Toll‐like receptor‐dependent activation of several human blood cell types by protamine‐condensed mRNA

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Abstract
We reported that RNA condensed on protamine is protected from RNase‐mediated degradation and can be used for vaccination. Here, we show that such complexes are also danger signals that activate mouse cells through a MyD88‐dependent pathway. Moreover, mRNA‐protamine complexes stimulate human blood cells. They strongly activate DC and monocytes, leading to TNF‐α and IFN‐α secretion. In addition, protamine‐RNA complexes directly activate B cells, NK cells and granulocytes. The detailed analysis of the activated cell types, the study of the cytokines released from PBMC cultured with protamine‐RNA complexes and recently published results suggest that TLR‐7 and TLR‐8 may be involved in the recognition of protamine‐stabilized RNA. Our data indicate that protamine‐stabilized RNA, which may be similar to RNA condensed in the nucleocapsids of RNA viruses, is a strong danger signal. Thus, similarly to plasmid DNA, protamine‐RNA combines antigen production and non‐specific immunostimulation. The studies presented here explain the capacity of protamine‐RNA to act as a vaccine, and pave the way towards the development of safe and efficient mRNA‐based immunotherapies.