Masahiko Ohtsuki

TRAIL-R2 (DR5) Mediates Apoptosis of Synovial Fibroblasts in Rheumatoid Arthritis

TRAIL has been proposed as an anti-inflammatory cytokine in animal models of rheumatoid arthritis (RA). Using two agonistic mAbs specific for TRAIL-R1 (DR4) and TRAIL-R2 (DR5), we examined the expression and function of these death receptors in RA synovial fibroblast cells. The synovial tissues and primary synovial fibroblast cells isolated from patients with RA, but not those isolated from patients with osteoarthritis, selectively expressed high levels of cell surface DR5 and were highly susceptible to anti-DR5 Ab (TRA-8)-mediated apoptosis. In contrast, RA synoviocytes did not show increased expression of TRAIL-R1 (DR4), nor was there any difference in expression of Fas between RA and osteoarthritis synovial cells. In vitro TRA-8 induced apoptosis of RA synovial cells and inhibited production of matrix metalloproteinases induced by pro-inflammatory cytokines. In vivo TRA-8 effectively inhibited hypercellularity of a SV40-transformed RA synovial cell line and completely prevented bone erosion and cartilage destruction induced by these cells. These results indicate that increased DR5 expression and susceptibility to DR5-mediated apoptosis are characteristic of the proliferating synovial cells in RA. As highly proliferative transformed-appearing RA synovial cells play a crucial role in bone erosion and cartilage destruction in RA, the specific targeting of DR5 on RA synovial cells with an agonistic anti-DR5 Ab may be a potential therapy for RA.

Caspase‐Independent Cell Death by Fas Ligation in Human Thymus‐Derived T Cell Line, HPB‐ALL Cells

In HPB‐ALL cells, a human thymus‐derived T‐cell line, Fas (CD95)‐mediated cell death was inhibited by about only 50% as a result of treatment with an amount of benzyloxycarbonyl‐Val‐Ala‐Asp‐(O‐methyl)‐CH2F (zVAD‐fmk) sufficient to block the caspase activity. Fas‐mediated caspase‐independent cell death was not observed in other lymphoblast cell lines or mouse activated splenocytes, but this type of cell death was observed in mouse and rat thymocytes, the same as for HPB‐ALL cells. This suggests that Fas‐mediated caspase‐independent cell death is a common feature in thymocytes. The signaling pathway of caspase‐independent cell death has not yet been fully elucidated. In HPB‐ALL cells, DNA fragmentation, one of the features of apoptotic cells, did not occur in the caspase‐independent cell death after Fas ligation. On the other hand, this type of cell death and the surface exposure of phosphatidylserine were recovered by pretreatment with geldanamycin, which brought about a decrease in receptor interacting protein (RIP) kinase expression. These results suggested that HPB‐ALL cells have a caspase‐independent RIP kinase‐dependent pathway for Fas ligation.