ARG1 and neoplasm: It is widely accepted that macrophages have two distinct activation states, namely the classical pro-inflammatory M1 state (with markers iNOS and CD86) and the anti-inflammatory M2 state (with markers CD206 and Arg1).21,22 M2 tumor-associated macrophages (TAMs) have been proven to play a crucial role in tumor progression and metastasis.23,24 Moreover, Watabe et al. demonstrated that M2 microglia suppress local immunity and promote breast cancer cell colonization in the brain.25 These findings suggest that microglia may be potential therapeutic targets for NSCLC-BM.