Loss of myeloid S1PR1 makes dysfunctional alveolar macrophages and vascular injury by inducing myeloid bias

Abstract

Abstract Alveolar macrophages (AM) are the most abundant resident innate immune cells in the lungs and directly communicate with both the non-sterile external environment and the pulmonary epithelium. AM initially trigger protective inflammation upon sensing pathogen but later induce lung repair by clearing dead cells, known as efferocytosis, and dampening of inflammation. Studies show that monocytes recruited in the airspace during injury gain AM signature and contribute in lung repair. Sphingosine 1 phosphate receptor 1 (S1PR1), is widely studied in lung endothelium as a barrier protective mechanism and in immune cells in context of trafficking and inflammation but has not yet extensively studied in respect to origin and function of AM. Here, we deleted S1PR1 in mice by crossing S1PR1f/f with LysMCre (S1PR1ΔLyz) mice to investigate the role of S1PR1 in myeloid cells in the mechanism of lung injury. We show that loss of S1PR1 in myeloid cells of mice (S1PR1ΔLyz mice) has 3-fold higher AM pool but surprisingly these mice showed defective lung fluid balance basally or post LPS challenge (intratracheal 0.5 mg/kg). We found that S1PR1−/−AM produced markedly less pro-inflammatory cytokines (IL1b and TNFa) and could not efferocytose. Intriguingly, S1PR1 deficiency induced myeloid biased haematopoiesis as evident from less erythroid progenitor (~0.75 fold) and B cells (~0.82 fold) in bone marrow of S1PR1ΔLyz mice leading to increased circulating monocytes. panRNAseq of S1PR1−/− AM showed markedly reduced Sphk1 and NFκB1 expression and increased expression of KLF4, Myb, Myc, TLR7 & 8. These findings indicate that loss of S1PR1 misfires the generation of incompetent monocytes leading to dysfunctional AM and defective lung repair.