NPPA and major depressive disorder: While IDO is induced during active disease states in RRMS (Kwidzinski et al., 2005; Mancuso et al., 2015), there is also evidence of rising KP activity in new mothers with PPD, showing a higher KYN/TRP ratio in those with escalated self-reported anxiety and depression in the puerperium (Maes et al., 2002).