Large Peritoneal Macrophages Play No Role in the Pathogenesis of Postoperative Ileus Induced by Intestinal Manipulation.

Introduction: Postoperative ileus (POI) is an iatrogenic disorder marked by temporary impaired gastrointestinal (GI) motility post-abdominal surgery. Surgical handling of the intestine activates resident macrophages (Mfs), leading to inflammatory cytokine release and leukocyte recruitment into the muscularis, which compromises intestinal contractility. The mechanisms behind this activation are unclear. Recent studies suggest peritoneal Mfs, particularly large peritoneal macrophages (LPMs), might play a role in sterile intestinal inflammation by rapidly recruiting to the serosal layer of the gut and aiding in tissue damage resolution.

Methods: To identify immune cells involved in the early phase of POI, single-cell RNA sequencing (scRNA-seq) was conducted. The migration of LPMs post-surgery was studied using adoptive transfer techniques. LPMs were depleted via intraperitoneal injection of clodronate liposomes. Subsequently, flow cytometry, quantitative PCR (qPCR), and immunofluorescence were performed to assess the impact of LPM depletion and analyze cell populations and inflammatory effects.

Results: (1) Intestinal manipulation (IM) leads to the accumulation of monocytes, neutrophils, mature Mfs, CD8+ T cells, and LPMs within 2 h post-surgery. (2) Heparin treatment does not affect gut transit or reduce IL-6, IL-1a, and IL-1b expression in the early phase of POI. (3) Depletion of LPMs via clodronate liposome does not prevent monocyte, neutrophil, and Mfs infiltration in the muscularis externa, nor does it improve gut transit or reduce cytokine expression. (4) LPMs migrate to the serosa after IM but do not enter the muscularis externa.

Conclusion And Inferences: LPMs adhere to the intestinal serosa following intestinal manipulation but do not migrate into the intestinal muscularis or participate in the inflammatory response and delayed transit. Consequently, LPMs are not involved in the pathogenesis of POI.