Objective: Granzyme B, an apoptosis-inducing factor, is expressed in natural killer (NK) cells, an important factor in innate immunity. We previously reported that granzyme B is expressed in arthritic cartilage and chondrocytes, and suggested that granzyme B expression is related to apoptosis distribution. We have now investigated whether granzyme B directly induces apoptosis in chondrocytes and whether chondrocytes possess NK cell-like function.
Methods: Chondrocytes included the human C-28/12 chondrocyte cell line, normal chondrocytes, and rheumatoid arthritis (RA) chondrocytes. Apoptosis was analyzed by ELISA and TUNEL after C-28/12 cells were incubated with active granzyme B. NK cell markers were examined in chondrocytes by FACS and immunohistochemistry. Chondrocytes with or without Z-AAD-CMK, a known granzyme B inhibitor, were stimulated with PHA (20 microg/ml), followed by coculture with K562 cells in order to test chondrocyte cytotoxity.
Results: Granzyme B was successfully introduced into C-28/12 chondrocytes, and was confirmed to dose-dependently induce apoptosis. Immunohistochemically, chondrocytes expressed the surface antigens of NK cells and exhibited cytotoxicity against K562 cells, which served as an indicator of cytotoxicity. Z-AAD-CMK inhibited cytotoxicity against K562 cells in a dose-dependent manner, thus confirming that chondrocyte cytotoxicity against K562 cells is dependent on granzyme B.
Conclusion: Our findings indicate that chondrocytes possess NK cell-like activity related to innate immunity, and that apoptosis is induced in these cells by granzyme B. Our findings suggest that inflammation activates granzyme B, which participates in the destruction of RA-affected joints.