HTRA1 and rheumatoid arthritis: However, another cytokine, IFN-γ, has been reported to significantly inhibit basal and LPS-induced Htra1 expression in macrophages and fibroblasts, two cell types mainly involved in Htra1 synthesis in rheumatoid arthritis, through activation of the p38 MAPK pathway and subsequent activation and binding of STAT1 to the Htra1 promoter [57].