Compartmental responses of the respiratory tract to Staphylococcus aureus

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is an important nosocomial pathogen associated with significant morbidity and mortality. Previous colonisation with this pathogen is a risk factor for the development of subsequent infection. Tolllike receptors (TLRs) are a family of transmembrane receptors of the innate immune system that recognize pathogen-associated molecular patterns. The role of nasal colonisation of S. aureus has started to receive more attention. In spite of this, there are not enough studies looking at its effects on human primary nasal epithelial cells and their response to TLR ligands. The respiratory tract itself seems to pose a contradiction given by the clinical observation that its upper portion (nasal compartment) allows the growth of bacteria, acting like a reservoir, whereas the lower portion (lung compartment) reacts with an exuberant inflammatory response to the same organisms, as noted during pneumonia. The mechanism related with this phenomenon remains to be elucidated. A negative regulator of the TLR signalling cascade called toll-interacting protein (tollip) has been demonstrated to induce hyporesponsiveness in the gastrointestinal tract in the presence of bacteria. So far, tollip has not been demonstrated in the respiratory tract.

AIMS: To compare the responses of the upper and lower respiratory tract to TLR ligands, to characterise the role of tollip in the respiratory tract and its effects in the induction of tolerance, and to determine the cellular response to nasal carriage of S. aureus.Materials and Methods: The cell line RPMI 2650 (representative of nasal epithelium) and the cell line A549 (representative of type II alveolar epithelium) were used to establish the cytokine response to stimulation with TLR ligands and to demonstrate the presence of tollip protein by immunocytochemistry and enzymelinked immunosorbent assay (ELISA). Primary human nasal epithelial and type II alveolar epithelial cells were isolated and cultured from consented subjects. The cytokine response to stimulation was measured using cytokine bead array and the presence of tollip was determined by immunofluorescence and quantitative polymerase chain reaction. The presence of TLRs was assessed by immunocytochemistry in primary nasal and type II alveolar epithelial cells and the response to stimulation with the TLR9 agonist CpG-C ODN was assessed in these cells as well as in primary human type II alveolar epithelial cells. Subjects were also assessed for nasal carriage of S. aureus and their associated cytokine responses.

RESULTS: The RPMI 2650 cell line, despite retaining phenotypic characteristics of the nasal epithelium, appears unresponsive to stimulation with TLR ligands. In contrast, the A549 cell line responded significantly to stimulation with TLR ligands.

Primary human nasal epithelial cells responded by secreting higher amounts of interleukin (IL)-8 and IL-6 in response to stimulation with S. aureus peptidoglycan (PGN) and tumour necrosis factor alpha (TNF-α) with a strong trend toward statistical significance. These cells did not respond to stimulation with Pseudomonas aeruginosa LPS. Primary type II alveolar epithelial cells responded significantly to stimulation with S. aureus PGN by increasing the secretion of IL-8, IL-6, IL-1β, TNF-α and IL-10 into cultured supernatant. Cells from the upper respiratory tract displayed a more tolerant phenotype given by the lower levels in cytokine production in response to stimulation with S. aureus PGN, in contrast to alveolar epithelial cells.

TLRs were identified in primary nasal epithelial cells. The negative regulator tollip was identified in cell lines as well as primary cells of the respiratory tract in its three segments: nasal, bronchial and type II alveolar. It was not possible to demonstrate an up-regulation of tollip after stimulation with TLR ligands in any of the cell types studied, although, it was possible to observe a significantly higher constitutive level in tollip mRNA transcripts from primary nasal epithelial cells in comparison to type II alveolar epithelial cells. TLR9 was identified in human primary nasal epithelial cells, although it was not possible to observe an increase in cytokine production after stimulation with a TLR9 agonist. TLR9 was expressed strongly in primary type II alveolar epithelial cells which responded by significantly increasing IL-8 production after stimulation with CpG-C ODN.

Primary nasal epithelial cells from individuals who carry S. aureus exhibit a proinflammatory profile, as evidenced by higher basal levels of IL-8 and IL-6 in comparison to non-colonised controls.Conclusion: The upper respiratory tract epithelium displays a tolerant phenotype in response to stimulation with TLR ligands in comparison to the lower respiratory epithelium, potentially favouring nasal colonisation by S. aureus. Tollip m-RNA transcripts appear to be up-regulated constitutively in the nasal epithelium which might favour this response. Staphylococcus aureus colonisation is however associated with a local pro-inflammatory state in the nasal epithelium of carrier individuals.