More effective discrimination between CD4+CD25+ regulatory T cells (Treg) and activated T cells would significantly improve the current level of purification of Treg and their therapeutic application. We observed that ∼90% of Treg (positive for the nuclear transcription factor Forkhead winged helix protein-3 and able to inhibit naive T cell proliferation) isolated from the spleens or lymph nodes of normal mice did not express significant levels of the inhibitory receptor programmed cell death-1 (PD-1) on their surface, but retained PD-1 intracellularly. An identical phenotype was also identified for human CD4+CD25high T cells isolated from peripheral blood of healthy volunteers. By contrast, activated T cells expressed high levels of surface PD-1 that paralleled up-regulation of CD25 during effector cell expansion. This distinction allowed us to isolate CD4+CD25+PD-1− T cells with suppressive activity from mice immunized with mature allogeneic dendritic cells. Although purification was limited to resting Treg because TCR ligation induced up-regulation of surface PD-1, this strategy nevertheless represents a valuable step toward more definitive characterization of Treg and their improved purification for therapeutic assessment.

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These novel findings suggest a possible functional relationship between the enhanced incidence of precursor plasmacytoid DC, their comparatively high relative expression of the coinhibitory molecule PD-L1, and the elevated frequency of Treg in operational liver transplant tolerance.

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The nucleotide-binding oligomerization domain (NOD)2/CARD15 protein, which senses muramyl dipeptide (MDP), a product of bacterial peptidoglycan, appears to play an important role in regulating intestinal immunity. Although the liver is exposed to gut-derived MDP, the influence of NOD2 ligation on hepatic APC, in particular dendritic cells (DC), is unknown. Freshly isolated mouse liver and spleen plasmacytoid (p)DC expressed higher levels of NOD2 message than conventional myeloid (m)DC. Following MDP stimulation in vivo, liver pDC, but not mDC, up-regulated expression of IFN regulatory factor 4 (IRF-4), a negative regulator of TLR signaling, and induced less allogeneic T cell proliferation and IFN-γ production. The adoptive transfer of liver pDC from MDP-treated mice failed to prime allogeneic T cells in vivo. By contrast, splenic DC IRF-4 levels and T cell stimulatory activity remained unchanged. Liver pDC from MDP-stimulated mice also displayed greater IκBα, cell surface B7-H1, and B7-H1 relative to CD86 than control liver pDC. No similar effects were observed for liver mDC or spleen DC. Absence of B7-H1 on liver pDC reversed the inhibitory effect of MDP. After ex vivo stimulation with LPS or CpG, liver pDC but not mDC from MDP-treated animals secreted less IL-12p70, IL-6, and TNF-α and induced weaker allogeneic T cell proliferation than those from controls. Moreover, CpG-stimulated liver pDC from MDP-treated mice secreted less IFN-α than their splenic counterparts, and systemic levels of IFN-α were reduced in MDP-treated animals after CpG administration. These findings suggest that differential effects of NOD2 ligation on liver pDC may play a role in regulating hepatic innate and adaptive immunity.