Peripheral blood monocytes (PBMo) originate from the bone marrow, circulate in the blood, and emigrate into various organs where PBMo differentiate into tissue resident cellular phenotypes of the mononuclear phagocyte system (MPS), including macrophages (MQ) and dendritic cells (DC). As in other organs, this emigration and differentiation process is essential to replenish the mononuclear phagocyte pool in the lung under both inflammatory and non-inflammatory steady-state conditions. While many studies have addressed inflammation-driven monocyte trafficking to the lung, the emigration and pulmonary differentiation of PBMo under non-inflammatory conditions is comparatively poorly understood. In order to assess the transcriptional profile of circulating and lung-resident MPS phenotypes, PBMo, lung MQ and lung DC from naïve mice were flow-sorted to high purity, and cellular gene expression was compared by DNA microarray on a genome wide scale. Differential regulation of selected genes was validated by quantitative polymerase chain reaction (qPCR) and at the protein level by flow cytometry. Differentially-expressed genes related to cell trafficking were selected and grouped into three clusters: (i) matrix metallopeptidases, (ii) chemokines/chemokine receptors, and (iii) integrins. Expression profiles of gene clusters were further examined at the mRNA and protein levels in subsets of circulating PBMo (GR1- versus GR1+) and lung MQ (alveolar MQ versus interstitial MQ). These data identify differentially activated genetic programs in circulating monocytes and their lung descendents. Lung DC activate an extremely diverse set of genes, but largely preserve a mobile cell profile with high expression levels of integrins, chemokines and chemokine receptors. In contrast, interstitial and alveolar MQ, downregulate gene expression of these traffic-relevant communication molecules, but strongly upregulate a distinct set of matrix metalloproteinases potentially involved in tissue invasion and remodeling. In addition, in this study, the first evidence for functional expression of 5-HT2C receptors on resident alveolar MQ (rAMQ;) is provided. The inventory of 5-HT type 2 receptors in mouse lung MQ and alveolar epithelial cells (AEC) was investigated by qPCR, which revealed the expression of receptor subunits 5-HT2A and 5-HT2B on AEC and 5-HT2C on rAMQ. Upon 5-HT stimulation, freshly isolated rAMQ increased expression of CCL2 mRNA as assessed by qPCR. Moreover, rAMQ stimulated with 5-HT augmented production of CCL2 protein as indicated by dot-blot assay and enzyme-linked immunosorbent assay. In rAMQ isolated from 5HT2C-deficient mice, CCL2 production at both the mRNA and protein level was not affected by 5-HT stimulation. The response of rAMQ to 5-HT was also assessed at the whole mouse genome level, by microarray. Genes that were identified by microarray as being regulated were involved in cytokine activity, signal transduction, biosynthetic processes or cell differentiation. Taken together, these data demonstrate the presence of functional 5-HT2C receptors on rAMQ and identify 5-HT as a novel modulator of rAMQ function.Collectively, these data provide new insight into the changes of the genetic profiles of PBMo and their lung descendents, namely DC and MQ under non-inflammatory, steady-state conditions. Genes involved in cell trafficking expressed on DC were validated at the protein level, while expression of the 5-HT2C receptor in rAMQ was investigated for its functionality. These findings will help to better understand the complex relations within the mononuclear phagocyte pool of the lung.
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