Fibroblast Activation Protein Expression Differs Between Inflamed and Fibrostenotic Inflammatory Bowel Disease

Background Intestinal fibrosis is a well‐known complication of inflammatory bowel disease (IBD). The development of anti‐fibrotic agents is hampered by limited validated outcome measures. Fibroblast activation protein (FAP) has been proposed as a fibrosis marker in immune‐mediated inflammatory diseases, including IBD. Our aim was to assess FAP expression in both Crohn's disease (CD) and therapy‐refractory ulcerative colitis (UC) and to characterise its spatial and cellular distribution. Methods We combined qPCR, immunohistochemistry (IHC), and imaging mass cytometry (IMC) to investigate FAP expression in transmural resection specimens from CD and UC patients and non‐IBD controls. Fibrosis and inflammation were evaluated and scored using H&E and Masson's trichrome staining. Primary intestinal fibroblasts were cultured for 3 weeks to allow matrix deposition, after which FAP was visualised using immunofluorescence staining. Results FAP mRNA and total FAP protein levels were increased in inflamed and fibrostenotic CD and therapy‐refractory UC and were mainly found in the submucosa and serosa. IMC revealed that FAP expression was predominantly expressed by CD90 + stromal cells, showing higher cellular FAP levels in inflamed tissues compared to fibrostenotic tissues. This discrepancy between total and cellular FAP in fibrostenotic tissue could potentially be explained by the observations suggesting that fibrostenotic segments showed increased extracellular FAP expression. In vitro experiments supported the finding of extracellular FAP in matrix deposited by primary intestinal fibroblasts. Conclusions FAP is strongly expressed in inflamed and fibrostenotic CD and, albeit to a lesser extent, in therapy‐refractory UC, with total FAP levels being highest in fibrostenotic disease. Cellular FAP expression is predominantly confined to CD90 + stromal cells in inflamed tissues, whereas fibrostenotic lesions are characterised by increased extracellular FAP.