ARG1 and neoplasm: Arlauckas et al. (2018) found that anti-PD-1 therapy decreases the number of Arg1+ TAM while increasing Arg1− TAM, and Arg1 expression is triggered via interleukin-4 and lactic acid, which are synthesized via tumor cells.51 Arg1 converts L-arginine into urea and L-ornithine, which results in the production of proline and polyamines crucial for cell growth and collagen synthesis in tumors.