Although innate immune cells are typically present inside tumors, they often have an inactive phenotype such that they are ineffective at killing the cancer cells or even promote tumor growth. Sarkar et al. discovered that it may be possible to reprogram these cells to a more active type using niacin (vitamin B3). The authors showed that niacin-exposed monocytes can inhibit the growth of brain tumor–initiating cells. Moreover, niacin treatment of intracranial mouse models of glioblastoma increased monocyte and macrophage infiltration into the tumors, stimulated antitumor immune responses,...…Although innate immune cells are typically present inside tumors, they often have an inactive phenotype such that they are ineffective at killing the cancer cells or even promote tumor growth. Sarkar et al. discovered that it may be possible to reprogram these cells to a more active type using niacin (vitamin B3). The authors showed that niacin-exposed monocytes can inhibit the growth of brain tumor–initiating cells. Moreover, niacin treatment of intracranial mouse models of glioblastoma increased monocyte and macrophage infiltration into the tumors, stimulated antitumor immune responses,...WW…