Loss of H2R signaling disrupts neutrophil homeostasis and promotes inflammation-associated colonic tumorigenesis in mice.

Abstract Background and aims We previously demonstrated that histamine suppressed inflammation-associated colonic tumorigenesis through histamine type 2 receptor (H2R) signaling in mice. This study aimed to precisely elucidate the downstream effects of H2R activation in innate immune cells. Methods Analyses using online databases of single-cell RNA sequencing of intestinal epithelial cells in mice and RNA sequencing of mouse immune cells were performed to determine the relative abundances of four histamine receptors among different cell types. Mouse neutrophils, which expressed greater amounts of H2R, were collected from the peritoneum of wild-type (WT) and H2R-deficient mice, of which low-density and high-density neutrophils were extracted by centrifugation and were subjected to RNA sequencing. The effects of H2R activation on neutrophil differentiation and its functions in colitis and inflammation-associated colon tumors were investigated in a mouse model of dextran sulfate sodium (DSS)-induced colitis. Results Data analysis of RNA sequencing and quantitative reverse transcription-PCR (qRT-PCR) revealed that Hrh2 (H2R gene) is highly expressed in neutrophils, but barely detectable in intestinal epithelial cells. In mice, the absence of H2R activation promoted infiltration of neutrophils into both sites of inflammation and colonic tumors. H2R-deficient high-density neutrophils yielded pro-inflammatory features via NF-kB and MAPK signaling pathways, and suppressed T cell proliferation. On the other hand, low-density neutrophils (LDNs) which totally lack H2R activation displayed an immature phenotype compared to WT LDNs, with enhanced MYC pathway signaling and reduced expression of the maturation marker Toll-like receptor 4 (TLR4). Conclusion Blocking H2R signaling enhanced pro-inflammatory responses of mature neutrophils and suppressed neutrophil maturation, leading to accelerated progression of inflammation-associated colonic tumorigenesis.