title: Superantigen- and TLR-dependent activation of tonsillar B cells after receptor-mediated endocytosis.
abstract: Classical B lymphocyte activation is dependent on BCR cross-linking in combination with physical interaction with Th cells. Other B cell molecules that contribute to the activation are complement, cytokine, and TLRs recognizing specific pathogen-associated molecular patterns. Moraxella (Branhamella) catarrhalis is a common Gram-negative respiratory pathogen that induces proliferation in human IgD-expressing B cells independently of T cell help. The activation is initiated by the B cell superantigen Moraxella IgD-binding protein (MID) through a nonimmune cross-linking of IgD. However, IgD cross-linking alone is not sufficient to induce proliferation. In this study, we characterized the significance of TLRs in superantigen-dependent B cell activation using whole bacteria or rMID in the presence or absence of TLR ligands. IgD cross-linking by MID sensitized B cells obtained from children with tonsillar hyperplasia for mainly TLR9, whereas TLRs 1, 2, 6, and 7 were less important. The Moraxella-induced activation was inhibited when a dominant-negative TLR9 ligand was added. Interestingly, BCR-mediated endocytosis of whole Moraxella and degradation of live bacteria in naive B cells were observed with fluorescence, confocal, and transmission electron microscopy. This unique observation proved the strong intracellular TLR9 response as well as highlighted the Ag-presenting function of B cells. In conclusion, our findings suggest an important role of TLRs in the adaptive immune response and reveal novel insights into the T cell-independent B cell activation induced by bacteria.
authors.affiliation: Department of Laboratory Medicine, University Hospital Malmö, Lund University, Malmö, Sweden.
authors.forename: Johan
authors.initials: J
authors.lastname: Jendholm
authors.forename: Matthias
authors.initials: M
authors.lastname: Mörgelin
authors.forename: Maria Laura A
authors.initials: ML
authors.lastname: Perez Vidakovics
authors.forename: Michael
authors.initials: M
authors.lastname: Carlsson
authors.forename: Hakon
authors.initials: H
authors.lastname: Leffler
authors.forename: Lars-Olaf
authors.initials: LO
authors.lastname: Cardell
authors.forename: Kristian
authors.initials: K
authors.lastname: Riesbeck