Background/Purpose: Understanding the pathogenesis of inflammation-induced pathologic bone formation remains a significant challenge in spondyloarthritis patients. We thought to uncover the pivotal events driving the inflammation-induced pathologic bone formation process. We hypothesized that innate immune sensing pathways activate bone anabolic factors in macrophages. This hypothesis was guided by our previous work that linked the accumulation of DNA and the cGAS/STING DNA-sensing pathway to pathologic bone accrual (1). In addition, in a mouse model of inflammatory arthritis, we aimed to characterize macrophage subsets and identify the signals responsible for mediating pathologic bone formation.

Methods: In in vitro studies, we transfected macrophages with poly(dA:dT) to activate cytosolic DNA-sensing pathways and assessed the impact of cGAS or STING deficiency on the production of bone anabolic factors, including Bone Morphogenic Protein (BMP2). We used Flt3^Cre;Rosa26^LSL-YFP as a cell lineage tracing system in mice. This system allowed for the separate identification of synovial macrophages derived from hematopoietic stem cell progenitors (HSC, F4/80 positive, YFP positive) or erythromyeloid progenitors (EMP, F4/80 positive, YFP negative). These mice were induced with serum transfer arthritis (STA), a model in which entheseal bone formation occurs at tibial sites. We purified HSC and EMP-derived synovial macrophages by fluorescence-activated cell sorting (FACS) and performed single-cell sequencing to characterize these macrophages.

Results: In in vitro transfection experiments, we demonstrated that the cGAS-STING DNA-sensing pathway plays a role in regulating BMP2 expression in macrophages. BMP2 mRNA was upregulated in macrophages transfected with poly(dA:dT), but not in transfected macrophages in which either cGAS or STING was deleted. In STA, we identified EMP-derived macrophages within inflamed synovial tissues as key producers of BMP2 and other cartilage and bone-anabolic proteins. In addition, we identified a distinct subset of macrophages shared by both EMP and HSC lineages. This subset of macrophages is highly enriched in the production of interferon-stimulated genes (ISGs), suggesting their potential modulation by the cGAS-STING or other DNA-sensing pathways.

Conclusion: Our data indicate that synovial macrophages have the potential to produce bone anabolic factors such as BMP2. This process may be activated through innate mechanisms, including the DNA-sensing pathway cGAS/STING. Furthermore, the discovery of macrophages within inflamed synovial tissues that respond to Type I interferons may provide key insights into pathogenesis and help explain why patients with inflammatory arthritis respond to JAK inhibition.

References: 1) Baum et al. STING Contributes to Abnormal Bone Formation Induced by Deficiency of DNase II in Mice. Arthritis Rheumatol. 69, 460-471 (2017).

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/macrophages-produce-bone-anabolic-factors-in-settings-of-inflammation-induced-bone-formation/