Vaccines that prime Wilms' tumor 1 (WT1)‐specific CD8⁺ T cells are attractive cancer immunotherapies. However, immunogenicity and clinical response rates may be enhanced by delivering WT1 to CD141⁺ dendritic cells (DCs). The C‐type lectin‐like receptor CLEC9A is expressed exclusively by CD141⁺ DCs and regulates CD8⁺ T‐cell responses. We developed a new vaccine comprising a human anti‐CLEC9A antibody fused to WT1 and investigated its capacity to target human CD141⁺ DCs and activate naïve and memory WT1‐specific CD8⁺ T cells.

WT1 was genetically fused to antibodies specific for human CLEC9A, DEC‐205 or β‐galactosidase (untargeted control). Activation of WT1‐specific CD8⁺ T‐cell lines following cross‐presentation by CD141⁺ DCs was quantified by IFNγ ELISPOT. Humanised mice reconstituted with human immune cell subsets, including a repertoire of naïve WT1‐specific CD8⁺ T cells, were used to investigate naïve WT1‐specific CD8⁺ T‐cell priming.

The CLEC9A‐WT1 vaccine promoted cross‐presentation of WT1 epitopes to CD8⁺ T cells and mediated priming of naïve CD8⁺ T cells more effectively than the DEC‐205‐WT1 and untargeted control‐WT1 vaccines.

Delivery of WT1 to CD141⁺ DCs via CLEC9A stimulates CD8⁺ T cells more potently than either untargeted delivery or widespread delivery to all Ag‐presenting cells via DEC‐205, suggesting that cross‐presentation by CD141⁺ DCs is sufficient for effective CD8⁺ T‐cell priming in humans. The CLEC9A‐WT1 vaccine is a promising candidate immunotherapy for malignancies that express WT1.