PTGER4 and rheumatoid arthritis: We found that EP4-specific activation significantly drives the differentiation of Th17 cells from patients with AS while the percentage of Th17 cells from healthy controls or patients with RA is not increased by EP4-specific activation (4.9 ± 0.5% without stimulation vs. 7.0 ± 0.8% with an EP4 agonist and 7.8 ± 0.9% with PGE2 in AS; p = 0.0498 and p = 0.0201, respectively; Fig. 2a, b).