Participants will receive standard-of-care ECP treatment two times a week for 16 weeks. Each treatment will last approximately 2-3 hours. Starting after Week 8 of the ECP treatments, participants will give themselves or be given IL-2 through an injection under their skin. Participants will do this once every day for 8 weeks until the end of the 16-week ECP treatment. If a participant's GVHD worsens during the initial 8 weeks of ECP treatment, he or she has the option of starting IL-2 early.