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2016 ; 169
(4
): 203-15
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Tryptophan Metabolism in Allergic Disorders
#MMPMID27161289
Gostner JM
; Becker K
; Kofler H
; Strasser B
; Fuchs D
Int Arch Allergy Immunol
2016[]; 169
(4
): 203-15
PMID27161289
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Allergic diseases such as asthma and rhinitis, as well the early phase of atopic
dermatitis, are characterized by a Th2-skewed immune environment. Th2-type
cytokines are upregulated in allergic inflammation, whereas there is
downregulation of the Th1-type immune response and related cytokines, such as
interferon-x03B3; (IFN-x03B3;). The latter is a strong inducer of indoleamine
2,3-dioxygenase-1 (IDO-1), which degrades the essential amino acid tryptophan, as
part of an antiproliferative strategy of immunocompetent cells to halt the growth
of infected and malignant cells, and also of T cells - an immunoregulatory
intervention to avoid overactivation of the immune system. Raised serum
tryptophan concentrations have been reported in patients with pollen allergy
compared to healthy blood donors. Moreover, higher baseline tryptophan
concentrations have been associated with a poor response to specific
immunotherapy. It has been shown that the increase in tryptophan concentrations
in patients with pollen allergy only exists outside the pollen season, and not
during the season. Interestingly, there is only a minor alteration of the
kynurenine to tryptophan ratio (Kyn/Trp, an index of tryptophan breakdown). The
reason for the higher tryptophan concentrations in patients with pollen allergy
outside the season remains a matter of discussion. To this regard, the specific
interaction of nitric oxide (NO?) with the tryptophan-degrading enzyme IDO-1
could be important, because an enhanced formation of NO? has been reported in
patients with asthma and allergic rhinitis. Importantly, NO? suppresses the
activity of the heme enzyme IDO-1, which could explain the higher tryptophan
levels. Thus, inhibitors of inducible NO? synthase should be reconsidered as
candidates for antiallergic therapy out of season that may abrogate the arrest of
IDO-1 by decreasing the production of NO?. Considering its association with the
pathophysiology of atopic disease, tryptophan metabolism may play a relevant role
in the pathophysiology of allergic disorders.
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