TNF and ossification of the posterior longitudinal ligament of the spine: In an in vitro cell model of OPLL, TNF-α was not only expressed in degenerated and ossified ligament tissues but also enhanced ALP activity and collagen synthesis in posterior longitudinal ligament cells, suggesting the important function of TNF-α in OPLL pathogenesis.143 IL-6 is capable of promoting the osteogenic differentiation of MSCs.