Differential regulation of PD-1 and its ligands in allergic asthma.

BACKGROUND: Targeting PD-1/PD-1 ligand signalling is an established treatment option for cancer. The role of these molecules in allergic asthma has been investigated in several mouse studies yielding conflicting results. However, human studies investigating the expression and regulation of PD-1 and its ligands in allergic inflammation are lacking.
OBJECTIVE: To analyse the expression and regulation of PD-1 and its ligands in human allergic asthma.
METHODS: The well-established human asthma model of segmental allergen challenge (SAC) was used to analyse the regulation of PD-1 and its ligands PD-L1 and PD-L2 on T lymphocytes and dendritic cells by flow cytometry. The impact of immunoglobulin E (IgE)-mediated signalling on PD-L1 expression was analysed on isolated plasmacytoid dendritic cells (pDCs).
RESULTS: PD-1 expression by blood CD4+ T cells was negatively associated with total and specific (against the allergen used for provocation) IgE serum concentrations. Twenty-four hours after SAC, a small decrease in endobronchial PD-1+ CD4+ T cells was accompanied by an increase in PD-L1 expression on endobronchial myeloid dendritic cells (mDCs) and pDCs. The PD-L1 up-regulation on pDCs was not induced by IgE-mediated mechanisms. In contrast, PD-L2 was only detected on endobronchial mDCs and was significantly down-regulated 24 hours after SAC. CONCLUSION AND CLINICAL RELEVANCE: This study shows, for the first time, an association of a low PD-1 expression by circulating CD4+ T cells with high total and specific (against the allergen used for provocation) IgE concentrations in allergic asthma. In addition, we demonstrate a differential regulation of PD-1 ligands on endobronchial DCs after allergen challenge which may favour Th2 inflammation. Therefore, modulating PD-1 ligand-mediated pathways might be a promising target in allergic asthma.