Mice reconstituted with human haematopoietic stem cells are valuable models to study aspects of the human immune system in vivo. We describe a model in which fully functional human CD141⁺ and CD1c⁺ conventional and CD123⁺ plasmacytoid dendritic cells (DC) develop from human cord blood CD34⁺ HSC in immunodeficient mice. CD141⁺ DC are the human equivalents of murine CD8⁺/CD103⁺ DC which are essential for the induction of tumour-inhibitory cytotoxic T lymphocyte responses, making them attractive targets for the development of new cancer immunotherapies. We used CD34⁺-engrafted NSG-A2 mice to investigate activation of DC subsets by synthetic dsRNA analogues polyinosinic-polycytidylic acid/poly I:C and Resiquimod/R848, agonists for TLR3 and TLR 7/8 respectively, both of which are expressed by CD141⁺ DC. Injection of humanised mice with these agonists resulted in upregulation of costimulatory molecules CD80, CD83 and CD86 by CD141⁺ and CD1c⁺ DC alike, and their combination further enhanced expression of these molecules by both subsets. When combined, poly I:C and R848 synergistically enhanced serum levels of key cytokines associated with the anti-tumour immune response: type I, II and III IFNs, IL-12, proinflammatory cytokines TNF-α, IL1-β, IL-6, and CXCR3-binding chemokines known to be involved in the recruitment of T lymphocytes to the tumour microenvironment. These data advocate a combination of PIC and R848 TLR agonists as an optimal effective adjuvant strategy for DC-targeting immunotherapies.