Dendritic cells (DC) are a heterogeneous family of leucocytes with important functions in immunity. Little is known about the role of distinct DC subtypes in vivo. In the mouse, a subset known as CD8alpha+ DC has been argued to represent a discrete DC lineage with specialised properties. These include a superior capacity for presenting exogenous antigens to CD8+ and CD4+ T cells, which makes CD8alpha+ DC an attractive target in vaccination and tolerisation. However, it is unclear whether CD8alpha+ DC fulfill unique and non-redundant roles in the immune system. In addition, the reported restriction of CD8alpha+ DC to thymus and secondary lymphoid organs is hard to reconcile with their documented capacity to act as presenting cells for antigens outside those organs. Finally, CD8alpha+ DC have not been identified in humans, greatly restricting their use in immunotherapy. In this proposal, we exploit the recent finding that DNGR-1 (CLEC9A) acts as a new and specific marker for the CD8alpha+ lineage to address these issues. We will generate DNGR-1-Cre mice as a universal tool to manipulate gene expression in the subset. We will use such mice to render CD8alpha+ DC sensitive to toxic proteins that permit constitutive or transient ablation of the subset for functional studies. DNGR-1-Cre mice will further be used to express fluorescent proteins in CD8alpha+ DC, allowing tracing of the lineage in vivo, both in lymphoid and non-lymphoid organs. Finally, we will use the DNGR-1 marker to identify and characterise putative CD8alpha+ DC equivalents in humans. The results from this proposal will illuminate the function of CD8alpha+ DC across species and open the door for using this intriguing DC subset in immunotherapy of cancer, infectious and autoimmune diseases.