FSTL1 and rheumatoid arthritis: Moreover, it was observed that FSTL1 functioned in a species-specific manner: recombinant human but not mouse protein, both produced in human cells (HEK293 and HT-1080), induces IL-6 expression in cultured synovial cells from RA patients but not in mouse NIH-3T3 cells and vice versa [83].