mRNA synthesis
The murine IL-21 gene (GenBank Accession Number: NM_021782.2) was inserted into plasmid vector pCZ (Jirui Biotechnology, Shenzhen, China) which incorporates 5’ and 3’ untranslated regions (UTRs) and includes a polyA tail with 120 nucleotides in length. The plasmid was linearized with BspQI and the mRNA was synthesized in vitro using T7 polymerase. The UTP was substituted with 1-methylpseudo UTP and a cap structure was added to increase the translation efficiency of IL-21 mRNA. The capped mRNA was purified using lithium chloride, and then Nanodrop (Thermo Fisher Scientific) was used to determine its concentration. The purified mRNA was subjected to 1% agarose electrophoresis for size check (Figure S1 ).