Psoriatic arthritis (PsA) is clinically heterogeneous, showing inflammation of the peripheral joints, spine, entheses, fingers and toes, skin or nails. Whilst there is substantial overlap in the underlying immunobiology driving inflammation in any of these sites, there are also marked differences. However, the reasons for these differences are unclear. The BRIGHT consortium hypothesizes that intestinal microbiota shape immune responses in the early stages of PsA thereby driving clinical heterogeneity. To address this, the group will investigate (i) how gut barrier integrity, intestinal dysbiosis, disease subtype and severity relate in patients; (ii) whether there are differences in the cellular sources that produce or respond to IL-23 and IL-17 production, using deep phenotyping of blood, gut, joint and skin; and (iii) whether PsA intestinal microbiota shape IL-23 and/or IL-17-dependent responses and treatment, using animal models of axial and peripheral forms of PsA.