Differential Requirements for l -Citrulline and l -Arginine during Antimycobacterial Macrophage Activity

Abstract

Microbicidal NO production is reliant on inducible NO synthase–mediated l-arginine metabolism in macrophages (MΦs). However, l-arginine supply can be restricted by arginase activity, resulting in inefficient NO output and inhibition of antimicrobial MΦ function. MΦs circumvent this by converting l-citrulline to l-arginine, thereby resupplying substrate for NO production. In this article, we define the metabolic signature of mycobacteria-infected murine MΦs supplied l-arginine, l-citrulline, or both amino acids. Using liquid chromatography–tandem mass spectrometry, we determined that l-arginine synthesized from l-citrulline was less effective as a substrate for arginase-mediated l-ornithine production compared with l-arginine directly imported from the extracellular milieu. Following Mycobacterium bovis bacillus Calmette–Guérin infection and costimulation with IFN-γ, we observed that MΦ arginase activity did not inhibit production of NO derived from l-citrulline, contrary to NO inhibition witnessed when MΦs were cultured in l-arginine. Furthermore, we found that arginase-expressing MΦs preferred l-citrulline over l-arginine for the promotion of antimycobacterial activity. We expect that defining the consequences of l-citrulline metabolism in MΦs will provide novel approaches for enhancing immunity, especially in the context of mycobacterial disease.