Prostate cancer's one-two punch

Tumors can avoid immune destruction despite being invaded by tumor-specific CD8⁺ T cells. On page 1257, Bronte et al. describe one way that prostate tumors subdue T cells—they make two enzymes that combine forces to cripple T cell signaling components.

Tumors use a variety of tactics to avoid attack by cytotoxic CD8⁺ T cells; one is to establish an inhospitable environment that inhibits T cell activation. In recent mouse studies, myeloid suppressor cells infiltrated tumors and set up an inhibitory environment, apparently by making two enzymes—arginase and nitric oxide synthase 2 (NOS2).

Both enzymes can inhibit T cell functions independently. But when expressed together, the enzyme duo triggers a cascade of reactions that culminates in the production of peroxynitrite—an oxidizing agent that nitrates tyrosine residues...