Axatilimab Ameliorates Inflammation and Fibrosis By Targeting Macrophages in a Preclinical Model of Chronic GVHD

Introduction: Chronic graft-versus-host disease (cGVHD) is an immune-mediated serious and life-threatening complication after allogeneic hematopoietic stem cell transplantation, occurring in 30%–70% of patients. The pathophysiology of cGVHD may involve inflammation, cell-mediated immunity, humoral immunity, and fibrosis. Axatilimab is a high-affinity humanized IgG4 (kappa light chain) monoclonal antibody targeting colony-stimulating factor 1 receptor (CSF-1R) that is under investigation for the treatment of cGVHD. CSF-1R is a member of the receptor protein tyrosine kinase family of growth factor receptors and is expressed on monocyte precursors, monocytes, and macrophages. Signaling by colony-stimulating factor 1 (CSF-1) or interleukin-34 (IL-34) through CSF-1R has been shown to be the primary regulatory pathway for determining development and differentiation of the mononuclear phagocytic cell lineage. Recent work has established a role for monocyte-derived macrophages as the pathogenic population of cells that establish the fibrotic niche and drive the fibrotic process. Upon entry of monocytes into tissues, the tissue microenvironment regulates their differentiation, resulting in a heterogeneous pool of cells ranging from pro-inflammatory (often referred to as M1 macrophages) to immune suppressive/wound healing (M2 macrophages). An imbalance in the number and activity of these macrophage populations is believed to result in the excess production of fibrotic growth factors, such as transforming growth factor-β and platelet-derived growth factor, leading to uncontrolled fibroblast activation and subsequent fibrosis. Blocking CSF-1R signaling with axatilimab is expected to reduce the circulating levels of pathogenic monocyte-derived macrophage precursors and inhibit their activation in tissues, thereby providing an opportunity to therapeutically intervene in diseases such as cGVHD. The results described herein support this assertion by demonstrating the direct impact of axatilimab exposure to skewing differentiating monocytes away from a pro-inflammatory and pro-fibrotic phenotype.

Methods: Primary human monocytes were differentiated in the presence or absence of ascending doses of axatilimab (0.07–7 μM) into distinct phenotypes: activated (M0), pro-inflammatory (M1-like), and suppressive (M2-like) macrophages. Supernatants and cell pellets were then assessed for secreted protein (Luminex Corporation) and gene expression (NanoString Technologies) profiles, respectively. Data were analyzed in R (version 4.1.1).

Results: Gene expression profiles of pro-inflammatory cytokines and chemokines (eg, IL-1β, chemokine CC motif ligand 2 [CCL2], CCL7, CCL24, and CCL22) from primary differentiated macrophage subsets were dramatically reduced in the presence of ascending concentrations of axatilimab. Further, macrophages exposed to axatilimab while differentiated under pro-inflammatory conditions showed increased levels of c-MYC expression compared with control cultures. c-MYC is a transcription factor associated with suppressive M2-like macrophages, which suggests that axatilimab interferes with pro-inflammatory macrophage differentiation. Consistent with this, expression of M2-associated cytokines, including IL-10, IL-5, and IL-4, was enhanced in axatilimab-treated macrophage cultures.

Conclusions: Blocking CSF-1R signaling in primary, differentiated macrophages dramatically alters the secretomes and gene expression profiles of these cells. Macrophages are key inducers of fibroblast growth factors, which in turn promote fibrotic disease. These data confirm that production of inflammatory and pro-fibrotic factors by pro-inflammatory macrophages is inhibited in the presence of axatilimab, thereby supporting the notion that axatilimab provides a unique opportunity to therapeutically intervene or prevent fibrosis in cGVHD.