TLR4 and schizophrenia: In schizophrenia, immune activation upregulates pro-inflammatory cytokines (e.g., IFN-γ, IL-1, TNF-α), which activate indoleamine 2,3-dioxygenase (IDO) via both IFN-γ receptor (IFN-γR)-dependent and independent pathways (e.g., synergistic action of TLR4, IL-1R, TNF-αR) (215).