Metabolites of Tryptophan Catabolism Are Elevated in Sera of Patients with Myelodysplastic Syndromes and Inhibit Hematopoietic Progenitor Amplification

Tryptophan catabolism, which is mediated by the enzymes indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO), produces kynurenine, which blocks T-cell activation and induces immunosuppression. Kynurenine itself is converted by downstream enzymes into secondary catabolites that also have toxic effects on T cells. Tryptophan catabolism is elevated in many cancers, including acute myeloid leukemia (AML). However, tryptophan catabolites that are downstream of kynurenine have never been investigated in hematological malignancies.

We evaluated the serum levels of primary and secondary tryptophan catabolites in a cohort of patients with myelodysplastic syndromes (MDS). Sera were isolated from 132 adult MDS patients after informed consent was obtained in accordance with the Helsinki Declaration. The levels of tryptophan, kynurenine, kynurenic acid, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, and anthranilic acid in the sera were quantified using HPLC. For erythroid cell expansion, CD34+ cells were collected and isolated from the mononuclear cell fractions of cytapheresis products from 3 healthy donors and cultured in liquid medium under erythroid conditions with tryptophan catabolites.

The MDS patients showed significantly lower levels of tryptophan and higher levels of kynurenine, kynurenic acid, 3-hydroxyanthranilic acid, and anthranilic acid compared with the healthy controls. We also compared the kynurenine and tryptophan levels in the MDS patients with our previous cohort of 112 patients with primary AML. The kynurenine/tryptophan ratios were significantly higher in the MDS patients (median 0.0468 vs. 0.0676). The tryptophan catabolites correlated with cytopenia; higher kynurenine levels were associated with lower hemoglobin levels and higher blast counts and were associated with presence of dysgranulopoiesis. Lower tryptophan levels were found in patients with platelet transfusion dependency. Kynurenic acid levels were higher in patients with dysmegakaryopoiesis. High 3-hydroxyanthranilic and kynurenic acid levels were associated with severe thrombopenia below 20 G/L. IPSS score, cytogenetic, and WHO diagnosis did not associated with any tryptophan catabolite level. The tryptophan catabolites inhibited progenitor expansion during the in vitro culture of hematopoietic cells and reduced the numbers of granulocytes and erythroblasts.

Thus, MDS patients are characterized by high tryptophan catabolism resulting in elevated primary and secondary metabolites, which both have inhibitory effects on hematopoiesis. These results suggest that IDO or TDO inhibitors should be investigated in MDS.

No relevant conflicts of interest to declare.