SPP1 and rheumatoid arthritis: Thus, OPN plays a crucial role in the pathogenesis and progression of RA by affecting the balance of Th1/Th2 cells and also inducing the differentiation of Th17 lymphocytes, affecting IL-17 levels and generating an inflammatory response, with its effects being more pronounced in their phosphorylated form [57,62,70,73,75,76,81,82].