A Study to Evaluate the Safety and Efficacy of Axatilimab in Combination With Ruxolitinib in Participants With Newly Diagnosed Chronic Graft-Versus-Host Disease

Inclusion Criteria: * ≥ 12 years of age at the time of informed consent. * New-onset moderate or severe cGVHD, as defined by the 2014 NIH Consensus Development Project Criteria for Clinical Trials in cGVHD, requiring systemic therapy. * History of 1 allo-SCT (any type of stem cell donor, any conditioning regimen, and source of hematopoietic stem cells). * Adequate hematologic function independent of platelet transfusion and growth factors for at least 7 days prior to study entry: ANC ≥ 0.75 × 109/L and platelet count ≥ 20 × 109/L. * Willingness to avoid pregnancy or fathering children. Exclusion Criteria: * Received more than 1 prior allo-SCT. Prior autologous HCT is allowed. * Has overlap cGVHD, defined as simultaneous presence of features or characteristics of aGVHD in a patient with cGVHD. * Received previous systemic treatment for cGVHD, including systemic corticosteroids and extracorporeal photopheresis. * Received systemic corticosteroids within 2 weeks prior to C1D1, regardless of indication. * Initiated systemic treatment with CNIs or mTOR inhibitors within 2 weeks prior to C1D1. * Prior treatment with a JAK inhibitor within 8 weeks before randomization. Participants who received a JAK inhibitor for the treatment of aGVHD are eligible only if they achieved a response (CR or PR) to JAK inhibitor treatment and did not discontinue due to toxicity. * Evidence of relapse of the primary hematologic disease or treatment for relapse after the allo-SCT was performed, including DLIs for the treatment of molecular relapse. * History of acute or chronic pancreatitis. * History of thromboembolic events (such as deep vein thrombosis, pulmonary embolism, stroke, myocardial infarction) in the 6 months prior to study entry. * Active symptomatic myositis. * Severe renal impairment, that is, estimated CrCl \< 30 mL/min measured or calculated by Cockcroft-Gault equation in adults and Schwartz formula in pediatric participants, or end-stage renal disease on dialysis. Participants with CrCl of 30 to 59 mL/min on treatment with fluconazole are not eligible. * Impaired liver function, defined as total bilirubin \> 1.5 × ULN and/or ALT and AST \> 3 × ULN in participants with no evidence of liver cGVHD. * Currently active significant cardiac disease, such as uncontrolled arrhythmias, uncontrolled hypertension, or Class 3 or 4 congestive heart failure as defined by New York Heart Association, or a history of myocardial infarction or unstable angina within 6 months prior to randomization. * Pregnant or breastfeeding. Other protocol-defined Inclusion/Exclusion Criteria may apply.