Transient in vivo activation of rat brain macrophages/microglial cells and astrocytes by immunostimulatory multiple CpG oligonucleotides

Abstract Certain DNA sequences containing motifs of unmethylated CpG nucleotides are immunostimulatory and might contribute to the development of inflammatory lesions after infections. CpG motifs might further contribute to side effects of oligonucleotide-based therapeutic approaches. Here we have analyzed the effects of intracranial injections of synthetic CpG oligonucleotides. We observed that oligonucleotides with several unmethylated CpG motifs, but not methylated or inverted GpC motifs, stimulated microglial cells and astrocytes of the rat brain. This transient, self-limiting response is maximal at day 3 after injection and subsides until day 5. Activated microglial cells could be identified to produce two novel monocytic peptides, the allograft inflammatory factor-1 (AIF-1) and endothelial monocyte activating polypeptide II (EMAP II). Astrocytes were similarily activated as shown by expression of the enzyme heme-oxygenase-1 (HO-1). Glial cell proliferation (expression of PCNA) or aptosis was not observed. Thus immunostimulatory DNA activates the local innate immune defense system of the brain, and might contribute transiently to infectious, inflammatory and degenerative responses of the central nervous system.