This study explores the effects of **axatilimab** (a drug) combined with **corticosteroids** (a type of medicine that reduces inflammation) as the first treatment for chronic graft-versus-host disease (cGVHD). cGVHD is a condition that can occur after a person receives a bone marrow or stem cell transplant from another person. The study compares axatilimab to a placebo, which is a harmless substance that looks like the real medicine but has no effect. Participants in this study need to be at least 12 years old and have a new diagnosis of moderate or severe cGVHD. They must have had only one previous transplant from another person and have not received certain other treatments for cGVHD. Also, they must not have any serious infections or other severe health problems. **Key Points:** - **Study Duration & Visits:** Participants will need to attend multiple visits over the study period. - **Compensation:** There might be reimbursement for travel-related expenses. - **Potential Risks:** Risks may include side effects from the drugs used in the study. Participants must avoid pregnancy during the study. Always consult with your doctor to ensure this study is a good fit for you.