Peritoneal exosomes mediate activation of CD4(+) T cells and enhancement of vaccine efficacy in teleost.

BACKGROUND: Exosomes have emerged as promising vaccine candidates due to their role in intercellular communication and bioactive cargo-carrying capacity. However, their intrinsic properties in activating adaptive immunity have not yet been fully addressed. This study aims to elucidate how teleost peritoneal cell-derived exosomes (PDEs) elicit CD4(+) T cell activation and contribute to vaccine-mediated adaptive immunity. METHODS: PDEs were isolated from flounder immunized with inactivated Vibrio anguillarum (Exo(FKC)) or non-immunized flounder (Exo(Con)) via differential centrifugation. Tissue and cellular distribution of DiO-labeled exosomes was visualized and analyzed via small animal imaging, fluorescence microscopy, and flow cytometry. The capacity of PDEs to promote CD4(+) T cell activation was assessed using flow cytometry and CFSE assay. Multi-omics approaches were used to analyze Exo(FKC)-mediated CD4(+) T cell activation mechanisms at protein, mRNA, and miRNA levels. CD4(+) T cell levels and antibody production were analyzed to assess the impact of Exo(FKC) on enhancing vaccine potency. RESULTS: After intraperitoneal vaccination, PDEs increased markedly. These exosomes fused with peritoneal cells to facilitate the transport of antigen-presenting molecule locally and were transferred to key lymphoid organs. PDEs carrying T cell-associated RNA (jun/p38a) and miRNA (novel-m0034-3p), with elevated MHCII and CD80/86 levels, promoted CD4⁺ T cell activation. Exo(FKC), when used as an adjuvant with the vaccine, enhanced vaccine efficacy by activating CD4⁺ T cells and increasing antibody production. CONCLUSIONS: This study highlights a novel mechanism by which exosomes directly contribute to vaccine-induced immunity, as demonstrated in a lower vertebrate model. It is plausible that similar processes may occur in higher vertebrates, including humans, providing new insights into the role of exosomes in adaptive immune responses. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12964-026-02752-z.