SpA synovitis was characterized by higher vascularity and infiltration with CD163+ macrophages and polymorphonuclear leukocytes (PMNs) and by lower values for lining-layer hyperplasia, lymphoid aggregates, CD1a+ cells, intracellular citrullinated proteins, and MHC–HC gp39 complexes than RA synovitis. Here, HLA-C is linked to synovitis.