Phase I/II Clinical Trial of Axatilimab, a CSF1R Monoclonal Antibody, in Combination With Ruxolitinib as Therapy for Patients With Myelofibrosis (MF) and Chronic Myelomonocytic Leukemia (CMML)

* To find the recommended dose of axatilimab given alone and in combination with ruxolitinib in patients with MF and CMML. * To learn if axatilimab given in combination with ruxolitinib can help to control MF and CMML.

Inclusion Criteria: * Age .18 years as MF and CMML are very rare diseases in the pediatric population. * Diagnosis of MF by WHO or ICC and: 1. Phase 1 dose escalation (Cohort A and B): at least 1 prior therapy for MF or with suboptimal response after at least 3 months of therapy with a JAK inhibitor. 2. Phase 2 dose expansion: i. Relapsed cohort (Cohort C): Patients with at least 1 prior MF therapy. or ii. JAKi-naive cohort (Cohort D): patients with newly diagnosed and treatment naive MF with intermediate .1 risk by DIPSS-plus (Appendix 2) * Diagnosis of CMML refractory to treatment with hydroxyurea (for patients with proliferative CMML defined as WBC.13x109/L) or at least 4 cycles of treatment with hypomethylating agent, with relapse/progression after any number of cycles of hypomethylating agent therapy or who are intolerant of treatment with either therapy. Patients may have received prior therapy with other investigational agents or cytotoxic regimens. * ECOG performance status ≤2 * Adequate hepatic function with total bilirubin \