Tryptophan Metabolism in Allergic Disorders

Area of Science:

• Immunology
• Allergy Research
• Tryptophan Metabolism

Background:

• Allergic diseases like asthma and rhinitis feature a Th2-skewed immune response with upregulated Th2 cytokines and downregulated Th1 cytokines, including interferon-gamma (IFN-γ).
• IFN-γ induces indoleamine 2,3-dioxygenase-1 (IDO-1), an enzyme that degrades tryptophan, impacting T cell proliferation and immune regulation.
• Elevated serum tryptophan and a minor change in the kynurenine/tryptophan ratio (Kyn/Trp) have been observed in pollen allergy patients, particularly outside the pollen season.

Purpose of the Study:

• To investigate the reasons for higher tryptophan concentrations in pollen allergy patients outside the pollen season.
• To explore the role of nitric oxide (NO∙) and its interaction with IDO-1 in regulating tryptophan levels in allergic conditions.
• To assess the potential of targeting IDO-1 or NO∙ production for antiallergic therapies.

Main Methods:

• Review of existing literature on allergic inflammation, immune responses, and tryptophan metabolism.
• Analysis of reported associations between tryptophan levels, pollen allergy, and immunotherapy response.
• Consideration of the interplay between nitric oxide (NO∙) and indoleamine 2,3-dioxygenase-1 (IDO-1) activity.

Main Results:

• Increased tryptophan concentrations are found in pollen allergy patients outside the pollen season, not during.
• Nitric oxide (NO∙) has been reported to be elevated in asthma and allergic rhinitis patients.
• NO∙ suppresses the activity of the tryptophan-degrading enzyme IDO-1, potentially explaining higher tryptophan levels.

Conclusions:

• The suppression of IDO-1 by NO∙ may explain elevated tryptophan levels in allergic patients outside the pollen season.
• Inhibitors of inducible NO∙ synthase could be reconsidered for out-of-season antiallergic therapy by reducing NO∙ production and thus IDO-1 inhibition.
• Tryptophan metabolism is a relevant factor in the pathophysiology of allergic disorders and warrants further investigation.