AHR and brain neoplasm: Opitz et al., found that TDO2 expressing (i.e. KYN producing) brain tumour xenografts in AhR-proficient mice had an enhanced tumour growth rate, increased levels of the inflammatory cytokines (IL-1β, IL-6 and IL-8) and a decreased number of infiltrating CD8+ cells around tumours with high AhR and TDO2 expression [57].