Hanging in the balance: endogenous anti-inflammatory mechanisms in tissue repair and fibrosis

Inflammation is the physiological response to tissue injury caused by pathogens or trauma. Nevertheless, inflammation should be resolved in a timely manner, resulting in elimination of the inflammatory cells and mediators from the injured tissue, to avoid its deleterious consequences. Uncontrolled inflammation can lead to inflammatory, autoimmune, and cancerous disorders that are the result of improper resolution. The healing of the injured tissue during the termination of inflammation must also be tightly controlled since excessive tissue repair can lead to fibrosis and scarring of the affected organ. In the last three decades, it has been revealed that the resolution of inflammation is tightly orchestrated by specific cells, protein, and lipid mediators that are produced at proper timing and distinct locations. The bioactivity of these anti-inflammatory, pro-resolving, and immunoregulatory agents results in clearance of the tissue from inflammatory leukocytes and their products, and the return of homeostatic tissue architecture and function. Here, we will survey the current endogenous mechanisms governing the resolution of inflammation and directing it towards injury healing and halting of acquired immune responses while preventing excessive tissue repair and fibrosis. We focus on the role played by apoptotic polymorphonuclear cells (PMNs), 15-lipoxygenase (LO)-derived lipid mediators, and TGF beta in this macrophage-governed decision-making process and suggest new modes of action for fibrosis prevention and return to homeostasis.