CLDN3 and inflammatory bowel disease: The main TJ proteins occludin and tricellulin, together with sealing claudins (such as claudins-3, -4, -5, -7 and -8) are downregulated in colonic and rectal tissue of individuals with IBD, whereas claudin-1 and the pore-forming claudin-2 are upregulated, leading to reduced barrier function (Heller et al., 2005; Prasad et al., 2005; Zeissig et al., 2007; Oshima et al., 2008; Weber et al., 2008; Breugelmans et al., 2023).