The role of cell surface receptors in the activation of human B cells by phosphorothioate oligonucleotides

Phosphorothioate oligodeoxynucleotides (sODN) containing the CpG motif or T CG repeats induce T cell-independent polyclonal activation of human B cells . To elucidate the mechanism of this response, the role of cell surface rec eptors was investigated. Sepharose beads coated with stimulatory but not no nstimulatory sODNs induced B cell proliferation comparably with soluble sOD Ns. The B cell stimulatory activity of Sepharose-bound sODN did not result from free sODN released from the beads since media incubated with coated be ads were inactive. Using FITC-labeled sODNs as probes, binding to human B c ells could be detected by flow cytometry, Binding was rapid, saturable, ini tially temperature independent, but with a rapid off-rate. Competition stud ies indicated that both stimulatory sODNs and minimally stimulatory sODNs b ound to the same receptor. By contrast, phosphodiester oligonucleotides wit h the same nucleotide sequence as sODNs and bacterial DNA inhibited the bin ding of sODNs to B cells minimally, Charge appeared to contribute to the bi nding of sODNs to B cells since binding of sODNs was competitively inhibite d by negatively charged molecules, including fucoidan, poly I, and polyviny l sulfate, These data indicate that human B cells bind sODNs by a receptor- mediated mechanism that is necessary but not sufficient for polyclonal acti vation.