Prolonged antigen storage endows merocytic dendritic cells with enhanced capacity to prime anti-tumor responses in tumor-bearing mice

Tumor cell vaccination with irradiated autologous tumor cells is a promising approach to activate tumor-specific T cell responses without the need for tumor Ag identification. However, uptake of dying cells by dendritic cells (DCs) is generally a noninflammatory or tolerizing event to prevent the development of autoreactive immune responses. In this study, we describe the mechanisms that confer the potent T cell priming capacity of a recently identified a population of DCs (merocytic DCs [mcDCs]) that potently primes both CD8⁺ and CD4⁺ T cells to cell-associated Ags upon uptake of apoptotic cells. mcDCs acquired cell-associated materials through a process of merocytosis that is defined by the uptake of small particles that are stored in nonacidic compartments for prolonged periods, sustained Ag presentation, and the induction of type I IFN. T cells primed by mcDCs to cell-associated Ags exhibit increased primary expansion, enhanced effector function, and increased memory formation. By using transgenic T cell transfer models and endogenous models, we show that treatment of tumor-bearing mice with mcDCs that have been exposed to dying tumor cells results in tumor suppression and increased host survival through the activation of naive tumor-specific CD8⁺ T cells as well as the reinvigoration of tumor-specific T cells that had been rendered nonresponsive by the tumor in vivo. The potent capacity of mcDCs to prime both CD4⁺ and CD8⁺ T cells to cell-associated Ags under immunosuppressive conditions makes this DC subset an attractive target for tumor therapies as well as interventional strategies for autoimmunity and transplantation.