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

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. PDEs were isolated from flounder immunized with inactivated Vibrio anguillarum (ExoFKC) or non-immunized flounder (ExoCon) 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 ExoFKC-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 ExoFKC on enhancing vaccine potency. 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. ExoFKC, when used as an adjuvant with the vaccine, enhanced vaccine efficacy by activating CD4⁺ T cells and increasing antibody production. 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.