Bronchopulmonary dysplasia (BPD) is a severe complication of preterm births, which develops due to exposure to supplemental oxygen and mechanical ventilation. Published studies demonstrated that the number of endothelial progenitor cells (EPC) is decreased in mouse and human BPD lungs and that adoptive transfer of EPC is an effective approach in reversing the hyperoxia-induced lung damage in mouse model of BPD. Recent advancements in macrophage biology identified the specific subtypes of circulating and resident macrophages mediating the developmental and regenerative functions in the lungs. Several studies reported the successful application of macrophage therapy in accelerating the regenerative capacity of damaged tissues and enhancing the therapeutic efficacy of other transplantable progenitor cells. In the present study, we explored the efficacy of combined cell therapy with EPC and resident alveolar macrophages (rAM) in hyperoxia-induced BPD mouse model. rAM and EPC were purified from neonatal mouse lungs and were used for adoptive transfer to the recipient neonatal mice exposed to hyperoxia. Adoptive transfer of rAM alone did not result in engraftment of donor rAM into the lung tissue but increased the mRNA level and protein concentration of proangiogenic CXCL12 chemokine in recipient mouse lungs. Depletion of rAM by chlodronate-liposomes decreased the retention of donor EPC after their transplantation into hyperoxia-injured lungs. Adoptive transfer of rAM in combination with EPC enhanced the therapeutic efficacy of EPC as evidenced by increased retention of EPC, increased capillary density, improved arterial oxygenation, and alveolarization in hyperoxia-injured lungs. Dual therapy with EPC and rAM has promise in human BPD.NEW & NOTEWORTHY Recent studies demonstrated that transplantation of lung-resident endothelial progenitor cells (EPC) is an effective therapy in mouse model of bronchopulmonary dysplasia (BPD). However, key factors regulating the efficacy of EPC are unknown. Herein, we demonstrate that transplantation of tissue-resident alveolar macrophages (rAM) increases CXCL12 expression in neonatal mouse lungs. rAM are required for retention of donor EPC in hyperoxia-injured lungs. Co-transplantation of rAM and EPC improves the efficacy of EPC therapy in mouse BPD model.

BACKGROUND: Acute respiratory distress syndrome (ARDS) is a pulmonary inflammatory process primarily caused by sepsis. The resolution of inflammation is an active process involving the endogenous biosynthesis of specialized pro-resolving mediators, including resolvin D1 (RvD1). Resident alveolar macrophages (RAMs) maintain pulmonary homeostasis and play a key role in the resolution phase. However, the role of RAMs in promoting the resolution of inflammation by RvD1 is unclear.
OBJECTIVE: Here, we investigated the mechanisms of RvD1 on regulating RAMs to promote the resolution of ARDS.
METHODS: Mice were administered lipopolysaccharide and/or Escherichia coli via aerosol inhalation to establish a self-limited ARDS model. Then, RvD1 was administered at the peak inflammatory response. RAMs self-renewal was measured by flow cytometry, RAM phagocytosis was measured by two-photon fluorescence imaging. In addition, plasma was collected from intensive care unit patients on days 0-2, 3-5, and 6-9 to measure RvD1 and S100A8/A9 levels using triple quadrupole/linear ion trap mass spectrometry.
RESULTS: RAMs were found to play a pivotal role in resolving inflammation during ARDS, and RvD1 enhanced RAM proliferation and phagocytosis, which was abrogated by a lipoxin A4 receptor (ALX, RvD1 receptor) inhibitor. Both primary RAMs transfected with rS100A8/A9 and/or S100A8/A9 siRNA and S100A9-/- mice (also deficient in S100A8 function) showed higher turnover and phagocytic function, indicating that RvD1 exerted its effects on RAMs by inhibiting S100A8/A9 production in the resolution phase. RvD1 reduced S100A8/A9 and its upstream MAPK14 levels in vivo and in vitro. Finally, in the patients, RvD1 levels were lower, but S100A8/A9 levels were higher.
CONCLUSIONS: We propose that RvD1 improved RAM self-renewal and phagocytosis via the ALX/MAPK14/S100A8/A9 signaling pathway. Plasma RvD1 and S100A8/A9 levels were negatively correlated, and associated with the outcome of sepsis-induced ARDS.

At the early stages of life development, alveoli are colonized by embryonic macrophages, which become resident alveolar macrophages (ResAM) and self-sustain by local division. Genetic and epigenetic signatures and, to some extent, the functions of ResAM are dictated by the lung microenvironment, which uses cytokines, ligand-receptor interactions, and stroma cells to orchestrate lung homeostasis. In resting conditions, the lung microenvironment induces in ResAM a tolerogenic programming that prevents unnecessary and potentially harmful inflammation responses to the foreign bodies, which continuously challenge the airways. Throughout life, any episode of acute inflammation, pneumonia being likely the most frequent cause, depletes the pool of ResAM, leaving space for the recruitment of inflammatory monocytes that locally develop in monocyte-derived alveolar macrophages (InfAM). During lung infection, the local microenvironment induces a temporary inflammatory signature to the recruited InfAM to handle the tissue injury and eliminate the pathogens. After a few days, the recruited InfAM, which locally self-sustain and develop as new ResAM, gain profibrotic functions required for tissue healing. After the complete resolution of the infectious episode, the functional programming of both embryonic and monocyte-derived ResAM remains altered for months and possibly for the entire life. Adult lungs thus contain a wide diversity of ResAM since every infection brings new waves of InfAM which fill the room left open by the inflammatory process. The memory of these innate cells called trained immunity constitutes an immunologic scar left by inflammation, notably pneumonia. This memory of ResAM has advantages and drawbacks. In some cases, lung-trained immunity offers better defense capacities against autoimmune disorders and the long-term risk of infection. At the opposite, it can perpetuate a harmful process and lead to a pathological state, as is the case among critically ill patients who have immune paralysis and are highly susceptible to hospital-acquired pneumonia and acute respiratory distress syndrome. The progress in understanding the kinetics of response of alveolar macrophages (AM) to lung inflammation is paving the way to new treatments of pneumonia and lung inflammatory process.

Acute lung injury (ALI) and/or acute respiratory distress syndrome (ARDS) are life-threatening critical syndromes characterized by the infiltration of a large number of inflammatory cells that lead to an excessive inflammatory response. Resolvin D1 (RvD1), an endogenous lipid mediator, is believed to have anti-inflammatory and proresolving effects. In the present study, we examined the impact of RvD1 on the pulmonary inflammatory response, neutrophil influx, and lung damage in a murine model of lipopolysaccharide (LPS)-induced ALI. Treatment with RvD1 protected mice against LPS-induced ALI, and compared to untreated mice, RvD1-treated mice exhibited significantly ameliorated lung pathological changes, decreased tumor necrosis factor-α (TNF-α) concentrations and attenuated neutrophil infiltration. In addition, treatment with RvD1 attenuated LPS-induced neutrophil infiltration via the downregulation of CXCL2 expression on resident alveolar macrophages. Finally, BOC-2, which inhibits the RvD1 receptor lipoxin A4 receptor/formyl peptide receptor 2 (ALX/FPR2), reversed the protective effects of RvD1. These data demonstrate that RvD1 ameliorates LPS-induced ALI via the suppression of neutrophil infiltration by an ALX/FPR2-dependent reduction in CXCL2 expression on resident alveolar macrophages.