Antibody-mediated targeting of antigen to C-type lectin-like receptors Clec9A and Clec12A elicits different vaccination outcomes

Targeting antigen (Ag) to dendritic cell (DC) surface receptors is a potential new mode of vaccination. C-type lectin-like receptors Clec9A and Clec12A are attractive receptor targets however their targeting in vivo elicits significantly different outcomes for unknown reasons. To gain insight into the mechanisms responsible, we have examined the intrinsic capacity of Clec9A and Clec12A to elicit MHC I and MHC II Ag presentation following ex vivo targeting with primary murine DC. Both receptors exhibited high rates of internalization by CD8⁺ DCs, while Clec12A delivered a significantly higher Ag owing to its higher expression level. Targeting Ag to immature CD8⁺ DCs via both Clec9A and Clec12A failed to elicit MHC I cross-presentation above that of controls, while Clec12A was the superior receptor to target following CD8⁺ DC maturation. CD8⁻ DCs were unable to elicit MHC I cross-presentation regardless of the receptor targeted. For MHC II presentation, targeting Ag to Clec12A enabled significant responses by both immature CD8⁺ and CD8⁻ DCs, whereas Clec9A did not elicit significant MHC II Ag presentation by either DC subset, resting or mature. Therefore, Clec9A and Clec12A exhibit different intrinsic capacities to elicit MHC I and MHC II presentation following direct Ag targeting, though they can only elicit MHC I responses if the DC expressing the receptor is equipped with the capacity to cross-present. Our conclusions have consequences for the exploitation of these receptors for vaccination purposes, in addition to providing insight into their roles as Ag targets in vivo.