YAP1 and inflammatory bowel disease: On the other hand, YAP in macrophages was shown to deteriorate the IBD, since it negatively affects M2 polarization of macrophages, which is induced by IL-4/IL-13 and promotes the activation of M1 macrophages that is caused by lipopolysaccharide (LPS) or interferon-γ (IFN-γ) (19).