Macrophages in Chronic Rejection: The Shapeshifters Behind Transplant Survival

Background: Organ transplant offers patients a second chance at life, yet chronic rejection remains a formidable barrier to long-term success. Unlike the instantaneous storm of acute rejection, chronic rejection is a slow, unremitting process that silently remodels vessels, scars tissues, and diminishes graft function. At the center of this process are macrophages, immune “shapeshifters” that can heal or harm depending on their cues. Methods: This manuscript systematically reviews and synthesizes the current evidence from experimental studies and clinical observations, as well as molecular insights, to unravel how macrophages orchestrate chronic rejection. It travels over macrophage origins alongside their dynamic polarization into pro-inflammatory (M1) or pro-repair yet fibrotic (M2) states. The discussion integrates mechanisms of recruitment, antigen presentation, vascular injury, and fibrosis, while highlighting the molecular pathways (NF-κB, inflammasomes, STAT signaling, metabolic rewiring) that shape macrophage fate. Results: Macrophages play a central role in chronic rejection. Resident macrophages, once tissue peacekeepers, amplify inflammation, while recruited monocyte-derived macrophages fuel acute injury or dysfunctional repair. Together, they initiate transplant vasculopathy through cytokines, growth factors, and matrix metalloproteinases, slowly narrowing vessels and starving grafts. Donor-derived macrophages, often overlooked, act as early sentinels and long-term architects of fibrosis, blurring the line between donor and host immunity. At the molecular level, macrophages lock into destructive programs, perpetuating a cycle of inflammation, vascular remodeling, and scarring. Conclusions: Macrophages are not passive bystanders but pivotal decision makers in chronic rejection. Their plasticity, while a source of pathology, also opens therapeutic opportunities. Emerging strategies like macrophage-targeted drugs, immune tolerance approaches, gene and exosome therapies currently offer ways to reprogram these cells and preserve graft function. By shifting the macrophage narrative from saboteurs to guardians, transplantation medicine may transform chronic rejection from an inevitability into a preventable complication, extending graft survival from fleeting years into enduring decades.