There are various hypotheses that explain the rise in asthma and allergies. From home moderations (increased insulation leads to more exposure to indoor allergens) [1], to rising obesity [2], to the ‘hygiene hypothesis’ [3](the theory that an increasingly sanitary world has not exposed children to enough germs to allow their immune systems to tell the difference between what is harmful and harmless, as shown by the number of people with allergies that grew up on farms than those that did not), and many more hypotheses of varying levels of evidence, it is clear that complex physiological factors are causing these increasingly prevalent diseases [4]. Among recent focus, has been the microbiome.
Although most associate “microbiome” with the gut, the skin, urinary tract and other areas of the body have important functions and impact on health, including the lungs. The gut-lung microbiome axis has just recently been getting attention, and studies have shown that what was previously thought to be a sterile environment, the lung microbiome is actually dynamic and plays an important role in diseases of the respiratory tract [5]. The lung microbiome is comprised of different species than the lower gastrointestinal microbiome. In fact, it most closely resembles that of the throat [6]. In numerous recent studies, it has become clear that a dysbiosis (specifically a microbial community shift from Bacteriodetes to Proteobacteria), is associated with chronic lung disease [7]. Various causes have been linked to this phenomenon (age, nutrition, lifestyle, pollution are among some of them [8]), along with presence of pro-inflammatory cytokines regulated by the immune system [7].
On a cellular level, asthmatic sufferers have been shown to have markedly less gut and lung microbiome diversity [10] and higher IgE concentrations, the immune cell associated with allergies [11]. The gut microbiome, which has been studied extensively over the last few decades, has profound effects on immune system inflammatory regulation, and an underdeveloped microbiome has proven a risk factor in the early development of allergies and asthma [12].
While there have not been clinical studies directly aimed at altering the lung microbiome in asthma and allergies, clinical attempts to improve allergies and asthma with probiotics directed to improve gut microbiome health have been evaluated with mixed results [13,14]. Logically, it makes sense that a probiotic containing species proven for healthy gut flora (those in Actinobacteria and Firmicutes phylums), may not have optimal results if given for the purposes of a healthy lung environment (which is healthiest when Bacteroidetes dominant). Many avenues are open for medicinal aims at the lung microbiome.
This being said, speculatively, considering the ability of the gut microbiome to down-regulate the inflammatory response, it can be considered a good adjunct therapeutic approach in the treatment of asthma to improve gut flora. It will be interesting to see where new developments in treatment of the gut-lung axis proceeds and what improved clinical approaches to asthma and allergies may result.