IL15 and rheumatoid arthritis: Exogenous IL-15 up-regulated CD158b expression on both NK cells from RA patients (40.0 ± 2.5% vs. 35.3 ± 2.4%, p < 0.001) and healthy volunteers (9.7 ± 2.4% vs. 4.4 ± 1.5%, p = 0.018).