miRNA expression signatures in human alveolar macrophages after toll-like receptor activation

We have employed miRNA microarray expression profiling to identify miRNAs that are differentially regulated by toll-like receptor activation in alveolar macrophages. Macrophages are strategically distributed all over the body and represent the first line of defense to invading pathogens. In response to bacterial or viral infections, these phagocytic cells engulf and destroy the infectious agent. Macrophages recognize a variety of microbial products, such as bacterial cell wall components and nucleic acids, via pathogen recognition receptors (PRRs), such as toll-like receptors (TLRs). TLR4 represents the receptor for bacterial lipopolysaccharide (LPS). The extracellular TLR1/2 heterodimer recognizes bacterial triacetylated lipopeptides and their mimic, the synthetic compound Pam3CSK4. In contrast, intracellular TLR3 detects double-stranded RNA, i.e., an intermediate in viral replication, as well as its synthetic analogon polyinosinic:polycytidylic acid (Poly(I:C)). TLRs differentially activate transcription factors due to the varying involvement of the adapter molecules. However, little is known about TLR-mediated miRNA induction in macrophages. In this study, several microRNAs were identified that were specifically associated with a certain treatment scheme, suggesting that macrophage responses highly depend on the nature of the stimulus. Human alveolar macrophages isolated from lung tissue of patients undergoing lung resection were either left untreated or treated as follows: 1 µg/mL LPS for 2 h; 1 µg/mL Pam3CSK4 for 2 h; 10 µg/mL Poly(I:C) for 2 h; 0.1 µg LPS for 24 h (chronic low-dose exposure to induce LPS tolerance); 0.1 µg LPS for 24 h followed by rechallenge with 1 µg/mL LPS for 2 h; 1 µg Poly(I:C) for 24 h (chronic low-dose exposure).