Activation of human CD141 ⁺ and CD1c ⁺ dendritic cells in vivo with combined TLR3 and TLR7/8 ligation

Mice reconstituted with human hematopoietic stem cells are valuable models to study aspects of the human immune system in vivo. We describe a humanized mouse model (hu mice) in which fully functional human CD141 ⁺ and CD1c ⁺ myeloid and CD123 ⁺ plasmacytoid dendritic cells (DC) develop from human cord blood CD34 ⁺ cells in immunodeficient mice. CD141 ⁺ DC are the human equivalents of murine CD8 ⁺ /CD103 ⁺ DC which are essential for the induction of tumor-inhibitory cytotoxic T lymphocyte responses, making them attractive targets to exploit for the development of new cancer immunotherapies. We used CD34 ⁺ -engrafted NSG-A2 mice to investigate activation of DC subsets by synthetic dsRNA or ssRNA analogs polyinosinic-polycytidylic acid/poly I:C and Resiquimod/R848, agonists for TLR3 and TLR8, respectively, both of which are expressed by CD141 ⁺ DC. Injection of hu 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 enhanced serum levels of key cytokines associated with cross-presentation and the induction of cytotoxic T lymphocyte responses including IFN-α, IFN-β, IL-12 and CXCL10. These data advocate a combination of poly I:C and R848 TLR agonists as means of activating human DC for immunotherapy.