CpG oligonucleotides induce acute murine thrombocytopenia dependent on toll-like receptor 9 and spleen tyrosine kinase pathways

Background: CpG oligonucleotides (ODNs) are synthetic single-stranded DNA sequences that act as immunostimulants. They have been increasingly used to treat several cancers; however, thrombocytopenia is a potential recognized side effect of some sequences. Objectives: We tested the ability of 2 CpG ODNs (ODN 2395 and ISIS 120704) to induce thrombocytopenia when administered to BALB/c mice and determined mechanisms associated with thrombocytopenia. Methods: BALB/c mice were prebled and then injected with titrated doses of CpG ODNs, and platelet counts were determined. The mice were treated with intravenous immunoglobulin (IVIg) or various inhibitors and antagonists of toll-like receptor 9 (TLR9) and spleen tyrosine kinase (Syk) to determine their effects on thrombocytopenia. Results: Compared with saline-treated mice or mice treated with 2 '-O-methoxyethyl- modified antisense ODN, both ODN 2395 and ISIS 120704 induced acute dosedependent thrombocytopenia within 3 and 24 hours, respectively. The thrombocytopenia was associated with significant increases in plasma monocyte chemoattractant protein 1. IVIg administration significantly rescued the CpG ODN-induced thrombocytopenia, as did treatment with either a Syk inhibitor or TLR9 antagonists. In vitro, CpG ODN could activate human platelets and this correlated significantly with enhanced IVIg- and Syk-dependent phagocytosis by THP-1 monocytes. Conclusion: These results suggest that CpG ODNs induce acute inflammatoryassociated (IVIg-sensitive) thrombocytopenia that can be alleviated by Syk- or TLR9blockade, and an IVIg- and Syk-dependent platelet clearance pathway appears primarily responsible for the thrombocytopenia.