TNF and rheumatoid arthritis: Interestingly, interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), which increase under pathological conditions, such as rheumatoid arthritis (RA) and osteoporosis, induce the expression of RANKL in osteoblasts and stromal cells, eventually enhancing osteoclast formation (5, 6).