Reduction in the resident intestinal myelomonocytic cell population occurs during ApcMin/+ mouse intestinal tumorigenesis

Abstract

With its significant contribution to cancer mortality globally, advanced colorectal cancer (CRC) requires new treatment strategies. However, despite recent good results for mismatch repair (MMR)-deficient CRC and other malignan- cies, such as melanoma, the vast majority of MMR-proficient CRCs are resistant to checkpoint inhibitor (CKI) therapy. MMR-proficient CRCs commonly develop from precursor adenomas with enhanced Wnt-signalling due to adenomatous polyposis coli (APC) mutations. In melanomas with enhanced Wnt signalling due to stabilized β-catenin, immune anergy and resistance to CKI therapy has been observed, which is dependent on micro-environmental myelomonocytic (MM) cell depletion in melanoma models. However, MM popula- tions of colorectal adenomas or CRC have not been studied. To characterize resident intestinal MM cell populations during the early stages of tumorigenesis, the present study utilized the ApcMin/+ mouse as a model of MMR-proficient CRC, using enhanced green fluorescent protein (EGFP) expression in the mouse lysozyme (M-lys)lys-EGFP/+ mouse as a pan-myelo- monocytic cell marker and a panel of murine macrophage surface markers. Total intestinal lamina propria mononuclear cell (LPMNC) numbers significantly decreased with age (2.32±1.39x107 [n=4] at 33 days of age vs. 1.06±0.24x107 [n=8] at 109 days of age) during intestinal adenoma development in ApcMin/+ mice (P=0.05; unpaired Student's t-test), but not in wild-type littermates (P=0.35). Decreased total LPMNC numbers were associated with atrophy of intestinal lymphoid follicles and the absence of MM/lymphoid cell aggregates in ApcMin/+ mouse intestine, but not spleen, compared with wild-type mice. Furthermore, during the early stage of intes- tinal adenoma development, there was a two-fold reduction of M-lys expressing cells (P=0.05) and four-fold reduction of ER-HR3 (macrophage sub-set) expressing cells (P=0.05; two tailed Mann-Whitney U test) in mice with reduced total intestinal LPMNCs (n=3). Further studies are necessary to determine the relevance of these findings to immune-surveil- lance of colorectal adenomas or MMR-proficient CRC CKI therapy resistance.