This study is a prospective randomized placebo-controlled phase 2 study to compare CYP-001 plus corticosteroids (CS) to placebo plus CS in allogeneic hematologic stem cell transplant recipients with HR-aGvHD. Severity of GvHD will be assessed at screening and throughout the study using Mount Sinai Acute GvHD International Consortium (MAGIC) guidelines. Eligible subjects will be randomized to receive either CYP-001 IV infusion on Days 0 and 4 or placebo on the same days. All subjects will receive ongoing CS therapy as appropriate per institutional guidelines. Subjects will have study visits up to Day 100 during the Primary Evaluation Period. During the Follow-Up Period, subjects will have study visits up to 24 months.