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Summary: Interleukin-35 (IL-35) is an immunosuppressive cytokine composed of Epstein-Barr-virus-induced protein 3 (Ebi3) and IL-12α chain (p35) subunits, yet the forms that IL-35 assume and its role in peripheral tolerance remain elusive. We induce CBA-specific, IL-35-producing T regulatory (Treg) cells in TregEbi3WT C57BL/6 reporter mice and identify IL-35 producers by expression of Ebi3TdTom gene reporter plus Ebi3 and p35 proteins. Curiously, both subunits of IL-35 are displayed on the surface of tolerogen-specific Foxp3+ and Foxp3neg (iTr35) T cells. Furthermore, IL-35 producers, although rare, secrete Ebi3 and p35 on extracellular vesicles (EVs) targeting a 25- to 100-fold higher number of T and B lymphocytes, causing them to acquire surface IL-35. This surface IL-35 is absent when EV production is inhibited or if Ebi3 is genetically deleted in Treg cells. The unique ability of EVs to coat bystander lymphocytes with IL-35, promoting exhaustion in, and secondary suppression by, non-Treg cells identifies a novel mechanism of infectious tolerance. : Sullivan et al. show that while many factors and cytokines contribute to primary immunosuppression, EV-associated IL-35 uniquely promotes “infectious” tolerance not only by inducing IL-35 production in non-Treg cells but also by causing an immunosuppressive phenotype in EV-acquiring T and B cells, leading to secondary suppression of immune responses. Keywords: IL-35, extracellular vesicles, cytokines, tolerance, Treg, tetraspanin, Ebi3, p35, CD81