Up-regulation of indoleamine 2,3-dioxygenase 1 (IDO1) expression and catalytic activity is associated with immunosuppression and poor prognosis in penile squamous cell carcinoma patients

Abstract

Background: Indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan (Trp) catabolism have been demonstrated to play an important role in tumor immunosuppression. This study examined the expression and catalytic activity of IDO1 in penile squamous cell carcinoma (PSCC) and explored their clinical significance.

Methods: IDO1 expression level, serum concentrations of Trp and kynurenine (Kyn) were examined in 114 PSCC patients by immunohistonchemistry and solid-phase extraction-liquid chromatography-tandem mass spectrometry. The survival was analyzed using Kaplan-Meier method and the log-rank test. Hazard ratio of death was analyzed via univariate and multivariate Cox regression. Immune cell types were defined by principal component analysis. The correlativity was assessed by Pearson's correlation analysis.

Results: The expression level of IDO1 in PSCC cells was positively correlated with serum Kyn concentration and Kyn/Trp radio (KTR; both P < 0.001) but negatively correlated with serum Trp concentration (P = 0.001). Additionally, IDO1 up-regulation in cancer cells and the increase of serum KTR were significantly associated with advanced N stage (both P < 0.001) and high pathologic grade (P = 0.008 and 0.032, respectively). High expression level of IDO1 in cancer cells and serum KTR were associated with short disease-specific survival (both P < 0.001). However, besides N stage (hazard radio [HR], 6.926; 95% confidence interval [CI], 2.458-19.068; P < 0.001) and pathologic grade (HR, 2.194; 95% CI, 1.021-4.529; P = 0.038), only serum KTR (HR, 2.780; 95% CI, 1.066-7.215; P = 0.036) was an independent predictor for PSCC prognosis. IDO1 expression was positively correlated with the expression of interferon-γ (IFNγ, P < 0.001) and immunosuppressive markers (programmed cell death protein 1, cytotoxic T-lymphocyte-associated protein 4 and programmed death-ligand 1 and 2; all P < 0.05), and the infiltration of immune cells (including cytotoxic T lymphocytes, regulatory T lymphocytes, tumor-associated macrophages, and myeloid-derived suppressor cells; all P < 0.001) in PSCC tissues. Furthermore, the expression of IDO1 was induced by IFNγ in a dose-dependent manner in PSCC cells.

Conclusions: IFNγ-induced IDO1 plays a crucial role in immunoediting and immunosuppression in PSCC. Additionally, serum KTR, an indicator of IDO1 catabolic activity, can be utilized as an independent prognostic factor for PSCC.

Keywords: cytotoxic T-lymphocyte-associated protein 4; immunosuppression; indoleamine 2,3-dioxygenase 1; interferon-gamma; kynurenine/tryptophan ratio; penile cancer; programmed cell death protein 1; programmed death-ligand 1; tumor-infiltrating immune cells.

Figure 1

Kaplan‐Meier DSS curves stratified by different variables in 114 patients with PSCC. DSS curves stratified by IDO1 H‐score in cancer cells (A), IDO1⁺ TIIC density (B), CD8+ TIL density (C), serum Trp concentration (D), Kyn concentration (E) and KTR (F). DSS was different among subgroups stratified by all variables (all P < 0.05) except IDO1⁺ TIIC density (P = 0.196). Abbreviations: DSS, disease‐special survival; IDO1, Indoleamine 2,3‐dioxygenase 1; PSCC, penile squamous cell carcinoma; Kyn, kynurenine; Trp, tryptophan; TIIC, tumor‐infiltrating immune cell; KTR, Kyn/Trp ratio

Figure 2

IDO1 expression is associated with immune‐related gene expression and immune cell infiltrates in PSCC. Pearson's correlation analysis between the mRNA levels of IDO1 and the mRNA levels of immune‐related gene PD‐1, CTLA‐4, PD‐L1, and PD‐L2 (A) and the infiltration of CTL, Treg, TAM and MDSC (B), defined by a principal component analysis of the corresponding signature marker genes in PSCC. Abbreviations: IDO1, Indoleamine 2,3‐dioxygenase 1; PSCC, penile squamous cell carcinoma; PD‐1, programmed cell death protein‐1; CTLA‐4, cytotoxic T lymphocyte‐associated protein 4; PD‐L1, programmed death‐ligand 1; PD‐L2, programmed death‐ligand 2; CTL, cytotoxic T lymphocyte; Treg, regulatory T lymphocyte; TAM, tumor‐associated macrophage; MDSC, myeloid‐derived suppressor cell

Figure 3

IFNγ induce IDO1 expression in PSCC cells lines. (A) Pearson's correlation analysis between the mRNA levels of IDO1 and IFNγ. (B) Western blotting analysis of the intrinsic IDO1 expression in PSCC cell lines (H596 cells were served as positive control). (C) The inducible IDO1 expression in Penl1 and 149RCa cells treated with IFNγ for 48 hours. Abbreviations: IDO1, Indoleamine 2,3‐dioxygenase 1; PSCC, penile squamous cell carcinoma; IFNγ, interferon‐gamma