Granzyme B cleaves tenascin-C to release its C-terminal domain in rheumatoid arthritis

Rheumatoid arthritis (RA) is a common autoimmune disorder characterized by exacerbated joint inflammation. Despite the well-documented accumulation of the serine protease granzyme B (GzmB) in RA patient biospecimens, little is understood pertaining to its role in pathobiology. In the present study, tenascin-C (TNC) - a large, pro-inflammatory extracellular matrix glycoprotein - was identified as a substrate for GzmB in RA. GzmB cleaves TNC to generate 3 fragments in vitro: a 130 kDa fragment that remains anchored to the matrix and 2 solubilized fragments of 70 and 30 kDa. Mass spectrometry results suggested that the 30 kDa fragment contained the pro-inflammatory TNC C-terminal fibrinogen-like domain. In the synovial fluids of patients with RA, soluble levels of GzmB and TNC were significantly elevated compared with healthy controls. Further, immunoblotting revealed soluble 70 and 30 kDa TNC fragments in the synovial fluids of patients with RA, matching TNC fragment sizes generated by GzmB cleavage in vitro. Granzyme K (GzmK), another serine protease of the granzyme family, also cleaves TNC in vitro; however, the molecular weights of GzmK-generated TNC fragments did not correspond to TNC fragment sizes detected in patients. Our data support that GzmB, but not GzmK, contributes to RA through the cleavage of TNC.