The HLA–B27/β2-microglobulin (β2m)–transgenic (Tg) rat is a leading model of B27-associated spondyloarthritis (SpA), and the disease is dependent on the presence of intestinal bacteria. Previous studies have shown that adult HLA–B27/β2m–Tg rats have an altered intestinal microbiota. This study sought to better define the age-dependent changes to both mucosal immune function and dysbiosis in this rat model of SpA.