Poor allostimulatory function of liver plasmacytoid DC is associated with pro-apoptotic activity, dependent on regulatory T cells

Background/Aims: The liver is comparatively rich in plasmacytoid (p) dendritic cells (DC), - innate immune effector cells that are also thought to play key roles in the induction and regulation of adaptive immunity. Methods: Liver and spleen pDC were purified from fms-like tyrosine kinase ligand-treated control or lipopolysaccharide-injected C57BL/10 mice. Flow cytometric and molecular biologic assays were used to characterize their function and interaction with naturally occurring regulatory T cells (Treg). Results: While IL-10 production was greater for freshly isolated liver compared with splenic pDC, the former produced less bioactive IL-12p70. Moreover, liver pDC expressed a low Delta4/Jagged1 Notch ligand ratio, skewed towards T helper 2 cell differentiation/cytokine production, and promoted allogeneic CD4⁺T cell apoptosis. T cell proliferation in response to liver pDC was, however, enhanced by blocking IL-10 function at the initiation of cultures. In the absence of naturally occurring CD4⁺CD25⁺ regulatory T cells, similar levels of T cell proliferation were induced by liver and spleen pDC and the pro-apoptotic activity of liver pDC was reversed. Conclusions: The inferior T cell allostimulatory activity of in vivo-stimulated liver pDC may depend on the presence and function of Treg, a property that may contribute to inherent liver tolerogenicity.