For a more comprehensive description of airway inflammation in CF, the authors recommend a number of previous reviews of this subject matter (1
). The normal host inflammatory response consists of two phases: (1
) activation by an inciting stimulus resulting in signal transduction, gene transcription, and altered cell function (proinflammatory mediator production), and (2
) termination, which also includes signal transduction, gene transcription, and altered cell function (increased antiinflammatory and decreased proinflammatory mediator production, receptor endocytosis, production of soluble receptors, and return to baseline state). Abnormalities have been described for CF in both the activation and termination of the inflammatory response.
Although the lungs of newborns with CF are structurally normal, plugging of the bronchioles with secretions and hypertrophy of submucosal gland ducts appear by 4 months of age (13
). Once challenged by viral or bacterial infection, high numbers of neutrophils are persistently recruited to the airway. Typically, normal host defense mechanisms are meant to eliminate an infection. This ultimately fails in the CF airway, and the subsequent inflammatory response becomes an independent and significant pathologic force (2
). The CF airway contains large concentrations of inflammatory mediators and abundant amounts of neutrophils and neutrophil products, including oxidants and proteases. These overwhelm local defenses and contribute to lung injury. Lung secretions from patients with CF contain large concentrations of tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, and IL-8 (14
). All of these cytokines have the common characteristic that their synthesis is promoted by transcription factor nuclear factor (NF)-κB, which is activated by cellular interaction with bacteria, bacterial products, and proinflammatory cytokines (15
). Other pathways that may be involved in promoting the inflammatory response in CF include the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) and activated protein (AP)-1 pathways (18
Although the neutrophil is the predominant inflammatory cell present in the CF airway and any effective antiinflammatory therapy must address the neutrophil and its products directly or indirectly, it is not the only cell participating in the inflammatory response. The airway epithelial cell, which stands sentry between the host and its environment, clearly plays a major role in orchestrating the inflammatory response in the airway (22
). The epithelial cell has important defense functions in the airway besides the obvious physical barrier that an intact epithelium provides and the basement membrane material that it secretes. Epithelial cells are the source of innate defense molecules, and they also direct the recruitment and activation of other cells of defense when appropriately stimulated. Some cytokine products of epithelial cells, such as TNF-α, IL-1β, IL-6, IL-8, granulocyte-macrophage colony–stimulating factor, and granulocyte colony–stimulating factor, are produced in excess in CF, whereas some, such as IL-10 and RANTES (regulated on activation, normal T-cell expressed and secreted), appear to be reduced (14
Because airway epithelial cells normally express CFTR, absence of its normal function might have consequences that extend beyond ion transport in these cells. Defective cytokine production in airway epithelial cells, which are dependent on CFTR for normal secretory function, could explain why the disorders of the inflammatory response in CF seem limited to the respiratory tract (23
). Importantly, there is no increase in the frequency or severity of infection outside the respiratory tract, as would be found in patients with neutrophil defects or other primary immune deficiencies. Furthermore, patients with CF generally have normal immune responses to standard immunizations and often have elevated immunoglobulin concentrations.
In addition to airway epithelial cells, other inflammatory cells, such as macrophages, T lymphocytes, B lymphocytes, dendritic cells, and mast cells, are present in the CF airway in relative abundance and also are likely to contribute to the inflammatory response (37
). Many of these cells express CFTR, but the pathophysiological link between dysfunctional CFTR and the enhanced inflammatory response in CF is not understood. There is supportive evidence suggesting a direct linkage between abnormal CFTR and the excessive airway inflammatory response. Inhibition of CFTR production by antisense oligonucleotides or inhibition of CFTR function by overexpression of the regulatory (R) domain of CFTR in epithelial cell lines results in increased activation of NF-κB and increased secretion of IL-8 upon stimulation of these cells with proinflammatory stimuli (23
). Further studies using a pharmacologic inhibitor of CFTR demonstrate that intact CFTR channel function is necessary for normal regulation of inflammatory cascades in epithelial cells (25
). These results suggest that alterations in cellular physiology due to defective or deficient CFTR cause dysregulation of inflammatory pathways. Regardless of the relationship between CFTR and airway inflammation, it is clear that once set in motion, the inflammatory response is responsible for many of the secondary pathologic effects in CF and ultimately impacts survival. Until that time when a cure is realized, controlling the inflammatory response with drugs will be an important cornerstone of CF therapy. Because of the amplification that occurs in the inflammatory pathways, efficacy of an antiinflammatory agent targeted at any one step in the inflammatory cascade may result in attenuation of the overall inflammatory response.
Because of the excessive nature of the inflammatory response in the CF lung, many investigators have sought to determine abnormalities in either the activation or inhibition of the airway inflammatory response. Additional regulatory abnormalities may also occur that lead to excessive transcription of proinflammatory cytokine genes (18
). Clinical and research observations that the inflammatory response occurs in the absence of detectable pathogens might suggest that the inflammatory response in the CF lung operates independently of an infectious stimulus. However, these findings may also be explained by failure to terminate the inflammatory response once the inciting stimulus has been eradicated and may actually represent an abnormal persistence of inflammation after clearance of early, transient infection (45
). Interestingly, some studies found that CF airways are relatively deficient in the counterregulatory molecules IL-10 and nitric oxide (NO) (29
), both of which preserve the function of inhibitory protein-κB (IκB), the inhibitor of NF-κB. The decrease in NO production by CF airway epithelial cells appears to be secondary to a specific lesion in the signal transducer and activator of transcription (STAT)-1 pathway that leads to the activation of NO synthetase-2 (43
). In situations in which either IL-10 or NO or both might be deficient, less IκB would be available to inhibit NF-κB and proinflammatory mediator production would increase (18
). Therefore, an imbalance between IκB and NF-κB in airway epithelia would result in the prolonged and excessive production of the mediators responsible for the damaging inflammation.
In addition, CF lung tissues appear to be deficient in peroxisome proliferator activating receptor (PPAR) (51
). When activated, PPAR forms a heterodimer with activated retinoid X receptor (RXR), which is able to modulate inflammation in addition to other actions (52
). PPAR exerts its attenuating effects typically by inhibiting NF-κB activity via up-regulation of IκB or by competition with NF-κB for helicases (53
). CF airway epithelial cell lines appear to have less PPARγ activity than do non-CF airway epithelial cell lines (54
). Thus, decreased PPAR expression also likely contributes to the imbalance between IκB and NF-κB that favors increased inflammation in CF. This has led some investigators to evaluate PPAR agonists in clinical trials in CF.
It seems apparent that the inflammatory lung damage in CF is much more complicated than an exuberant response to a persistent pathogen. Although bacterial infection plays an important role in stimulating the inflammatory response in CF, it also seems apparent that the inflammatory response itself is abnormal in several ways. These involve more than just overproduction of any one proinflammatory mediator. Several recent studies emphasize the concept that decreased antiinflammatory and counterregulatory activities are at least as important as increases in proinflammatory mediators in the overall pathophysiology of CF lung disease. The mechanisms proposed above are not mutually exclusive. It is likely that a combination of the above processes fuel the aggressive and damaging inflammatory cascade. Therefore, a direct attack on inflammation itself seems warranted.
Given the life-limiting nature of CF, speeding drug development is imperative. Many trials are underway concurrently in search of therapies that will slow the progression of the lung disease. These studies often compete for a limited number of patients, and thus may result in the unintended consequence of slowing the development of potentially effective therapies. Biomarkers that more rapidly identify biologic activity and predict clinical response in CF would be very useful. Because the excessive inflammatory response in CF is primarily confined to the lung, investigators must necessarily study secretions and cells from the airway to better describe the association between defective CFTR and inflammation and the impact of antiinflammatory agents on the disease pathophysiology. Two different drugs may target completely separate areas of the inflammatory cascade, yet both may be effective in attenuating secondary disease processes. Clearly, some understanding of a drug's mechanism of action is necessary in selecting the most ideal outcome measures for a given clinical trial. It is unlikely that any one biomarker will be used as the sole surrogate outcome measure in a clinical study. Rather, it is more likely that a clinical trial will have to use a panel of biologic markers. The optimal array of biomarkers is likely to change from study to study depending on a drug's mechanism of action as described in later sections.