Altered tryptophan metabolism via the kynurenine pathway as a result of increased indoleamine 2,3-dioxygenase 1 activity is a well-established consequence of various inflammatory conditions, including HIV [43–45, 70–72], TB [36, 58, 68, 73, 74] and HIV/TB co-infection [16, 17]. This evidence concerns the gene IDO1 and coinfection.