The role of peripheral blood dendritic cell subsets in the house dust mite-allergic immune response

Abstract

Background Plasmacytoid dendritic cells (pDC) and CD1c+ myeloid dendritic cells (mDC) are the two major dendritic cell (DC) subsets in the human peripheral blood. Both subsets are not only equipped with a range of mechanisms for pathogen recognition and immune response induction but also express immunoregulatory molecules. Through the production of indoleamine 2,3-dioxygenase (IDO), granzyme B (GrB) and the programmed-death ligand 1 (PD-L1) DC can inhibit T cell expansion and induce regulatory T cells. House dust mite (HDM) allergy is among the most common respiratory allergies worldwide. The only available treatment capable of restoring allergen tolerance is allergen-specific immunotherapy (AIT). The mechanisms underlying the dysregulation of the immune response in allergic disease, as well as those involved in successful AIT, are not fully understood. This study aims to assess the role of DC subsets in the HDM-allergic immune response and in restoring allergen tolerance through AIT.

Methods Dendritic cells were obtained from peripheral blood samples, and their surface phenotype was assessed using flow cytometry. The uptake of the major HDM allergen Der p 1 was additionally analyzed using confocal microscopy. To assess IDO activity, substrate (tryptophan) and metabolite (kynurenine) concentrations were measured by enzyme-linked immunosorbent assay (ELISA) in the serum of healthy controls (HC), HDM-allergic (HDMA) individuals, and HDM-AIT patients.

Results and conclusions A time- and concentration-dependent uptake of the major HDM allergen Der p 1 by pDC was observed. Confocal microscopy demonstrated a partial colocalization of intracellular Der p 1 with early- and late-endosomal markers in pDC. However, the receptors and mechanisms involved in the uptake of Der p 1 remain to be elucidated. In unstimulated pDC, the surface phenotype appeared to be potentially more immature in HDMA with a lower expression of MHC class I. Incubation with the HDM allergen Der p 1 did not affect the pDC surface phenotype. Upon incubation in IL-3-containing medium, pDC from HC significantly upregulated PD-L1 and GrB while pDC from HDMA did not. Similarly, the upregulation of PD-L1 in mDC was by trend lower in HDMA, suggesting a less tolerogenic phenotype of DC from HDMA. AIT induced minor changes in the pDC surface phenotype, including an enhancement in CD86 expression. Additionally, PD-L1 expression in response to toll-like receptor (TLR) 9 stimulation increased with the duration of AIT. IDO expression by pDC and mDC, as assessed by flow cytometry, was found to be similar in HC, HDMA and HDM-AIT groups. Serum levels of IDO substrate and metabolite concentrations were similar in HC and HDMA, but metabolite levels were increased in individuals undergoing AIT. Together, this suggests that increased IDO activity in cells other than DC might be involved in the restoration of allergen tolerance through AIT.