Functional expression of granzyme B in human plasmacytoid dendritic cells: a role in allergic inflammation.

ABSTRACT

BACKGROUND: Plasmacytoid dendritic cells (pDCs) are involved in a variety of immune functions. However, the expression of cytotoxic granule proteins like granzymes and perforin in human pDCs is still poorly understood. OBJECTIVE: The aim of this study was to systematically analyse the expression and regulation of cytotoxic granule proteins in human pDCs. METHODS: The expression of cytotoxic proteins was analysed by RT-PCR, flow cytometry, and fluorescence microscopy. The functional expression of these proteins was confirmed in a flow-cytometry-based cytotoxicity assay using K562 cells as targets. In order to analyse the regulation of pDC-derived cytotoxic proteins in infectious and allergic diseases, human pDCs were analysed after stimulation with toll-like receptor (TLR)7/9 ligands and in the human asthma model of segmental allergen challenge. RESULTS: Granzyme B (GrB), but not the granzymes A, H, K, M or perforin, was specifically expressed by human pDCs and this GrB expression was up-regulated by IL-3 stimulation. In addition, IL-3-stimulated pDCs were found to kill K562 cells in a GrB- and caspase-dependent manner. TLR7/9 ligands significantly suppressed GrB expression in pDCs. In contrast, there was an up-regulation of GrB in endobronchial pDCs 24 h after allergen challenge, and this was accompanied by enhanced GrB concentrations in bronchoalveolar lavage fluid. CONCLUSION: We report the selective expression of GrB in human pDCs and show for the first time pDC-mediated GrB- and caspase-dependent cytotoxicity against target cells. In addition, the regulation of GrB expression was investigated in vitro and in vivo providing an evidence for a specific role of pDC-derived GrB in allergic inflammation.