CD4 and infection: Furthermore, the Kyn metabolites of IDO impair phago/lysosome fusion and autophagy, which can kill intracellular Mtb [33,66]; inhibit the function of CD4+ T cells by expanding Tregs and MDSCs; deplete Trp, which is essential for the rapidly proliferating T cells during an infection; and reduce the levels of the key anti-Mtb molecule indole propionic acid (IPA) [67,68].