Background/Purpose: The intracellular DNA sensing Stimulator of Interferon Genes (STING) pathway is critical for detection of viral and bacterial pathogen DNA. Hyperactivating mutations in this innate immune pathway are associated with autoinflammatory and autoimmune diseases, including gain of function mutations in STING that lead to STING-Associated Vasculopathy with onset in Infancy (SAVI). Type I interferons (IFN-I) are downstream of STING and regulate many cellular processes. An often under-appreciated aspect of IFN-I signaling, tonic (constitutive) IFN-I signaling, has been proposed as a critical element of cellular homeostasis. Vital for maintaining a balanced threshold of immune genes, tonic interferon-stimulated genes (ISGs) serve to regulate immune responses. We have focused on the role of the STING pathway in both tonic and inducible type I interferon (IFN-I) signaling and its impact on bone homeostasis.

Methods: STING deficient, myeloid specific STING-deficient (STING^flox/floxLysM cre^+/), and SAVI mice (activating murine point mutants STING V154M) were studied. Osteoclastogenesis assays were performed in vitro, osteoclasts were enumerated in vivo, and serum C terminal pyridinoline cross‐linked peptide (CTX), a marker of bone resorption, was determined by ELISA. RNA sequencing was performed on osteoclast precursors before and during differentiation with M-CSF and RANKL. In SAVI osteoclastogenesis assays, differentiation was tested in the absence and presence of an IFN-I blocking antibody.

Results: We previously reported that STING-deficient and STING^LysM-deficient osteoclast precursors form greater numbers of osteoclasts in vitro and that STING-deficient mice lose trabecular bone faster than littermate controls, an effect consistent with increased osteoclast activity. Myeloid STING is sufficient for this effect, as ovariectomized STING^LysM-deficient mice exhibit a significantly greater loss of bone compared to controls. We extend these findings by showing that both STING-deficient and STING^LysM-deficient mice have increased osteoclast numbers in vivo, with evidence of increased bone resorption by serum CTX assays. To address mechanism, bulk RNAseq was performed and showed that STING-deficient osteoclast precursors exhibit a marked reduction in both tonic ISG expression prior to and following RANKL stimulation (Figure 1). To further test whether STING-mediated IFN-I limits osteoclast differentiation, osteoclastogenesis assays were performed using precursor cells from STING gain of function (SAVI) mice. SAVI osteoclast precursor cells showed significantly inhibited osteoclastogenic potential, associated with a decreased expression of differentiation factors. Importantly, blockade of IFNAR1 restored the ability of SAVI precursor cells to form osteoclasts, indicating that STING-regulated IFN-I limits osteoclastogenesis (Figure 2).

Conclusion: STING is an upstream mediator of both tonic and RANKL-induced IFN-I signaling, providing a break on osteoclast formation and protection from bone loss. These data also demonstrate that in settings in which the STING pathway is abnormal or is therapeutically altered, there may be important effects on bone.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-stimulator-of-interferon-genes-sting-protects-from-bone-loss-through-regulation-of-tonic-and-induced-type-i-interferon-pathways/