A new mechanistic study from Washington University in St. Louis, published in Nature, has identified an immune cell “trick” that helps explain how mRNA vaccines generate such strong killer T cell responses — and the explanation turns on a phenomenon immunologists call “cross-dressing.”
The paper, “mRNA vaccines engage unconventional pathways in CD8+ T cell priming”, comes from a WashU Medicine team led by senior authors Kenneth M. Murphy and William E. Gillanders, with Suin Jo as first author, alongside collaborators including Robert D. Schreiber and Gwendalyn J. Randolph. Using genetically engineered mouse models, the researchers set out to answer a basic but unresolved question about mRNA-lipid nanoparticle (LNP) vaccines: exactly which immune cells prime the CD8+ “killer” T cells that clear infected or abnormal cells, and how.
What “cross-dressing” means in immunology
Dendritic cells are the immune system’s professional messengers — they capture antigens, process them into short peptide fragments, load those fragments onto MHC class I molecules, and present the resulting peptide-MHC complex to T cells to “teach” them what to attack. That conventional route is called cross-presentation, and for years it was assumed to be the main way vaccines activate CD8+ T cells.
Cross-dressing is a different, less-processed route. Instead of digesting an antigen itself, a dendritic cell simply acquires an already-assembled peptide-MHC-I complex directly off the surface of another cell — essentially borrowing someone else’s finished “outfit” rather than making its own. It’s a known phenomenon in immunology, but the WashU team’s contribution is showing how central it is to the way mRNA vaccines actually work.
What the study found
Two dendritic cell subsets are of particular interest here: cDC1 cells, long considered the specialists in cross-presentation, and cDC2 cells, a more numerous but historically less-credited subset. The WashU team found that mRNA-LNP vaccines did not require cDC1 cells or the WDFY4-dependent cross-presentation pathway to prime CD8+ T cells — a genuinely surprising result given how much prior antigen-presentation research has centered on that route. Instead, both cDC1 and cDC2 cells were able to prime CD8+ T cell responses redundantly, and a substantial share of that priming came through cross-dressing: dendritic cells picking up peptide-MHC-I complexes from non-blood, non-immune (non-haematopoietic) cells — likely the muscle or other tissue cells where the injected mRNA-LNP is first taken up and translated into protein.
Critically, the researchers found this cross-dressing route depends on type I interferon signaling, the same innate-immune alarm system that mRNA-LNP vaccines are already known to trigger. In other words, the vaccine’s own adjuvant-like innate immune activation appears to help set up the conditions for dendritic cells to cross-dress in the first place.
The study also raises a further possibility worth flagging carefully: because cross-dressing lets a dendritic cell display MHC-I complexes it didn’t generate itself, this route could help explain why mRNA vaccines sometimes appear to activate CD8+ T cells against antigens that were never actually encoded by the vaccine. That’s a notable observation for vaccine immunology broadly, though the paper’s own findings on this point are described as an explanatory possibility arising from the mechanism, not a demonstrated clinical effect.
Why it matters — and what it doesn’t yet mean
This is preclinical, mechanistic research conducted in mouse models, not a human clinical trial, and it should be read that way. It doesn’t describe a new vaccine, and it isn’t evidence that current mRNA vaccines need to change. What it does is fill in a real gap in the basic science of how mRNA-LNP vaccines generate cellular immunity — specifically, which dendritic cells matter and by what molecular route they prime CD8+ T cells.
That kind of foundational mechanistic clarity matters for vaccine platform design generally: if cross-dressing via cDC2 cells (rather than cDC1-restricted cross-presentation) is a major, type I interferon-dependent contributor to CD8+ T cell priming, that reshapes how future mRNA vaccine adjuvants and formulations might be optimized to strengthen T cell responses — including in contexts like cancer immunotherapy and vaccines against intracellular pathogens, where robust CD8+ T cell activity is the whole point. Translating a mouse-model mechanistic finding into any actual formulation change would require substantial further preclinical and, eventually, clinical work.
Source
Jo, S., Li, L., Thakur, C., et al. “mRNA vaccines engage unconventional pathways in CD8+ T cell priming.” Nature (2026). DOI: 10.1038/s41586-026-10353-6. Research conducted at Washington University in St. Louis School of Medicine.
