Arginine sensing is essential for virulence of the human pathogen Leishmania

The intracellular pathogen Leishmania resides and proliferates inside phagolysosomes of the human macrophage. During infection, intracellular Leishmania competes with the host for arginine, creating a situation that endangers parasite survival. We discovered that upon arginine starvation, intracellular Leishmania promptly activates a MAPK2-mediated Arginine Deprivation Response (ADR) pathway, resulting in up-regulation of the Leishmania arginine transporter (AAP3), as well as a small group of other transporters. Moreover, we found that the conserved amidino group at the distal cap of the arginine side chain is the ligand that activates ADR, in both promastigotes and intracellular amastigotes. We hypothesized that ADR-mediated up-regulation of AAP3 activity is necessary to withstand the resultant arginine starvation. A CRISPR/Cas9-mediated disruption of the AAP3 locus yielded mutants that retain a basal level of arginine transport but were not responsive to arginine starvation. These mutants were impaired in their ability to develop inside THP1 macrophages grown under physiological concentrations of arginine (0.1 mM). However, flooding the macrophage growth medium with arginine (1.5 mM) restored parasite infectivity and intracellular growth to that of the wild type. The results indicated that inside the host macrophage, Leishmania must overcome the arginine “Hunger Games” by up-regulating transport of arginine via the ADR. Furthermore, the AAP3 mutants were ~70-80% less virulent in Balb/C mice, showing, for the first time, that the ability to monitor and respond to changes in host metabolite levels is essential for pathogenesis.