Significantly, the maintenance of a clinically unresponsive state following aeroallergen exposure in atopic individuals has been associated with increased IDO activity and IL-10 production. Overall it appears that the ability of certain microbes to promote IDO activity, in addition to IL-10 expression by DCs, may be important in the generation of a regulatory immune response and the establishment of tolerance.
The increase in antibiotic resistance and need for new and improved strategies to tackle infectious disease have led to an examination of the therapeutic potential of commensal induced modulation of the mucosal immune response. Consequently, it has been discovered that certain LAB do have protective effects against bacterial and viral infections in the GI and respiratory systems. Administration of probiotics has been associated with lower incidence of ventilator-associated pneumonia, reduced respiratory infections in healthy and hospitalized children and reduced duration of common cold infection. It should be noted that in addition to causing morbidity and mortality directly there is good evidence that respiratory infections, particularly viral infections, are a contributing factor not only to the exacerbation of asthma, but also to development of the disease Indeed, it has been suggested that the focus of potential beneficial effects of probiotics in asthma should be directed at identifying organisms capable of reducing viral infections in early life.
In mouse studies, intranasal administration of LAB protects against respiratory pathogens. However, direct exposure of the probiotic organism to the airway mucosa is not required, and LAB can protect host animals from airway infection through an interaction with GALT, such as Peyer patch cells, and indirect enhancement of respiratory immunity (Fig 1). The protective effects of both intranasal and oral probiotics are generally associated with upregulation of natural killer (NK) cell and/or macrophage activity in the airway mucosa.