ABSTRACT Tumor-associated macrophages (TAMs) are the most prevalent immune cell in the tumor microenvironment. TAMs adopt an M2-like phenotype that supports angiogenesis, attenuates anti-cancer immune responses, and facilitates metastatic dissemination. Studies in humans and experimental animal models support targeting TAMs for anti-cancer therapy. However, the environmental conditions triggering M2 polarization and molecular mechanisms mediating this process are poorly understood. This knowledge is required to target TAMs therapeutically and to identify patients that would benefit from such therapies. One potential pathway for TAM polarization is via metabolic reprogramming. Previous studies showed that glycolysis supports a pro-inflammatory, anti-tumor M1 phenotype in macrophages, while mitochondrial respiration is required for the M2 phenotype. Our recently published work challenged this paradigm. We showed that treating macrophages with LPS or bacteria (M1 stimuli) induce lactate production which in turn, promotes a late phase switch to an M2-like phenotype. The mechanism underlying this surprising observation involves a novel lactate-induced epigenetic modification (H3 lysine lactylation (Kla)) at promoters of genes associated with the M2-phenotype that directly promotes transcription. Hypoxic conditions, such as those found in tumors, also induce lactate production by macrophages. In preliminary work, we show that hypoxia induces expression of M2-like macrophage genes, and these genes are marked by Kla at their promoters. We further show that TAMs isolated from tumors with high hypoxia have elevated levels of histone Kla and M2-like proteins in comparison to TAMs tumors with low hypoxia. Finally, we show that inhibiting endogenous lactate production by TAMs (via myeloid cell specific deletion of Ldha) attenuates the M2-like phenotype of TAMs, lessens tumor growth, and increases CD8+ effector T cells in tumors with high hypoxia, but not in tumors with low hypoxia. Based on these studies, we hypothesize that a lactate-Kla pathway induces the M2-like phenotype of TAMs during hypoxia and promotes tumor growth by suppressing adaptive immunity. To test this hypothesis, we propose three aims: (1) determine the contribution of TAM lactate production to its M2-like phenotype during hypoxia, (2) dissect mechanisms by which lactate production by TAMs promotes tumorigenesis, and (3) determine the contribution of histone lactyltransferases to histone Kla and M2-like phenotype and function of macrophages. By integrating human studies with mechanistic animal, cell-based, epigenetic, and biochemical studies, our proposed work seeks to delineate the mechanisms that promote M2- like TAMs and their effects on tumor development. Delineating these mechanisms may identify potential molecular targets for TAM-based therapeutics that improve anti-tumor immunity and reduce tumorigenesis.