A novel monoclonal antibody against FbaA can inhibit the binding of the complement regulatory protein factor H to group A streptococcus.

Some microbial pathogens utilize human complement regulatory proteins, such as factor H (FH) and factor H-like protein 1 (FHL-1), for immune evasion. FbaA is an FHL-1 and FH binding protein expressed on the surface of group A streptococcus (GAS), a common agent of pharyngeal, skin, and soft tissue infections. In this study, we prepared monoclonal antibodies (MAbs) against FbaA, assayed them for specificity, and located their binding domains in FbaA. We found an MAb called FbaA MAb2, which demonstrated the highest affinity to GAS among all of the MAbs. Based on the binding with component peptides, the detected epitope, which was specific for FbaA MAb2, was the amino acid residues 95 to 118 of FbaA; on the other hand, it did not bind with the truncated protein of the internally deleted residues of the segment from 95 to 118 of FbaA. Furthermore, the predominant amino acids specific for FbaA MAb2 screened by phage display epitope library were I, T, P, D, and L, corresponding to the amino acid residues 101, 103, 105, 106, and 110 of FbaA, respectively. The binding location of FbaA with FH and FHL-1 was a 16-amino-acid region corresponding to amino acid residues 97 to 112 of FbaA, which overlapped the FbaA MAb2 binding domain, as confirmed by competitive inhibition enzyme-linked immunosorbent assay and immunofluorescence microscopy. Based on the results of the invasion assay, FbaA MAb2 can inhibit the binding of FH to GAS.