Human β-defensin-3 alters, but does not inhibit, the binding of Porphyromonas gingivalis haemagglutinin B to the surface of human dendritic cells

Human β-defensin-3 (HBD3) is a small, cationic, host defence peptide with broad antimicrobial activities and diverse innate immune functions. HBD3 binds to many microbial antigens and, in this study, we hypothesised that the known binding of HBD3 to Porphyromonas gingivalis recombinant haemagglutinin B (rHagB) alters, but does not inhibit, the binding of rHagB to human dendritic cells. To test this, human myeloid dendritic cells were incubated for 5 min with rHagB, HBD3 + rHagB (10:1 molar ratio), HBD3 or 0.1 M phosphate-buffered saline (PBS) (pH 7.2) and were then rapidly fixed and processed for confocal microscopy and ultramicrotomy. rHagB and HBD3 could be detected with primary monoclonal mouse antibody to rHagB (MoAb 1858) or polyclonal rabbit antibody to HBD3 (P241) and secondary fluorescent-labelled anti-mouse or anti-rabbit antibodies (confocal microscopy) or protein A-colloidal gold (immunoelectron microscopy). In cells incubated with rHagB only, fluorescence and protein A-colloidal gold were seen at the cell surface and throughout the cytoplasm. In cells incubated with HBD3 + rHagB, fluorescence was observed only at the cell surface in a 'string of pearls' configuration. Overall, these results suggest that HBD3 binding to rHagB alters, but does not inhibit, the binding of rHagB to human myeloid dendritic cells.