Abstract Body (Do not enter title and authors here): Introduction: Efficient clearance of dead/dying cells by macrophages (MΦs) (termed efferocytosis) is critical for timely repairing the injured heart after ischemia/reperfusion (I/R). Prior work by us and others shows that Lipocalin 10 (Lcn10), a secreted protein of lipocalin family, can regulate MΦ polarization and is greatly downregulated in cardiac tissue of patients with heart failure. However, the potential role of Lcn10 in MΦ efferocytosis during cardiac I/R has never been investigated.

Methods: Wild-type (WT) and Lcn10-knockout (KO) mice were subjected to 45 minutes of ischemia via the ligation of left anterior descending artery (LAD) followed by reperfusion. Cardiac function was measured by echocardiography. Flow cytometry was utilized to assess MΦ efferocytosis. Bone marrow-derived MΦs (BMDMs) were used to test efferocytosis in vitro and dissect mechanism. Dead cells were prepared by treating red-dye-labelled H9C2 cells with H₂O₂ (1mM, 2h).

Results: After co-culturing BMDMs with dead H9C2 cells for 2h, we observed a 25% reduction of efferocytosis in Lcn10-KO BMDMs, compared to WTs. Using Lcn10-KO^mCherry mouse model (cardiomyocyte-specific overexpression of mCherry in Lcn10-KO background), we consistently found that cardiac MΦs in Lcn10-KO mice taken up fewer dead mCherry-myocytes at 4 days post-I/R surgery, leading to an increased accumulation of cardiac dead cells, higher serum levels of troponin I, greater myocardial fibrosis, and dramatically impaired cardiac function, compared to WT^mCherry controls. In contrast, pre-treatment of BMDMs with recombinant Lcn10 protein (rLcn10) enhanced efferocytosis by 50%. Mechanistically, RNA-seq analysis revealed that many efferocytosis-related genes were dysregulated (e.g., downregulation of Id1, Id3, Itga8, Itgav, Cd44, Cx3cr1, Msr1, and upregulation of Sirpα) in Lcn10-KO BMDMs, compared to WTs. Interestingly, most of these dysregulated genes are controlled by transcription factors Id1/3. Further studies showed that rLcn10 interacted with Cd44 and activated the expression of its downstream Id1/3, leading to enhanced MΦ efferocytosis. Importantly, rLcn10-mediated efferocytosis was greatly impaired by pre-treatment of MΦs with either anti-CD44 blocking antibody (KM201, 10µg/ml) or Id1/3 inhibitor (AGX51, 25µM).

Conclusion: Our data indicate that Lcn10 is essential for MΦ efficient efferocytosis to repair I/R-induced cardiac damage though affecting the Cd44-Id1/3 signaling and its related MΦ surface receptors.