Introduction
Asthma is one of the common respiratory diseases characterized by persistent or non-resolving allergen-driven inflammation. Toll-like receptors (TLRs), a member of pattern recognition receptors (PRRs), have been suggested to contribute to the initiation and perpetuation of airway inflammation, by virtue of their ability as the host's first line defense to recognize invading pathogens and aeroallergens (1). Within the TLRs family, TLR2 is regarded as the major one responsible for the sustaining airway inflammation and thus be most relevant to the onset of asthma (2–4). However, despite more and more advances in our understanding of the role of TLR2 in allergic asthma, the mechanism underlying TLR2 regulation in allergic airway inflammation is still elusive.
Recently, the other important type of intracellular PRRs, nucleotide-binding oligomerization domain-like receptors (NLRs) has been reported to be strongly associated with inflammatory diseases (5, 6). Particularly, as the best-characterized subtype shown to be expressed in airway, nucleotide-binding domain and leucine-rich repeat protein 3 (NLRP3) inflammasome is considered to be involved in the progress of asthma (7, 8). The NLRP3 inflammasome is a cytosolic protein complex composed of NLRP3, ASC, and pro-caspase-1. The activation of NLRP3 inflammasome proteolytically cleaves pro-caspase-1 into active caspase-1, which in turn promotes maturation of IL-1β and IL-18 (9). And these two NLRP3-associated cytokines are critical in the initiation and amplification of inflammatory process (5). Although NLRP3 inflammasome has been extensively investigated, its role in allergic airway inflammation is still controversial (10–14), and its regulatory networks remain elusive. TLRs and NLRs are two important kinds of PRRs, the formation and activation of NLRP3 inflammasome has been suggested to be initiated by TLRs (8, 15, 16). However, the crosstalk between TLR2 and NLRP3 inflammasome activity in the allergic airway diseases, as well as the underlying mechanism is not clear.
Melatonin (N-acetyl-5-methoxytrytamine), which is synthesized by the pineal gland or extra-pineal tissues (17), has been identified as a powerful immune-modulatory and anti-inflammatory molecule (18). Exogenous administration of melatonin has been reported to pronouncedly ameliorate airway inflammation (19, 20). Similarly, endogenous melatonin, which is produced from tryptophan (Trp) by converting 5-hydroxytryptamine (5-HT) successively by two key enzymes AANAT and ASMT, is also tightly associated with the pathogenesis of asthma (21, 22). It has been reported that endogenous melatonin synthesis is suppressed by activation of TLR9 (22), while other study shows that melatonin is able to inhibit TLRs-mediated inflammation (23), suggesting there may be a feedback loop between TLRs system and endogenous melatonin synthesis. More notably, it has been recently identified that a novel molecular target for melatonin is NLRP3 in murine model of septic response, liver injury and acute lung injury (24–26). However, it is uncertain whether TLR2-melatonin feedback loop exists in allergic airway disease, and regulates NLRP3 inflammasome activity.
To address these questions, we first examined whether TLR2 modulated melatonin biosynthesis and NLRP3 inflammasome activity by using TLR2−/− mice, and further revealed whether this effect of TLR2 was feedback regulated by melatonin by administrating exogenous melatonin or melatonin receptor antagonist luzindole in OVA-induced allergic airway inflammation.