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

Microbicidal NO production is reliant on inducible NO synthase-mediated L-arginine metabolism in macrophages (M Phi s). However, L-arginine supply can be restricted by arginase activity, resulting in inefficient NO output and inhibition of antimicrobial M Phi function. M Phi s 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 M Phi s 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-Guerin infection and costimulation with IFN-gamma, we observed that M Phi arginase activity did not inhibit production of NO derived from L-citrulline, contrary to NO inhibition witnessed when M Phi s were cultured in L-arginine. Furthermore, we found that arginase-expressing M Phi s preferred L-citrulline over L-arginine for the promotion of antimycobacterial activity. We expect that defining the consequences of L-citrulline metabolism in M Phi s will provide novel approaches for enhancing immunity, especially in the context of mycobacterial disease.