Objective To observe the effect of LIGHT on synovial macrophage-osteoclast difier entiation in rheumatoid arthritis.Methods Synovial macrophages were collected from 8 synovial tissues har vested from RA patients by digestion with conagenase.The macrophages from each patient were divided into 5 subgroups:group 1 was co-cultured with Macrophage Colony Stimulating Factor(MCSF).group 2 was co cultured with MCSF and LIGHT.group 3 was co-cultured with MCSF and Receptor activator for Nuclear Fac tor kB ligand(RANKL),group 4 was co-cultured with MCSF and LIGHT and RANKL group 5 was co-cul tared with LIGHT.After two weeks of culture.tartrate resistant acid phosphatase(TRAP)and F-actin staining were used to detect the formation of osteoclast on cover slip(CS).Functional evidence of osteoclasts was as sessed by the formation of resorption pits ou dentine slice(DS).Results TRAP and F-actin were both nega tive in group 1 and group 5 and no pit on dentine slice could be observed.In group 2.there were some small round and ovoid osteoclasts with TRAP(+)and F-actin(+).Pits on DS were small and discrete.In group 3. there were many large irregular osteoclasts with TRAP(++)and F-actin(++).The number and volume of pits were both increased.In group 4.there were even more and larger osteoclasts with TRAP(+++)and F-actin (+++).The pits were even larger and became confluent,ie,pits(++++).Conclusion LIGHT can promote RANKL-mediated osteoclastogenesis in RA synovial macrophages.and it can induce osteoclast formation through a mechanism independent of RANKL.