Greater than a third of CVD related deaths are attributed to myocardial infarction. Myocardial infarction is the result of a blocked coronary artery, and is most common in those aged over 65 years. Without the capacity for cellular regeneration, the cardiac tissue must remodel at the cellular level in order to meet the physiological demands of the system. The signaling pathways which are activated following myocyte death initiate acute reparative changes in the heart which can be broken into initial and late phases. The initial phase involves the replacement of necrotic tissue with fibrotic scar formation, elongation of myocytes and thinning of the infarct zone. The volume of the ventricle increases as an adaptive mechanism to maintain normal cardiac output and stroke volume (Sutton and Sharpe, 2000; Konstam et al., 2011). The late phase of ventricular remodeling involves hypertrophic myocyte elongation in the non-infarcted areas, increased wall mass, and chamber enlargement. The ventricle morphologically elongates and the performance begins to decline. The late phase will progress indefinitely and ultimately results in heart failure (Konstam et al., 2011). Replacement of damaged muscle mass with fibrotic tissue is entirely dependent on the physiological functioning of cardiac fibroblasts (Furtado et al., 2016).

Improved medical care has resulted in a larger population surviving CVD events, and therefore, the prevalence of secondary complications such as heart failure has increased. Currently, one in five people globally will develop heart failure and survival estimates at 5 and 10 years post-diagnosis are 50 and 10%, respectively (Taylor et al., 2012). Heart failure is defined by the American Heart Association as “a complex clinical syndrome that results from any structural or functional impairment of ventricular filling or ejection of blood” (Yancy et al., 2013). Heart failure can be broadly classified as systolic heart failure or diastolic heart failure. Systolic heart failure is the result of a reduced ejection fraction and is characterized by reduced efficiency in the contraction and ejection of blood by the myocardium. Increased collagen deposition in systolic dysfunction is largely due to the need for cardiomyocyte replacement after cardiac damage. This increased ventricular stiffness results in reduced contraction and cardiac output, and a reduced systemic perfusion capacity (Khan and Sheppard, 2006). Disruption of myocardial excitation-contraction coupling, uncoordinated contraction of cardiomyocyte bundles, disrupted endomysial homeostasis, and sliding displacement of cardiomyocytes resulting in ventricular dilation have been proposed as mechanisms of fibrosis-induced systolic dysfunction (Biernacka and Frangogiannis, 2011). In diastolic heart failure, the ejection fraction is preserved, however there is a disturbance in the accommodation of blood volume during diastole at low filling pressures. Diastolic heart failure is largely a consequence of impaired ventricular relaxation or increased myocardial stiffness caused by excessive cardiac fibrosis (Gomes et al., 2013). The mechanisms underlying diastolic heart failure involve abnormal ECM composition and cell-ECM interactions, as well as alterations in the myocyte cytoskeleton, both of which impair the passive properties of the ventricular wall (Gomes et al., 2013).

Interstitial lung diseases including IPF are characterized by abnormalities between the capillaries and alveolar spaces of parenchymal tissue. In IPF, the aberration is a relentlessly progressive fibrosis that causes irreversible damage to the lung structure and function (Ryu et al., 2014). Patients with IPF have a median survival of only 2–5 years from diagnosis. The prevalence of IPF in the United States is ~40 cases per 100,000, and accounts for over 16,000 deaths per year (Lynch et al., 2016; Raimundo et al., 2016). The health burden of IPF in other developed and developing countries is no less significant. IPF is diagnosed primarily in the elderly, and more so in males and/or patients with a history of smoking (Smith et al., 2013; Wuyts et al., 2014). Histologically, the spatially heterogeneous pattern of fibrosis in IPF, referred to as “usual interstitial pneumonia” (UIP), comprises architectural distortion, interstitial thickening, presence of fibroblastic foci and honeycombing cystic remodeling (Rabeyrin et al., 2015). UIP is primarily associated with IPF, but also occurs in other less common interstitial lung diseases, including connective tissue-disease associated interstitial lung diseases and chronic hypersensitivity pneumonitis (Wuyts et al., 2014). Whilst of unknown etiology (hence the term “idiopathic”), IPF is now widely considered to be initiated by persistent injuries to the alveolar epithelium (Selman et al., 2004). Furthermore, for reasons that remain incompletely understood, the subsequent injury-repair response to alveolar injury is highly dysregulated, resulting in the persistence of fibroblasts and the excessive accumulation of ECM proteins that scar the lung in IPF. Following an acute exacerbation, IPF patients display additional damage known as diffuse alveolar damage, which is characterized by patterns of inter and intra-alveolar accumulation of fibroblasts and ECM. This additional damage involves regions of lung previously spared from fibrosis and scarring (Papiris et al., 2010; Bhatti et al., 2013). Coupled with the existing UIP pattern, these exacerbations, typically of unknown cause, accelerate the normally gradual decline of respiratory function into a rapidly deteriorating state.

ARDS is caused by injuries to the alveolar capillary barrier, comprising an acute inflammation phase and a subsequent fibroproliferation phase leading to interstitial fibrosis (Frenzel et al., 2011). ARDS afflicts ~200,000 people annually in the US, accounting for about 75,000 deaths per annum (Rubenfeld et al., 2005). ARDS prevalence and mortality increases with age, and is higher in men than women (Rubenfeld et al., 2005; Cheifetz, 2016; Cochi et al., 2016). ARDS is initiated by increases in systemic inflammation of pulmonary or extra-pulmonary origin. Insults range from direct forms, including viral, fungal, or bacterial pulmonary infections to indirect forms such as sepsis, peritonitis, or severe acute pancreatitis (Meduri and Eltorky, 2015). Injury is followed by an acute inflammation phase with leak of edema fluid into the interstitium and alveoli spaces. Resolution occurs if the damage from the initial injury is not severe and ongoing, and when repair processes which restore alveolar capillary barrier integrity occur in a timely manner. Any adaptive repair response involves a degree of fibroblast activation and migration to guide tissue repair by supporting epithelial and vascular regeneration. However, in many cases of ARDS, an excessive and persistent fibro-proliferation response occurs leading to interstitial fibrosis (Frenzel et al., 2011). Mechanical injury as a consequence of low tidal volume ventilation treatment strategies heightens the maladaptive fibrotic repair response. ARDS mortality, which is ~25% in the US, is strongly associated with the magnitude of pulmonary fibrosis (Rocco et al., 2009). Histologically, the pattern of tissue damage in ARDS is referred to as diffuse alveolar damage, which is differentiated in exudative and fibro-proliferative phases (Meduri and Eltorky, 2015). The exudative phase shows spatially uniform alveolar damage, fibrin deposition (hyaline membranes) along the alveolar wall, lung edema and hemorrhage. The later phase exhibits patterns of inter and intra-alveolar accumulation of fibroblasts and ECM.

Fibroblasts are a spindle shaped population of mesenchymal cells that are found in the connective tissue of most organs (Shinde and Frangogiannis, 2014). Physiological injury or insult activates fibroblasts near the site of injury to efficiently undergo dynamic phenotypic changes that aide in scar formation (Shinde and Frangogiannis, 2014). Recent data has demonstrated that there is extensive heterogeneity in fibroblasts from different tissues, as well as those from the same tissue type (Lajiness and Conway, 2014). Fibroblasts are regarded primarily as the producers of the ECM, and do so via their extensive endoplasmic reticulum. The ECM scaffold they produce serves multiple roles including the support of adjacent cells, cell signaling, and cell adhesion (Hynes, 2009; Frantz et al., 2010; Lajiness and Conway, 2014; Shinde and Frangogiannis, 2014). Maintaining tissue homeostasis under normal conditions involves a fine balance in the expression of the ECM network constituents (Meneghin and Hogaboam, 2007). Under pathological conditions, fibroblast activation and proliferation is augmented, leading to increased production of ECM at the site of injury. Over-production of ECM components is a commonly observed hallmark of fibrotic diseases, and ultimately results in a severely scarred, inflexible and functionally compromised organ.

The origin of fibroblasts in both the heart and lung remain unclear. In the heart it has been proposed that they may originate either from local interstitial fibroblasts or from circulating precursor cells from the bone marrow (Yano et al., 2005). In the lung, the origins of the fibroblasts may also be derived from resident interstitial fibroblasts or circulating bone marrow-derived fibrocytes, or could be derived from epithelial-mesenchymal transdifferentiation (Mubarak et al., 2012; Maharaj et al., 2013; Burnham et al., 2014). Aside from epithelial cells, transdifferentiation of pleural mesothelial cells also possibly contribute to the increased number of myofibroblasts, particularly as fibrosis in IPF emanates from the sub-pleura region (Mubarak et al., 2012). Regardless of source, fibroblasts contribute to fibrosis by proliferating and differentiating into collagen producing, contractile myofibroblasts as observed in cardiac and pulmonary pathologies.

Myocardial infarction induced fibrosis occurs in three distinct and controlled phases: the inflammatory, proliferative, and maturation phases (Zymek et al., 2006; Frangogiannis, 2014; Table 1). Myocardial necrosis induced by sudden acute death of cardiomyocytes initiates the inflammatory phase, in which innate immune pathways are activated, inflammatory mediators are released, leukocytes are recruited to the infarcted site and neutrophils are activated (Zymek et al., 2006; Frangogiannis, 2014). The primary inflammatory phase is then actively suppressed to prepare the site for the proliferative phase. In this phase, mononuclear cells and macrophage subpopulations secrete growth factors that recruit and activate reparative cells including fibroblasts and myofibroblasts (Frangogiannis, 2014). These cells accumulate in granulation tissue, depositing ECM proteins that contribute to scar formation and preserve the structural integrity of the tissue (Zymek et al., 2006). The reparative cells then undergo apoptosis and the wound transitions into the maturation phase and inflammation begins to resolve. The resolution of the inflammatory phase results in the replacement of the vascular granular tissue with a collagen scar (Zymek et al., 2006; Frangogiannis, 2014). As the scar matures, significant collagen cross-linking ensues, increasing the tensile strength of the scar (Czubryt, 2012). This scarring impairs cardiac contractility and relaxation, leading to ventricular remodeling and ultimately heart failure, as well as impairments in electrical signaling synchronicity which can lead to arrhythmogenesis (Miragoli et al., 2007; Czubryt, 2012). It was long thought that, unlike other organs, the heart did not contain endogenous stem cells, and thus, the cells of the heart could not regenerate nor recellularize after damage, rendering the damage and scarring irreversible. In 2003, however, a small, yet biologically important population of endogenous cardiac stem cells were discovered in rats and subsequently in humans (Beltrami et al., 2003; Torella et al., 2007). These findings suggested the heart also has regenerative capacity, albeit limited, and exciting ongoing research, whilst in its infancy, is exploring the impact on cardiac fibrosis.

Whilst cardiac fibrosis is largely an adaptive response to myocyte loss and/or cardiac pressure overload, pulmonary fibrosis is a response to epithelial injury. The onset and extent of pulmonary fibrosis depends on the severity and persistence of the initial injury, as well as the regenerative capacity of the epithelium. Inadequate re-epithelialization following injury triggers a cascade of events leading to pathological fibrosis. Increased type II alveolar epithelial cell (AECII) apoptosis and senescence are considered to have important roles in the development of pathological fibrosis in IPF and ARDS (Matute-Bello et al., 1999; Plataki et al., 2005; Hecker et al., 2014; Lopez et al., 2015). Following injury, the stages of fibrosis in cardiac and respiratory disease are similar, as are the cellular and molecular mechanisms that lead to scar formation. However, an important difference distinguishing the fibrosis of IPF to that of many other respiratory and cardiac diseases is the nature and extent of inflammation. Whilst both the innate and adaptive immune systems have roles in IPF, the inflammatory response is far less active than in other forms of fibrosis (Feghali-Bostwick et al., 2007; Gilani et al., 2010; Margaritopoulos et al., 2016). As in cardiac fibrosis, pulmonary fibrosis has consecutive and overlapping fibroproliferative and maturation stages (Table 1). During these stages, epithelial, endothelial, and fibroblast cells undergo migration and/or proliferation in a coordinated process to repair the damaged tissue. The proliferation and transdifferentiation of fibroblasts leads to an increase in the myofibroblast population at the site of injury. These cells facilitate the exaggerated deposition of ECM in aberrant tissue repair, leading to the formation of an irreversible scar. As previously outlined, this scar significantly impairs organ function and ultimately leads to dysfunction and failure. Distinct from cardiac fibrosis, epithelial cells in IPF and ARDS are a major source of mediators essential to the migration, proliferation, and differentiation of resident fibroblasts.

It is well documented that fibrosis is closely linked to the inflammatory response (Krenning et al., 2010; Biernacka and Frangogiannis, 2011). Emerging evidence suggests that cardiac fibrosis in general is primarily due to complications associated with acute and prolonged inflammation (Biernacka and Frangogiannis, 2011; Van Linthout et al., 2014), suggesting that even low-grade, persistent inflammation is enough to promote cardiac fibrosis. The cellular and molecular events that underpin the inflammatory processes in cardiac disease are complex and poorly understood, with many tissue and disease specific mechanisms. In pulmonary fibrosis however, the role of inflammation is less clear. Certainly, an active inflammatory response following injury to the alveolar epithelium contributes to fibrosis in ARDS. However, the role of inflammation in the development of IPF pathology is less certain. Whether an initial injury event(s) and a subsequent acute inflammatory response occurs is difficult to establish because of the lengthy delay before the diagnosis of IPF is made (Wuyts et al., 2014). The role of inflammation in the progression of fibrosis in IPF has been challenged because anti-inflammatory and immunosuppressive agents are ineffective as therapies (Davies et al., 2003; Richeldi et al., 2003). The current prevailing hypothesis is that IPF is a result of persistent micro-injuries to the alveolar epithelium, rather than a phenomenon driven by inflammation (Selman et al., 2004). Despite this conception, there are increased numbers of inflammatory cells, including mast cells, macrophages, and lymphocytes in the fibrotic lung and circulation of IPF patients (Nagai et al., 2007; Peteranderl et al., 2016). Furthermore, the lung immune cell profiles in IPF and ARDS are similar (Schupp et al., 2015).

Following cardiac tissue injury, inflammatory and immune cells such as monocytes, granulocytes, macrophages, mast cells, dendritic cells, as well as B and T lymphocytes are recruited to the site of injury. These infiltrating cells are responsible for producing and secreting an array of both inflammatory and anti-inflammatory cytokines such as interleukins (e.g., IL-1 and IL-13), tumor necrosis factor-α (TNF-α) and transforming growth factor-β (TGF-β; Afanasyeva et al., 2004; Blyszczuk et al., 2008; Kania et al., 2009; Van Linthout et al., 2014). More recently, these inflammatory cells have been identified to release fibrogenic cytokines and growth factors which promote the fibrotic tissue formation following the initial inflammatory cell infiltrate (Frangogiannis, 2006; Wynn, 2008; Biernacka and Frangogiannis, 2011). Studies aimed at elucidating the role of inflammatory cells in the development of cardiac fibrosis, have focused on the involvement of T helper (Th) lymphocytes, specifically Th1, Th2, and Th17 lymphocytes (Van Linthout et al., 2014). Such studies identify that in cardiac inflammatory wound healing, Th2 lymphocytes act directly and indirectly (via expression of factors such as IL-4, -5, and -13) with fibroblasts to promote myofibroblast activation and suppress matrix metalloproteinase (MMP) activity leading to the accumulation of ECM proteins and formation of fibrotic tissue (Yu et al., 2006; Marra et al., 2009; Wei, 2011; Bailey et al., 2012). Similarly, studies suggest that Th1 lymphocytes have the ability to play both a pro- and anti-fibrotic role. The anti-fibrotic role of Th1 lymphocytes has long been recognized to act via induction of interferon-γ (IFN-γ), which reduces TGF-β expression (Ulloa et al., 1999; Eickelberg et al., 2001), and inhibits IL-4 and -13 (Fairweather et al., 2004), suppressing the formation of fibrotic tissue (Van Linthout et al., 2014). The pro-fibrotic activity of Th1 lymphocytes is also achieved via secretion of IFN-γ, which results in production of pro-fibrotic and pro-inflammatory mediators such as TNF-α (Nathan et al., 1984; Piguet et al., 1989; Chen et al., 2001; Marko et al., 2012). Further studies suggest Th1 cells control activation, migration and differentiation of macrophages, and macrophage dependent monocyte chemoattractant protein-1 expression, leading to further inflammation and fibrosis (Wynn, 2004; Han et al., 2012; Van Linthout et al., 2014). Finally, Th17 cells play an essential role in the development of cardiac fibrosis via the expression of IL-17 (Feng et al., 2009). The pro-fibrotic characteristics of IL-17 are derived from its ability to induce the expression of pro-inflammatory factors, which in turn, promote the recruitment and activation of neutrophils (Ouyang et al., 2008; Pelletier et al., 2010), promote MMP-1 expression in cardiac fibroblasts (Cortez et al., 2007), and induce cardiac fibrosis directly via protein kinase C activation (Liu et al., 2012).

Interactions between inflammatory cells and resident structural cells are likely to contribute to pulmonary fibrosis. Increased numbers of mast cells in the lung of IPF and ARDS patients elicit fibrogenic actions in a manner involving direct contact with fibroblasts (Liebler et al., 1998; Wygrecka et al., 2013). Alternatively activated (M2) macrophages, the predominant macrophage phenotype in the lungs of IPF patients, have an impaired innate immune function but produce fibrogenic mediators such as TGF-β, IL-13, and chemokine (C-C motif) ligand 18 (CCL18; Pechkovsky et al., 2010). The latter mediates a fibrogenic axis between M2 macrophages and fibroblasts in IPF by inducing fibroblast collagen production, which in turn up-regulates M2 macrophage CCL18 expression (Prasse et al., 2006; Stahl et al., 2013). Increased numbers of non-proliferating, antigen-activated T and B cells are also detected in lung tissue and the circulation of IPF patients (Daniil et al., 2005; Marchal-Sommé et al., 2006; Kahloon et al., 2013; Xue et al., 2013). CD8+ T lymphocyte numbers in particular correlate with measures of respiratory disease severity in IPF, implicating a role in pulmonary fibrosis (Daniil et al., 2005). Autoantibodies produced by activated B cells may also contribute to the development of IPF (Donahoe et al., 2015). Whilst most autoantibodies are benign, some, such as those against heat shock protein 70 are pathological (Kahloon et al., 2013). Therapies that reduce autoantibody production, including the B cell targeting rituximab, show potential as treatments for IPF (Donahoe et al., 2015). In lungs of IPF patients, mature lymphocytes, and dendritic cells form aggregates resembling lymphoid follicles, which act autonomously to recruit maturing dendritic cells and recently activated T cells (Marchal-Sommé et al., 2006). One possible explanation for why IPF is refractory to anti-inflammatory therapeutics is that these agents primarily target immature naïve immune cells, not the mature lymphocyte and dendritic cells detected in IPF.

In fibrosis, the ECM is substantially thickened by an increase in the abundance of fibrillar collagens I and III. Fibril formation is facilitated by increases in fibronectin, a nucleator of fibrillogenesis (i.e., collagen fibril assembly), whereas collagen XII and XIV have roles in collagen distribution and organization (Fukuda et al., 1998). Crosslinking of collagen catalyzed by lysyl oxidase and glycation contributes to the increased rigidity and stiffness of fibrotic tissue (Tzortzaki et al., 2006; Barry-Hamilton et al., 2010). Interestingly, alterations in lysyl oxidase family members have been linked to IPF (Barry-Hamilton et al., 2010; Aumiller et al., 2017). Collectively, these biophysical changes to the ECM in a disease context possibly perpetuate fibrosis as the proliferative and contractile responses of pulmonary fibroblasts increase when they are attached to rigid matrices (Marinkovic et al., 2013; Zhou et al., 2013). Such a detrimental positive feedback loop involving increased ECM stiffness is thought to contribute the distinctive expansive pulmonary fibrosis of IPF (Marinkovic et al., 2013; Zhou et al., 2013).

Homeostatic balance of the ECM network involves complex modulation by multiple factors, and includes the ongoing process of production and deposition of ECM proteins. Two of the major ECM modulators are localized non-structural proteins that facilitate the rearrangement of the ECM under physiological and pathological conditions. These proteins are the MMPs which degrade ECM proteins and their inhibitors: tissue inhibitors of metalloproteinases (TIMPs). MMPs and TIMPs are released throughout the process of tissue healing by fibroblasts, and they work together to control the remodeling and degradation of the existing ECM in and around the site of injury (Fan et al., 2012; Rienks et al., 2014). Several chemokines, cytokines and growth factors have been shown capable of regulating the production of MMPs and TIMPs by fibroblasts. For example, pro-inflammatory cytokines such as TNFα and IL-1β can induce the transcription of several MMPs along with TIMP-1 and -2 in the myocardium (Fan et al., 2012).

MMPs are synthesized and secreted as inactive zymogens (pro-MMPs) which are activated by removal of an amino-terminal propeptide domain, which exposes a catalytic domain (Vanhoutte and Heymans, 2010; Fan et al., 2012). There are ~26 identified MMPs to date, and of these MMP-1, -2, -3, -8, -9, -12, -13, and -28 have been implicated in myocardial remodeling (Fan et al., 2012). MMP-1, -8, and -13 cleave native (fibrillar) forms of collagen types I, II and III, whereas the gelatinolytic MMP-2 and -9 cleave collagen types I, IV, and V (Fan et al., 2012). MMPs are additionally responsible for the release of growth factors from the ECM, cleavage of receptors for these growth factors at the cell's surface and activation of other MMPs (Vanhoutte and Heymans, 2010). In addition, the membrane-type MMP (MT1-MMP/MMP14), which cleaves several ECM proteins, has been highlighted to be of importance in the heart, as a reduction in MT1-MMP improves survival and heart function post myocardial infarction in mice, whilst overexpression lowers survival and function (Zavadzkas et al., 2011). In IPF and ARDS, the levels of MMP-1, -3, -7, -8, -9, -13, and/or -28 in lung tissue are increased, correlating with disease severity (Selman et al., 2000; Henry et al., 2002; Fligiel et al., 2006; Craig et al., 2015). Furthermore, several membrane- type MMPs have been shown to be associated with IPF. MT1-MMP and MT2-MMP are increased in the lungs of IPF patients and murine lung fibrosis models, and MT3-MMP was expressed by fibroblasts and AECs in IPF (Craig et al., 2015; Pardo et al., 2016). MT6-MMP was also shown to be decreased in lung tissue of patients with IPF (Pardo et al., 2016). Other proteases involved in collagen remodeling including urokinase plasminogen activator and fibroblast activation protein α are increased in the lung interstitium of IPF patients (Acharya et al., 2006; Schuliga et al., 2017). A potential mechanism by which MMPs and other matrix degrading proteases contribute to pulmonary fibrosis is by remodeling collagen into forms that evoke fibrogenic actions. MMP-1, -8, and -13 and fibroblast activation protein α are collagenolytic enzymes that cleave and subsequently release native collagen I and III fibrils. The resulting collagen fragments retain the triple helical conformation that binds α2β1 integrin on pulmonary fibroblasts to augment proliferation (Schuliga et al., 2009). Conversely, native collagen I and III bind α2β1 integrin in a different manner that suppresses pulmonary fibroblast proliferation (Schuliga et al., 2009, 2010). The pericellular proteolytic denaturation of collagen immediately surrounding pulmonary fibroblasts possibly preserves the proliferative, synthetic phenotype of fibroblasts with little impact on net tissue stiffness. The spread of fibrosis from discrete regions of the lower lung, as happens in IPF, may occur because of pericellular proteolysis at the leading edge, where fibroblast proliferation is most active (Acharya et al., 2006; Nkyimbeng et al., 2013). Asides from their direct role in ECM remodeling, MMPs possibly contribute to fibrosis by inducing epithelial-mesenchymal transdifferentiation, fibrocyte infiltration, and the polarization of alveolar macrophages into a M2 phenotype via activation of fibrogenic cytokines (Craig et al., 2015).

TIMPs, as their name suggests, inhibit MMP activity by binding to active sites on their respective MMPs, thus preventing them from cleaving ECM components (Vanhoutte and Heymans, 2010). TIMP-2, -3, and -4 are expressed in the healthy heart, whilst TIMP-1 expression is low, although it is upregulated under pathological conditions (Fan et al., 2012). It is becoming increasingly apparent however that TIMPs have MMP-independent functions including the regulation of apoptosis, cell survival, growth, migration, differentiation, angiogenesis, and inflammation (Vanhoutte and Heymans, 2010). Adenovirus-mediated overexpression of TIMP-1, -2, -3, or -4 in cardiac fibroblasts increases proliferation and myofibroblast differentiation, whereas TIMP-2 overexpression alone increases collagen production, and TIMP-3 induces apoptosis (Lovelock et al., 2005). TIMPs are also important regulators of ECM turnover in the lung. However, information regarding TIMP levels and distribution in fibrotic respiratory disease is either limited or inconsistent. To our knowledge, no study has assessed TIMP levels in human fibrotic ARDS, besides measurements in bronchoalveolar lavage fluid (BALF), tracheal aspirates or serum, none of which accurately reflect tissue levels (Miller et al., 2006). Initial studies indicated that ECM accumulation in IPF is a consequence of increased TIMP production and a resulting non-degrading collagen environment. Immunohistochemical analysis of lung tissue of IPF patients showed an association between TIMP-2 antigen and fibroblasts, whereas TIMP-1 antigen was present in interstitial macrophages and TIMP-3 and -4 antigens were confined to the epithelium (Selman et al., 2000; Garcia-Alvarez et al., 2006). Garcia-Alvarez et al. also detected TIMP-3 in fibrotic regions (including fibroblasts) of IPF lung tissue in situ, and showed that IPF-derived pulmonary fibroblasts expressed higher levels of TIMP-1, -2, and -3 in vitro characterized by compared to fibroblasts characterized by from control donors (Garcia-Alvarez et al., 2006). However, Nkyimbeng et al. (2013) recently showed that mRNA and protein levels of TIMPs in the lung of IPF patients and controls were similar, whereas the levels of MMP-1, -7, and -13 were higher in IPF. They also showed that MMP-1 and -13 antigenicity in lung of IPF patients' in situ overlap with collagenolytic activity in certain regions of fibrosis, particularly the airways where honeycombing cysts develop. Based on these findings, the authors suggested that the levels of TIMPs in regions of IPF lung are inadequate to counter increased collagenolytic MMP production. However, despite these local increases in collagenolytic activity, overwhelming matrix production in IPF results in a net increase in ECM. Another potentially important role of TIMPs in IPF and ARDS is the regulation of inflammatory responses that drive fibrosis. TIMP-3 in particular has important roles in M2 macrophage polarization in lung injury and disease (Gill et al., 2010, 2013).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.