Research Article: Pseudomonas aeruginosa Exploits Lipid A and Muropeptides Modification as a Strategy to Lower Innate Immunity during Cystic Fibrosis Lung Infection

Date Published: December 23, 2009

Publisher: Public Library of Science

Author(s): Cristina Cigana, Laura Curcurù, Maria Rosaria Leone, Teresa Ieranò, Nicola Ivan Lorè, Irene Bianconi, Alba Silipo, Flora Cozzolino, Rosa Lanzetta, Antonio Molinaro, Maria Lina Bernardini, Alessandra Bragonzi, Edgardo Moreno.

Abstract: Pseudomonas aeruginosa can establish life-long airways chronic infection in patients with cystic fibrosis (CF) with pathogenic variants distinguished from initially acquired strain. Here, we analysed chemical and biological activity of P. aeruginosa Pathogen-Associated Molecular Patterns (PAMPs) in clonal strains, including mucoid and non-mucoid phenotypes, isolated during a period of up to 7.5 years from a CF patient. Chemical structure by MS spectrometry defined lipopolysaccharide (LPS) lipid A and peptidoglycan (PGN) muropeptides with specific structural modifications temporally associated with CF lung infection. Gene sequence analysis revealed novel mutation in pagL, which supported lipid A changes. Both LPS and PGN had different potencies when activating host innate immunity via binding TLR4 and Nod1. Significantly higher NF-kB activation, IL-8 expression and production were detected in HEK293hTLR4/MD2-CD14 and HEK293hNod1 after stimulation with LPS and PGN respectively, purified from early P. aeruginosa strain as compared to late strains. Similar results were obtained in macrophages-like cells THP-1, epithelial cells of CF origin IB3-1 and their isogenic cells C38, corrected by insertion of cystic fibrosis transmembrane conductance regulator (CFTR). In murine model, altered LPS structure of P. aeruginosa late strains induces lower leukocyte recruitment in bronchoalveolar lavage and MIP-2, KC and IL-1β cytokine levels in lung homogenates when compared with early strain. Histopathological analysis of lung tissue sections confirmed differences between LPS from early and late P. aeruginosa. Finally, in this study for the first time we unveil how P. aeruginosa has evolved the capacity to evade immune system detection, thus promoting survival and establishing favourable conditions for chronic persistence. Our findings provide relevant information with respect to chronic infections in CF.

Partial Text: The strategy of innate immune recognition is based on the detection of constitutive and conserved products of microbial metabolism called pathogen-associated molecular patterns (PAMPs) or alternatively microbe-associated molecular patterns (MAMPs), since they are also common to all microbe and not only to the pathogen version [1]. These molecular signatures are recognized by the host through a family of pattern recognition receptors (PRRs), which includes Toll-like (TLRs) and nucleotide binding and oligomerization domain-like receptors NLR (Nod-Like receptor) [2]. For example, lipid A contained in bacterial lipopolysaccharide (LPS) acts as a PAMP and is sensed by the cognate PRR TLR4-MD2, while different motifs contained in peptidoglycan (PGN) of Gram-positive or Gram-negative bacteria are recognized by the intracellular PRRs Nod1 and Nod2 [3], [4]. Interaction of PAMPs with PRRs results in activation of antimicrobial responses [1], such as production of antimicrobial peptides and secretion of pro-inflammatory cytokines, necessary for pathogen’s eradication. However, many pathogens have evolved adaptive strategies for subverting the host innate immune system by evading detection by PRRs and/or impairing the downstream cellular signalling pathway [5]. In this study, we present evidence that Pseudomonas aeruginosa exploits PAMPs modification as a strategy to lower innate immune system detection and signalling during chronic stages of lung infection in fibrosis cystic (CF) patients.

In this work, we analysed the PAMPs chemical modification as a strategy of P. aeruginosa to hijack genes involved in innate immune responses and to favour survival in patients with CF. A major question, derived from previous reports [7], [8], was whether P. aeruginosa could establish life-long chronic infections in CF hosts according with strategies used by a number of other bacterial pathogens. Persistence is normally established after an acute period of infection involving activation of both the innate and acquired immune system. While the acute infection is usually fully resolved by eliminating the invading bacteria, some bacteria including Salmonella enterica sv typhi, Helicobacter pylori, Mycobacterium tuberculosis and others survive and cause persistent life-long infection by evading immune surveillance [19].



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