Tweaking lung inflammation in COPD: the “mirky” ways of miRNAs

The complexity of a living organism, especially in the context of a multifactorial disease such as chronic obstructive pulmonary disease (COPD), is not driven by gene number but gene regulation. MicroRNA (miRNA) has risen to be one of the most intricate and complex gene expression regulators, not only by inducing direct messenger RNA (mRNA) degradation and translation inhibition but also by participating in methylation switches and tackling key transcription factors (1). miRNAs are small strings of RNA, 20–25 nucleotides, usually encoded in introns and exons of host genes, with some miRNA families encoded as clusters (2). miRNAs have been involved in a plethora of processes and responses, both in normal tissue development and in disease. In the last few years, several miRNAs have been identified as culprits in cigarette smoke (CS)-induced COPD.

Roffel et al. (3) performed an elegant evaluation of the expression and function of miR-223-3p in COPD. To describe the pattern of miR-223-3p expression in humans, they used two independent screening and validation cohorts of patients with COPD and controls, rendering an unbiased profile of miR-223-3p expression. To confirm miR-223-3p expression pattern and run further correlation analyses, a third separate COPD group from the GLUCOLD (Groningen and Leiden Universities Corticosteroids in Obstructive Lung Disease) study was used. The expression of miR-223-3p was found to be increased in lung tissue from patients with COPD when compared with controls, and higher pulmonary levels of miR-223-3p were associated with lower lung function and increased neutrophilic inflammation in large airway samples.

To further describe the role of miR-223-3p in COPD development, the authors exposed wild-type (WT) mice acutely and subacutely to CS, reporting miR-223-3p-increased expression in both lung tissue and bronchoalveolar lavage (BAL) upon CS exposure. An increased number of neutrophils and monocytes was found in the lungs from CS-exposed miR-223-deficient mice after acute CS exposure. Interestingly, these effects on neutrophils waned after 4 wk of CS exposure, whereas the numbers of dendritic cells, macrophages, and CD8 T cells increased. The authors suggest that miR-223 might be a negative regulator of innate inflammation upon acute CS exposure and that subchronic CS exposure might lead to exhausted myelopoiesis and hematopoiesis in miR-223-deficient mice. However, the mechanisms underlying this biphasic response to CS are yet to be clarified. Given the observed time-dependent changes in miR-223 expression upon CS exposure, it would be worth investigating whether chronic (4–6 mo) CS exposure in miR-223-deficient mice can further shift the immune responses toward an adaptive profile, or lead to an immune exhaustion. A chronic CS-exposed murine model would be particularly informative, as chronic CS exposure causes oxidative stress and apoptosis of alveolar epithelial cells that have been linked both to miRNA activities and to lung emphysematous changes.

Hence, another question that arises from these findings is whether the activities of other miRNAs associated with COPD immunopathogenesis (4, 5) are somehow overlapping or complementary to miR-223’s activities. The levels of another key miRNA, miR-24-3p, have been recently correlated with radiographic emphysema in multiple COPD cohorts. Also, the inhibition of miR-24-3p increased susceptibility to alveolar type 2 (ATII, that have stem cell potential) epithelial cell apoptosis, hinting at the fact that miRNAs might be crucial not only in immune regulation but also in lung structural cell homeostasis (5). Roffel et al. (3) performed ex vivo experiments using COPD-derived airway epithelial cells (AECs) to test whether miR-223 regulates proinflammatory responses by suppressing the production of neutrophil attractants by AECs. Overexpression of miR-223-3p in AECs from patients with COPD, stimulated with CS extract, resulted in suppression of the neutrophil chemoattractants CXCL8 and GM-CSF, which may further explain the increase in neutrophil inflammation in the miR-223-deficient mice upon CS stimulation. These data suggest that COPD AECs have lost the potential to control neutrophilic recruitment, which could explain the association between high miR-223 levels and low lung function observed in the human COPD cohorts published in this and previous studies (6, 7). The authors do not investigate miR-223 expression in small airways. Given the central role of small airways in the pathogenesis of COPD, it would be very informative to investigate the effect of lack or overexpression of miR-223 in the small airways from CS-exposed humans and mice. Transcriptomic analyses of the small airways might reveal surprising pathways through which lung epithelial-derived miRNAs orchestrate immune responses in the context of CS-induced lung inflammation.

Roffel et al. (3) also show that the overexpression of miR-223-3p significantly upregulated tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) expression in COPD-derived AECs. This finding, although it has not been emphasized by the authors, provides a concrete clue about the involvement of miR-223 in COPD pathogenesis and a link between lung immune and structural cell regulation operated by miR-223. In fact, the TRAF6 signaling pathways in the epithelial tissues play a pivotal role in IL-17-mediated inflammation. IL-17 amplifies tissue immune activity by promoting, partially through TRAF6, the production of inflammatory mediators, including neutrophil recruitment (8). Furthermore, the miR-223-induced TRAF6 upregulation might chronically activate NF-κB, which, in the lung, leads to lung inflammation, whereas in the bone marrow, it could potentially lead to a skewed myeloid differentiation, with diminished monocytic lineage and blunted overall hematopoietic self-renewal (9). Thus, the miR-223-induced upregulation of TRAF6 in AECs might represent a potential mechanism by which epithelial cells trigger both innate (neutrophils) and adaptive (T cells) immune activation, leading to chronic inflammation, airways obstruction, and emphysema. These novel findings trigger more questions. Are the effects of mRNAs, and miR-223 in particular, limited to innate and T cell immunity? What about the B cell and humoral immune responses? B cells have been recently found to be key culprits in COPD pathogenesis, playing a crucial role in COPD progression and CS-induced emphysematous changes mediated by (self)-reactive antibody production (10). Thus, it is feasible that miRNAs act as a modulator of B cell responses as well, serving as the tip of the balance between T cell-dependent and T cell-independent immune responses upon CS stimulation. These hypotheses are yet to be investigated.

Overall, this study elegantly demonstrates the complicated interaction that occurs in expression control systems like miRNA, and offers new knowledge on the interactions between the lung structural and immune compartments in response to CS exposure and in COPD. Although epigenetics has been recently capturing the center stage of lung biology, the yet unexplored field of miRNAs has the potential to connect such epigenetic changes to the tissue and immune-related COPD pathobiology. We are ready to enjoy the journey this will take us on.

GRANTS

This work was supported in part by National Heart, Lung, and Blood Institute Grant No. HL149744 (to F.P).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.R.-Q. drafted manuscript; J.R.-Q. and F.P. edited and revised manuscript; F.P. approved final version of manuscript.