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We demonstrated induction of circulating exosomes in lung transplant recipients (LTxRs) with acute/chronic rejection (bronchiolitis obliterans syndrome [BOS]) containing donor human leukocyte antigen and lung self-antigens (SAgs) (Kα-1 Tubulin, Collagen V), costimulatory molecules (CD80, CD86), transcription factors (NFkB, 20S proteasome) (Am J Transplant 17(2):474-484, 2017). In the current study, we identified unique proteomic signatures in circulating extracellular vesicles (EVs) that can differentiate human LTxRs with acute rejection (AR), BOS, respiratory viral infection (RVI), and stable. EVs were isolated using ultracentrifugation from LTxRs plasma (stable, AR, BOS or RVI) following transplant. EVs were characterized for the presence of lung SAgs (Kα-1 Tubulin, Collagen V) and size (50-200nm) was determined using NanoSight. EV protein cargoes were prepared for shotgun proteomics using liquid chromatography-tandem mass spectrometry (LC-MS/MS). We identified unique proteins (2 AR, 24 BOS, 4 RVI, 8 stable cohort) using LC-MS/MS. Differential analysis of the proteins from AR, BOS and RVI to stable yielded significant unique proteins (p-value <.05, fold-change >2) in each clinical cohort (27 AR, 2 BOS, 2 RVI). EVs from AR contained proteins involved in immunoglobulin, complement regulation, innate and adaptive immune response pathways. EVs from BOS revealed enrichment of immunoglobulin receptors and carboxypeptidase N catalytic chain. EVs from RVI were enriched for macrophage stimulating factor. Our results demonstrate unique signatures in acute/chronic rejection and RVI highlighting complex immune mechanisms underlying lung allograft rejection. Presence of complement proteins and immunoglobulins in circulating EVs from AR can be helpful in differentiating cellular rejection with a mixed phenotype (cellular and humoral). We identified immunoglobulins and complement proteins in BOS in support of significance of Abs in the pathogenesis of BOS. Macrophage stimulating factor contained in EVs specific to RVI can be useful for the differential diagnosis of RVI from AR. To conclude, proteomic signatures of circulating EVs from LTxRs provided insights into immunological mechanisms at play in graft rejection and RVI leading to increased risk for chronic lung allograft dysfunction. [ABSTRACT FROM AUTHOR]