IDO1 and major depressive disorder: A recent review found increased IDO activation (using K:T ratio as a surrogate marker) is associated with age and chronic inflammation in PWH.30 Moreover, in the IDOze study, increased IDO activation (higher K:T ratios) in VS-WWH was associated with worse objective and subjective sleep outcomes.22,23 Tryptophan dysmetabolism and increased IDO activation have also been linked to depression and neurodegenerative disorders.16,24,31,32