Abstract 295: Fibroblast-specific endoglin deletion alters colonic immune infiltrate in premalignant colorectal lesions

Abstract Background: Endoglin expression on cancer-associated fibroblasts has been reported to promote tumor growth and metastasis, especially in colorectal cancer (CRC). However, little research has been done on the function of endoglin-expressing fibroblasts in the early stages of CRC and how it interacts with the host immune system. Here, we investigated the role of endoglin expression on fibroblasts during intestinal inflammation and tumorigenesis in a chemically induced colitis-associated CRC model. Methods: We generated two inducible fibroblast-specific endoglin (ENG) knockout mice Collagen1a1 CreERT2.ENGfl/fl (ENGCol1a1-/−) and Collagen1a2-CreERT.ENGfl/fl (ENGCol1a2-/−). Cre-mediated recombination in mice was induced by oral administration of tamoxifen for three consecutive days. Polyp formation was induced by a single injection of azoxymethane (AOM), followed by three cycles of dextran sodium sulphate (DSS). Analysis of the immune cell composition in the colons and polyps was done using flow cytometry. Hematoxylin and eosin (H&E) staining was performed to evaluate intestinal inflammation. Results: ENGCol1a1-/− mice showed a considerably higher number of colonic lesions than non-induced controls, whereas in the ENGCol1a2-/−, surprisingly no significant difference in the number of lesions was observed. Flow cytometry analysis revealed, compared to controls, the lesions of ENGCol1a1-/− mice had a significant increase in F4/80+ Ly6C− macrophages and Ly6G+ neutrophils, while those of ENGCol1a2-/− mice had a marked decrease in CD11C+ dendritic cells. Given the changes in immune cell infiltration, we also assessed acute inflammation, using a short-term DSS experiment. Interestingly, ENGCol1a1-/− mice lost significantly more weight compared to controls, indicating increased inflammation. Although weight loss did not differ in Col1a2 mice between the two groups, ENGCol1a2-/− mice had a much lower H&E staining score, indicating less colonic damage and inflammation. Furthermore, a decrease in F4/80+ Ly6C− macrophages was observed in the colons of both ENGCol1a1-/- and ENGCol1a2-/− mice, despite no change in the overall CD45+ immune cell population. Notably, the number of Ly6G+ neutrophils was decreased in the colons of ENGCol1a1-/− mice but increased in ENGCol1a2-/− mice. In addition, a decrease in CD11C+ dendritic cells was seen in the colons ENGCol1a2-/− mice. Conclusion: Our results suggest that AOM/DSS-induced polyp formation and infiltrating immune cells differ in ENGCol1a1-/− and ENGCol1a2-/− mice. Fibroblast-specific deletion of endoglin can reduce macrophage and dendritic cell infiltration, colonic damage, and protect mice from DSS colitis. Citation Format: Subinuer Abudukelimu, Mark J. Schoonderwoerd, Madelon Paauwe, Eveline S. de Jonge-Muller, Stef G. Janson, Nadine van Montfoort, Lukas JAC Hawinkels. Fibroblast-specific endoglin deletion alters colonic immune infiltrate in premalignant colorectal lesions [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 295.