Mycoplasma immunoglobulin binding protein universally binds human antibodies, thereby reducing Fab arm flexibility and displacing antigens in immune complexes

Mycoplasma immunoglobulin binding (MIB) protein plays a key role in immune evasion across several Mycoplasma species. MIB tightly interacts with the Fab domain of immunoglobulin G, inducing conformational changes that disrupt the antigen-binding site. Here, we reveal that MIB binds strongly and universally to all major isotypes and subclasses of human antibodies and can thus serve as a bait protein to efficiently deplete antibodies from serum. Our results demonstrate strong interactions between MIB and a diverse set of both recombinant monoclonal and endogenous polyclonal antibodies, the latter purified from serum or colostrum. All antibodies, including IgG and IgA monomers, J-chain coupled IgA dimers, and IgM pentamers, consistently bind two copies of MIB per antibody protomer. Using cross-linking mass spectrometry, we pinpoint that antibody-MIB interactions involve conserved regions of the variable and constant antibody domains, independent of isotype. Furthermore, we demonstrate that MIB not only disrupts the Fab-antigen interaction but also restricts Fab flexibility, as visualized by atomic force microscopy. Conversely, MIB does not affect Fc-mediated protein interactions, as exemplified by the successful reconstitution of a 36-subunit immune complex, (IgG)6-(MIB)12-C1q.