Antigen loaded monocytes prime CD4+ T cell responses through Notch2 dependent cDC2

Abstract Cellular vaccines, primarily monocyte-derived dendritic cells (MoDC), have recently been realized as therapeutic cancer vaccines with modest clinical efficacy. Understanding the mechanism by which these vaccines lead to antigen presentation is key to improving their potency. A significant advancement was the discovery that MoDC vaccines do not directly present antigen, but instead rely on endogenous dendritic cells for their antigen presenting cell (APC) function. Subsequent research has also demonstrated that primary monocytes can be loaded with antigen and used as cellular vaccines, outperforming MoDC vaccines in mouse models of melanoma and glioblastoma. In this study, we characterized how monocyte vaccines generate CD4+ T cell responses. We utilized the CD11c-Cre RBPJfl/fl mouse model, which lack a subset of Notch2 dependent cDC2 characterized by expression of Endothelial cell-selective adhesion molecule (ESAMhi). We demonstrate that the ESAMhicDC2 subset is required for effective CD4+ OT-II T cell priming after vaccination with ovalbumin (OVA) loaded monocytes. Interestingly, depletion of the ESAMhi DC population does not affect monocyte vaccine-induced CD8+ T cell priming, a process previously shown to be dependent on cDC1. This indicates that monocyte vaccines generate T cell responses by independently transferring antigen to both ESAMhi and CD8+ DCs, leveraging their APC function to prime CD4+ and CD8+ T cell responses, respectively. This work furthers the mechanistic understanding of monocyte vaccines, and will inform the design of future monocyte vaccines as they enter clinical trials as therapeutic cancer vaccines. Supported by NCI P50 CA 190991 and funding from the Stead Scholarship Program of the Duke University Department of Medicine.