mRNA vaccines engage unconventional pathways in CD8 T cell priming

Vaccines comprised of mRNA and lipid nanoparticles (LNP) activate B and T cells by inducing in vivo production of specific protein antigens. While B cells can be activated directly by antigens, activation of T cells requires antigen processing and presentation by MHC molecules on specialized antigen presenting cells (APCs). In response to viral infections, tumors and current vaccines, antigen presentation to CD8 T cells is particularly dependent on type 1 conventional dendritic cells (cDC1s) which are specialized for efficient antigen cross-presentation of exogenous antigens. However, whether similar mechanisms are employed by mRNA-LNP vaccination is currently unknown. Here, we show that mRNA LNP vaccines do not depend on cDC1 to induce CD8 T cell responses but engage cDC1 and cDC2 redundantly. In addition, these responses do not involve the normal WDFY4-dependent cross-presentation pathway. Rather, cDC2 are a prominent component driving CD8 T cells response against mRNA-LNP vaccination and engage cross-dressing of peptide-MHC-I complexes derived from non-hematopoietic cells. Notably, multimodal single cells analysis revealed that CD8 T cells primed either by cDC1 or cDC2 exhibit phenotypical differences. These results suggest that mRNA-LNP vaccination should be further evaluated for its ability to sustain normal functional memory that emerges from natural infections or vaccines that engage cDC1 for CD8 T cell priming.