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

BACKGROUND: CpG ODN 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 two CpG ODN (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 pre-bled and then injected with titrated doses of CpG ODNs and platelet counts were determined. The mice were treated IVIg or with various inhibitors and antagonists of toll-like receptor 9 (TLR9) and spleen tyrosine kinase (Syk) to determine their effects on the thrombocytopenia.

RESULTS AND CONCLUSIONS: Compared with saline-treated mice or mice treated with 2'-O-methoxyethyl (MOE)-modified antisense (ASO) ODN, both ODN 2395 and ISIS 120704 induced an acute dose-dependent thrombocytopenia within 3 and 24 hours respectively. The thrombocytopenia was associated with significant increases in plasma monocyte chemoattractant protein 1 (MCP1). Intravenous immunoglobulin (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. These results suggest that CpG ODN induce an acute inflammatory-associated (IVIg-sensitive) thrombocytopenia that can be alleviated by Syk- or TLR9-blockade, and an IVIg- and Syk-dependent platelet clearance pathway appears primarily responsible for the thrombocytopenia. Whether these results are applicable to humans still has to be elucidated.