The inflammatory versus constitutive trafficking of mononuclear phagocytes into the alveolar space of mice is associated with drastic changes in their gene expression profiles

Mononuclear phagocytes enter the lungs both constitutively to maintain alveolar macrophage and dendritic cell homeostasis as well as during lung inflammation, where the role of these cells is less well defined. We used a transgenic mouse strain (CX₃CR1^+/GFP) that harbors a green fluorescent protein (GFP) label in circulating monocytes to identify and sort these cells from the vascular and alveolar compartments under both constitutive and acute lung inflammatory conditions. Using nylon arrays combined with real-time RT-PCR for gene expression profiling, we found that flow-sorted, highly purified mononuclear phagocytes recruited to acutely inflamed mouse lungs showed strongly upregulated mRNA levels of the neutrophil chemoattractants KC, MIP-2 and IP-10, which contrasted with alveolar mononuclear phagocytes that immigrated in steady-state. Similar observations were made for the lysosomal cathepsins B, L and K being strongly upregulated in mononuclear phagocytes upon recruitment to inflamed lungs but not during constitutive alveolar immigration. Inflammatory elicited mononuclear phagocytes also demonstrated significantly increased mRNA levels of the cytokine TNF-α and the PRR associated molecules CD14, TLR-4 and Syndecan-4. Together, inflammatory elicited mononuclear phagocytes exhibit strongly increased neutrophil chemoattractants, lysosomal proteases and LPS signaling mRNA transcripts, suggesting that these cells may play a major role in acute lung inflammatory processes.