After systemic administration of microbial β-1,3-glucan (curdlan), SKG mice raised in specific pathogen–free (SPF) but not germ-free (GF) conditions develop IL-23/IL-22/IL-17 axis-dependent arthritis of spine and joints, psoriasiform skin inflammation and ileal goblet cell loss, gut barrier disruption, and ileitis (9, 13–15). Here, IL17A is linked to arthritic joint disease.