Abdominal surgeries are lifesaving procedures in many clinical scenarios. The medical breakthrough of abdominal surgeries dates back to the early 19th century and goes along with the story of Mrs. Jane Crawford who suffered from an ovarian cancer. Thanks to abdominal surgery, and notwithstanding the lack of anesthesia and antiseptic measures, Dr. Ephraim McDowell was able to successfully remove the tumor and Mrs. Crawford fully recovered thereafter 1. Yet, this substantial progress in surgery was soon reported to entail the risk of a previously unknown complication - post-surgical peritoneal adhesion. Till today, surgical interventions in the abdominal cavity are frequently associated with the formation of post-surgical peritoneal adhesions. Peritoneal adhesions are irreversible fibrotic scar bands. They derive from the injured mesothelium and connect abdominal organs and the abdominal wall at non-anatomic locations 2–4. Thus, adhesions impede free organ movement and promote severe complications, such as small bowel obstruction, chronic pelvic pain, and infertility 3. Despite major health burden for patients and associated raising costs for health care systems 5,6, to date no cure for patients suffering from post-surgical adhesions exists and the understanding of underlying molecular mechanisms of adhesion formation is incomplete.
The current paradigm states that peritoneal injury initiates inflammation and coagulation resulting in fibrin deposition 3. We recently showed how fibrin deposition is preceded by macrophage aggregation 7. The primary fibrin/macrophage aggregate is suggested to serve as a scaffold for a continuing fibrotic reaction, which is characterized by extracellular matrix deposition by myofibroblasts migrating to injury sites 4,8.
Adhesion research and many previous findings gained from the use of sterile injury mouse models. However, in a clinical setting, abdominal surgeries are rarely sterile procedures. In fact, manipulation of intestines bears a high risk of gut microbe contamination of the peritoneal cavity and studies have linked bacterial peritonitis with the occurrence of post-surgical adhesions 3. Here we set out to experimentally determine how microbial contamination factors into the formation of adhesions.
On December 16th, 2021, we published the paper “Intraperitoneal microbial contamination drives post-surgical peritoneal adhesions by mesothelial EGFR-signaling” in Nature Communications. As a foundation for this project, a modular mouse animal system of surgical peritoneal injury and microbial contamination was developed, enabling to study effects of surgical trauma and bacterial contamination in combination and separately. First findings showed that surgical injury and microbial contamination indeed promote adhesion formation in peritoneal cavities of mice, particularly indicated by increased collagen deposition. Secretion of collagen was assigned to myofibroblasts, thereby raising the next question. Where do myofibroblasts in adhesions come from? Using Wt1-based genetic lineage tracing, we demonstrated that the main source of myofibroblasts, responsible for the increased extracellular matrix deposition at injury sites, are mesothelial cells undergoing mesenchymal transition. This supports recent findings from Fischer et al. 9. To further elucidate the effect of microbial contamination on mesothelial cells involved in adhesion formation, we studied potential alterations in signaling pathways of mesothelial cells challenged by injury and microbial contamination. Our RNA-sequencing data revealed that mesothelial cell activation and trans-differentiation is driven by epidermal growth factor receptor (EGFR) signaling, which is significantly elevated upon microbial contamination of mouse peritoneal cavities. We then asked whether same processes occur in humans. Therefore, we compared mesothelial cells from patients suffering from bacterial peritonitis with mesothelial cells from healthy donors. Mesothelial EGFR expression in peritonitis cases was significantly increased over the healthy baseline, where EGFR was hardly detectable. Encouraged by this, we went back to the mouse model and inhibited EGFR signaling using the FDA-approved small molecule inhibitor Gefitinib, which resulted in a significant reduction of adhesions in mice.
Taken together, our data shows that microbial contamination leads to EGFR activation on mesothelial cells which induces their conversion into fibroblasts eventually promoting increased adhesion formation. Furthermore, EGFR is a target with potential for future therapies.
If you are interested to read more about our work, please find the complete study in Nature Communications: https://doi.org/10.1038/s41467-021-27612-x
- Benigno, B. B. Ephraim McDowell and Jane Todd Crawford: The Bicentennial of a Surgical Masterpiece. Obstet. Gynecol. 113, 1141–1144 (2009).
- Ellis, H. et al. Adhesion-related hospital readmissions after abdominal and pelvic surgery: a retrospective cohort study. The Lancet 353, 1476–1480 (1999).
- Hellebrekers, B. W. J. & Kooistra, T. Pathogenesis of postoperative adhesion formation. Br. J. Surg. 98, 1503–1516 (2011).
- Zwicky, S. N., Stroka, D. & Zindel, J. Sterile Injury Repair and Adhesion Formation at Serosal Surfaces. Front. Immunol. 12, 684967 (2021).
- Sikirica, V. et al. The inpatient burden of abdominal and gynecological adhesiolysis in the US. BMC Surg. 11, 13 (2011).
- ten Broek, R. P. G. et al. Burden of adhesions in abdominal and pelvic surgery: systematic review and met-analysis. BMJ 347, f5588–f5588 (2013).
- Zindel, J. et al. Primordial GATA6 macrophages function as extravascular platelets in sterile injury. Science 371, eabe0595 (2021).
- Sandoval, P. et al. Mesothelial-to-mesenchymal transition in the pathogenesis of post-surgical peritoneal adhesions: Mesothelial-to-mesenchymal transition and peritoneal adhesions. J. Pathol. 239, 48–59 (2016).
- Fischer, A. et al. Post-surgical adhesions are triggered by calcium-dependent membrane bridges between mesothelial surfaces. Nat. Commun. 11, 3068 (2020).