Plasminogen and plasmin induce specialized pro-resolving mediators and promote efferocytosis via 5-lipoxygenase.

Background: The plasminogen (Plg)/plasmin (Pla) system has been recognized for its pro-resolving actions, such as promoting efferocytosis. However, the role of specialized pro-resolving mediators (SPMs) in these Plg/Pla effects remains unexplored.

Objectives: To investigate whether Plg/Pla promotes SPMs production in macrophages and to elucidate the role of the 5-lipoxygenase (5-LO) pathway in Pla-induced efferocytosis.

Methods: Bone marrow-derived macrophages (BMDMs) from wild-type (WT) and Plg-knockout (KO) mice were treated or not with Plg/Pla, with or without leupeptin (protease inhibitor) and tranexamic acid (lysine analog), for 24 hours. SPMs and leukotriene B4 (LTB₄) production, and the mRNA expression of 5-, 12-, and 15-LO (SPMs biosynthetic enzymes) were assessed in WT-BMDMs. Additionally, 12/15-LO expression was analyzed in pleural macrophages from Pla-injected mice. Efferocytosis of apoptotic neutrophils was examined in Pla-injected WT and 5-LO KO mice and using the 5-LO inhibitor MK886 in Pla-stimulated BMDMs.

Results: Plg/Pla enhanced the production of LXA₄, MaR1, and RvD1, as well as the mRNA expression of 5-, 12-, and 15-LO in macrophages, a process dependent on proteolytic activity and lysine binding sites. Macrophages from Plg KO mice produced lower LXA4 levels, while LTB₄ levels remained comparable to WT cells. Pla injection promoted macrophage recruitment to the pleural cavity alongside 12/15-LO protein upregulation. Furthermore, Pla increased the efferocytosis of apoptotic neutrophils in WT mice, but not in 5-LO KO mice, and pharmacological inhibition of 5-LO reduced Pla-induced efferocytosis in vitro.

Conclusion: In summary, Plg/Pla pro-resolving actions are associated with SPMs production in macrophages and a 5-LO-dependent pro-efferocytosis mechanism.