Our recent studies have documented the presence of high numbers of recipient antigen presenting cells (APCs) displaying donor MHC molecules (cross-dressed) on their surface after skin in the lymphoid organs of mice having received a skin, heart or pancreatic islet transplant. In addition, we showed that acquisition of allogeneic MHC molecules by host APCs is mediated by donor-derived vesicles (exosomes) trafficking through blood and lymphatic vessels.

In the present study, we investigated whether allogeneic exosomes could activate a direct T cell alloresponse in vitro and in vivo.Exosomes were isolated (using an Exoquick kit, SBI) from C57BL/6J activated splenocytes. These exosomes were used to cross-dress BALB/c cells in vitro. The transfer of donor MHC class I and II (K^b and A^b) on BALB/c cells was ascertained using flow imaging. Next, T cells from naïve BALB/c mice were cultured in vitro with either allogeneic B6 exosomes or BALB/c spleen cells cross-dressed with allogeneic B6 MHC. Allogeneic exosomes were unable on their own to activate T cells while cross-dressed cells induced potent pro-inflammatory direct T cell responses in vitro. Subsequently, 1.7 x 10⁹ B6 exosomes were injected intraperitoneally to BALB/c mice. High frequencies of lymph node and spleen recipient T cells producing gIFN through the direct allorecognition pathway were detected in these mice, as compared to unmanipulated recipients or recipients injected with syngeneic exosomes. We conclude that during in vitro conditions, cross-dressing of recipient APCs with allogeneic MHC molecules can trigger a pro-inflammatory direct alloresponse by T cells. Furthermore, injection of allogeneic exosomes alone can induce the alloresponse in vivo.

CITATION INFORMATION: Babiker-Mohamed M, Marino J, Crosby P, Benichou G. Activation of Alloreactive T Cells by Allogeneic Exosomes and Cells Cross-Dressed with Allogeneic MHC Molecules. Am J Transplant. 2016;16 (suppl 3).

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