IL-12 mRNA-LNP promotes dermal resident memory CD4+ T cell development.

Dermal resident memory CD4⁺ T cells (dTrm) provide protection against vector-borne infections. However, the factors that promote their development remain unclear. We tested if an mRNA vaccine, encoding a protective leishmanial antigen, induced dTrm cells. The mRNA vaccine induced robust systemic T-cell responses, but few Trm cells were found in the skin. Since IL-12 promotes Th1 responses, we tested whether IL-12 mRNA combined with the mRNA vaccine could enhance dTrm cell development. This combination significantly expanded Leishmania-specific Th1 cells expressing skin-homing molecules and memory T cell markers in the draining lymph node. Additionally, higher numbers of dTrm cells were maintained in the skin, and mice exhibited functional immunity indicated by a delayed hypersensitivity response and protection upon challenge with Leishmania. These findings highlight IL-12 as a key driver of CD4⁺ dTrm development, enabling their global seeding across the skin, and underscore the potential of IL-12-enhanced mRNA vaccines to generate durable immunity against cutaneous leishmaniasis and other skin-targeted infections.