We demonstrated that the serum levels of IL-35 and IL-37 were significantly decreased in the COPD patients with severe emphysema subtype compared to non-emphysema or mild-to-moderate emphysema subtype. The serum IL-35 levels in COPD patients were correlated with BMI, mMRC score, and the concentrations of IL-33 and IL-37. The lower IL-35 group exhibited decreased BMI, lower FEV1/FVC, severe extent of emphysema and worse mMRC. Moreover, multivariable regression showed that the serum IL-35 level, but not IL-37 level, was independently associated with the severity of emphysema.
Emphysema and chronic bronchitis are the most important conditions that compose COPD and several studies have indicated the different clinical characteristics between these two subtypes of COPD[2, 19]. Castaldi et al’s study investigated 3427 COPDGene participants and discovered significant variability in characteristics and disease progression between emphysema-dominant subtype and non-emphysema subtype. They found lower FEV1, more acute exacerbations and higher mortality risk in emphysema-dominant subtype[20]. Lin et a’ study also showed that emphysema-predominant COPD had a greater 5-year mortality and a worse annual decline in lung function than airway obstruction-predominant COPD[21]. In addition, they showed the gender differences between two groups that more male patients were observed in the emphysema-dominant subtype[20, 21], which was consistent with our findings. Our results showed that lower BMI, worse mMRC scale, lower lung function parameters including FEV1 and FEV1/FVC in severe emphysema group compared to non-emphysema and mild-to-moderate emphysema group. Multivariable regression analysis further indicated that lower FEV1/FVC and increased mMRC score were independently associated with severe emphysema. These results were in accordance with the previous studies, indicating severe emphysema discriminate a more advanced stage of COPD, associated with worse quality of life, more severe airway obstruction and worse prognosis.
Chronic and persistent inflammation can produce gradual changes in the small airways that lead to structural damage in the wall, which was considered to play critical role in the progression of emphysema in COPD[4, 5, 22]. The activated inflammatory cells including neutrophils, macrophages, and T lymphocytes produce pro-inflammatory cytokines, reactive oxygen species, and tissue degrading proteases, which further expand the inflammation and cause tissue damage[23]. Wilgus et al’s study discovered that panlobular emphysema was associated with greater extent of emphysema and elevated systemic inflammation compared to centrilobular emphysema[24]. Several studies have revealed several pro-inflammatory cytokines including IL-1β and IL-6 was associated with the severity of emphysema[25, 26], while Th17/IL-17 may be particularly important in the pathogenesis of emphysema[22, 27]. In our study, we demonstrated that the serum levels of anti-inflammatory cytokines IL-35 and IL-37 were decreased in severe emphysema subtype. Several recent studies suggested the effect of IL-35 and IL-37 in the pathogenesis of asthma. They reported that the plasma levels of IL-35 and IL-37 decreased significantly in stable or exacerbated asthmatic patients than in healthy controls.[28–31] Abushouk et al’s study on asthmatic children patients showed that the expressions of IL-35 and IL-37 were negatively correlated with IL-4, IL-17 and IgE levels[32]. Mechanism researches on the asthmatic mice model revealed that administration of IL-35 could reduce the counts of inflammatory cells and the levels of IL-4, IL-13 and IL-17[33], while elevated the numbers of Treg cells to inhibited allergic inflammation[29]. Besides, the administration of IL-37 could also downregulate allergic airway inflammation by attenuating pro-inflammatory mediators’ expressions induced by IL-1β, IL-33 and IL-24[34, 35]. Although there are several studies supporting the significant anti-inflammation effect of IL-35 and Il-37 in the progression of asthma, the studies of their effects during the expanding inflammation response in COPD were limited. Liu et al’s study found that cigarette smoke (CS) and house dust mite (HDM) exposure suppressed IL-35 level in vivo and in vitro[36]. Wu et al’s study elucidated that intranasally with IL-37-expressing lentivirus significantly alleviated CS-induced inflammatory cell recruitment, alveolar septum enlargement and alveolar wall attenuation[14]. Chen et al’s study showed that plasma IL-35 decreased in COPD patients compared to the healthy counterparts[13]. Jiang et al reported decreased serum IL-35 levels in COPD patients which were associated with mMRC score and GOLD grade[37]. In our study, we reported more pronounced decrease of serum IL-35 and IL-37 level in the severe emphysema subtype of COPD compared to non-emphysema and mild-to moderate emphysema, which might be another explanation for the robust inflammation in severe emphysema.
We found the association of IL-35 level with the BMI, FEV1/FVC and mMRC score. In the lower IL-35 group, patients presented with significant lower BMI, decreased FEV1/FVC, upregulated mMRC score and increased extent of emphysema. BMI has been demonstrated as a useful indicator to predict the prognosis of COPD patients. Several studies have reported that BMI was correlated with lower survival, more exacerbations and worse pulmonary function[38, 39]. There were also reports demonstrated that the combination of BMI, airflow obstruction and dyspnea evaluated by mMRC score was a multidimensional index of the severity of COPD[40]. Therefore, according to our results, lower IL-35 level might indicate patients in a more advanced stage of COPD with worse prognosis and worse quality of life. Multivariable regression analysis further indicated that serum IL-35 level, but not IL-37 level, was independently associated with the severity of emphysema. We considered that IL-35 might be a more important anti-inflammatory cytokine during the progression of COPD than IL-37.
IL-33, a member of IL-1 famliy, has been found to upregulate in COPD and to correlate with an increased risk of exacerbations and lung function impairment.[41] Previous studies showed that exposure to CS can induce a chain of systemic responses not only IL-33 production in epithelial cells but also in peripheral blood mononuclear cells, which induces persistent activation of immune system favoring COPD progression[42]. However, in our study, we didn’t find the association of IL-33 level with the extent of emphysema. A weak correlation was observed between the serum expression of IL-33 and IL-35. Several previous studies have observed the counterbalance of IL-33 and IL-35 in the inflammatory diseases. Shamji et al observed that IL-35 inhibited IL-5 and IL-13 production by ILC2s in the presence of IL-33[43]. Nie et al’s study on the nasal epithelial-derived proinflammatory cytokines showed that IL-35 inhibited the production of IL-33 from nasal epithelial cells[44]. Liu et al’s study on the CS and HDM-exposure mice model considered the interplay of IL-33 and IL-35 in the development of lung inflammation[36]. Based on the weak positive correlation between IL-33 and IL-35, we considered there might be a more complex cytokines’ network including IL-33 and IL-35 in the chronic inflammation progression in COPD which need further investigation.
There were some limitations in this study. First, the number of patients enrolled with COPD was relatively small. However, all enrolled patients met inclusion criteria and provided integrated clinical data. Second, we haven’t measured the concentrations of IL-35 and IL-37 in lung tissues or sputum samples. Also, our assessment was performed in patients with stable COPD and we did not evaluate serum IL-35 and IL-37 levels in exacerbated conditions of COPD. These questions could be investigated in future to make more profound understanding of complex networks of anti-inflammatory cytokines in the progression of chronic inflammation in COPD.
In conclusion, we found that the serum IL-35 and IL-37 were decreased significantly in severe emphysema subtype. IL-35, but not IL-37, was correlated with BMI and mMRC score. Lower IL-35 concentrations, lower BMI and worse mMRC score were independently associated with severe emphysema subtype.