The roles of CD4⁺ T cell help, sex, and dose in the induction of protective CD8⁺ T cells against a lethal poxvirus by mRNA-LNP vaccines

The role of CD4⁺ T cells in the induction of protective CD8⁺ T cells by mRNA lipid nanoparticle (LNP) vaccines is unknown. We used B6 or Tlr9 ^-/- mice depleted or not of CD4⁺ T cells and LNP vaccines loaded with mRNAs encoding the ectromelia virus (ECTV) MHC class I H-2 K^b-restricted immunodominant CD8⁺ T cell epitope TSYKFESV (TSYKFESV mRNA-LNPs) or the ECTV EVM158 protein, which contains TSYKFESV (EVM-158 mRNA-LNPs). Following prime and boost with 10 μg of either vaccine, K^b-TSYKFESV-specific CD8⁺ T cells fully protected male and female mice from ECTV at 29 (both mRNA-LNPs) or 90 days (EVM158 mRNA-LNPs) post boost (dpb) independently of CD4⁺ T cells. However, at 29 dpb with 1 μg mRNA-LNPs, males had lower frequencies of K^b-TSYKFESV-specific CD8⁺ T cells and were much less well protected than females from ECTV, also independently of CD4⁺ T cells. At 90 dpb with 1 μg EVM158 mRNA-LNPs, the frequencies of K^b-TSYKFESV-specific CD8⁺ T cells in males and females were similar, and both were similarly partially protected from ECTV, independently of CD4⁺ T cells. Therefore, at optimal or suboptimal doses of mRNA-LNP vaccines, CD4⁺ T cell help is unnecessary to induce protective anti-poxvirus CD8⁺ T cells specific to a dominant epitope. At suboptimal doses, protection of males requires more time to develop.