P24 Layer-specific quantification of immune cells and fibrotic changes in fibrostenosing crohn’s disease

Introduction The pathological effects of Crohn’s Disease (CD) are due to chronic transmural inflammation and may eventually lead to progressive wall fibrosis with stenotic partial luminal obstruction, forming fibrostenosing lesions (FSL). Such fibrosis remains a significant therapeutic challenge. A better understanding of the cellular interactions and composition of CD FSL could identify potential targetable pathways. Our objective was to accurately quantify cells and fibrosis involved in CD FSL within each intestinal wall layer.

Methods Formalin-fixed, paraffin-embedded terminal ileal resections from 30 normal controls and 30 CD FSL patients were stained using immunohistochemistry (IHC) to identify specific cell types including lymphocytes, macrophages, and smooth muscle actin positive cells. Collagen deposits were visualised using Picro-Sirius Red staining. Collagen deposits and IHC-detected positive cells were quantified using QuPath and analysed for their distribution within each ileal layer (mucosa, muscularis mucosae, submucosa, muscularis propria, serosa).

Results Significantly increased fibrosis and inflammatory cellular infiltration were observed in most intestinal wall layers within CD FSL. There was ulcerative mucosal loss with expansion of the muscularis mucosae due to increased fibrosis and smooth muscle hyperplasia. The serosa was expanded by fibrosis and fat wrapping/creeping fat. Increased infiltration of CD3, CD4, CD8 T and CD20-B lymphocytes, CD68 macrophages and CD31 endothelial cells was determined in all layers, except for the mucosa. The intestinal obstruction of Crohn’s FSL is due to extensive fibrosis and muscle hyperplasia in the inner ileal layers together with inflammatory cell infiltration, providing targetable pathways for future drug development and testing.

Conclusion This is the first study annotating all the intestinal wall layers in surgically resected whole ileal sections of Crohn’s FSL allowing layer-specific quantification for better understanding, diagnosis and correlation with other spatial analyses (such as single cell and spatial transcriptomics) of the immune cellular changes occurring in Crohn’s Disease FSLs.