Mechanisms of liver regeneration in RAPID – a novel model of auxiliary liver transplantation : The role of endothelial progenitor cells

Details

Supervisors

Leclercq, Isabelle

Faculty

Faculté de médecine et médecine dentaire

Degree label

Master [180] en médecine, à finalité spécialisée

Abstract

Background The RAPID surgery (Resection And Partial liver segment 2–3 transplantation with Delayed total hepatectomy) represents a significant breakthrough in liver transplantation, enabling successful transplantation of small grafts well below conventional size thresholds (Graft-to-Recipient Weight Ratio (GRWR) =0.8-1%), without inducing Small-for-Size Syndrome (SFSS). This innovative two-stage procedure begins with the resection of the recipient’s left liver, which is replaced by a small graft (GRWR<0.5%) from a living donor. Simultaneously, the right portal branch is ligated, subjecting the graft to pro-regenerative portal hyperflow. Approximately 14 days later, the graft regains sufficient volume and functionality to allow completion hepatectomy without triggering SFSS. SFSS is a form of post-transplant hepatocellular failure arising from two primary vascular phenomena: initially, the abrupt diversion of portal flow towards a reduced parenchymal mass induces shear stress, damaging the sinusoids. Subsequently, liver regeneration is marked by an imbalance between excessive hepatocyte proliferation relative to insufficient or delayed neoangiogenesis. Therefore, the success of RAPID in avoiding liver dysfunction despite significant graft growth and portal hyperflow necessitates a comprehensive exploration of the mechanisms underlying sinusoidal restoration. Within this context, endothelial progenitor cells, renowned for their potent pro-angiogenic activity, emerge as potential crucial mediators. This umbrella term encompasses myeloid angiogenic cells (MAC) from the bone marrow, which contribute paracrinally to angiogenesis, and endothelial colony-forming cells (ECFC) from vascular walls, which physically integrate into new vessels. Methods RAPID transplant recipients were included in our prospective single-centre research protocol. They were transplanted for liver metastases from colorectal or neuroendocrine cancer. Biological samples, including liver biopsies and peripheral blood, were collected between the two operative stages. Living donors undergoing a 40% hepatectomy served as a control group for conventional liver regeneration. During the critical interstage time frame of the RAPID, systemic levels of pro-angiogenic and chemotactic cytokines were measured by ELISA. The number of circulating endothelial progenitor cells was assessed by flow cytometry, and their ability to form colonies by cell culture. The integration of exogenous sinusoidal cells into the graft was assessed in a sex-mismatched transplantation setting, using a dual FISH X and Y-chromosome labelling combined with endothelial immunostaining. Results After RAPID transplantation, recipients exhibited a significant increase in G-CSF levels compared to the donors (+8.29pg/mL, p=.02), particularly on the first post-operative day (POD) (p=.04). However, no systemic elevation in VEGFa or chemotactic cytokines was observed. Additionally, the count of circulating MACs increased as early as POD2 (p=.03), with more robust recruitment occurring in the recipients than the donors (absolute increase +14.7% of blood mononuclear cells, p=.04). Similarly, ex-vivo MAC colony-formation demonstrated a significant increase, albeit with some delay, starting around POD5 (p=.03). Colony counts remained comparable between donors and recipients. In contrast, ECFC levels remained unchanged, and scarcely engrafted into graft vascular walls, accounting for only 3.06% of identified endothelial nuclei. In conclusion, our study highlights a systemic pro-angiogenic conditioning in RAPID recipients, marked by a predominance of paracrine MACs over engrafting ECFC. In the innovative context of this transplantation, our results encourage future in-depth characterisation of endothelial physiology, which could orchestrate both graft regeneration and function.