S100A8 and juvenile idiopathic arthritis: Pro-inflammatory S100 proteins, S100A8/9 (AKA calprotectin, myeloid-related protein, MRP8/14) and S100A12 are considered sensitive indicators of disease severity, activity, and predictors for response to methotrexate therapy in patients with JIA [46]. In studies, the MRP8/14 level was found to be lower or normal in patients who responded to the methotrexate therapy vs. those who did not [44].