A Study to Evaluate Axatilimab Versus Best Available Therapy in Pediatric Participants With Chronic Graft-Versus-Host Disease After at Least 2 Prior Lines of Systemic Therapy (AGAVE-256)

Inclusion Criteria: * Aged ≥ 2 to \< 18 years at the time of signing the informed consent. * Active, moderate to severe cGVHD, requiring systemic immune suppression. * Participants with refractory or recurrent cGVHD who have received at least 2 lines of systemic therapy, including corticosteroids and ruxolitinib. * Concomitant use of systemic corticosteroids is allowed. Participants on systemic corticosteroids must be on a stable dose of corticosteroids for at least 2 weeks prior to C1D1. Topical and inhaled corticosteroid agents are allowed. * Participants must accept to be treated with one of the following BAT options on C1D1: CNI (cyclosporine or tacrolimus), ECP, MMF, an mTOR inhibitor (everolimus or sirolimus), rituximab, imatinib, methotrexate, or ibrutinib. * History of allo-HCT from any donor HLA type (related or unrelated donor with any degree of HLA matching) using any graft source (bone marrow, peripheral blood stem cells, or cord blood). Recipients of myeloablative, nonmyeloablative, or reduced-intensity conditioning are eligible. Exclusion Criteria: * Receipt of more than 1 prior allo-HCT. Prior autologous HCT is allowed. * Evidence of relapse of hematologic disease or treatment for relapse after the allo-SCT was performed, including DLI for the treatment of molecular relapse. Note: Participants who have received a scheduled DLI as part of their transplant procedure and not for management of malignancy relapse are eligible. * Systemic treatment with CNIs or mT…