Dendritic cells (DC) are extremely proficient inducers of naïve CD4+ T cell activation due to their high expression level of peptide-MHC and an array of accessory molecules involved in cell migration, adhesion and co-signaling, including PD-1 ligand 1 (PD-L1) and PD-1 ligand 2 (PD-L2). Whether PD-L1 and PD-L2 have a stimulatory or inhibitory function is a matter of debate, and could be partially dependent on the model system used. In this study we examined the role of PD-L1 and PD-L2 expressed by DC in naïve CD4+ T cell activation in a more physiologically relevant model system, using OVA-specific T cells in combination with various levels of TCR stimulation. Overexpression of PD-L1 or PD-L2 by DC did not inhibit T cell proliferation, even when B7-1 and B7-2 mediated costimulation was absent, although IL-2 production was consistently decreased. Surprisingly, blocking PD-L1 and PD-L2 with soluble programmed death-1 (sPD-1) also inhibited T cell activation, probably via reverse signaling via PD-L1 and/or PD-L2 into DC, leading to reduced DC maturation. This study suggests a relatively minor contribution of PD-1 ligands in DC-driven CD4+ T cell activation and provides evidence for reverse signaling by PD-L1 and PD-L2 into DC, resulting in a suppressive DC phenotype.
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