Title: Notch signaling facilitates in vitro generation of cross-presenting classical dendritic cells
Description: Dendritic cells (DCs) comprise a heterogeneous population of mononuclear phagocytes that play a critical role in both innate and adaptive immunity. DCs in mice can be divided into two main types. Plasmacytoid DCs (pDCs) secrete type I interferons (IFN-α/β) in response to viruses. Classical or conventional dendritic cells (cDCs) are highly adept at Ag presentation. There are two main subsets of cDCs; the CD11b+ cDC subset presents exogenous Ag to CD4+ T cells on major histocompatibility complex class II (MHCII) and the CD8α+/CD103+ cDCs uniquely capable of cross-presenting exogenous Ag to CD8+ T cells on MHCI. Functional equivalents of both subsets have been identified in humans and have been designated cDC2 and cDC1, respectively. All DCs develop from progenitors found in the bone marrow (BM) by a process directed primarily by the cytokine Fms-like tyrosine kinase 3 ligand (FL). The specification of DC types is driven by several transcription factors such as IRF8, while terminal cDC differentiation is guided by tissue-specific signals mediated through signaling pathways such as Notch and lymphotoxin-β.

Notch is an evolutionarily conserved pathway of cell-cell communication that plays an essential role in the development of immune cell types, including T and B lymphocytes. DC-specific gene targeting, has been used to establish the role of Notch2 receptor signaling in the differentiation of cDC2 subset in the spleen and intestine and splenic cDC1. Because primary cDCs, particularly cDC1, are rare in vivo their study and use in translational applications require methods to generate functional cDC subsets in vitro. Commonly used cultures of primary BM with the cytokines FL or granulocyte-monocyte colony stimulating factor (GM-CSF) produce a mixture of pDC, cDC2 and cDC1-like cells, or cDC2-like cells and macrophages, respectively. Thus, new approaches are needed to yield high numbers of fully differentiated cDCs, particularly of mature cDC1. Given the critical role of Notch signaling in cDC differentiation in vivo, I hypothesized that it would facilitate cDC differentiation in vitro. Indeed, coculture of murine primary BM cells with OP9 stromal cells expressing Notch ligand Delta-like 1 (OP9-DL1) facilitated the generation of bona fide, IRF8-dependent CD8α+ CD103+ Dec205+ cDC1 with an expression profile resembling ex vivo cDC1.

Critically, the resulting cDC1 showed improved Ccr7-dependent migration, superior T cell cross-priming capacity and antitumor vaccination in vivo. Further, OP9-DL1 cocultures of immortalized progenitors allowed for the de novo generation CD8α+ cDC1. This discovery can help further our understanding of the mechanisms of DC differentiation while providing a tool to allow for the generation of unlimited numbers of cDCs for functional studies. Further, as cDC1 are essential for the cross-priming of cytotoxic T cells against tumors, they hold great promise as cellular vaccines. However, the use of DCs in clinical applications has been hampered by inadequate methods to generate large quantities of functionally mature cDC1 in vitro. As such, these findings should help to advance the use of cDCs in translational and therapeutic applications, such as antitumor vaccination and immunotherapy.