Synthesis and <i>in Vitro</i> Characterization of Antigen-Conjugated Polysaccharide as a CpG DNA Carrier

Oligodeoxynucleotides containing unmethylated CpG sequences (CpG DNAs) are known as an immune adjuvant. CpG DNAs coupled with a particular antigen enabling both CpG DNA and antigen delivery to the same antigen-presenting cell have been shown to be more effective. Based on our previous finding that β-(1→3)-d-glucan schizophyllan (SPG) can be used as a CpG DNA carrier, here we present the synthesis of an antigen-conjugated SPG and the characterization of the conjugate. Ovalbumin (OVA, 43 kDa) was used as a model antigen, and two OVA were conjugated to one SPG molecule (Mw = 150 000), denoted by OVA−SPG. Circular dichroism and gel electrophoresis showed that OVA−SPG could form a complex with a (dA)40-tailed CpG DNA at the 3' end (1668-(dA)40). When OVA−SPG was added to macrophages (J774.A1), the amount of the ingested OVA−SPG was increased compared with that of OVA itself, suggesting that Dectin-1 (proinflammatory nonopsonic receptor for β-glucans) is involved to ingest OVA−SPG. Furthermore, the complex of the conjugate and DNA was co-localized in the same vesicles, implying that OVA (antigen) and CpG DNA (adjuvant) were ingested into the cell at the same time. This paper shows that OVA−SPG can be used as a CpG DNA carrier to induce antigen-specific immune responses.