Serum analysis of tryptophan catabolism pathway: correlation with Crohn's disease activity

Abstract

Background: Indoleamine 2,3 dioxygenase-1 (IDO1) is a tryptophan catabolizing enzyme with immunotolerance-promoting functions. We sought to determine if increased gut expression of IDO1 in Crohn's disease (CD) would result in detectable changes in serum levels of tryptophan and the initial IDO1 pathway catabolite, kynurenine.

Methods: Individuals were prospectively enrolled through the Washington University Digestive Diseases Research Center. The Montreal Classification was used for disease phenotyping. Disease severity was categorized by the Physician's Global Assessment. Serum tryptophan and kynurenine were measured by high-pressure liquid chromatography. IDO1 immunohistochemical staining was performed on formalin-fixed tissue blocks.

Results: In all, 25 CD patients and 11 controls were enrolled. Eight CD patients had serum collected at two different timepoints and levels of disease activity compared. Strong IDO1 expression exists in both the lamina propria and epithelium during active CD compared to controls. Suppressed serum tryptophan levels and an elevated kynurenine/tryptophan (K/T) ratio were found in individuals with active CD as compared to those in remission or the control population. K/T ratios correlated positively with disease activity as well as with C-reactive protein and erythrocyte sedimentation rate. In the subgroup of CD patients with two serum measurements, tryptophan levels were elevated while kynurenine levels and the K/T ratio lowered as the disease activity lessened.

Conclusions: IDO1 expression in CD is associated with lower serum tryptophan and an elevated K/T ratio. These levels may serve as a reasonable objective marker of gut mucosal immune activation and as a surrogate for CD activity.

Figure 1

Immunohistochemical staining for IDO1 in human biopsy samples. Selected representative images are shown from healthy controls [A–C] and active Crohn’s disease [D–F]. Staining from control populations showed no significant IDO1 staining in the epithelium (A, 400×) and only faint staining (arrowheads) in lamina propria mononuclear cells of the colon (B, 630×) and small intestine (C, 630×). This was in contrast to the strong staining present in biopsies from Crohn’s disease patients in the colon (D, 400× and E, 630×) and small intestine (F, 630×). Isotype control of inflamed small intestine (G, 400×).

Figure 2

Serum IDO substrate/metabolite stratified by Global Physician Assessment of Disease Activity. P-values across patient groups were calculated by 1-way ANOVA. Table shows P-value for K/T ratio difference between groups calculated by unpaired Student’s T-test.

Figure 3

Individuals’ serum levels of IDO1 substrate/metabolite change with Crohn’s disease severity. P-value calculated by paired Student’s T-test.

Figure 4

Kynurenine/Tryptophan ratio correlates with inflammation biomarkers in Crohn’s disease. Statistical comparison performed by linear regression analysis.