Introduction
Cystic fibrosis (CF) is an autosomal recessive hereditary disease caused by mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene. Mutations of CFTR lead to non- or mal-function of all exocrine glands and mucosal surfaces of the human body. Thus, the disease affects various organs including intestine, pancreas and lung (1). Life expectancy of CF patients is severely decreased and this is nowadays mainly dictated by the pulmonary phenotype: CF patients suffer from thickened respiratory mucus causing mucus plugging of the airways, chronic inflammation as well as increased incidence of pulmonary bacterial infections. Infections are of polymicrobial nature yet H. influenzae, S. aureus, and P. aeruginosa are clinically important pathogens in CF lung disease (2). Bacterial airway infection and inflammation associated with reduced mucociliary clearance mediate progressive lung damage and a decline in lung function over time, finally resulting in death due to respiratory failure.
Especially chronic airway infections with P. aeruginosa have been correlated with an accelerated loss of lung function (3, 4). P. aeruginosa infections typically start as intermittent infection with environmental strains that initially are sensitive to antibiotic eradication. However, over time P. aeruginosa undergoes adaptive mutations including gain of antibiotic resistance, loss of virulence factors, e.g., proteases or pyocyanin production, and increased alginate synthesis. This favors the establishment of chronic infection and resistance to antibiotic treatment that results in failure of eradication (5). Several secreted proteases of P. aeruginosa have been described modulating the inflammatory response of the host. As such, LasB, a protease under the control of the quorum sensing receptor LasR, has been demonstrated to degrade IL-6 and IL-8. This helps P. aeruginosa to establish an infection since it blocks the recruitment of leukocytes (6). Also other LasR regulated proteases (7, 8), like LasA or AprA, have been demonstrated to degrade cytokines and might act in the same way as LasB (9). Interestingly, as soon as P. aeruginosa infection has been established, LasR often acquires loss of function mutations during the transition of intermittent to chronic infections and thereby further boosts pulmonary inflammation (6).
However, the decline in lung function that is associated with development of chronic infection with P. aeruginosa is not constant or linear. Instead, periods of relatively stable lung function are interrupted by episodes with an acute drop in lung function, from which full recovery might not be achieved by antibiotic treatment (10). Causes and pathological mechanisms involved in these pulmonary exacerbations are often unclear and bacterial and viral infections have been attributed to it (11). Virus-induced pulmonary exacerbations are well-known in other lung diseases like COPD or asthma (12). Yet, the importance of viral induced pulmonary exacerbations in CF patients is still unclear (13, 14). However, it has been shown that the lung microbiome composition itself is quite resilient and does not change to great extent in most cases of exacerbation (15, 16) and therefore the involvement of non-bacterial organisms, including viruses, is likely. The antiviral response is triggered by intracellular recognition of viruses via nucleic acid pattern receptors including TLR3 and RIG-I. Activation of these receptors induces an initial type I/III IFN synthesis which subsequently boosts its own production in a positive feedback loop (17). It has been shown that respiratory epithelial cells produce mainly type III IFN and the importance of these proteins in the airways is well-documented (18, 19). Moreover, manipulation of type III IFN has been linked to increased susceptibility of asthmatic patients toward human rhinoviruses (hRV) and a contribution to pulmonary exacerbation has been suggested (20, 21). Since P. aeruginosa and respiratory viruses have been linked to pulmonary exacerbations and in addition, respiratory viruses have been associated with the transition from transient to chronic airway infections with P. aeruginosa (22, 23) a link between both microorganisms is likely. Therefore, we investigated in this study if P. aeruginosa is able to modulate the antiviral response of bronchial epithelial cells and how such interplay might happen at the mechanistical level. In addition we analyzed sputa of CF patients for the presence of respiratory viruses and determined the levels of virus RNA in order to link P. aeruginosa to virus infection thus identifying clinical importance of the experimental findings.