Specific immunotherapy normalizes tryptophan concentrations in patients with allergic rhinitis

An immune shift towards Th2-type immunity seems to be critical in the pathogenesis of allergic asthma and rhinitis. In a previous study, we found higher serum tryptophan concentrations in patients with seasonal tree or grass pollen rhinoconjunctivitis who underwent specific immunotherapy (SCIT) than in controls, and those with the highest levels at baseline responded less well to SCIT. In the present study, we examined whether 'booster immunotherapy' after cessation of SCIT had any influence on tryptophan metabolism during follow-up.

Serum concentrations of tryptophan, kynurenine and neopterin were assayed in 19 patients (mean age: 26.2 years; 6 females) allergic to grass and/or tree pollen before and after they had received a booster immunotherapy with 4 injections of an allergoid vaccine (Pollinex Quattro; Bencard Vienna, Austria) over 8 ± 3 months outside the pollen season.

Serum tryptophan and kynurenine concentrations decreased after booster immunotherapy (mean ± SD, before immunotherapy: 81.1 ± 14.2 µmol/l, after immunotherapy: 61.4 ± 20.9 µmol/l and before immunotherapy: 2.25 ± 0.44, after immunotherapy: 1.69 ± 0.70 µmol/l, respectively; both p < 0.01); this was especially true in those responders who also tended to have lower baseline kynurenine concentrations as compared with nonresponders (p = 0.05). Finally, a correlation between changes in tryptophan metabolism and neopterin concentrations was observed after immunotherapy.

The decrease in tryptophan and kynurenine concentrations following booster immunotherapy in hay fever patients strengthens the hypothesis that tryptophan metabolism might be involved in the course of allergic responses. However, it is still unclear whether the abnormal tryptophan metabolism in pollinosis patients is related to indoleamine 2,3-dioxygenase and/or to a specific cytokine background.