3.4-Immunomodulatory in vitro and animal data
Azithromycin exerts its immunomodulatory effects on different points in the inflammatory cascade, modulating cell functions and cell signaling processes[7,37,38].
In airway epithelial cells macrolides can maintain cell integrity by stabilizing the cell membrane, increasing the transepithelial electrical barrier and inducing processing of the tight junction proteins claudins and junctional adhesion molecule-A [37,39,40]. They can also decrease mucus hypersecretion in vitro and in vivo, even when not produced by bacteria, which may improve mucociliary clearance [37,41,42]. Azithromycin use directly relaxed pre-contracted airway smooth muscle cells [7].
This macrolide can decrease the hypersecretion of pro-inflammatory cytokines and chemokines by acting in many inflammatory cells as monocytes, macrophages and fibroblasts [7]. Its use has been related with a reduction of IL-1β, IL-4, IL-5, IL-6, IL-8, IL-12, IFN-γ, IP-10, TNF-α, and GM-CSF [7,37,38,43]. In alveolar macrophages, azithromycin attenuated Th-1 cell responses, shifting polarization of alveolar macrophages to their alternative activated anti-inflammatory M2 phenotype [7]. It also increased phagocytosis of apoptotic bronchial epithelial cells by macrophages [44]. In fibroblasts, macrolides have demonstrated to inhibit fibroblast proliferation, collagen production and to decrease transforming growth factor (TGF-β) levels [45]. In lymphocytes, azithromycin has shown to suppress CD4+ T-cell activation [46] All these findings have been demonstrated in vitro. On the contrary, azithromycin can increase the release of IL-10, an anti-inflammatory cytokine related to the reparation of the inflamed tissues [7,37,43,45].
In animal models, the treatment with azithromycin reduced mortality in pneumococcal pneumonia, viral bronchiolitis and polymicrobial sepsis in mice[47–49]. These findings were found even in the setting of macrolide-resistant strains, suggesting that the immunomodulatory properties, including the aversion of cytokine storm, may explain these benefits. Azithromycin reduced the accumulation of inflammatory cells (macrophages, lymphocytes, and neutrophils) in bronchoalveolar lavage and in lung tissue[47]. In addition, downregulated the expression of chemokines (G-CSF, CCL3/MIP-1α, CCL4/MIP-1β) and cytokines (IL-1β, IL-6, IL-12, TNF-α and IFN-γ) in the lung[47,49].
In the late fibroproliferative-fibrotic phase of ARDS, azithromycin may suppress lung fibrosis[19]. In a murine model of acute lung injury caused by bleomycin, it significantly reduced fibrosis and restrictive lung function pattern[50]. Once fibrosis has been established, azithromycin could also have antifibrotic and proapoptotic effects on primary fibroblasts[51].