PECAM1 and rheumatoid arthritis: To evaluate whether angiogenesis and non-canonical NF-κB signaling in particular might be important for bone formation and thus different between SpA and RA, we analyzed synovial cryosections with IF for vascular markers CD31 (pan-endothelial marker), αSMA (pericyte marker) and NF-κB Inducing Kinase (NIK) (immature vessels) to assess vessel distribution and size (Fig. 2A).