Addition of IRAK4i reduced the MTX-mediated increase in CSF2 expression in activated RA-FLS (fold change to untreated activated control 0.4 ± 0.2 with MTX + IRAK4i versus 3.2 ± 0.9 with MTX alone, p = 0.054) (Fig. 4E), indicating that MTX acts early on the IL-1 pathway upstream of this kinase. Here, CSF2 is linked to rheumatoid arthritis.