Background/Purpose: Spondyloarthritis (SpA) is characterized by pathological osteogenesis, inflammation and extensive angiogenesis in inflamed tissues. TNF plays a central role in SpA pathology and transgenic mice that specifically overexpress transmembrane ™TNF exhibit features similar to SpA. tmTNF ligation to TNF receptors (TNF-R) in endothelial cells (ECs) can induce signal transduction pathways, that may promote osteogenesis and angiogenesis. Of note, osteogenesis and angiogenesis are coupled by EC differentiation towards a type H (CD31^hiendomucin^hi) blood vessel phenotype. We investigated the link between pathological osteogenesis, inflammation and angiogenesis in experimental SpA in tmTNF tg mice and the contribution of TNF-R signaling to these processes.

Methods: In this study, tmTNF tg mice, non-tg littermates, and tmTNF tg on either a TNF-RI or TNF-RII deficient background were analyzed for aforementioned SpA features. Cryosections were prepared from vertebrae isolated of these mice at various stages of diseases ranging from 6 weeks till 8 months. Markers to characterize and quantify tissue and cells involved in osteogenesis, angiogenesis and inflammation were analyzed by immune fluorescence confocal microscopy and flow cytometry.

Results: tmTNF tg mice from 6 week onwards contained an increase in type H vessels and osterix⁺ osteoprogenitor cells in the bone diaphysis (osterix mean fluorescence intensity (MFI): 82.8±7.6) compared to non-tg littermates (MFI: 46±9.1). These differences in type H vessels and osteogenesis within the vertebrae of tmTNF tg mice were maintained in mice followed up to 8 months. Non-tg littermate vertebrae only exhibited physiological osteogenesis, i.e. in the metaphysis and periosteum. tmTNF tg mice also exhibited ectopic osteogenesis at their entheses, which was not observed in non-tg littermates. Immunostainings demonstrated presence of type H vessels and osterix⁺ osteoprogenitors outside of the bone marrow (BM) at sites of ectopic osteogenesis. This was not observed in non-tg littermates. Interestingly, tmTNF tg mice also displayed altered BM architecture characterized by extensive lymphoid aggregates, which predominantly consisted of B220⁺ B cell aggregates in combination with high endothelial venules (HEV). tmTNF tg mice on a TNF-RI deficient background did not display lymphoid aggregates or HEVs, while tmTNF tg on a TNF-RII deficient background did, although to a lesser extent than tmTNF tg mice, suggesting that particularly TNF-RI-induced signaling events are necessary for the inflammatory phenotype in BM.

Conclusion: tmTNF overexpression in mice results in increased diaphyseal type H vessels and ectopic development of type H vessels associated with enhanced numbers of osteoprogenitors and pathological bone formation. In addition, extensive lymphoid aggregates associated with HEVs develop in the BM. The identification of specialized blood vessels contributing to key features of SpA pathogenesis may reveal potential novel therapeutic targets for SpA.

« Back to 2018 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/pathological-osteogenesis-and-inflammation-in-experimental-spondyloarthritis-are-associated-with-aberrant-type-h-blood-vessels-and-development-of-high-endothelial-venules-accompanied-by-ectopic-lympho/