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Published online before print November 21, 2005

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(Journal of Leukocyte Biology. 2006;79:319-329.)
© 2006 by Society for Leukocyte Biology

The IgD-binding domain of the Moraxella IgD-binding protein MID (MID^962-1200) activates human B cells in the presence of T cell cytokines

Medical Microbiology, Department of Laboratory Medicine, Malmö University Hospital, Lund University, Malmö, Sweden

¹Correspondence: Medical Microbiology, Department of Laboratory Medicine, Malmö University Hospital, Lund University, SE-205 02, Malmö, Sweden. E-mail: kristian.riesbeck{at}med.lu.se

Moraxella catarrhalis immunoglobulin D (IgD)-binding protein (MID) is an outer membrane protein with specific affinity for soluble and cell-bound human IgD. Here, we demonstrate that mutated M. catarrhalis strains devoid of MID show a 75% decreased activation of human B cells as compared with wild-type bacteria. In contrast to MID-expressing Moraxella, the MID-deficient Moraxella mutants did not bind to human CD19⁺ IgD⁺ B cells. The smallest MID fragment with preserved IgD-binding capacity comprises 238 amino acids (MID^962-1200). To prove the specificity of MID^962-1200 for IgD, a Chinese hamster ovary (CHO) cell line expressing membrane-anchored human IgD was manufactured. MID^962-1200 bound strongly to the recombinant IgD on CHO cells. Moreover, MID^962-1200 stimulated peripheral blood lymphocyte (PBL) proliferation 5- and 15-fold at 0.1 and 1.0 µg/ml, respectively. This activation could be blocked completely by antibodies directed against the CD40 ligand (CD154). MID^962-1200 also activated purified B cells in the presence of interleukin (IL)-2 or IL-4. An increased IL-6 production was seen after stimulation with MID^962-1200, as revealed by a human cytokine protein array. MID^962-1200 fused to green fluorescent protein (GFP) bound to human B cells and activated PBL to the same degree as MID^962-1200. Taken together, MID is the only IgD-binding protein in Moraxella. Furthermore, the novel T cell-independent antigen MID^962-1200 may, together with MID^962-1200–GFP, be considered as promising reagents in the study of IgD-dependent B cell activation.

Key Words: B lymphocyte • immunoglobulin D • GFP • Moraxella catarrhalis

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