination with systemic corticosteroids in subjects who have undergone Hematopoietic Stem Cell Transplantation (HSCT) and have newly diagnosed grade II-IV lower GI acute Graft Verses Host Disease (aGVHD). Treatment with F-652 will be once a week for 4 weeks, with post treatment follow up visits on days 28, 56, 180 and 365.