Tryptophan metabolism promotes immune evasion in human pancreatic β cells

BACKGROUND: To resist the autoimmune attack characteristic of type 1 diabetes, insulin producing pancreatic β cells need to evade T-cell recognition. Such escape mechanisms may be conferred by low HLA class I (HLA-I) expression and upregulation of immune inhibitory molecules such as Programmed cell Death Ligand 1 (PD-L1). METHODS: The expression of PD-L1, HLA-I and CXCL10 was evaluated in the human β cell line, ECN90, and in primary human and mouse pancreatic islets. Most genes were determined by real-time RT-PCR, flow cytometry and Western blot. Activator and inhibitor of the AKT signaling were used to modulate PD-L1 induction. Key results were validated by monitoring activity of CD8+ Jurkat T cells presenting β cell specific T-cell receptor and transduced with reporter genes in contact culture with the human β cell line, ECN90. FINDINGS: In this study, we identify tryptophan (TRP) as an agonist of PD-L1 induction through the AKT signaling pathway. TRP also synergistically enhanced PD-L1 expression on β cells exposed to interferon-γ. Conversely, interferon-γ-mediated induction of HLA-I and CXCL10 genes was down-regulated upon TRP treatment. Finally, TRP and its derivatives inhibited the activation of islet-reactive CD8+ T cells by β cells. INTERPRETATION: Collectively, our findings indicate that TRP could induce immune tolerance to β cells by promoting their immune evasion through HLA-I downregulation and PD-L1 upregulation. FUNDING: 10.13039/501100003092Dutch Diabetes Research Foundation, 10.13039/501100011570DON Foundation, the Laboratoire d’Excellence consortium Revive (ANR-10-LABX-0073), 10.13039/501100001665Agence Nationale de la Recherche (ANR-19-CE15-0014-01), 10.13039/501100002915Fondation pour la Recherche Médicale (EQ U201903007793–EQU20193007831), 10.13039/501100010767Innovative Medicines InitiativeINNODIA and INNODIA HARVEST, Aides aux Jeunes Diabetiques (AJD) and Juvenile Diabetes Research Foundation Ltd (JDRF).