Restoring antigen presentation for efficient and durable activation of tumor-specific CD8+ T cell responses is pivotal to immunotherapy, yet the mechanisms that cause subversion of dendritic cells (DC) functions are not entirely understood, limiting the development of targeted approaches. In this study, we show that bone fide DC resident in lung tumor tissues or DC exposed to factors derived from whole lung tumors become refractory to endosomal and cytosolic sensor stimulation and fail to secrete IL-12 and IFN-I. Tumor conditioned DC exhibited downregulation of the SNARE VAMP3, a regulator of endosomes trafficking we found to be required for cross-presentation of tumor antigens and DC-mediated tumor rejection. Dissection of cell-extrinsic suppressive pathways identified lactic acid in the tumor microenvironment as sufficient to inhibit type-I interferon downstream of TLR3 and STING. DC conditioning by lactate also impacted adaptive function, accelerating antigen degradation and impairing cross-presentation. Importantly, DC conditioned by lactate failed to prime anti-tumor responses in vivo. These findings provide a new mechanistic viewpoint to the concept of DC suppression and hold potential for future therapeutic approaches.