ARG1 and infection: As IL-4 can drive alternatively activated macrophage differentiation in infection,31 we tested for genes correlated with this phenotype in RNA from infected lungs on day 3 postinfection: arginase 1 (Arg1), an amino acid metabolism (AAM) effector gene, was expressed in infected lungs, and significantly more in isotype than anti-IL-4-treated animals (Fig. 6E).