Regulation of macrophage anti-pathogen defenses by L-arginine

Similar to how we need certain nutrients to maintain a healthy diet and lifestyle, cells of our immune system require nutrients for important cellular and molecular functions. Our lab has studied how the nutrient L-arginine (an amino acid) contributes to macrophage-mediated host defense. L-arginine, like many other amino acids, can combine to form proteins in our cells and tissues. In macrophages, however, L-arginine has an additional role in serving as a precursor for microbicidal agents. We are actively studying the consequences of L-arginine availability and synthesis on the antimicrobial activity of macrophages. Macrophage antimicrobial function that relies on L-arginine is partially dependent on the production of nitric oxide (NO), a toxic free radical that is damaging to microbial pathogens (and host tissue, in high concentrations). Still, we have recently found that only ~50% of the antimicrobial macrophage activity against Mycobacterium tuberculosis that is dependent on L-arginine requires NO. Our lab is actively uncovering the NO-dependent mechanism(s) for L-arginine-mediated antimicrobial function in mouse and human macrophages.

Exploring L-citrulline metabolism in macrophages and anti-tuberculosis host defense

Cells can acquire L-arginine in one of two ways: importing from the extracellular milieu, or synthesizing from a related amino acid, L-citrulline. Our lab focuses on the latter, and have engineered genetic-based approaches for addressing the necessity of L-arginine synthesis in cell culture and mouse models. With these models, we have found that mice lacking either argininosuccinate synthase (Ass1) or argininosuccinate lyase (Asl) in their myeloid cells (including macrophages) are more susceptible to virulent Mycobacterium tuberculosis. From this, we have been actively addressing 1) how L-arginine, L-citrulline, and related amino acids fluctuate during mycobacterial infection, 2) how the enzymes that synthesize and utilize L-citrulline are regulated in macrophages following infection, and 3) if L-citrulline metabolism can be therapeutically targeted to assist in host defense against M. tuberculosis infection.

L-arginine synthesis from L-citrulline and the anti-mycobacterial T cell response

Similar to macrophages, L-arginine is necessary for certain T cell functions. These include, but are not limited to, cell division and cytokine production. L-citrulline rescues T cell function in L-arginine deplete microenvironments – a phenomenon dependent on L-arginine biosynthesis from L-citrulline. In this project, we have observed T cells deficient in this process have a defect in tissue accumulation following live mycobacterial BCG vaccine administration. We propose L-citrulline metabolism is essential for – and might be targeted to enhance – vaccine-mediated host defenses against tuberculosis. We are poised to 1) demonstrate how L-arginine synthesis provides a competitive advantage in establishing anti-mycobacterial T cell responses, 2) examine “peripheral” cell L-citrulline metabolism, and how this impacts T cell-mediated immunity to mycobacteria, and 3) determine the necessity of L-arginine synthesis, and its therapeutic potential, to enhance protection during tuberculosis challenge following BCG vaccination strategies.