Structural basis for evasion of IgA immunity by Staphylococcus aureus revealed in the complex of SSL7 with Fc of human IgA1

Abstract

Infection by Staphylococcus aureus can result in severe conditions such as septicemia, toxic shock, pneumonia, and endocarditis with antibiotic resistance and persistent nasal carriage in normal individuals being key drivers of the medical impact of this virulent pathogen. In both virulent infection and nasal colonization, S. aureus encounters the host immune system and produces a wide array of factors that frustrate host immunity. One in particular, the prototypical staphylococcal superantigen-like protein SSL7, potently binds IgA and C5, thereby inhibiting immune responses dependent on these major immune mediators. We report here the three-dimensional structure of the complex of SSL7 with Fc of human IgA1 at 3.2 A resolution. Two SSL7 molecules interact with the Fc (one per heavy chain) primarily at the junction between the Calpha2 and Calpha3 domains. The binding site on each IgA chain is extensive, with SSL7 shielding most of the lateral surface of the Calpha3 domain. However, the SSL7 molecules are positioned such that they should allow binding to secretory IgA. The key IgA residues interacting with SSL7 are also bound by the leukocyte IgA receptor, FcalphaRI (CD89), thereby explaining how SSL7 potently inhibits IgA-dependent cellular effector functions mediated by FcalphaRI, such as phagocytosis, degranulation, and respiratory burst. Thus, the ability of S. aureus to subvert IgA-mediated immunity is likely to facilitate survival in mucosal environments such as the nasal passage and may contribute to systemic infections.

Fig. 1.

Crystal structure of SSL7 bound to Fc of human IgA1. (A) Ribbons-style representation with the IgA-Fc homodimer (heavy chains, magenta and cyan; carbohydrates, orange CPK spheres) and the two SSL7 molecules (yellow and red) with secondary structure displayed. (B) Solvent-accessible surface view of the SSL7 complex with IgA-Fc.

Fig. 2.

Conserved SSL7 interactions with the Fc of human IgA1. Only those residues that participated in atomic contacts or hydrogen bonding (dashed lines) for both SSL7 (yellow) molecules bound to IgA-Fc (magenta) are displayed. A full list of residues involved in the different interfaces is presented in Table 2.

Fig. 3.

Surface views of binding regions on SSL7 and the Fc of human IgA1. (A) Residues at the interface (≤4 Å) are mapped to the molecular surfaces of the Fc (chain A, cyan; chain B, orange) and SSL7 (chain D, yellow) for one complex of SSL7 and the Fc. (B) Side view of the interaction between SSL7 and the Fc.

Fig. 4.

Mutagenesis of key residues on the Fc of human IgA1 confirms overlapping binding sites for SSL7 and FcαRI. (A–C) The IgA-Fc fusion proteins (WT and mutants, L256A, L257A, L258A, N316A, and H317A) were transiently expressed on CHOP cells and were analyzed by flow cytometry for binding of SSL7 (A), FcαRI-Ig fusion (B), and anti-IgA (C). The anti-IgA polyclonal antiserum binding of the various IgA-Fc fusion proteins was comparable with WT (90–103%) or modestly reduced (L256A mutant, 87%; L257A mutant, 71%). Binding data (mean ± SD; n = 4) are expressed as percent normalized by WT IgA Fc. ANOVA (ns, not significant P > 0.05; *, P < 0.05; ***, P < 0.001) was performed with a Dunnett multiple comparison test (Prism 5 version 5.00, GraphPad Software, Inc., San Diego, CA) on data before normalization.

Fig. 5.

Comparison of the structures of SSL7 and FcαRI bound to Fc of human IgA1. (A and B) Surface views of the SSL7 complex (A) and FcαRI complex (B) (PDB code 1OW0) bound to IgA-Fc are shown. (C) Electron density (F_o − F_c at 2.5 σ) for the SSL7-bound Fc with the regions of polypeptide containing the Cα2 domain disulfides omitted during map calculation. (D and E) Disulfide bridges are shown for the Cα2 domain pairings of IgA-Fc structures bound to SSL7 (D) and FcαRI (E).