Exosomal miR - 21 causes neurotoxicity via TLR7 signaling in SIV neurological disease.

Exosomes from the brains of rhesus macaques were isolated and characterized, both with and without simian immunodeficiency virus (SIV) induced central nervous system (CNS) disease. Small RNA sequencing revealed that exosomes from the brains of animals with CNS disease contained significantly increased miR-21, a microRNA we have previously shown to be significantly increased in both SIV and HIV induced CNS disease. In addition to neurons in the brain, macrophage/microglial cells/nodules also express miR-21 during SIV induced CNS disease. In vitro culture of macrophages revealed that miR-21 is released into exosomes, and exosomes from macrophage cultures produced from WT, but not miR-21-/- KO, animals are neurotoxic. We find this neurotoxicity is dependent upon exosome carriage of miR-21, and mutation of the sequence within miR-21 predicted to bind TLR7 eliminates this neurotoxicity. Indeed exosomal miR-21 activates TLR7 in a reporter cell line, and the neurotoxicity of exosomal miR-21 is dependent upon TLR7, as neurons isolated from TLR7-/- KO mice are protected from neurotoxicity. Finally, we show that exosomes isolated from the brains of monkeys with SIV induced CNS disease both activate TLR7 and are neurotoxic when compared to exosomes from control animals. Exosomes were isolated from n=4 SIV infected without encepholitis, n=4 SIV infected with encepholitis, and n=3 control Macaca mulatta.