Differential contribution of bone marrow derived-monocytes to the lung tissue resident alveolar macrophages and persistent lung inflammation with chronic air pollution exposure

Chronic exposure to ambient particulate matter <2.5µ (PM2.5) has been linked to cardiopulmonary disease. Tissue-resident (TR) alveolar macrophages (AΦ) are long lived, self-renew and critical to the health impact of inhalational insults. There is inadequate understanding of the impact of PM2.5 exposure on nature/time course of transcriptional responses and the proliferation/maintenance of AΦ including the contribution from bone marrow (BM) over chronic time periods. We investigated the effects of exposure to real-world concentrated PM2.5 or filtered air (FA) in chimeric (CD45.2/CD45.1) mice. Here, we show that PM2.5 exposure induces an influx of BM-derived monocytes to lungs at 4-weeks, with no contribution to TR-AΦ population. Chronic (32-weeks) PM2.5 exposure resulted in enhanced apoptosis (Annexin V+) and decreased proliferation (BrdU+) of TR-AΦ and presence of BM-AΦ in inflamed lungs. RNA-seq analysis of flow sorted TR-AΦ and BM-AΦ from 4 and 32-weeks exposed mice, revealed a unique time dependent pattern of differentially expressed genes, with PM2.5 exposure with a pro-inflammatory bias. PM2.5 exposure resulted in pulmonary fibrosis and reduced alveolar fraction which corresponded to protracted lung inflammation. Our findings suggest a time dependent PM2.5 entrainment of a BM-derived monocytes infiltration into PM2.5 exposed lungs with an inflammatory phenotype, that together with enhanced apoptosis of TR-AΦ and pro-inflammatory polarization may contribute to perpetuation of chronic inflammation and lung fibrosis. RNA sequencing of alveolar macrophages and monocytes of CD45.1 and CD45.2 origin from PM2.5 or FA exposed mice for 4 and 32 weeks.