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The IgD CH1 region contains the binding site for the human respiratory pathogen Moraxella catarrhalis IgD‐binding protein MID

The Moraxella catarrhalis IgD‐binding protein (MID) has a unique specificity for human IgD, and the sequence with maximal IgD binding is located within the amino acids MID962–1200. In the present paper, we examined the MID binding site on IgD using a series of recombinant Ig. Full‐length IgD, IgD F(ab’)2, and an IgD F(ab’) C290R mutant lacking the inter‐heavy‐chain cysteine 290 were manufactured. Furthermore, a series of IgD/IgG chimeras were constructed. ELISA, dot blot and flow cytometry were used to study the binding of purified Ig to native MID, recombinant MID962–1200 or to Moraxella with or without MID. MID962–1200 bound both the IgD F(ab’)2 and F(ab’) C290R, indicating that the binding occurred independently of antibody structure. When amino acids 157–224 of the IgD CH1 region were substituted with IgG sequences, binding by M. catarrhalis or recombinant MID962–1200 was abolished. Subsequent smaller substitutions of IgD CH1 157–224 with IgG sequences led us to conclude that IgD CH1 amino acids 198–206 were crucial for the interaction between MID and IgD.