Pulmonary tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis, a slow growing bacteria without chitin. Over one third of the world's population has latent tuberculosis infection leading to 1.5 million deaths per year. The gold standard of TB diagnosis is demonstration of mycobacteria in tissues or body fluids.32 Necrotizing granuloma characterizes the pathology of active infection. Activated macrophages are involved in granuloma development as are other innate cells, such as natural killer T cells. The cellular immune response, however, is required to control the infection. Antigen-specific T lymphocytes participate with macrophages in forming granulomas which produce cytokines, chemokines, and other soluble immune mediators.33,34 While there is some variability, most studies have demonstrated CHIT1 levels are increased in the serum of TB patients, correlate with disease severity and decline with treatment.35,36 These studies suggest that pulmonary granuloma lead to release of CHIT1 into the serum and that resolution of the inflammation diminish the release. It appears, then, that CHIT1 can be used as a marker of tuberculosis activity, severity, and response to treatment.

Sarcoidosis is a multisystem granulomatous disease of unknown origin. While there has been an intense search for an infectious etiology of this disease, none has been reproducibly found. The pathology is characterized by non-necrotizing ("non-caseating") granulomas that result from accumulation of activated proliferating mononuclear phagocytes and T lymphocytes primarily affecting the lungs.37 Sarcoidosis presents many different clinical forms, and its onset may be acute, subacute, or chronic; it may be asymptomatic and remit spontaneously.38,39 Spontaneous remission is more common in patients with acute sarcoidosis (about 50% of all cases). Chronic sarcoidosis has insidious onset and slow progression with wide variations between individuals and almost invariable lung involvement. In progressive advanced forms with fibrotic lung involvement, there may be severe hypoxemia and pulmonary hypertension.40 In recent years, the unpredictable clinical course of sarcoidosis has prompted several investigators to look for reliable indicators of disease outcome. Biomarkers proposed for clinical evaluation of sarcoidosis severity have included radiological features, bronchoalveolar lavage (BAL) cell profile, CD4/CD8 ratio, neopterin, lysozyme, angiotensin-converting enzyme (ACE), and cytokines/chemokines and their receptors, such as soluble interleukin-2 receptor (sIL-2R).41,42,43,44,45,46,47

The idea of evaluating CHIT1 from serum and BAL as a biomarker of sarcoidosis disease progression arose from evidence of direct involvement of activated macrophages in the pathogenesis of sarcoidosis and granuloma formation.48,49 CHIT1 concentrations are elevated in sarcoid patients when compared to controls, and levels correlate with disease severity and progression.50,51,52 The lowest concentrations of CHIT1 were found in untreated patients in remission, while the highest enzyme concentrations were found in symptomatic patients with persistent disease on steroids and with functional deterioration in the last year. In this symptomatic group, CHIT1 decreased significantly after the increasing of steroid dose or the introduction of a new immunosuppressant therapy (P<0.01).52

Mechanistically, granuloma formation requires activation of both T cells and macrophages.53 A pivotal step in the generation of specific T cell responses in sarcoidosis is the activation of T cells by antigen-presenting cells, including alveolar macrophages. Several groups independently found that alveolar macrophages from patients with sarcoidosis disclosed an increased antigen-presenting capacity compared to controls.54,55,56 Nevertheless, immunopathogenesis of sarcoidosis remains poorly understood. Challenges to understanding disease mechanisms in sarcoidosis include: (1) the fact that a causative agent is unknown; (2) the lack of a universally accepted animal model, and (3) the various manifestations of the disease, as well as the marked heterogeneity of the disease between different individuals and ethnic groups.57 Despite of these limitations, an immunological response triggered by an as-yet unknown environmental antigen(s) in a genetically susceptible individual is a generally accepted concept as a potential mechanism of sarcoidosis.58

Serum CHIT1 levels correlate with sIL-2R concentration,59 a known marker of T-cell activation and sarcoidosis severity.42,60 Sarcoid macrophages secrete high quantities of both CHIT1 and chemokine (C-C motif) ligand 18 (CCL18).61 CCL18 is expressed by a broad range of monocytes/macrophages and dentritic cells, and enhanced CCL18 production has been demonstrated in various malignancies and inflammatory diseases.62 The relative changes in CHIT1 and CCL18 during the course of disease closely mimicked each other. Therapy with steroids resulted in clear reductions, in plasma CHIT1 and CCL18 and relapse of disease activity was preceded by increases in these parameters. The coordinated regulation of CCL18 and CHIT1 is consistent with the hypothesis that they are components of the same immune pathway in this granulomatous lung disease. The establishment of an appropriate animal model of sarcoidosis together with CHIT1-specific targeting will be instrumental to test this possibility. If this is the case, CHIT1 could be a therapeutic target for the intervention of inflammation and/or tissue remodeling associated with sarcoidosis.

ILD is a heterogeneous group of lung disorders with varying clinical courses and pathogenetic mechanisms. Other than sarcoidosis, the most common ILDs include idiopathic pulmonary fibrosis (IPF). IPF is rapidly progressive and typically unresponsive to therapy.63,64 The etiology and pathogenesis of IPF are unknown, although IPF is thought to stem from epithelial lung injury.64,65 The unpredictable clinical course of IPF has motivated research to identify the potential prognostic biomarkers for the disease.66 A study showed that CHIT1 activity was elevated in the BAL of IPF patients compare to controls, but not in serum.51 This result suggests that CHIT1 expression in specific macrophage subsets might be responsible for remodeling and tissue damage seen in the lung from IPF patients. As such, it is conceivable that CHIT1 could be involved in fibrogenesis of other ILD. In systemic sclerosis (SSc), ILD represents the leading cause of death and the incidence of ILD is up to two-third of SSc patients.67 Because of the extremely variable incidence and outcome of ILD in connective tissue disease, there are huge unmet medical needs to develop biomarker(s) for diagnosis, prognosis, patient subtyping, response to treatment, or as surrogate endpoints in clinical trials. A study by Lee et al.11 demonstrated that, in 2 different SSc patient cohorts, CHIT1 bioactivity and protein levels were significantly increased in the circulation and lung of patients compared to demographically matched controls. Compared to SSc patients without lung involvement, patients with ILD had higher levels of circulating CHIT1 activity, and CHIT1 activity was correlated with disease severity. Using animal models of fibrosis and transgenic mice, they further demonstrated that CHIT1 enhances bleomycin- or IL-13-stimulated pulmonary fibrosis. They further identified that CHIT1 enhances TGF-β1 receptor expression and its signaling in fibroblasts, suggesting CHIT1 is a modifier of pathologic tissue fibrosis31 in that TGF-β1 play a significant role. The potential CHIT1 regulation of TGF-β1 pathway associated with tissue phenotypes is schematically illustrated in Figure. These studies, for the first time, identifies CHIT1 as a biomarker of subset of SSc patients with pulmonary fibrosis and also raises the possibility that CHIT1 and/or the interaction between CHIT1 and TGF-β1 are potential therapeutic targets of this disease. In this regard, identification of specific CHIT1 interacting molecule(s) that regulate TGF-β1 signaling would be instrumental for further development of therapeutic strategy on SSc patients with ILD.

COPD is characterized by chronic airway inflammation and emphysematous alveolar destruction. A dominant hallmark of COPD is an abnormal inflammatory response to inhaled particles which have the potential to produce lung injury.68,69,70 The significantly higher levels of chitinase activity in habitual smokers compared to controls have been shown to result from upregulation of CHIT1 gene expression, especially in macrophages.71 Smokers with COPD have increased CHIT1 in BAL, more CHIT1-positive cells in bronchial biopsies, and an elevated proportion of alveolar macrophages expressing CHIT1 when compared to smokers without COPD or never-smokers.29 CHIT1 levels in BAL correlated with airflow obstruction and emphysema. CHIT1 also correlated with Interleukin (IL)-1β, IL-8, TNF-α, and its type II soluble receptor. In this study, alveolar macrophages from smokers with COPD released CHIT1 after TNF-α-stimulation while alveolar macrophages from controls did not. Further, CHIT1 promoted the release of IL-8, monocyte chemoattractant protein-1 (MCP-1), and matrix metalloproteinase (MMP-9). Finally, CHIT1 overexpression in the lung of normal mice promoted macrophage recruitment and the synthesis of the murine homologue of IL-8, keratinocyte-derived cytokine, and MCP-1. This data demonstrates that CHIT1 is a functional component of the inflammatory pathway produced by cigarette smoke.

Whereas the above studies have shown CHIT1 to be a risk factor for COPD, we have published a study demonstrating that, depending on the clinical population, CHIT1 may represent a potential biomarker for resistance to obstructive, COPD-like lung injury.30 In a nested case-control study involving firefighters from the 9/11 World Trade Center disaster who developed particulate matter-induced lung disease, serum CHIT1 was found not only to reduce the risk of airway obstruction as defined by abnormal forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) and FEV1, but also it was associated with recovery of lung function. The mechanism of CHIT1's potentially protective role against particulate matter-induced airway obstruction is unknown and need to be further defined in future studies.

The role of chitin, chitinases or chitinase-like proteins in allergy and asthma have been greatly elucidated in recent studies.72 The expression and activity of AMCase were upregulated in an ovalbumin mouse model of asthma independent of IL-13.73

Furthermore, inhibition of AMCase enzymatic activity prevents OVA allergen-induced airway hyperresponsiveness and inflammation present in these mice after challenge. Using a similar acute mouse asthma model, upregulation of both lung AMCase and Ym1 gene expressions was detected by DNA microarray,74 and concurrent elevated levels of AMCase, Ym1 and Ym2 in the BAL fluid was also observed using 2D gel electrophoresis proteomics.75 Studies have additionally demonstrated an association between chitinase-like proteins, such as YKL-40 and asthma.76

While a plethora of investigations have linked AMCase and chitinase-like protein levels to asthma, numerous negative studies have demonstrated lack of association between specific CHIT1 and reactive airway disease.29,77 One study, however, reported that the CHIT1 gene's rs3831317 24-base pair duplication allele, which results in reduced CHIT1 activity, was prevalent in patients with severe asthmatics with fungal sensitization.78 The importance of CHIT1 duplication in cystic fibrosis patients appears less pronounced.78 However, there is an interesting inverse correlation between CHIT1 duplication and W1282 X CFTR mutation that produce few or no function CFTR chloride channel through premature translation termination. It is speculated that the combination of reduced CHIT1 activity, higher rates of Aspergillus colonization, and the use of immune suppressive drug including corticosteroids, may contribute to the high levels of allergic broncho-pulmonary aspergillosis (ABPA) in cystic fibrosis patients.78 The potential protective effect of CHIT1 in fungal infection in cystic fibrosis needs to be further proved through prospective studies including large number of patients and controls.