Figure 3. PD-1: PD-L1 signaling in allograft tolerance.

This figure illustrates some of the potential interactions of this pathway in lymphoid organs and in a cardiac allograft. (A), Tolerogenic DCs interact with naïve T cells and may induce either alloantigen-specific Tregs (iTregs) or decreased activation of effector T cell (T eff), depending on the presence (or absence) of TGF-β. (B), After activation by DCs in secondary lymphoid organs, alloantigen-specific effector T cells migrate to the graft where initial interaction with PD-L1 in the endothelium might inhibit T effector responses. Additional T eff cells that infiltrate the graft may be suppressed by iTregs or local cells expressing PD-L1 (APCs or non-hematopoieitic cells). Whether PD1/PDL1 on iTregs play a significant role in the direct suppression of T eff remains to be determined. Other potential costimulatory interactions are not depicted in this figure for simplicity.