Background/Purpose: We have previously identified a role for cytosolic DNA sensors in bone by analyzing mice that develop inflammatory polyarthritis and trabecular bone accrual in the setting of DNA accumulation. In this model, DNA accrues in macrophages due to deletion of DNase II and is detected by cytosolic sensors that signal through stimulator of interferon genes (STING). This results in inflammatory cytokine and Type I IFN production. Type I IFNs in DNase II^-/-mice lead to anemia-driven embryonic lethality; thus co-deletion of the type I IFNR is essential (DNase II/IFN-IR double deficient (DKO) mouse). Although macrophages are implicated as a critical cell type in this model, new evidence suggests that a stromal component also plays an important role in the observed bone phenotype.

Methods: Histologic analysis of long bones was performed to confirm phenotypes. Macrophages were expanded from bone marrow (BM) of wild type (WT) and STING-deficient mice, transfected with poly(dA:dT) and RNA was analyzed by Nanostring using probe sets for genes in both innate immune and bone-related pathways. RNA was also prepared from whole BM from control and DKO mice, and similar Nanostring analyses were performed. We utilized bone marrow chimeras to determine the contribution of hematopoietic vs. radioresistant host cells to the bone phenotype. Lethally irradiated Het (DNase II^+/- IFNaR^-/-) or DKO mice were reconstituted with Het or DKO stem cells to generate four groups: Het (donor)→Het (recipient), DKO→DKO, Het→DKO and DKO→Het.  Complete reconstitution was verified by FACS analysis.

Results: As previously reported (A&R 2014; Vol 66:11S, 788), we found that trabecular bone accumulates in long bone and spleen in DKO mice, two sites of DNA accrual in macrophages. MicroCT, CFU assays for osteoblast precursors, and histomorphometry all supported a predominant role for osteoblasts in this bone phenotype. We showed that STING deficiency abrogated both arthritis and bone accrual in DKO mice. We now show that in total BM from DKO mice, expression of STING itself as well as of Ifi204, a cytosolic DNA sensor in the STING pathway, is up-regulated. BM-derived macrophages were prepared from WT and STING-deficient mice and transfected with poly(dA:dT), resulting in a trend toward up-regulated expression of genes known to promote osteoblast differentiation. STING deficiency abrogates up-regulated expression of these genes. Finally, analysis of bone marrow chimeras demonstrated that only DKO→DKO mice developed trabecular bone accrual, demonstrating that both hematopoeitic and stromal elements are required for expression of the bone phenotype.

Conclusion: Trabecular bone accrual is a feature of the autoimmune DKO mouse model. We have identified candidate factors that may drive osteoblast differentiation and function in this model, the expression of which depend on STING. Furthermore, macrophages contribute to the bone phenotype. However, bone marrow chimeras demonstrate that both hematopoietic and stromal elements are required. Identification of novel pathways linking innate immunity and bone may allow for new targets to treat bone loss in autoimmune disease.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/bone-accrual-in-the-dnase-ii-deficient-model-of-autoimmunity-requires-sting-as-well-as-hematopoietic-and-stromal-elements/