Stereo-selective binding of monoclonal antibodies to the poly-gamma-d-glutamic acid capsular antigen of Bacillus anthracis

• We determined the binding affinities and specificities of several murine mAbs that react with the capsule of Bacillus anthracis.
• Binding of all mAbs was not solely dependent on non-directional ionic forces.
• All mAbs preferentially bound γ-linked d glutamic acid over γ-linked l glutamic acid.
• The murine mAb F26G3 was the most selective for γ-linked d glutamic acid.
• A molecular model of mAb F26G3 shows five glutamic acid residues optimally fill the binding site and that two conformations have 30 ps lifetimes.

Bacillus anthracis is surrounded by an anti-phagocytic capsule that is entirely composed of γ-linked d-glutamic acid (γDPGA). γDPGA is required for virulence and is produced in large quantities following spore germination. We have previously described the isolation of several γDPGA-reactive mAbs. The reagents are effective in both immunoprotection and diagnostic applications. The current work was done to further investigate the specificity of γDPGA-reactive mAbs. The specificity of each mAb was characterized using surface plasmon resonance. Our results indicate that each mAb is stereoselective for binding to d-glutamic acid oligomers, but to varying degrees. In particular, mAb F26G3 is highly selective for γDPGA; alterations in stereochemistry disrupted recognition. These differences in mAb reactivity suggest that binding of γDPGA by mAb F26G3 is more specific than non-directional ionic interactions between a negatively charged antigen and a positively charged antibody.