Scientists are still exploring exactly how gut bacteria contributes to optimal human health and is involved in many mechanisms including digestion, vitamin synthesis and host defense. A new study by the Institut Pasteur, published in the Science journal, has determined how microbiota acts on the immune system and that the presence of specific microbes blocks the immune cells that contribute to triggering allergies.
The results are based on the hygiene hypothesis which specifies that infectious diseases decline in industrialized countries whereas allergic diseases increase as improvements in hygiene levels lead to a reduction of microbes. Previous epidemiological studies have shown that children living in contact with farm animals develop fewer allergies during their lifetime. Administering antibiotics to mice within the first days of life results in a loss of microbiota, and subsequently, in an increased incidence in allergy.
The study is one of the first to detail the metabolic pathway involved and that several types of immune responses are generated to defend the organism. The presence of bacterial or fungal microbes involves immune cells known as type 3 cells. These immune cells coordinate the phagocytosis and killing of the microbes. However, in the case of infection by pathogenic agents that are too large to be handled by type 3 cells (such as parasitic worms and certain allergens), the cells that organize the elimination of the pathogen, including allergic reactions, are known as type 2 cells.
The research study revealed that type 3 cells activated based on a bacterial attack act directly on type 2 cells and block their activity. Type 2 cells are consequently unable to generate allergic immune responses. Gut microbiota indirectly regulates type 2 immune responses by inducing type 3 cells. An imbalance in the gut microbiota triggers an exaggerated type 2 immune response normally used to fight large parasites leading to an allergic responses.
C. Ohnmacht, J.-H. Park, S. Cording, J. B. Wing, K. Atarashi, Y. Obata, V. Gaboriau-Routhiau, R. Marques, S. Dulauroy, M. Fedoseeva, M. Busslinger, N. Cerf-Bensussan, I. G. Boneca, D. Voehringer, K. Hase, K. Honda, S. Sakaguchi, G. Eberl. The microbiota regulates type 2 immunity through ROR t T cells. Science, 2015; DOI: 10.1126/science.aac4263