Moraxella catarrhalis-dependent tonsillar B cell activation does not lead to apoptosis but to vigorous proliferation resulting in nonspecific IgM production.

The respiratory pathogen Moraxella catarrhalis has a high affinity for human IgD and is mitogenic for peripheral blood B lymphocytes. Moraxella IgD-binding protein, which is a multifunctional outer membrane protein with adhesive properties, is responsible for the interaction. Previous experiments with the Ig-binding B cell superantigens protein A and protein L from Staphylococcus aureus and Peptostreptococcus magnus, respectively, have suggested that nonimmune BCR cross-linking induces B cell apoptosis through the intrinsic pathway. The goal of this study was to characterize early and late B cell events in the presence of M. catarrhalis in comparison with S. aureus. Despite an increased phosphatidyl serine translocation as revealed by Annexin V binding in flow cytometry analyses, neither M. catarrhalis nor S. aureus induced activation-associated apoptotic cell death in purified human tonsillar B cells. In contrast, a vigorous B cell proliferation, as quantified using thymidine incorporation and CFSE staining, was observed. An increased expression of an array of surface proteins (i.e., CD19, CD21, CD40, CD45, CD54, CD69, CD86, CD95, and HLA-DR) and IgM production was found upon activation with M. catarrhalis. In conclusion, M. catarrhalis-dependent B cell activation does not result in apoptosis but in cell division and nonspecific IgM synthesis, suggesting that the bacterial interaction with tonsillar B cells serves to redirect the early adaptive immune response.