Functional role of macrolides at different stages of resolution of inflammation. Pathogens such as bacteria and viruses invade through the epithelium into the underlying stroma. Proinflammatory cytokines released from the epithelium and by invading pathogens initially attract neutrophils to initiate an inflammatory response, involving other inflammatory cells such as macrophages and dendritic cells. The presence of macrolides in phagocytic cells in the inflamed or infected tissue ensures a prolonged effect. This is best exemplified by the long half-life of AZM, which has a number of time-dependent disease-modifying effects, including promotion of initial neutrophil bacterial killing, attenuation and subsequent resolution of chronic inflammation, and enhancement of epithelial barrier integrity. The macrolide erythromycin has been shown to inhibit the further diapedesis of neutrophils by upregulating DEL-1 in endothelial cells to help resolve the inflammatory process. DC_tol, tolerizing dendritic cell.

Enhancement of bronchial epithelial barrier integrity and epithelial-stromal interaction. Upon infectious insult by bacteria/viruses or injury by other toxic agents, such as exposure to smoking and other environmental oxidants like SO₂, the airway epithelial barrier is weakened, facilitating the paracellular entrance of infectious agents into the underlying stroma. Both the epithelium and stromal cells may release proinflammatory cytokines that attract neutrophils and macrophages. Inflammatory stroma may disturb the normal homeostasis, potentially causing fibrotic-like changes that may maintain and/or further weaken the epithelium. AZM enters the infectious area with neutrophils, and its actions (targets of which are indicated in the far-right panel) may be important in restoring normal homeostasis, including resolution of inflammation, reducing fibrosis, and enhancing the airway epithelial barrier.

Macrolides interfere with a number of signaling and cellular processes. Macrolides exert their effects via multiple mechanisms involving a variety of signaling cascades, as shown here. The complexity of this has been simplified, and the majority of pathways mentioned in this review are presented. Macrolides, in particular AZM, are known to target toll-like receptor (TLR) and tyrosine receptor kinase (TRK) pathways, inhibiting activation of their downstream signaling molecules, including ERK, JNK, P38, and mTOR. This, in turn, results in the downregulation of a number of genes, including those involved in inflammation (IL-1β, IL-6, IL-8, IL-17, and TNFα), thus dampening the inflammation response. Conversely, certain proinflammatory and anti-inflammatory cytokines (IL-10) are increased through activation of the transcription factors NF-κB and AP-1. It is also through these pathways that macrolides reduce expression of MUC5AC, MMP (in particular MMP2), MMP9, and induced nitric oxide synthase (iNOS). Macrolides also inhibit fibrosis and EMT by inhibiting signaling pathways related to Wnt and transforming growth factor-β. Macrolide activation of G-protein–coupled receptors, along with TRK, phospholipase C (PLC), and second messengers DAG and IP3, causes the release of calcium from the endoplasmic reticulum (ER). Mobilized calcium leads to a variety of cell type–dependent effects ranging from activation of TRK signaling, stabilizing calcium levels, and affecting ion channels and adhesion molecules, such as E-cadherin and TJs. Macrolides also inhibit ROS generation and can thus reduce cellular damaging cascades generated by ROS-associated activation of NLRP3 that triggers inflammasome formation and SASP. Accumulation of macrolides in lysosomes results in their enhanced stability through binding to lipids and reduction of phospholipase activity, with the subsequent release of enzymes such as cathepsin. Autophagy flux is also blocked by macrolides. AP-1, activator protein-1; ATF2, activating transcription factor-2; DAG, diacylglycerol; IP3, inositol trisphosphate; JNK, c-Jun N-terminal kinases; LEF, lymphoid enhancer factor; mTOR, mammalian target of rapamycin; Nrf2, nuclear factor-erythroid factor 2-related factor 2; PIP2, phosphatidylinositol 4,5-bisphosphate; PKC, protein kinase C; RAC-1, ras-related C3 botulinum toxin substrate 1; SMAD2, mothers against DPP homolog 2; SRF, serum response factor.