Optimal murine CD4⁺ T cell priming by mRNA-lipid nanoparticle vaccines requires endogenous antigen processing

Lipid nanoparticle (LNP)-encapsulated nucleoside-modified mRNA vaccines elicit robust CD4⁺ T cell responses, yet the mechanisms underlying this T cell priming remain unknown. Antigens presented to CD4⁺ T cells on major histocompatibility complex class II (MHC II) are traditionally acquired by antigen presenting cells (APCs) from extracellular sources. Here we show that vaccine-specific CD4⁺ T cell responses instead rely on antigen directly expressed within APCs, without extracellular transit. Murine APCs treated with mRNA-LNP vaccines activate T cells more efficiently when presenting antigen produced internally, rather than acquired externally. Immunization with mRNA-LNP vaccines engineered to inhibit antigen expression in APCs results in lower antigen-specific CD4⁺ T cell, T follicular helper cell, and antibody responses in mice. In contrast, excluding vaccine antigen from muscle cells minimally affects CD4⁺ T cell responses. Our findings demonstrate that endogenous antigen presentation is essential to mRNA-LNP vaccine-induced immune responses and refine paradigms of MHC II-restricted antigen processing and presentation.