CpG‐DNA aided cross‐presentation of soluble antigens by dendritic cells

For cross‐presentation immature dendritic cells (DC) require enhanced antigen (Ag) uptake and a maturation signal to prime for MHC class I‐restricted CTL responses in vivo. While immunostimulatory CpG‐DNA provides, via TLR9, the maturation signal, CpG‐DNA linked to Ag augments cellular Ag uptake. In this study we show that CpG‐DNA ovalbumin (OVA) conjugates trigger in vivo peptide‐specific CTL responses at tenfold lower Ag doses compared to a mixture of CpG‐DNA plus OVA. We provide evidence that CpG‐DNA‐OVA conjugates shift OVA uptake by immature DC from the presumably inefficient fluid phase pinocytosis to efficient DNA receptor‐mediated endocytosis. Since the DNA‐binding receptor mediating endocytosis lacks any sequence specificity, cellular uptake of OVA conjugated with either stimulatory or non‐stimulatory oligonucleotides (ODN) is equally enhanced. As a consequence cross‐linking of OVA with either stimulatory or non‐stimulatory DNA yields, via enhanced OVA uptake, efficient generation and presentation of the dominant OVA‐CTL epitope SIINFEKL. However, only stimulatory CpG‐ODN cross‐linked to OVA provide the DC maturation signal required to trigger robust primary CTL responses towards the cross‐presented MHC class I complexed T cell epitope SIINFEKL. Our studies show that stimulatory CpG‐ODN linked to Ag fulfill a dual role: enhancement of Ag uptake yielding efficient Ag cross‐presentation by DC and in addition, their activation into professional DC.