Probiotics and Mucosal Immunity
Commensal bacteria in the gastrointestinal tract play an integral role in both innate and humoral immunity. It is well established that this protective role can be maintained or modulated by the ingestion of probiotics. More recently, it has been shown that specific probiotic strains can influence the secretion of cytokines to help direct naïve helper T cells towards either a Th1 dominant, cell-mediated immune response or towards a Th2 dominant, humoral immune response. This paper will review current knowledge of the Th1: Th2: Th3/ Treg model of humoral immunity as well as introduce how strain-specific probiotics can be used therapeutically to help balance this immune response and therefore, help prevent and treat disease.
It is now well established that commensal intestinal microflora influence the physiology and pathology of the host. Moreover, it has been shown that the microbiological content of the gastrointestinal tract can be altered by the oral administration of health promoting microorganisms, namely probiotics. As a result, there is increased interest in the role of probiotics in maintaining optimal human health by preventing and treating disease. Probiotics are defined as living microorganisms that, upon ingestion in sufficient numbers, exert health benefits beyond basic nutrition.(1) Probiotic bacteria, like certain Lactobacillus and Bifidobacterium species, influence human health in a number of different ways.
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One of the most accepted and researched effects of supplementation with probiotics is immune system modulation. Probiotics influence several components of an immune response including humoral, cellular or innate immunity. A specific component of the humoral immune response involves the secretion of specific cytokines to influence naïve T-helper (Th) cells to become either Th1 or Th2 dominant and therefore, promote cellular or humoral immunity, respectively. Probiotics can help influence the release of specific cytokines by unique T cell subsets and therefore, play a fundamental role in mucosal immunity. In a healthy immune system, there is a balance between Th1 and Th2 cell activity. This balance is transiently shifted when necessary to support cell-mediated immunity or humoral immunity but usually is rebalanced quickly once the specific immune challenge is removed. However, if a cytokine imbalance persists, a specific T-cell pathway is maintained and immunopathological diseases like atopy, hypersensitivity reactions and chronic inflammation can occur. A recent hypothesis, termed the hygiene hypothesis, postulates that the increased prevalence of allergy in industrialized nations is due to inadequate microbial stimulation and therefore, development of a robust Th-1 response during childhood. This poorly developed Th-1 response occurs because of excessive hygiene, intensive food sterilization and modification of commensal gut flora in newborns from persistent feeding with artificial formulas. Without adequate feedback from Th-1 cell dependent cytokines, Th-2 cell dependent cytokines, like IL-4, IL-5, IL-9 and IL-13, predominate to promote the development of the allergic response.
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It recently has been observed that specific probiotic strains stimulate the secretion of specific cytokines and, therefore, facilitate the development of naïve T-cells towards a particular immune pathway. This paper, which is part of the proceedings of a symposium, will summarize the Th1/Th2 model of the immune system, the cytokines that regulate T cell development and the effects of specific probiotic strains on the modulation of mucosal immunity. An enhanced understanding of how specific strains of bacteria influence the immune system can be useful in selecting probiotics that can help correct the imbalances postulated by the hygiene hypothesis. Restoring the balance between Th-1 and Th-2 cells would help prevent and treat many diseases.