Introduction
Chronic obstructive pulmonary disease (COPD) is a leading cause of morbidity and mortality worldwide, but predictors of progression remain elusive [
1]. Alpha
1-antitrypsin inhibits neurophil elastase, and the discovery of alpha
1-antitrypsin deficiency (AATD) and its association with emphysema and COPD helped to establish the concept that an imbalance between proteases and antiproteases, exacerbated by exposure to cigarette smoke, can play a key role in the development of disease, at least in a subset of COPD [
2]. Evolving evidence suggests that a variety of proteinases, acting on diverse substrates of the extracellular matrix, play an important role in pulmonary parenchymal destruction in COPD [
3,
4]. The study of these proteinases has been complicated by the diversity of COPD phenotypes, which include components of emphysema and chronic bronchitis, as well as the common co-morbid cardiovascular and other systemic diseases associated with COPD, all of which potentially confound the analyses of biomarker-disease associations [
1,
5]. Because AATD-associated emphysema represents a more homogenous phenotype of COPD, with a more rapid decline in pulmonary status, generally at a younger age with fewer comorbidities, it offers a potential model for understanding COPD pathogenesis with respect to proteinases beyond those directly inhibited by alpha
1-antitrypsin [
2,
6].
Matrix metalloproteinase-9 (MMP-9) is one such proteinase that has received considerable attention in COPD[
7‐
18]. Clinical studies have suggested that, among the various biomarkers potentially associated with lung disease, MMP-9 may be particularly important, although there have been few longitudinal studies of MMP-9 and few pulmonary status outcomes examined [
7‐
10]. Of the 143 serologic biomarkers examined by Pinto-Plata and colleagues, MMP-9 was the most highly correlated with the BODE Index (cross-sectionally) and with COPD exacerbations (longitudinally over 12 months) [
7]. Additionally, the ratio of biomarker in COPD subjects to that in health controls was higher for MMP-9 than for any of the other biomarkers examined [
7]. Higashimoto and colleagues examined multiple biomarkers among 96 COPD patients, finding that only MMP-9 and C-reactive protein were statistically significantly associated with declines in FEV
1 over the following 12 months [
8]. Neither these nor other longitudinal studies of MMP-9 have investigated pulmonary status measured by transfer factor for carbon monoxide (TLco), radiographically-quantified lung density, or exercise capacity. Studies to date have furthermore not incorporated multiple longitudinal measurements of MMP-9. Finally, clinical studies thus far have not reported on the longitudinal associations between MMP-9 and outcomes within the context of AATD-associated emphysema, a phenotype of COPD that could be particularly relevant to understanding the role of MMP-9 in the natural history of COPD pathogenesis.
Utilizing data from the placebo arm of a clinical trial, we tested the hypotheses that plasma MMP-9 levels are cross-sectionally and prospectively associated with pulmonary status declines and with COPD exacerbations among emphysema patients with AATD.
Discussion
This study provides evidence that MMP-9 is a biomarker that predicts COPD progression. Our insights were facilitated by repeated longitudinal measurements of both MMP-9 and a variety of measures of pulmonary status. Moreover, we carried out this longitudinal analysis of MMP-9 in a relatively large cohort of AATD-associated emphysema, a well defined phenotype that offers a particularly valuable model of COPD progression [
2].
We found that higher plasma levels of MMP-9 were associated with poorer pulmonary status at baseline and that MMP-9 was longitudinally associated with certain decrements in pulmonary status and worse health outcomes. Of note, however, this was not universally the case for all of the pulmonary status measures we studied, suggesting that the role of MMP-9 is likely to be complex and, in particular, time-dependent. Nonetheless, the longitudinal associations between higher levels of MMP-9 and declines in TLco and lung density are both findings that support higher MMP-9 levels being a marker for pulmonary parenchymal destruction. Thus, our longitudinal findings with respect to lung density and TLco suggest that, in AATD-associated emphysema, further study of a direct involvement of MMP-9 in disease progression is warranted.
The magnitude of the prospective associations between MMP-9 and pulmonary status measurements was, in many cases, modest from a clinical perspective, but the time period examined was short relative to the multi-year timeframe over which emphysema progresses. Indeed, the longitudinal changes observed in the various pulmonary status measures, irrespective of MMP-9 levels, were themselves small. That many findings associated with MMP-9 were statistically significant, despite the relatively short period of follow-up, is thus all the more noteworthy.
Although MMP-9 was cross-sectionally associated with lower FEV
1, its longitudinal association with obstructive ventilatory defects was ambiguous, with higher MMP-9 levels being associated prospectively with a lower FEV
1/FVC while associated with a higher FEV
1, albeit of borderline statistical significance (p = 0.13). Research on MMP-9's association with airway pathology is limited [
14], but this finding does stand in contrast to that of Hashimoto and colleagues [
8]. On the other hand, Vignola and colleagues, in their cross-sectional study, also found a positive correlation between MMP-9 and higher FEV
1 [
48]. One theoretical explanation for a paradoxical association with decreased FEV
1/FVC ratio but increased FEV
1 would be that destruction of pulmonary parenchyma, and its associated airway tethering, may potentially result in greater reductions in longitudinal than radial airway traction and thus work to increase small airway caliber [
49]. It is also possible that this finding was simply a chance observation, especially given that the longitudinal association with increased FEV
1 did not meet criteria for statistical significance. Moreover, the annualized decline in FEV
1 of 36 ml in this cohort is substantially lower than the 54 ml annual decline that has previously been observed in AATD-associated emphysema [
50]. This finding magnifies the inherent limitations of studying FEV
1, an endpoint that can be problem-ridden in airways disease because of its variability and effort dependence. Indeed, this was part of the impetus for choosing lung density, rather than FEV
1, as the primary outcome in the REPAIR trial [
19].
The fact that the longitudinal associations observed with respect to MMP-9 persisted even after controlling for both leukocyte count and hs-CRP levels argues that these predictive observations are not merely reflective of MMP-9 as a non-specific inflammatory marker. Indeed, MMP-9 was poorly correlated with hs-CRP levels. Further, hs-CRP was itself a relatively poor predictor of changes in pulmonary status and did not predict COPD exacerbations. CRP has shown some variable ability to predict COPD progression and outcomes, although this has generally been examined in non-AATD cohorts which included active smokers, who were excluded here [
46,
47,
51]. Thus, the positive associations found with respect to MMP-9 are more remarkable in the context of an absence of consistently positive findings associated with CRP levels.
Our study was not designed to establish a causative role of MMP-9 in antitrypsin deficient emphysema progression. Evidence from other studies suggest that MMP-9 degrades basement membrane type IV collagen, insoluble elastin, and other matrix proteins important to the function and structural integrity of the lung parenchyma [
14‐
16]. MMP-9 has also been shown, through amino terminal processing, to potentiate interleukin-8 chemotactic activity for neutrophils ten-fold, providing an additional inflammatory pathway for lung damage and COPD exacerbations [
18]. Nonetheless, we cannot exclude the possibility that our observations could be attributable to a confounding correlation between MMP-9 and other factors that contribute to COPD exacerbations or the progression of emphysema. For example, MMP-9 is released primarily by neutrophils and macrophages; these leukocytes could by themselves be responsible for some of our observations, independent of MMP-9. Our analysis controlled for leukocyte count, and the presumed primary mechanism of action for MMP-9 is biologically coherent in playing a causative role in some of the effects we observed, but this study supports only the conclusion that MMP-9 is a biomarker that predicts disease progression but not necessarily that MMP-9 plays an etiologic role.
There are additional limitations of our research beyond those alluded to above. Despite the advantages in studying AATD-associated emphysema, our findings may not be generalizable to COPD in the absence of AATD and, indeed, the AATD registry from which patients were recruited for this study represents only a subset of AATD. Further, this study was also conducted in subjects who had been non-smokers for at least 6 months, and it is unclear whether current tobacco smoking might alter the relationship between MMP-9 and pulmonary outcomes. The assay used to quantify MMP-9 measured total MMP-9 concentration rather than MMP-9 enzymatic activity as measured by bioassay. While measurement of enzymatic activity may have been preferable in providing greater ability to find associations [
52], reduced accuracy from relying on total MMP-9 would tend to diminish our ability to demonstrate statistically significant associations, but does not argue against the validity of associations that we did observe. Finally, assessments of MMP-9 and pulmonary status and outcomes were not performed on an ideal schedule and frequency. For instance, some measurements, such as PFTs, were available every 3 months while others, such as lung density, were available every 6 months. The lack of an MMP-9 measurement at 9 months also places constraints on our analysis. In the best of all possible worlds, an MMP-9 assessment far more frequently than every 3 months (e.g., every two weeks) might have provided data clarifying MMP-9 fluctuations in relation to disease outcomes, in particular COPD exacerbations. Nonetheless, the repeated longitudinal measurements in this study, although imperfect, are a key strength of the analysis.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
TAO contributed to the study design, data analysis, and developed the first draft of the manuscript. MDE contributed to the study design, data analysis, and manuscript preparation. AR contributed to the study design and manuscript preparation. LM contributed to the data analysis and manuscript preparation. PDB contributed to the study design, data analysis, and manuscript preparation. All authors read and approved the manuscript.