IDO2 and tuberculosis: Tryptophan is a key amino acid required for protein biosynthesis, and a precursor for the synthesis of a diversity of other metabolites (Davis et al., 2019); the most reported on, in the case of TB (Campbell et al., 2014; Manyelo et al., 2019), is the kynurenine pathway, via the M. tb infection-induced pro-inflammatory cytokines IL-6, TNF-α, and IFN-γ, upregulating indoleamine 2,3-dioxygenase (IDO) (Campbell et al., 2014).