This is a humanized IgG4 antibody using the same sequences as the therapeutic antibody axatilimab. It is a high affinity antibody targeting the colony stimulating factor 1 receptors (CSF1R, also known as M-CSFR and CD115). Signaling through CSF1R regulates the proliferation and differentiation of cells in the monocytic lineage. Donor-derived, pro-inflammatory macrophages are dependent on CSF1R signaling and have been shown in preclinical studies to be responsible for symptoms associated with chronic graft-versus-host disease (cGVHD). CSF1R blockade with axatilimab reduces the number of these pro-inflammatory macrophages and plays a meaningful role in the treatment of cGVHD.