Suppression of Human T-Cell Activation and Expansion of Regulatory T Cells by Pig Cells Overexpressing PD-Ligands

Background. Genetic modification of pigs (e.g., transgenic expression of human complement regulatory molecules or inactivation of alpha 1,3galactosyltransferase) enabled the development of promising strategies to overcome hyperacute rejection after pig-to-primate xenotransplantation. However, cellular rejection still remains a hurdle for successful xenograft survival. This report tested the hypothesis that overexpression of human negative costimulatory PD-Ligands (PD-L) in pig antigen presenting cells might be an approach to prevent human anti-pig T-cell responses. Methods. The pig B-cell line L23 was transfected with the pIRES-AcGFP vector containing human PD-L1 or PD-L2. Stable transfectants (L23-PD-L1, L23-PD-L2 cells) were established and used for in vitro stimulation of purified human CD4(+) T cells. Results. Human CD4+ T cells responded with significantly reduced proliferation to L23-PD-L1 or L23-PD-L2 cells and produced less IL-2, IFN gamma, TNF alpha, IL-4, and IL-5 than cells stimulated with mock-transfected B cells. The concentration of IL-10, however, was increased in CD4+ T cells responding to stimulation with PD-L1 or PD-L2 transfectants. Furthermore, in cultures of CD4+ T cells stimulated for 3 weeks with PD-L1 or PD-L2 transfectants a CD4(+)CD(25high)Foxp(3+) subset showed up that effectively suppressed the activation of conventional CD4+ T cells. Conclusions. These findings imply that PD-1/PD-Ligand pathways are interesting targets to prevent human anti-pig T-cell responses after xenotransplantation, and also suggests that PD-1/PD-Ligand interactions may play a role in the control of the activity and/or homeostasis of regulatory T cells.