Lung–gut cross‐talk: evidence, mechanisms and implications for the mucosal inflammatory diseases

The mucosal immune system (including airway, intestinal, oral and cervical epithelium) is an integrated network of tissues, cells and effector molecules that protect the host from environmental insults and infections at mucous membrane surfaces. Dysregulation of immunity at mucosal surfaces is thought to be responsible for the alarming global increase in mucosal inflammatory diseases such as those affecting the gastrointestinal (Crohn's disease, ulcerative colitis and irritable bowel syndrome) and respiratory (asthma, allergy and chronic obstructive pulmonary disorder) system. Although immune regulation has been well-studied in isolated mucosal sites, the extent of the immune interaction between anatomically distant mucosal sites has been mostly circumstantial and the focus of much debate. With novel technology and more precise tools to examine histological and functional changes in tissues, today there is increased appreciation of the 'common mucosal immunological system' originally proposed by Bienenstock nearly 40 years ago. Evidence is amounting which shows that stimulation of one mucosal compartment can directly and significantly impact distant mucosal site, however the mechanisms are unknown. Today, we are only beginning to understand the complexity of relationships and communications that exist between different mucosal compartments. A holistic approach to studying the mucosal immune system as an integrated global organ is imperative for future advances in understanding mucosal immunology and for future treatment of chronic diseases. In this review, we particularly focus on the latest evidence and the mechanisms operational in driving the lung-gut cross-talk.