Emerging role of exosomes in allorecognition and allograft rejection.

PURPOSE OF REVIEW: This article reviews recent literature on the nature of extracellular vesicles released by allogeneic transplants and examine their role in T-cell alloimmunity involved in rejection and tolerance of these grafts.

RECENT FINDINGS: Donor cells release extracellular vesicles, including exosomes, after transplantation of allogeneic organs and tissues. Consequently, recipient APCs take up these exosomes and present donor MHC antigens on their surface (allo-MHC cross-dressing) thus, activating some alloreactive T cells via a mechanism called semi-direct pathway of allorecognition. In addition, one study shows that exosomes carrying noninherited maternal antigens are associated with maternal microchimerism and tolerance in offspring. Finally, a few studies describe potential utilization of exosomes as modulators of alloimmunity and biomarkers of rejection in allotransplantation.

SUMMARY: Extracellular vesicles, including exosomes, released by allografts contribute to recognition of donor antigens by T cells after allotransplantation. This occurs through cross-dressing of recipient APCs with donor MHC antigens and subsequent activation of T cells, a process called semi-direct alloreactivity. The relevance of this phenomenon in rejection and tolerance of allografts and the potential utilization of exosomes as biomarkers in transplantation are discussed.