The PGE2-EP4 signaling pathway, which binds to PTGER4 receptor, is known to enhance the expansion and activation of Th17 cells stimulated by IL-23 leading to inflammation in ankylosing spondylitis, atopic dermatitis, psoriasis, and alopecia areata [17, 18]. Here, PTGER4 is linked to ankylosing spondylitis.