Interstitial lung disease in connective tissue diseases: evolving concepts of pathogenesis and management

Interstitial lung disease (ILD) is a challenging clinical entity associated with multiple connective tissue diseases, and is a significant cause of morbidity and mortality. Effective therapies for connective tissue disease-associated interstitial lung disease (CTD-ILD) are still lacking. Multidisciplinary clinics dedicated to the early diagnosis and improved management of patients with CTD-ILD are now being established. There is rapid progress in understanding and identifying the effector cells, the proinflammatory and profibrotic mediators, and the pathways involved in the pathogenesis of CTD-ILD. Serum biomarkers may provide new insights as risk factors for pulmonary fibrosis and as measures of disease progression. Despite these recent advances, the management of patients with CTD-ILD remains suboptimal. Further studies are therefore urgently needed to better understand these conditions, and to develop effective therapeutic interventions.

and lymphocytic interstitial pneumonia. Table 2 outlines the characteristic histopathologic and radiologic features of the diff erent forms of ILD. Th e present review will primarily focus on the pathogenesis and treatment of SSc-associated ILD, with a brief overview of the other CTD-ILDs.

Systemic sclerosis
SSc is characterized by tissue injury leading to excessive collagen deposition, and pulmonary disease is a leading cause of death in these patients.
Most patients with SSc have evidence of ILD by HRCT of the chest or at autopsy. Close to one-half of cases develop clinically signifi cant ILD. In a multiethnic study, the risk for ILD in cases of SSc was greater in patients of African-American ethnicity and in those patients with more extensive skin and cardiac involvement [2]. Autoantibody expression is a predictor of internal organ involvement, particularly lung involvement. Th e presence of anti-topoisomerase antibodies (Scl-70) is strongly associated with development of signifi cant ILD, while anti-centromere antibodies appear to be protective -although patients with limited SSc are not excluded from developing ILD [2,3]. A recent European League Against Rheumatism Scleroderma Trials and Research analysis revealed in a cohort of 3,656 SSc patients that ILD was present in 53% of cases with diff use cutaneous SSc and in 35% of cases with limited cutaneous SSc [4].
Biomarkers, although currently not available for clinical testing, may serve as indicators of disease and as predictors of progression. Serum levels of surfactant proteins A and D (SP-A and SP-D) and the glycoprotein Krebs von den lungen-6 (KL-6), produced by type II alveolar epithelial cells, are elevated in sera of patients with ILD [5]. A comparison of SP-D and KL-6 serum concentrations showed that both markers were elevated in patients with SSc-associated ILD compared with healthy controls, with SP-D being more sensitive and KL-6 more specifi c [5]. Th ere is a great deal of current interest in novel biomarkers such as chitinase-like protein YKL-40, which is already shown to be useful in asthma [6].
Histologically, SSc-associated ILD is characterized by early pulmonary infi ltration of infl ammatory cells and subsequent fi brosis of the lung parenchyma. Th e most  Table 2. Characteristic histopathologic patterns and radiologic fi ndings in the interstitium of IPF and connective tissueassociated ILD common histologic pattern seen in SSc-associated ILD is NSIP; the UIP pattern is less common. Histologically, NSIP is characterized by varying degrees of infl ammation and fi brosis, with the majority of patients showing prominent infl ammation. In contrast, UIP is charac terized by dense patchy fi brosis with honeycombing, primarily in a subpleural distribution. Radiologic fi ndings associated with CTDs are summarized in Table 2. SSc-associated ILD is characterized on chest X-ray by hazy, reticular infi ltrates that are prominent in the lower lobes. HRCT characteristically reveals ground glass opacities, traction bronchiectasis, and minimal honeycombing consistent with an NSIP pattern ( Figure 1a). In contrast, the UIP pattern of IPF is characterized by patchy reticular opacities associated with traction bronchiectasis and honeycombing with a predominantly basal and peripheral reticular pattern ( Figure 1b). Th e utility of HRCT to detect histologic pattern is suffi cient to make the diagnosis of UIP/IPF in 50 to 60% of cases [7].

Mediators of lung fi brosis in systemic sclerosis
Interstitial lung involvement in SSc develops from an inter play of autoimmunity, infl ammation, and vascular injury. Endothelial or epithelial injury is thought to precede infl ammation and fi brosis, but the mechanisms that perpetuate pulmonary fi brosis are still not fully understood ( Figure 2).
A number of proinfl ammatory and profi brotic extracellular mediators have been implicated in the pathogenesis of interstitial lung diseases and IPF, and are also likely to have important roles in SSc-associated ILD. Th ese include chemokines, cytokines, growth factors, lipids, and prostanoids. Th e pivotal mediator of fi brosis is the multifunctional cytokine, transforming growth factor beta (TGFβ). Substantial evidence implicates TGFβalong with platelet-derived growth factor, endothelin-1 (ET-1), and other cytokines -in the pathogenesis of SSc. Accordingly, targeting the intracellular signaling pathways activated by TGFβ and other profi brotic mediators is a rational treatment strategy for controlling fi brosis and is an active area of current research.

Mediators of TGFβ responses Canonical Smad signaling
Th e canonical TGFβ signal transduction pathway involves sequential phosphorylation of the activin-like kinase-5 type I TGFβ receptor and a group of intracellular signaling proteins called Smads [8]. When bound by active TGFβ, the cell surface TGFβ receptors transmit signals through phosphorylation of cytoplasmic Smad proteins, which translocate into the cell nucleus and trigger transcription of genes such as type I collagen, fi bro nectin, α-smooth muscle actin and connective tissue growth factor (CTGF), each of which plays important roles in fi brogenesis [9]. Smad3 null mice are protected against bleomycin-induced fi brosis of the skin and lungs [10,11]. In addition, pharmacologic blockade of activinlike kinase-5 activity with small molecule inhibitors such as SB431542 and SD208 results in complete abrogation of profi brotic responses induced by TGF β, and normalization of the autonomously activated phenotype of SSc fi broblasts in vitro [12,13]. Selective blockade of Smad phosphorylation or of non-Smad signaling downstream of TGFβ using small molecules are promis ing novel approaches to the treatment of fi brosis that are under investigation.

c-Abelson tyrosine kinase
In normal fi broblasts TGFβ induces activation of c-Abelson tyrosine kinase (c-Abl), a member of the Src family of nonreceptor protein tyrosine kinases [14]. Trans forming mutations of c-Abl are found in 95% of patients with chronic myelogenous leukemia, and result in constitutive kinase activity that is directly responsible for myeloid cell hyperproliferation [15]. Recent studies of c-Abl function in nonmyeloid cells reveal that c-Abl is directly activated by TGFβ, and integrates serinethreonine kinase signaling with nonreceptor tyrosine kinase pathways [16].
Imatinib mesylate is a potent small molecule inhibitor of c-Abl, as well as of platelet-derived growth factor receptor activity. Inhibition of c-Abl kinase activity using imatinib was recently demonstrated to abrogate the stimulation of collagen gene expression in vitro, and to prevent the development of skin and lung fi brosis in vivo in animal models [14,17]. Preclinical studies show that, in explanted normal skin and lung fi broblasts, imatinib eff ectively blocked TGFβ-induced stimulation of collagen synthesis and myofi broblast transformation, which are key events in the fi brotic response [18]. Furthermore, imatinib partially reversed the abnormal phenotype of SSc fi broblasts [17]. Since one of the downstream targets of c-Abl is the profi brotic transcription factor Egr-1 (see below), blockade of c-Abl activity might prevent fi brosis by inhibiting Egr-1 activation [19].
Anecdotal reports indicate therapeutic effi cacy of imatinib in SSc, graft versus host disease, nephrogenic fi brosis, and other fi brosing conditions. Ongoing clinical trials are evaluating the effi cacy and safety of imatinib in SSc-associated ILD. Of note, however, a recently completed randomized controlled trial showed no benefi t of imatinib compared with placebo in patients with IPF [20]. Pathogenesis of pulmonary fi brosis is initiated by microvascular injury, which leads to endothelial cell damage and alveolar epithelial injury. This leads to activation of the coagulation cascade, release of various cytokines and growth factors, and ultimately activation of fi broblasts, a key event in the development of fi brosis. CTGF, connective tissue growth factor; IGF-1, insulin-like growth factor-1; LPA, lysophosphatidic acid; TGF-β, transforming growth factor beta.

Egr-1
Egr-1 is a zinc fi nger DNA binding transcription factor that is rapidly and transiently induced at sites of injury. Egr-1 is implicated in cell proliferation, diff erentiation and survival, and plays a central role in orchestrating acute tissue responses to injury [21]. Egr-1 null mice were protected from pulmonary and skin fi brosis induced by TGFβ or by bleomycin, and Egr-1 was shown to be suffi cient and necessary for the stimulation of type I collagen production in vitro [22]. Genome-wide expression profi ling using microarrays has demonstrated that abnormal Egr-1 expression in the lung was strongly associated with rapid progression of lung fi brosis in patients with IPF [23]. In addition, both Egr-1 mRNA and protein were elevated in explanted SSc skin fi broblasts in vitro [24]. Egr-1 was also shown to be a key mediator of lung fi broblast activation induced by insulin-like growth factor (IGF) binding protein 5 [25].
Th ese observations identify Egr-1 as a critical intracellular mediator of lung fi brosis in humans and in mouse models. Ongoing studies are investigating blocking Egr-1 expression or activity with drugs such as imatinib as potential strategies to control pathologic fi brosis.

Peroxisome proliferator-activated receptor gamma
Peroxisome proliferator-activated receptor gamma (PPARγ) is a nuclear steroid hormone receptor and a ligand-activated transcription factor. Originally described in adipocytes, it is now recognized that PPARγ is widely expressed in tissues and plays key regulatory roles not only in adipogenesis and insulin sensitivity, but also in infl ammation and immunity.
An emerging novel function for PPARγ is as an endogenous anti-fi brotic defense mechanism. Ligand activation of cellular PPARγ potently inhibited the activation of TGFβ-inducible responses in normal skin and lung fi broblasts [26]. It is notable that the expression of PPARγ is markedly reduced in lung biopsies from patients with SSc-associated ILD [27]. Ligands for inducing the activity of PPARγ include endogenous natural agonists such as fatty acids or prostaglandins (PGJ 2 ), and synthetic pharmacologic agents such as rosiglitazone and pioglitazone [28]. Th ese drugs are in wide use for the treatment of type 2 diabetes. Rosiglitazone was recently shown to attenuate bleomycin-induced dermal fi brosis and infl ammation in vivo. Furthermore, rosiglitazone prevents alveolar epithelial mesenchymal transition and also TGFβ-induced stimu lation of collagen gene transcription, myofi broblast transdiff erentiation, and cell migration in normal fi broblasts [29].
In light of its potent anti-infl ammatory and antifi brotic activities and relative safety in clinical practice, studies of existing PPARγ agonists -and novel selective agonists under development -are now warranted for treatment of ILD.

Endothelin-1
Endothelial injury in small and medium-sized arteries is a defi ning feature of SSc that leads to activation of the coagulation cascade followed by myofi broblast diff erentiation, activation of endothelial cells, and capillary loss. ET-1 is a potent vasoconstrictor released by endothelial cells, epithelial cells and mesenchymal cells. In lung injury, ET-1 binds to ET-1A and ET-1B receptors, recruits fi broblasts and stimulates matrix production [30]. Transgenic mice overexpressing ET-1 develop lung fi brosis [31] and ET-1 levels are elevated in mouse models of bleomycin-induced fi brosis [32]. ET-1 also has been found to stimulate TGFβ secretion in lung fi broblasts [33]. Studies with bosentan, a dual-receptor ET-1 antagonist, are underway for the treatment of IPF and SScassociated ILD.

Growth factors and chemokines Lysophosphatidic acid
Th e bioactive phospholipid lysophosphatidic acid (LPA) and its receptor LPA 1 have recently been implicated in the pathogenesis of IPF [34]. LPA is produced by activated platelets, as well as by fi broblasts. Th e LPA 1 receptor is expressed in fi broblasts, endothelial cells, and epithelial cells, and enables LPA to induce diverse biologic eff ects involved in tissue responses to injury.
Both LPA and its receptor are required for the development of lung fi brosis in a mouse model of IPF induced by bleomycin [34]. Th ese studies revealed that the fi broblast chemoattractant activity present in the lungs of IPF patients is largely attributable to LPA, suggest ing that LPA mediates fi broblast recruitment during the development of lung fi brosis. Preliminary results indicate that mice lacking the LPA 1 receptor are protected from bleomycin-induced dermal fi brosis com pared with wildtype mice (FV Castelino, AM Tager, un pub lished data). Th e LPA-LPA 1 pathway therefore appears to be a promising novel therapeutic target for SSc-associated pulmonary fi brosis.

Insulin-like growth factor
IGFs and their binding proteins have been implicated in the pathogenesis of pulmonary fi brosis and SSc. Increased levels of IGF-1 are detected in the serum as well as in the bronchoalveolar lavage of patients with SSc-associated ILD [35]. In addition, blockade of the IGF pathway leads to resolution of pulmonary fi brosis in a mouse model of pulmonary fi brosis [36]. Th ese observations raise the possibility that targeting the IGF pathway may be a potential treatment of CTD-ILD.

Connective tissue growth factor
CTGF, also known as CCN2, is a small cysteine-rich matri cellular protein with an important role in angio genesis and the formation of connective tissue [37]. Although the specifi c receptors for CTGF or the precise mechanism of action are poorly understood, CTGF acts as a downstream mediator of TGFβ, and may play a role in the stimulation of extracellular matrix production and myofi broblast diff erentiation.
Levels of CTGF are elevated in the skin and lungs from patients with SSc, as serum levels of CTGF refl ect disease severity. Lung fi broblasts explanted from bleomycininjected mice have a high expression of CTGF [38]. CTGF is therefore an attractive target for the treatment of pulmonary fi brosis, and clinical trials using a monoclonal anti-CTGF antibody are under preparation.

Treatment considerations
To date there is no cure or eff ective disease-modifying therapy for any form of CTD-ILD. d-Penicillamine and colchicine are largely ineff ective [39,40]. Because evidence of infl ammation is commonly present in earlystage disease, current therapies for SSc-associated ILD target the infl ammatory response. Th e immuno suppressive agents most widely used for this purpose are corticosteroids, cyclophosphamide, azathioprine, and myco phenolate mofetil. While corticosteroids are generally ineff ective, other agents have demonstrated a modest benefi cial eff ect.
In contrast to various rheumatic diseases where immuno suppressives have been helpful, immunosuppres sive therapies in CTD-ILD have not led to complete responses. Th ere is only limited experience with newer biologicals such as anti-TNF therapies or rituximab.

Cyclophosphamide
Multiple studies and uncontrolled trials of CTD-ILD have reported benefi cial eff ects of cyclophosphamide administered orally or intravenously [40][41][42]. Th ese studies showed improvement in respiratory symptoms, lung function, radiologic fi ndings, and bronchoalveolar lavage infl ammation, as well as survival.
Th e Scleroderma Lung Study was the fi rst multicenter, randomized placebo-controlled clinical trial to evaluate the eff ectiveness of oral cyclophosphamide in SScassociated ILD [43]. In the study, 158 patients with earlystage SSc and symptomatic ILD with radiologic or bronchoalveolar lavage evidence of alveolar infl ammation were randomized to cyclophosphamide or placebo. A 12-month course of active therapy was associated with a modest but statistically signifi cant improvement in forced vital capacity (FVC), but no change in diff using capacity for carbon monoxide. Furthermore, respiratory symptoms and chest radiologic abnormalities showed improvement [43,44]. Th e response in pulmonary function was most pronounced in those patients with the most advanced lung disease at baseline. At 24-month follow-up, the benefi cial eff ect of cyclophosphamide on pulmonary function largely disappeared. In contrast, benefi cial responses in skin score and quality of life measures persisted at 2 years.
A randomized, double-blind, placebo-controlled study from the UK compared the effi cacy of intravenous cyclophosphamide combined with corticosteroids and followed by azathioprine with that of placebo. Th is study of 45 SSc patients with early ILD demonstrated a favorable out come in the treatment group, but, due to the small size of the study, the results did not achieve statistical signifi cance [45].

Mycophenolate mofetil
Mycophenolate mofetil is an immunosuppressive drug with less toxicity compared with cyclophosphamide. One study evaluated 17 patients with SSc-associated ILD treated with mycophenolate mofetil for up to 24 months [46]. At 12 months the FVC and diff using capacity for carbon monoxide had improved by 2.6% and 1.4%, respec tively, while at 24 months the increase in FVC was 2.4% [46]. Gerbino and colleagues evaluated myco phenolate mofetil in 13 patients with early SSc-associated ILD [47]. Th e FVC improved by a mean of 4% predicted at a median of 21 months. Th e ongoing Scleroderma Lung Study II will compare the effi cacy and safety of mycophenolate mofetil with cyclophosphamide in patients with SSc-associated ILD.

Azathioprine
Azathioprine is an alternative agent for SSc-associated ILD. Patients with a milder form of ILD or those unable to tolerate cyclophosphamide may be potential candidates.
A retrospective analysis described 11 patients with SSc-associated ILD who received azathioprine and prednisone [48]. In this study, 8/11 patients showed an improvement in FVC and dyspnea scores at 12 months.
Data also suggest a role for azathioprine as maintenance therapy following intravenous cyclophosphamide. A retrospective series of 27 patients with SSc-associated ILD showed stabilization or improvement of lung function with a combination regimen of monthly intra venous cyclophosphamide given for 6 months followed by 18 months of azathioprine [49].

Endothelin-1 receptor antagonists
Bosentan is a dual ET-1 receptor antagonist approved for the treatment of pulmonary arterial hypertension. ET-1 is overexpressed in SSc skin and lungs, and can act as a profi brotic cytokine that promotes myofi broblast proliferation.
Th e Bosentan in Interstitial Lung Disease (BUILD 1) study examined the potential anti-fi brotic effi cacy of bosentan in IPF. One hundred and fi fty-eight patients with IPF were randomized to receive either bosentan or placebo [50]. With the 6-minute walk test as primary outcome at 12 months, bosentan was no better than placebo. Moreover, a recent placebo-controlled trial of bosentan for the treatment of SSc-associated ILD patients (BUILD 2) was terminated due to lack of effi cacy [51], and BUILD 3 -evaluating the safety and effi cacy of bosentan in IPF patients -did not meet the primary endpoint of a reduction in morbidity and mortality (unpublished data).

Tyrosine kinase inhibitors
Th e therapeutic use of small molecule kinase inhibitors for nonmalignant diseases has generated a great deal of interest, but their use is limited by toxicity. In contrast to inhibitors of ubiquitous protein kinases such as p38, imatinib mesylate (Gleevec®; Imatinib mesylate, Novartis, Basel, Switzerland) selectively blocks the activity of the c-Abl tyrosine kinase and, to a lesser degree, the plateletderived growth factor receptor and c-kit, and appears to have a relatively favorable long-term safety profi le in a large number of chronic myelogenous leukemia patients.
Individual case reports provide support for the use of imatinib in SSc. Van Daele and colleagues described a patient with SSc who had progressive pulmonary fi brosis despite treatment with intravenous cyclophosphamide [52]. After 5 months of imatinib, improvement in skin score (from 18 to 12) and pulmonary function was noted. Another report described a woman with longstanding and progressive SSc unresponsive to intravenous cyclophosphamide and mycophenolate mofetil [53]. Th is patient showed improved skin and stabilization in lung function after 6 months of imatinib therapy [53]. In contrast, a recent large, multicenter, randomized controlled trial of imatinib versus placebo in the treatment of IPF showed no signifi cant benefi t [20].

Pirfenidone
Pirfenidone is a pyridone with both anti-infl ammatory and anti-fi brotic eff ects. Pirfenidone was shown to inhibit collagen synthesis and TGFβ production in vivo in animal models of IPF [54]. In clinical studies, pirfenidone slowed a decline in lung function and exercise capacity [55]. A randomized, double-blind, placebo-controlled phase III trial in IPF (CAPACITY 1 trial) demonstrated a decrease in the rate of decline of vital capacity and an increase in progression-free survival time over 52 weeks; however, the primary endpoint of change in the percentage predicted FVC at week 72 was not met [56]. Th e CAPACITY 2 trial, using a lower dose of pirfenidone, reached its primary endpoint [57]. Pirfenidone was administered to two patients with SSc-associated ILD [58]. Th ese patients showed no signifi cant radiological progression or functional deterioration. One should note, however, that the Food and Drug Administration recently rejected the use of pirfenidone for the treatment of IPF, citing the need for an additional clinical trial to prove effi cacy given that the drug worked in one of the two trials and questioning whether the benefi t provided by the drug was meaningful (InterMune press release).

Lung transplantation
Lung transplantation remains an option for SSc patients with ILD who fail to respond to pharmacologic therapy. A recent study comparing lung transplantation in 29 patients presenting SSc-associated ILD with 70 patients presenting IPF showed comparable cumulative survival (64%) at 2 years [59]. In another retrospective analysis, 23 of 47 SSc patients were alive at 24 months post lung transplantation [60].

Prognosis of systemic sclerosis-associated interstitial lung disease
ILD is a leading cause of morbidity and mortality in SSc. Th e prognosis of SSc-associated ILD depends on the underlying pathology. Th e NSIP pattern has a more favorable outcome compared with the UIP pattern characteristically associated with IPF [61]. A recent retrospective review of 80 patients with SSc-associated ILD showed that 76% had an NSIP pattern and 11% had a UIP pattern [61,62]. In this study, the 5-year survival rates were similar for patients with the NSIP pattern and the UIP pattern (82% and 91%, respectively). It is diffi cult to predict whether IPF patients with a UIP pattern would have similar survival as SSc patients with a UIP pattern.

Rheumatoid arthritis
Lung disease is a leading cause of death in RA, second only to infection. Evidence of ILD is seen in 20 to 30% of patients, but the reported prevalence varies depending on the criteria used for diagnosis [63]. Most RA patients show pulmonary parenchymal abnormalities on HRCT, including bronchial wall thickening, bronchial dilation, micronodules, and opacities, along with pleural eff usions.
Th e main fi nding in patients with ILD is bibasilar symmetrical reticular infi ltrates followed by honeycombing. Th e histologic NSIP pattern was previously thought to be most frequent in RA, but recent studies of surgical lung biopsies reveal that the UIP pattern may be more common [64]. Th e 5-year survival in RA patients with UIP is less than 50%. In one study, RA patients had a greater number of CD4-positive T cells in the bronchoalveolar lavage fl uid than IPF patients [65].
Little is known regarding the optimal therapy of RAassociated lung disease, and randomized trials are lacking. Corticosteroids and immunosuppressive agents are widely used, but corticosteroids by themselves are of limited benefi t in RA UIP [66]. In IPF, N-acetylcysteine added to a regimen of prednisone and azathioprine slowed deterioration of the FVC and diff using capacity for carbon monoxide at 12 months [67]. Mycophenolate mofetil is another therapeutic consideration in patients with RA-associated ILD. One report of two RA patients with ILD showed benefi t of mycophenolate mofetil on pulmonary function and radiologic abnormalities [68]. While the course of RA-associated ILD varies from a slow progression to a fulminant course, the prognosis is generally better than that of IPF.

Polymyositis/dermatomyositis
Th e presence of ILD markedly infl uences the disease course in infl ammatory myositis. Th e reported incidence of ILD varies from 20 to 54% depending on the criteria used for diagnosis [69,70]. Th e strongest predictive factor is the presence of autoantibodies to aminoacyl tRNA synthetase, most commonly anti-Jo-1 [71]. Another serum marker of increased risk for ILD is antibody to KL-6, a glycoprotein expressed on type II alveolar and bronchiolar epithelial cells [72]. Amyopathic dermatomyositis is also associated with ILD and can have a poor prognosis [73]. In addition, anti-clinically amyo pathic dermatomyositis antibodies associated with this subset suggest rapidly progressive ILD [74].
Th e histopathology of ILD in PM/DM includes cryptogenic organizing pneumonia, diff use alveolar damage, and NSIP and UIP patterns [75]. One study suggested that patients with a cryptogenic organizing pneumonia pattern respond to corticosteroids, while those with diff use alveolar damage and UIP patterns do not [75].
Th ere are no controlled trials evaluating the treatment of PM/DM-associated ILD. Th e most common initial therapy uses corticosteroids, generally at a dose of 1 mg/kg/day prednisone for 6 to 8 weeks, followed by a gradual taper. Steroid-sparing immunosuppressive agents such as cyclophosphamide, azathioprine, and methotrexate are frequently used. For some patients whose disease is rapidly progressive, either oral steroids or pulse methylprednisolone combined with monthly intravenous cyclophosphamide has been reported to show a favorable response [76].
One report described the use of tacrolimus in two myositis patients with progressive ILD who had failed cyclophosphamide and high-dose corticosteroid treatment [77]. Th ese patients showed signifi cant improvement in symptoms and radiologic abnormalities. In another report, 12 out of 15 PM/DM patients treated with tacrolimus for up to 36 months showed signifi cant improvement in all pulmonary parameters [78].
Rituximab has also been used in the treatment of myositis and anti-synthetase syndromes. In a retro spec tive case series, rituximab appeared to stabilize ILD in seven out of 11 patients during the fi rst 6 months after treatment [79]. In addition, in a study of 49 patients with DM/PM, 75% showed a good response in myositis features after treatment with rituximab [80].
Th e lymphocytic interstitial pneumonia pattern was previously suggested to be the most characteristic histopathology in Sjögren's syndrome, but recent studies show that the NSIP pattern is more prevalent [81,82]. Lymphocytic interstitial pneumonia represents a benign polyclonal proliferation of mature B cells or T cells that can involve the lung either diff usely or focally. Lymphocytic interstitial pneumonia is also considered relatively respon sive to steroid therapy [81].
Th e optimal treatment for patients with Sjögren's syndrome-associated ILD is not known. Anecdotal reports and small case series suggest the disease is steroid responsive. While the majority of patients experienced rapid subjective improvement, pulmonary function tests and radiological abnormalities showed a slower response over several months [82]. Some patients require additional immunosuppressive agents such as azathio prine or cyclophosphamide.

Systemic lupus erythematosus
Pulmonary involvement is frequent in SLE, and can aff ect the pleura, pulmonary vasculature, and parenchyma. Th e prevalence and severity of ILD appears to be lower in SLE than in the other CTDs. Acute lupus pneumonitis is an uncommon manifestation of SLE [83]. Th e disease typically presents with acute dyspnea, cough, fever, and pleuritic pain, and occasionally with pulmonary hemorrhage. Diff use ILD or chronic pneumonitis in SLE occurs in 3 to 8% of patients [83].
Th e treatment for SLE-associated ILD is to some extent dictated by the predominant lung pathology. In patients with acute lupus pneumonitis, the mainstay of treatment is oral prednisone (1 mg/kg/day). If there is no prompt improvement, then intravenous methylprednisolone with an immunosuppressive agent such as cyclophosphamide is commonly used. One report described a patient with acute lupus pneumonitis who responded to weekly rituximab with a rapid improvement in subjective symptoms and pulmonary function test abnormalities [84].

Undiff erentiated connective tissue disease
Patients with undiff erentiated CTD often have some features of a rheumatic disease but do not have suffi cient fi ndings for a discrete rheumatic diagnosis [85]. Th ese patients may have a concomitant ILD that either precedes or occurs concomitantly with their rheumatic symptoms. In a case-control study evaluating 28 patients with idiopathic interstitial pneumonia, 88% of patients classifi ed with an initial histologic pattern of idiopathic NSIP had features of an undiff erentiated CTD [86]. In addition, patients with undiff erentiated CTD had a substantial improvement in FVC during a follow-up period of 8 months compared with IPF patients [87]. Treatment of undiff erentiated CTD-ILD is similar to other CTD-ILDs with an NSIP pattern.

Mixed connective tissue disease
Pulmonary involvement is a common complication of mixed CTD. Up to two-thirds of patients have a reduced diff using capacity for carbon monoxide, and approximately one-half have evidence of restrictive abnormalities on pulmonary function tests [88]. Th e predominant radiologic abnormality in the chest is ground glass opacities associated with septal thickening with a lower lobe predominance [89]. Th ese fi ndings are similar to those seen in SSc-associated ILD. Treatment of ILD in mixed CTD is similar to that of other CTD-ILDs. In one study, 47% of patients with mixed CTD-ILD responded to corticosteroids at a dose of 2 mg/kg/day [89].

Conclusion
ILD is now increasingly recognized as a frequent and serious complication of rheumatic diseases and CTDs. Eff ective disease-modifying therapies are still lacking, and many of the currently used treatments are largely ineff ective. Stem cell therapies and novel agents including rituximab, angiotensin II inhibitors, tyrosine kinase inhibitors, PPARγ agonists, intravenous immunoglobulin, and biologicals targeting chemokines, cytokines, and growth factors are in preclinical or clinical studies. Th ere is progress towards better understanding the pathogenesis of CTD-ILD, and the role of growth factors, chemokines, and lipid mediators. Serum biomarkers as either indicators of pulmonary fi brosis or indicators of disease progression are under active investigation. Despite these impressive recent advances, the management of patients with CTD-ILD remains unsatisfactory. Further study into the cell types, mediators, and pathways involved in lung fi brosis is urgently needed. Th ese further studies may lead to a better understanding of lung fi brosis, and to the development of safer and more eff ective rational therapies.