The development of better vaccines against cancer or intracellular pathogens is a major challenge of immunological research. Novel approaches are needed to induce efficient, long-lasting memory CD8 T cell (CTL) responses. The CD8α subset of mouse dendritic cells (mDC) is specialized in this function, excelling at antigen cross-presentation. Their targeting in vivo for vaccination yielded encouraging results. How to translate this strategy for human health was not obvious, because no human DC (hDC) subset equivalent to CD8α mDC had been discovered. Investigating the relationships between mDC and hDC subsets was hampered by the use of different markers for their identification. To overcome this problem, we used comparative genomics to unravel potential equivalences between mDC and hDC subsets based on the similarities of their gene expression programs. This led us to demonstrate that BDCA3 hDC are professional cross-presenting DC equivalent to CD8α mDC. However, we still do not know what the extent of the physiological functions of different DC subsets is, and we largely ignore how they are regulated. We propose a major and innovative effort to investigate in parallel in mouse and human the biology of two DC subsets thought to be important for antiviral and antitumoral defence: plasmacytoid DC and professional cross-presenting DC. We designed a Systems Biology approach to uncover key functions and regulatory pathways conserved in these cells. We will investigate DC subset functions and their regulation in vivo during infectious challenges, by using state-of-the-art technology to generate and analyse novel high throughput data and innovative mutant mice. We will directly translate to human the knowledge obtained in the mouse, through comparative genomics and in vitro experiments on hDC. This approach is the first of this kind, at the forefront of creating new knowledge to understand the pivotal role of DC subsets in immunity and to manipulate them for promoting health.