The IgD-binding domain of the Moraxella IgD-binding protein MID (MID962-1200) activates human B cells in the presence of T cell cytokines.

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 (MID962-1200). To prove the specificity of MID962-1200 for IgD, a Chinese hamster ovary (CHO) cell line expressing membrane-anchored human IgD was manufactured. MID962-1200 bound strongly to the recombinant IgD on CHO cells. Moreover, MID962-1200 stimulated peripheral blood lymphocyte (PBL) proliferation 5- and 15-fold at 0.1 and 1.0 mu g/ml, respectively. This activation could be blocked completely by antibodies directed against the CD40 ligand (CD154). MID962-1200 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 MID962-1200, as revealed by a human cytokine protein array. MID962-1200 fused to green fluorescent protein (GFP) bound to human B cells and activated PBL to the same degree as MID962-1200 Taken together, MID is the only IgD-binding protein in Moraxella. Furthermore, the novel T cell-independent antigen MID962-1200 may, together with MID962-1200-GFP, be considered as promising reagents in the study of IgD-dependent B cell activation