Recent findings suggest that symptoms of hay fever are linked to unusual composition of the nasal microbiome, particularly elevated levels of the bacteria Streptococcus salivarius. Could the cause of this chronic disorder be buried in the nasal passages, hiding amongst the commensal population?

Hay fever, also known as allergic rhinitis, is a chronic disorder characterised by an inflammatory response to allergens in the nasal passageways. Essentially, the immune system overacts to allergens which typically wouldn’t garner a response. Common allergens include pollen, mould, dust and pets. Responses to these may cause symptoms of itchiness, sneezing and excess mucus production, which can severely disrupt sleep and impact upon general quality of life.

Following detection of the allergens, the immune system embarks upon a two-phase response, via Ig-E mediated pathways. Initially mast cells are stimulated, which release inflammatory cytokines and recruit other immune cells. Following this, a range of immune cells infiltrate the nasal epithelium and launch a full- scale inflammatory armada.   

10-25% of the world’s population are affected by this pestilent plague!

Causes of the disease are likely to be both genetic and environmental. For example, twin studies have indicated that family history is a significant risk factor, whilst the hygiene hypothesis has been highlighted as an environmental cause. The hygiene hypothesis suggests that a lack of exposure to antigens during early life alters the composition of an infant’s microbiome, making it more susceptible to allergy in future life.

Through previous study on the disease, researchers have attempted to elucidate whether there is a link between commensal bacteria in the nose and hay fever. The microbiome defines a dynamic population of helpful microorganisms which have colonised our bodies. Rather unnervingly, humans can play host to over 100 trillion of these microorganisms, in immense diversity and complexity. Scientists are only beginning to appreciate the crucial role that these microorganisms play across a range of functions including immunity, development, and nutrition. Typically, they coexist and interact with the host peacefully, however disruption can result in pathogenic attacks. Dysfunction or dysbiosis in the microbiome has even been linked to autoimmune diseases such as inflammatory bowel disease and rheumatoid arthritis.

Streptococcus salivarius is one of these commensals, which typically benefits humans via its bacteriocin production; essentially fighting against its own kind to protect the host from infection. However, researchers have shown that S. salivarius can turn on its host and exacerbate symptoms of hay fever.

The paper used ribosomal RNA amplicon sequencing to compare the nasal microbiome of hay fever sufferers against healthy controls. It was found that hay fever sufferers have much less diversity among their nasal microbiota, and that one species, S. salivarius, was seventeen times more prevalent. Thus, the researchers postulate a causal link between the bacteria and hay fever symptoms.

Investigating further, in vitro experiments showed that S. salivarius promoted the inflammatory response in epithelial cells. Furthermore, the bacteria adhere to these cells especially well during an allergic response, due to mucins providing a suitable binding site. This close contact facilities pro-inflammatory factors within the bacteria reaching receptors on the epithelial cells, allowing for enhancement of the allergic reaction.

However, this finding is not all doom and gloom, as the authors optimistically highlight that this binding may prove an apt target for hay fever drugs. Interference with the binding could prevent aggravation of epithelial cells by S. salivarius and reduce the inflammatory response to allergens.

Such a discovery could prove groundbreaking for hay fever sufferers, as existing treatments tend to have moderate efficacy and bring with them significant side effects. For example, antihistamines can only improve nasal airflow in the short term and often induce drowsiness. Immunotherapy has been explored to build up a tolerance to a specific allergen, however this requires a long-term course of injections.

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