Using polychromatic reporter mice to decipher dendritic cell origin and heterogeneity

Dendritic cells (DCs) are sentinel immune cells endowed with the unique capacity to initiate antigen specific immunity and tolerance. Owning their unique attributes, dendritic cells have long been considered as the ideal candidates for the induction of an effective anti-tumour response. Yet their exploitation in clinical settings has been hindered by the limited knowledge on their origin and the intrinsic mechanism driving their functional specialisation. This research proposal exploits the development of unique genetic and molecular tools to decipher the molecular mechanisms underpinning dendritic cell diversity. The generation of polychromatic reporter mice enables to finely map the dynamic regulation of transcription factors in rare progenitors and mature DCs. These unprecedented genetic tools offer the unique opportunity to delineate DC restricted bone marrow progenitors based on the expression of lineage defining transcription factors rather than cell surface markers, and therefore have the potential to answer some of the current controversies regarding the origin of dendritic cells. This project will use high throughput immune approaches to define the molecular mechanisms underpinning DC diversity and functions, with the goal to uncover novel approaches to alter DC fate to tailor DC-based therapeutic approaches. Background and Aims Dendritic cells (DCs) are a heterogeneous group of sentinel immune cells. DCs patrol all tissue and capture exogenous antigens that are processed and presented via either the major histocompatibility complex class II (MHC-II) to CD4+ T cells (direct presentation) or shuttled through a specialised pathway to MHC-I to engage CD8+ T cells (cross-presentation) (. In absence of “danger” signal the lack of costimulatory molecules presented by dendritic cells will result in the production of a tolerogenic response. In contrast upon the recognition of danger (e.g pathogen, malignant tissue), DCs can fully mature resulting in the presentation of the foreign antigen to the T cells and the provision of additional costimulatory molecules to drive T cell mediated protective immunity. Given the multitude of immune challenges, DCs have evolved into phenotypically and transcriptionally distinct subsets. Broadly DCs can be separated into two types of conventional dendritic cells (cDC1s and cDC2s) and plasmacytoid dendritic cells (pDCs). cDCs specialize in presenting antigens to T cells eliciting a productive immune response, with the ability to licence T cell leading to their proliferation and differentiation into effector T cells. In contrast pDCs lack antigen presentation capacity but play a critical role in the early response to viral infection through the production of Type I interferon (IFN-I), inducing antiviral response in non-immune cells and activating antiviral pathways. While all DCs originate from the haematopoietic stem cells (HSCs) in the bone marrow, the recent advent of single cell technologies has revealed certain degree of linage restriction in cells that were previously described as pluripotent. In line with the above, an unexpected degree of diversity has been brought to light in DC progenitors that were considered as homogeneous based on the expression of some shared cell surface markers used to define them. These studies highlight that the molecular mechanisms underpinning progenitor state and lineage choice remains poorly defined and thus warrants further investigation. Derivation of DC subsets from progenitors has been shown to be dependent on the interplay of a complex network of transcription factors. Capitalising on the development of unique mouse models allowing to trace the expression of these transcription factors at a single cell level (dual reporters) and enabling their conditional or temporal deletion, this study will decipher the mechanisms through which these transcription factors control pDC differentiation and function.