Immunomodulatory Effects of Macrolide Antibiotics - Part 1: Biological Mechanisms

Macrolide antibiotics are well known for their antibacterial and anti-inflammatory properties. This article provides an overview of the biological mechanisms through which macrolides exert this ‘double effect’. Their antibacterial effect consists of the inhibition of bacterial protein synthesis, impaired bacterial biofilm synthesis, and the attenuation of other bacterial virulence factors. Apart from these direct antimicrobial effects, macrolides are known for their modulating effect on many components of the human immune system. By influencing the production of cytokines, they have a dampening effect on the proinflammatory response. Furthermore, the majority of cells involved in the immune response are, in one way or another, influenced when macrolide antibiotics are administered. Having such an obvious effect on the various aspects of the immune system, macrolides seem to be exceptionally suited for the treatment of chronic inflammatory diseases. Copyright © 2010 S. Karger AG, Basel Received: November 23, 2009 Accepted after revision: June 10, 2010 Published online: August 21, 2010 Drs. J. Altenburg, MD Department of Pulmonary Diseases Medical Centre Alkmaar, W ilhelminalaan 12 NL–1812 JD Alkmaar (The Netherlands) Tel. +31 72 548 2750, Fax +31 72 548 2167, E-Mail j.altenburg @ mca.nl © 2010 S. Karger AG, Basel 0025–7931/11/0811–0067$38.00/0 Accessible online at: www.karger.com/res D ow nl oa de d by : 54 .7 0. 40 .1 1 11 /4 /2 01 7 6: 11 :3 0 P M Altenburg /de Graaff /van der Werf / Boersma Respiration 2011;81:67–74 68 airway disorder associated with a very poor prognosis. After 1987, when erythromycin was introduced as standard therapy for DPB, an impressive increase in 10-year survival was seen, i.e. from 10–20% to over 90% [3–6] . The unexpected success was attributed to a previously unknown anti-inflammatory effect of erythromycin. This theory was supported by the fact that serum levels of erythromycin in these DPB patients were well below minimal inhibitory concentrations (MIC) for the detected pathogens and by the known lack of susceptibility of most Gram-negative organisms to erythromycin. In the last two decades, exhaustive evidence has shown that macrolides indeed have a direct antimicrobial effect but, more importantly, also modulate many components of the immune response. Because of this anti-inflammatory or ‘immune modulating’ effect, macrolide antibiotics have been widely used as maintenance treatment for various chronic inflammatory pulmonary diseases. Chronic inflammatory diseases generally feature a distorted inflammatory response. Instead of protecting the human body against exogenous attacks, the cascade of anti-inflammatory responses fails, damaging cells and making them more vulnerable to new attacks. In this article we aim to clarify the biological mechanisms through which macrolides exert their immunemodulating and antibacterial effect. These mechanisms are shown schematically in figure 1 . Effects on Host-Pathogen Interactions Most macrolides are active against Gram-positive cocci (including anaerobes) and have limited Gram-negative activity. They inhibit bacterial protein synthesis by binding to the 50S subunit of the ribosome [1, 7, 8] . Biofilm A biofilm is an aggregate of microorganisms immersed in a polysaccharide matrix adherent to each other and to the airway mucosa. Biofilm-forming bacteria are protected from phagocytosis, antimicrobial agents, and the ciliary action of the airway epithelial cells. Furthermore, microorganisms gathered in a biofilm develop significantly different genetic properties compared to planktonic species. Research on the biofilm effects of macrolides mainly focuses on Pseudomonas aeruginosa , which is one of the more virulent biofilm-forming microorganisms with a natural resistance to macrolides. However, effects of macrolides were also demonstrated on biofilm formation in Haemophilus influenzae and Staphylococcus epidermidis [9, 10] . Macrolides have been shown to alter the structure and architecture of the bacterial biofilm [11–13] . Results of Japanese in vitro studies indicate that azithromycin and clarithromycin change the structure of bacterial biofilms via the inhibition of polysaccharide synthesis [12, 14] . An insufficient biofilm allows for enhanced phagocytosis and clearance of bacteria by alveolar macrophages [11, 15] . Quorum Sensing During infection, bacteria employ mutual communication [quorum sensing (QS)] to coordinate the expression of genes, for instance, genes encoding for tissuedamaging factors [16] . Through the production of autoinducer molecules, genes can be switched on or off depending on the local pathogen density. Furthermore, activation of the QS cascade is claimed to promote biofilm formation and to stimulate IL-8 production, causing an enhanced neutrophil influx at the site of infection [6] . Several authors suggest that suppression of QS systems through reduced transcription of QS genes is also one of the mechanisms of macrolide action [16– 18] . Bacterial Adherence In vitro and in vivo evidence suggests that P. aeruginosa bacilli, when cultured in the presence of low levels of macrolides, e.g. erythromycin, demonstrate decreased adherence to cells of the airway epithelium [19–21] . Since the adherence of bacteria to mucosal surfaces is an important initial event in the pathogenesis of most bacterial infectious diseases, this could help explain the clinical efficacy of low-dose macrolide therapy in patients colonized with PA. Mobility The effect of macrolides on P. aeruginosa is accompanied by an impairment of the mobility of this microorganism. Pseudomonas spp. are mobile thanks to 2 distinctive modalities: flagella, which are tail-like structures that project from the cell body and move in a whip-like manner, and type IV pili (fimbriae) that provide twitching motility. Exposure to sub-MIC concentrations of macrolide antibiotics results in loss of mobility, partly due to the inhibition of flagellin production [22–24] , the principal constituent of bacterial flagella, and partly because some macrolides alter the assembly of type IV pili [13, 25] . This loss of mobility facilitates easier phagocytosis and the killing of bacteria by alveolar macrophages. D ow nl oa de d by : 54 .7 0. 40 .1 1 11 /4 /2 01 7 6: 11 :3 0 P M Immunomodulatory Effects of Macrolide Antibiotics – Part 1 Respiration 2011;81:67–74 69 Bacterial Toxins Cytotoxic enzymes produced by bacteria when causing infection, including exotoxin A, alkaline protease, elastase, and phospholipase C, are important factors in bacterial virulence. Erythromycin and, more recently, azithromycin have been shown to suppress the production of those enzymes and, consequently, to diminish bacterial virulence [23, 26–28] . Intracellular Effects Macrolides accumulate and show a prolonged retention in human cells after oral or intravenous administration, an effect that is augmented when macrolide treatment is given for a longer period of time [29–32] . In cystic fibrosis patients treated with azithromycin (500 mg dai ly) for at least 35 consecutive days, the concentration of azithromycin in neutrophils appeared to be up to 3,000 times higher as compared to the concentration in plasma [32] . Macrolides have also been shown to accumulate in alveolar macrophages [30, 33] . This suggests that tissue and intracellular concentrations may be more useful for assessing the antibacterial activity of azitromycin than serum concentrations [34, 35] . Because intracellular concentrations of macrolide + + +