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 alpha1,3galactosyltransferase) 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, IFNgamma, TNFalpha, 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+CD25(high)Foxp3+ 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.