Human CD4 T cells are a functional target for lipid nanoparticle-based mRNA vaccines

Billions of mRNA-based severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) lipid nanoparticle (LNP) vaccine doses have been administered globally and shown to be both safe and effective. However, the mechanisms by which mRNA vaccines facilitate protection remain unclear. Peripheral blood analyses from vaccinated individuals show B and T cell responses are elicited, but it remains unknown which cells contribute to particle uptake and protein expression to elicit protective immunity. Using mRNA LNP fluorescent reporters, we show that CD4 T cells are an unexpected target for LNP-based protein expression <i>in vitro</i> and <i>in vivo</i>. CD4 T cells transfected with commercial SARS-CoV-2 mRNA vaccines are sufficient as the sole antigen producer to generate specific antibody responses but cannot support antigen presentation to other CD4 T cells. This study demonstrates that non-antigen-presenting immune cells are a novel target for mRNA-based protein expression and can support effective adaptive immune responses.IMPORTANCEIn this work, we demonstrate that CD4 T cells are a major target for lipid nanoparticle (LNP) transfection <i>in vitro</i> and <i>in vivo</i>. Using a human lymph node-like organoid model, we show that CD4 T cells effectively express the corresponding protein and support a productive humoral antigen-specific vaccine response to severe acute respiratory syndrome coronavirus 2 mRNA LNPs. Similarly, lymph node CD4 T cells are a major transfection target <i>in vivo</i> in a murine immunization model. Overall, we conclude that cells outside of the injection site can be significant contributors to the pool of antigen production after mRNA LNP immunization. This discovery opens the door to new vaccination strategies that could target CD4 T cells directly to optimize immune responses or immunotherapies.