Asymptomatic atopy is associated with increased indoleamine 2,3-dioxygenase activity and interleukin-10 production during seasonal allergen exposure.

Indoleamine 2,3-dioxygenase (IDO) is a tryptophan (TRP)-catabolizing enzyme with regulatory effects on T cells. T cell inhibition is achieved through both TRP depletion and TRP metabolite accumulation in specific local tissue microenvironments. The expression of IDO activity by different types of antigen-presenting cells (APCs) has been shown to play a role in many instances of immunoregulation and tolerance induction. Induction of IDO after the engagement of the high-affinity receptor for IgE, FcepsilonRI, on atopic monocytes has been suggested to regulate T cell responses in atopic disorders. Interleukin-10 (IL-10), a cytokine known for its down-regulatory functions in the immune system, has recently been associated with the stable expression of IDO in mature tolerogenic dendritic cells. This study was devised to understand the role of systemic IDO and IL-10 in the prevention of clinical apparent allergy. The concentration of TRP and the break-down product kynurenine were measured by high-performance liquid chromatography in- and off-season in sera from patients with seasonal allergic rhinitis (n=12) and from clinically asymptomatic atopic patients sensitized to specific aeroallergens (n=12). Non-atopic (NA) individuals (n=12) served as control. The concentration of plasma IL-10 was determined in parallel from these donors by ELISA in- and off-season. In clinically unresponsive but aeroallergen-sensitized atopic individuals significantly higher systemic activity of IDO and increased plasma IL-10 levels were found during allergen exposure but not off-season compared to symptomatic atopic individuals with allergic rhinitis and NA individuals. Enhanced systemic IDO activity as well as increased systemic levels of IL-10 may contribute to the containment of allergic T cell responses and could be involved in the maintenance of a state of clinical unresponsiveness.