During the events of September 11^th, 2001, the collapse of the World Trade Center (WTC) pulverized much of the building's glass and metal structure and released an estimated 10 million tons of particulate matter (WTC-PM).[1]–[4] The bulk particulates were composed of cement, carbon, cellulose, and several fiber types including mineral wool, fiberglass and asbestos.[3], [4] WTC-PM was collected, sieved, aerosolized, and was found to range in size from PM2.5 to PM53.[4] The elemental analysis showed that WTC-PM was composed of high levels of calcium and sulfur (mostly in the form of sulfate) and it was found to be highly alkaline; pH 9–11.[4] Furthermore, induced sputum of firefighters up to 9 months after the event contained PM2.5–53.[5] Finally, biomass exposure is a major cause of lung function loss worldwide. In addition military cohorts exposed to products of combustion and PM due to close proximity to burn pits provide further insight into the negative health effects PM exposure. [6]

Approximately 92% of the exposed rescue workers were subsequently enrolled in the Fire Department of New York-World Trade Center-Medical Monitoring and Treatment Program.[7] Serum samples and spirometry were obtained within six months of exposure at medical monitoring entry (MME). Despite high levels of exposure the FDNY rescue worker cohort had a very heterogeneous response in regards to their lung function.[8]–[10] Most stabilized, a subset of rescue workers recovered lung function while others progressed and their FEV1 declined to less than their lower limit of normal (LLN). We have previously defined the persistent loss of FEV1 to less than LLN in this FDNY exposed population as World Trade Center Lung Injury (WTC-LI).[10]–[13] Workers with this progressive disease had bronchial wall thickening and air trapping on computed tomography (CT), suggesting airway injury as the predominant lung pathology.[14] Lower Manhattan residents with WTC exposure have similar findings.[8], [12], [14]–[21] This suggests that airway disease is the most common manifestation of WTC-LI.

We have previously investigated serum inflammatory, metabolic syndrome and cardiovascular biomarkers and found them to be predictors of FEV₁ decline and WTC-LI.[11], [13], [22], [23] More specifically, we found that myeloperoxidase (MPO) and soluble vascular cell adhesion molecule (sVCAM) can predict recovery from the original injury.[24] These results demonstrate that patho-physiological processes in the lung caused by dust and smoke exposure are reflected by biologically active protein expression in serum.

The role of proteases has been studied in the setting of many diseases including cancer, coronary disease, chronic obstructive pulmonary disease (COPD), and cigarette-induced chronic lung diseases.[25]–[29] Increased protease activity is a component of many diseases, including cigarette-induced chronic lung disease and other causes of accelerated lung function decline.[25]–[28] Of the known proteases, matrix metalloproteinases (MMPs) have been of particular interest in their role in lung remodeling, and have been found to be central to the pathogenesis of airway disease such as COPD. MMPs are a family of Zn²⁺-dependent proteases that can catabolize and degrade the extracellular matrix. Many are known intermediates of each other, and their levels are affected by environmental factors such as hypoxia, inflammation and oxidative stress. The utility of MMPs as biomarkers of lung disease severity and prognostic indicators are of key interest in many studies.[30] Genetic association studies with MMPs demonstrate a strong association with lung disease.[25], [31], [32] MMP-9 has been studied in induced sputum of WTC-dust-exposed firefighters, and was found to be related to the time spent during recovery efforts at the WTC site.[5] MMP-9 has also been used to predict lung function decline in the setting of α1-antitrypsin deficiency.[33] However, the overall importance of the proteases in the development and modulation of lung disease needs further study.[34]–[37]

This study investigates the levels of MMPs present in serum after exposure to WTC dust in rescue workers as potential systemic biomarkers predicting susceptibility to WTC-LI. It tests the hypothesis that proteases present within three months of 9/11/2001 can predict airway injury years later.

We report that early elevated expression of MMP-3 and MMP-12 in serum within 200 days after WTC exposure predicts protected lung function over the subsequent seven years (between 2001–2008). We found that MMP-3 and MMP-12 levels of the cohort were comparable to other published patient populations, including healthy controls and patient cohorts with emphysema or rheumatoid arthritis.[30], [42] MMP-3 was found to be more protective than MMP-12 with an odds ratio of 0.267 compared to 0.462. Both biomarker models displayed robust predictive ability by logistic regression with AUC>0.8. Presenting later to MME and blood draw predicted higher probability of developing WTC-LI. As time to blood draw increased, the probability of developing WTC-LI ranged from 70–90% in the MMP-3 model and from 40–70% for the MMP-12 model.

This cohort provides a unique opportunity identify biomarkers of susceptibility to dust induced lung injury. First, FEV₁ was measured prior to exposure. Second, after the WTC collapse produced a massive acute dust exposure, serum was drawn early during the evolution process to disease. Longitudinal FEV₁ measurements allowed us to define case status years after the serum was drawn. The cohort is also large enough to study only never-smokers, eliminating a major confounder of reduced FEV₁. The development of FEV₁ loss in obstructive lung disease is a multifactorial process. MMPs are central in the disease pathogenesis of obstructive lung disease, and are thus of interest in this population. MMPs are involved in parenchymal destruction and lung remodeling, and their quantification allows us to start to understand their role in the development of FEV₁ loss in this WTC exposed population. Furthermore, they are involved in separate pathways of disease attenuation than those of inflammation, cardiovascular disease, and metabolic syndrome that were the focus of our prior work.

Cases and controls differed in several ways that impacted future analyses. Incident cases gained significantly more weight than controls by the time of SPE. BMI is a known cause of decreased lung function, and is thus a possible confounder. Because our case definition of WTC-LI relies on FEV₁ at SPE, it is heavily impacted by pre-9/11 FEV₁ and BMI at SPE, which are both significantly different between cases and controls prior to model building. For this reason, we adjust for pre-9/11 FEV₁ and change in BMI (as a measure of weight gain) as confounders in the logistic regression model. The change in BMI in the logistic regression is simply used as a marker of weight gain. The model remains robust whether the weight gain is measured as change in BMI or BMI at SPE.

Cases and controls suffered similar acute reductions in FEV₁ from pre-9/11 to MME. This is consistent with the similar WTC exposure intensities in both case definitions. However, controls had higher pre-9/11 FEV₁ than susceptible cases (102 vs 88%). The differences in pre-9/11 FEV₁ could reflect predisposition to lung injury after non-tobacco smoke exposure during the population's 18 years of service in fighting fires. This is suggested by continued deterioration of cases in FEV₁ compared to the relative recovery of controls years after the acute WTC exposure. Because cases were defined by a threshold of FEV₁ of <77% at SPE, those with pre-9/11 FEV₁ close to threshold values were more likely to meet case definition. Cases also had markedly diminished airway function across multiple measures after exposure, including FEV₁, lower FEV₁/FVC ratio, more bronchodilator response, and steeper methacholine response slope.

We observe that increased MMP-3 and 12 expression reduces the odds of subsequent lung injury. Also, the time to medical monitoring enrollment and serum draw post-exposure was an independent predictor of lung injury. Those who had serum obtained two months post-9/11 (early presenters) had lower risk of lung injury than those who had serum obtained four months post-9/11 (late presenters). This result could be due to selection bias with individuals entering medical monitoring later at a more advanced stage of disease. Alternately, delay in early interventional treatment for those who enrolled in medical monitoring later may have inhibited their ability to recover from the exposure. We investigated this possibility, but did not find differences between early and late presenters in known confounders such as exposure intensity or pre-9/11 FEV₁. There was a significant increase in BMI in the late presenters, but the effect of time did not change with BMI as a covariate in the logistic models. Time to serum draw was not a significant covariate with all other serum biomarkers of risk or protection that we have reported.[11], [13], [23] This suggests that the predictive character of MMP expression over time is distinct from other biomarkers of metabolic syndrome, vascular injury or inflammation. Another possible interpretation of the effect of time is early elevation of MMP reflects normal healing, while persistent MMP elevation occurs with non-resolving inflammation. Since we do not have serial serum samples from each individual, we are unable to determine if the association of time to serum draw post-9/11 with risk of lung injury is due to selection bias or reflects underlying biology.

The protective role of MMP-12, also known as macrophage elastase, in this cohort was surprising since most studies on patients with established lung disease demonstrate elevated MMP-12 as a biomarker of disease severity in COPD patients.[25] Nevertheless, there exists murine data for potential protective roles of MMP-12 in the setting of non-small cell lung cancer.[43] The data is consistent with the hypothesis that after injury, elevated MMP-3 and MMP-12 reduce odds of developing WTC-LI. In this study, MMP-12 was assayed soon after injury, during the period of disease evolution, but years before disease was diagnosed. This makes reverse causation unlikely. It is possible that when irreversible pulmonary disease is established, increased MMP-12 is a consequence of disease severity and not its cause. Thus, MMPs would be elevated in more advanced disease due to induction of an infective or counter-productive repair response.

This is the first paper that describes a protective function of elevated MMP-3, stromelysin-1, in humans. This novel correlation needs to be shown in other lung injury cohorts prior to investigating the relationship of progressive injury pathways and MMP-3 expression. MMP-3 has been shown to be protective in murine models of atherosclerosis.[29] Murine models of intestinal sepsis also showed that deficiency in MMP-3 delayed clearance of bacteria and impaired migration of CD4⁺T-cells.[29]

This nested case control study has several limitations. The cohort was narrowly defined, reducing generalizability of these results. Specifically, the vast majority of FDNY workers were exposed at the time of the WTC destruction. The few unexposed workers were markedly less healthy than the exposed workers pre-9/11 and most were not on active duty at the time of the event. Therefore, all the study participants were highly exposed to WTC dust and products of combustion. Since there is no unexposed control group with comparable lung function, years of service, age, racial background and serum available it is therefore not feasible to construct an unexposed control. Also, using FEV₁ to define case status may produce misclassification of disease. However, the authors maintain that using FEV₁ is the simplest, most robust, and best validated single measure of airway injury in an obstructed cohort. Since the design reduced the potential for selection bias, misclassification of disease is non-differential. The reported associations therefore likely underestimate the true strength of association between the serum biomarkers and subsequent FEV₁. Although the serum biomarkers are expressed years before the disease is defined, the results are correlations and do not imply causation. This restricts our ability to assess the impact of WTC exposure to the observed biomarker disease relationship. Biomarker associations with lung injury must also be investigated in independent cohorts.

WTC exposure impacted multiple distinct injury and repair pathways. One interpretation of the findings is that biomarkers that reduce the risk of WTC-LI reflect effective repair after an acute dust induced lung injury. We report that elevated MMP-3 and MMP-12 are associated with a reduction in the odds of developing WTC-LI years later. Finally, we show that time to serum sample acquisition is independently associated with the probability of lung injury and highlights the utility of serum sampling efforts continuing for some time. We will investigate in future studies about the relationship between genetic polymorphisms that may improve or change the predictive ability of MMP-3 and MMP-12. Also, we will analyze longitudinal samples of serum on the same cohort to understand how chronicity of the biomarkers impact disease development.