Plasma MMP-9 and TIMP-1 levels on ICU admission are associated with 30-day survival

Matrix metalloproteinases (MMPs) are a group of zinc-dependent endopeptidases, which have the ability to disintegrate proteins of the extracellular matrix (ECM) [1]. They support migration of immune cells to infection sites and are further involved in a variety of endogenous proinflammatory and vasoactive cytokine responses, as well as in coagulation and fibrinolysis cascades [2, 3]. The activity of these enzymes is essential in several physiological processes, such as growth and wound healing but also in inflammatory and vascular pathophysiology (e.g. tissue remodeling, arteriosclerosis) [2]. Their activity is regulated in vivo by the specific endogenous tissue inhibitors of metalloproteinases (TIMP) [4‐6]. The balance of MMPs and TIMPs is important in order to maintain the integrity of the extracellular matrix [5].

The results of this study indicate a prognostic significance of elevated TIMP‑1 levels and, to some extent, of MMP‑9 levels in a heterogeneous group of critically ill patients as emphasized by the significantly higher circulating plasma MMP‑9 and TIMP‑1 levels on ICU admission in patients who died within 30 days. The MMPs are known to play a role in the cytokine storm following systemic immune activation. They support immune cell migration, have vasoactive effects and can induce vascular leakage in severe illness [40, 41]. Circulating MMP‑9 concentrations are increased in the first few hours of systemic inflammation [42] and were shown to correlate with severity of associated organ injury in animals [43]. In our study, mean MMP‑9 concentrations correlated with established inflammatory markers, whereas MMP‑2 did not. In acute proinflammatory processes, MMP‑9 is released from granules of leukocytes [22] stimulated by cytokines such as TNF-alpha. Moreover MMP‑9 promotes leukocyte recruitment and cleaves TNF‑alpha into its active form [11]. In the early phase after acute myocardial infarction MMP‑9 is secreted by neutrophils and potentially by macrophages [66]; it can modulate white blood cell function and further create a positive feedback loop for neutrophil activation and chemotaxis via IL‑1 beta (interleukin), i.e. lymphocyte activating factor and IL‑8 [67]. Consistently, white blood cell count and TNF‑alpha concentration significantly correlated with MMP‑9 levels in our patients.

Previous studies investigating the relation between MMPs and mortality in larger cohorts of critically ill patients were mainly conducted in patients suffering from sepsis [16, 18, 42, 47]. Our study sample comprised patients with heterogeneous underlying critical conditions, of which only one condition was sepsis. In line with our findings Nakamura et al. [47] reported significantly higher early MMP‑9 levels in patients who died of (septic) shock when compared to survivors. Hoffmann et al. also noted elevated MMP‑9 in patients who died of sepsis; however, their results did not reach a level of statistical significance [16]. Elevated MMP‑9 levels have further been reported in several other severe conditions such as severe brain injury [14] and stroke [45]. Plasma MMP‑9 activity has further been described as potentially predictive for lung injury and the development of ARDS (acute respiratory distress syndrome) in critically ill patients [46]. In our study sample, when performing subgroup analysis, MMP‑9 levels were an independent predictor of survival in patients with underlying cardiac disease. The MMP‑9 levels and associated inflammatory cytokines were previously related to acute cardiovascular events [44], disease severity in chronic heart failure and adverse outcomes in these patients [37]. Our findings are also in line with a recent report by Lahdentausta et al. supporting MMP‑9 as an early stage biomarker of poor outcome in cardiovascular disease potentially reflecting atherosclerotic plaque rupture and myocardial tissue destruction [49]. The MMP‑9 levels further correlate with left ventricular ejection fraction and were associated with perioperative myocardial injury in coronary bypass grafting [48], which was an underlying diagnosis in some of our patients. To our knowledge, this is the largest published study to date, demonstrating the prognostic value of MMP‑9 levels in a heterogeneous group of critically ill patients admitted to intensive care.

Further a clear association between elevated TIMP‑1 levels and 30-day mortality could be shown in our study population. The TIMP‑1 levels are known to be elevated in inflammatory and ischemic events; they are induced by proinflammatory and profibrotic stimuli and are to some extent independent of MMPs regulation [50]. Our findings are consistent with previously published study results investigating the impact of TIMP‑1 levels on mortality in critical care settings. Lorente et al. reported elevated circulating TIMP‑1 levels on admission in sepsis non-survivors and showed higher TIMP-1/MMP‑9 ratios at days 1, 4 and 8 to predict mortality in a large multicenter study [17, 18]. They concluded that a higher TIMP-1/MMP‑9 ratio was associated with severity, coagulation state, circulating cytokine levels and mortality and proposed it as an outcome biomarker of sepsis. Similarly, although in a smaller sample size, Hoffmann et al. described TIMP‑1 as an efficient prognostic marker for fatal outcome in septic patients [16]. Analogous findings were reported for sepsis-associated organ dysfunction after major abdominal surgery by another study group [51]. Taking into account positive and negative predictive values, however, Serrano-Gomez et al. found neither MMP‑9, MMP‑2, TIMP‑1, TIMP‑2 nor their respective ratios to have significant predictive values in mortality of sepsis patients [52]. The TIMP‑1 has been associated with survival in several other critical diseases, including traumatic brain injury [53, 54], cerebral infarction [55], graft versus host disease [56] and acute respiratory failure [57]. Possibly upregulated as a reaction to MMP release in the cytokine storm, elevated TIMP‑1 levels could favour microcapillary thrombosis and fatal multiple organ dysfunction, as previously proposed [17]. Interestingly, in a large longitudinal study, TIMP‑1 was a strong predictor of all-cause 10-year mortality, with most studied patients dying of cardiovascular disease [58]. Furthermore, higher TIMP‑1 levels were reportedly associated with cardiovascular events in a Chinese follow-up study of patients with coronary heart disease [59]. Episodes of ventricular tachyarrhythmia, potentially involved in sudden cardiac death, were associated with higher MMP‑9 levels, and particularly MMP-9/TIMP‑1 ratios in heart failure patients [60]. Disease progression in coronary artery disease was previously associated with an increasing MMP-9/TIMP‑1 ratio in circulating CD14+ monocytes. Disparity between TIMP‑1 and MMP‑9 levels was also shown to contribute to adverse events and mortality in heart failure patients [37]. In our patients, MMP-9/TIMP‑1 ratios were not statistically associated with survival. Other studies reporting prognostic relevance of MMP-9/TIMP‑1 ratios in a comparable setting of myocardial infarction [61] were conducted in serum, which might explain the disparity our findings [62]. Also, TIMP‑1 is involved in MMP-2/MMP-9-independent mechanisms, which might further explain the independent predictive value of TIMP‑1 in major adverse cardiovascular events, as proposed by Lindsey et al. [63]. In this context, levels of TIMP‑1 (and TIMP-2), correlated with acute phase proteins (CRP [C reactive protein], fibrinogen) in our patients, possibly indicating an aligned regulation. Taken together, our study provides further support for the prognostic potential of TIMP‑1 and contributes to expanding the predictive value of early TIMP‑1 levels to 30-day survival in a heterogeneous group of critically ill patients. Here survival significantly correlated with TIMP‑1 and MMP‑9 levels in line with known markers of systemic inflammation (WBC, CRP) and organ decompensation (proBNP, BUN). The excellent correlation between the well-established clinical score of survival, SAPSII, and TIMP‑1, but also MMP‑9 levels, further accentuates the significance of our main findings.

Concerning MMP‑2 and TIMP‑2 levels, however, no correlation with respect to the endpoints could be demonstrated. In agreement with our results, other authors consistently found elevated MMP‑9 levels in patients with acute systemic inflammation [22, 24, 35], whereas MMP‑2 levels did not differ between patients and controls in any of these studies. The use of MMP‑2 has been described as a possible long-term prognostic marker in heart failure patients by George et al. [34]. The authors reported an association between elevated MMP‑2 levels and mortality over a 24-month period. Higher plasma MMP‑2 levels in type 1 diabetes patients were further associated with cardiovascular events and all-cause mortality in a 12-year follow-up study [64]. In cases of severe Chagas cardiomyopathy MMP‑2 levels added to predictive value of other biomarkers with respect to 1‑year mortality [65]. Even though the role of MMP‑2 remains undisputed in cardiovascular disease [5, 22, 23] it appears to be more relevant as a predictor of long-term outcomes whereas MMP‑9 has been more frequently reported in (sub)acute systemic responses following cardiac and non-cardiac events [34, 35], which is in line with our findings.

In summary, 30-day survival was predicted by higher MMP‑9 and TIMP‑1 plasma levels on ICU admission in critically ill patients and TIMP‑1 and also MMP‑9 were significantly correlated with SAPS II, a scoring system specifically designed to estimate disease severity and predict fatal outcomes in ICU patients, further supporting our findings.

There are some limitations regarding the generalizability of our results in ICU patients and the outcome. Firstly, despite the large study sample, the size of our subgroups with non-cardiac diseases, the smallest of which was our sepsis subgroup, could be considered insufficient for general comparison between outcomes in cardiac and non-cardiac disease patients. It cannot be excluded that larger studies concerning MMP and mortality, conducted in respiratory failure patients, might find trends which were not detected in our setting. Sepsis, however, as well as infection-associated mortality and organ dysfunction have been extensively studied in this context with outcomes comparable to the findings in our subgroup. Secondly, our patients’ diagnoses were determined on admission to ICU and do not necessarily indicate the final causes of death. Thus, our findings cannot be definitely related to specific organ system failures without autopsy and histopathological results, which were not included for this study. Lastly, as our aim was the prognostic relevance of MMP and TIMP levels for short-term mortality, they were only assessed at one singular but critical time point, i.e. an acute setting leading to ICU admission. Follow-up trends in MMP and TIMP levels and long-term mortality were not addressed in this study.

The prognostic significance of (early) TIMP‑1 levels has been extended to a heterogeneous group of critically ill patients and further support the predictive value of MMP‑9 levels on short-term mortality, especially in patients with underlying cardiac disease. These findings indicate a value of MMP‑9 and TIMP‑1 as prognostic markers and also as potential candidates for a multibiomarker approach and therapeutic targets in systemic inflammation and in settings of acute organ failure.