Direct Reprogramming of Mouse and Human Fibroblasts into Conventional Dendritic Cells Type 1

Cell fate reprogramming of adult cells towards pluripotency or unrelated somatic cell-types has been explored in the context of regenerative medicine. Dendritic cells (DCs) are professional antigen presenting cells (APCs) specialized in the recognition, processing and presentation of antigens to T-cells, inducing adaptive immunity. In particular, the mouse conventional DCs type 1 (cDC1) subset or DC1 human equivalent excel on the ability to perform antigen cross-presentation, a critical step for inducing cytotoxic responses. We hypothesized that the unique properties of cDC1s could be induced in unrelated cell-types, allowing the direct control of immune responses with cell reprogramming. Here, the requirements to induce cDC1s were investigated using combinatorial overexpression of Transcription Factors (TFs) in Clec9a-tdTomato mouse fibroblasts. In the hematopoietic system, Clec9a specifically marks the DC lineage, including all conventional dendritic cells type 1 (cDC1). We have identified PU.1, IRF8 and BATF3 (PIB) as sufficient and necessary to induce Clec9a reporter activation, establish DC morphology and activate a cDC1 transcriptional program in mouse fibroblasts. The over- expression of PIB ignites the expression of DC markers including CD103, XCR1, MHC-I, MHC-II and co-stimulatory molecules. Functionally, Induced DCs (iDCs) secrete inflammatory cytokines and engulf, process, present and cross-present antigens to CD4+ and CD8+ T cells, respectively. Additionally, we have demonstrated that combined expression of PIB factors induces DC1 reprogramming in human fibroblasts. Human iDC1s acquire DC morphology, express DC1 markers, including Clec9a, CD141 and the co-stimulatory molecules CD40, CD80 and CD86, and acquire a DC1 transcriptional signature at the single cell level. Interestingly, DC1 reprogramming efficiency can be enhanced 70-fold by supplementing culture media with inflammatory cytokines, suggesting a regulatory role of inflammation during DC1 reprogramming. Hence, we provide evidence that antigen presentation and cross-presentation can be dynamically programmed by a small combination of TFs. These findings provide insights into cDC1 specification and a platform for developing cancer immunotherapies based on cell reprogramming.