Modulation of Xenogeneic T-cell Proliferation by B7 and mTOR Blockade of T Cells and Porcine Endothelial Cells

Background. Activation of porcine endothelial cells (PECs) is the mechanistic centerpiece of xenograft rejection. This study sought to characterize the immuno-phenotype of human T cells in response to PECs and to explore the immuno-modulation of B7 and mammalian target of rapamycin blockade of T cells and/or PECs during xeno-responses. Methods. Rapid memory T-cell (T_M) responses to PECs were assessed by an intracellular cytokine staining. T-cell proliferation to PEC with or without belatacept or rapamycin was evaluated by a mixed lymphocyte-endothelial cell reaction (MLER). Additionally, rapamycin-pretreated PECs were used in MLER. Cell phenotypes were analyzed by flow cytometry. Results. Tumor necrosis factor-α/interferon-γ producers were detected in CD8⁺cells stimulated by human endothelium but not PECs. MLER showed proliferation of CD4⁺and CD8⁺cells with predominantly memory subsets. Purified memory and naive cells proliferated following PEC stimulation with an increased frequency of T_Min PEC-stimulated naive cells. Proliferating cells upregulated programmed cell death-1 (PD-1) and CD2 expression. Belatacept partially inhibited T-cell proliferation with reduced CD2 expression and frequency of the CD8⁺CD2^highCD28^-subset. Rapamycin dramatically inhibited PEC-induced T-cell proliferation, and rapamycin-preconditioned PECs failed to induce T-cell proliferation. PD-1 blockade did not restore T-cell proliferation to rapamycin-preconditioned PECs. Conclusions. Humans lack rapid T_M-mediated responses to PECs but induce T-cell proliferative responses characterized largely as T_Mwith increasing CD2 and PD-1 expression. B7-CD28 and mammalian target of rapamycin blockade of T cells exhibit dramatic inhibitory effects in altering xeno-proliferating cells. Rapamycin alters PEC xeno-immunogenicity leading to inhibition of xeno-specific T-cell proliferation independent of PD-1-PD ligand interaction.