Apoptotic endothelial cells release small extracellular vesicles loaded with immunostimulatory viral-like RNAs

Purpose: Compare the RNA content of apoptotic human umbilical vein endothelial cells (HUVECs)-released vesicles: 1) Apoptotics bodies and 2) Apoptotic Exo-like vesicles, which are immunogenic and promote autoimmunity. Results: The transcriptome of ApoExos was unique with regards to its genomic origin, nucleotide composition and secondary structure. We show that ApoExos essentially contain non-ribosomal non-coding RNAs, among which 89% are derived from viral-like endogenous retroelements (EREs). ApoExos are also enriched in U-rich linear nucleotide sequences and Alus repeats. Conclusion: Our work demonstrates that apoptotic endothelial cells release EVs loaded with RNAs recognized by RLRs and endosomal TLRs (TLR3, TLR7 and TLR8), and are therefore tailor-made for initiation of innate immune responses and autoimmunity. Overall design: We performed whole RNA-sequencing in duplicate on normal and apoptotic human umbilical vein endothelial cells and on the two types of EVs released by these apoptotic HUVECs: apoptotic bodies and ApoExos.