TDO2 and viral infectious disease: However, in a recent research highlight from Giovannoni and Quintana [98], the authors, considering recent data, underline: (1) the upregulation of TDO/IDO in viral infection; (2) Kyn-induced AhR activation; (3) its limiting role in the immune response mediated by IFN-I and NF-κB in viral infections comprising Zika virus, influenza A virus and, with high probability, SARS-CoV-2 virus.