A Competitive Mechanism for Staphylococcal Toxin SSL7 Inhibiting the Leukocyte IgA Receptor, FcαRI, Is Revealed by SSL7 Binding at the Cα2/Cα3 Interface of IgA

Abstract

Leukocyte recruitment and effector functions like phagocytosis and respiratory burst are key elements of immunity to infection. Pathogen survival is dependent upon the ability to overwhelm, evade or inhibit the immune system. Pathogenic group A and group B streptococci are well known to produce virulence factors that block the binding of IgA to the leukocyte IgA receptor, Fc␣RI, thereby inhibiting IgA-mediated immunity. Recently we found Staphylococcus aureus also interferes with IgA-mediated effector functions as the putative virulence factor SSL7 also binds IgA and blocks binding to Fc␣RI. Herein we report that SSL7 and Fc␣RI bind many of the same key residues in the Fc region of human IgA. Residues Leu-257 and Leu-258 in domain C␣2 and residues 440 -443 PLAF in C␣3 of IgA lie at the C␣2/C␣3 interface and make major contributions to the binding of both the leukocyte receptor Fc␣RI and SSL7. It is remarkable this S. aureus IgA binding factor and unrelated factors from streptococci are functionally convergent, all targeting a number of the same residues in the IgA Fc, which comprise the binding site for the leukocyte IgA receptor, Fc␣RI.Staphylococcus aureus, a commensal organism of the human skin and nose (1), is also a significant human pathogen responsible for conditions such as Scarlet fever, toxin shock, septicemia, and endocarditis. The S. aureus genome contains three clusters of superantigen and superantigen-like genes, designated SaPIn1, -2, and -3 (2). SaPIn2 contains the Staphylococcus superantigen-like (ssl) genes, previously designated as staphylococcal enterotoxin-like (SET) genes (3). These genes are highly represented in clinical isolates of S. aureus and are inferred to contribute to pathogenicity of these strains (4). Crystallographic studies of SSL5 (SET3) (5) and SSL7 (SET1) (6, 7) proteins have indicated structural similarity to classical superantigens with the SSLs also comprising an OB-fold and ␤-grasp domain. We have recently defined two binding activities of the SSL7 protein demonstrating that SSL7 binds simultaneously to human complement factor C5 and IgA (8). The binding of IgA by SSL7 inhibited IgA binding to the leukocyte IgA receptor Fc␣RI (CD89) thus providing a potential mechanism for evasion of IgA-mediated cellular immunity, via the blockade of IgA-mediated leukocyte effector functions such as phagocytosis and respiratory burst (9 -11). General similarities with staphylococcal protein A are clear which, although an unrelated protein, is also capable of multiple interactions targeting the host immune response, including binding TNFR1 (12), C1qR (13), and IgG via V H 3 Fabs (14) and the Fc at the C␥2/C␥3 interface region (15). This study investigated the site of human IgA binding to SSL7. SSL7 was found to bind to the C␣2/C␣3 interface of IgA Fc and require residues essential for the interaction of Fc␣RI with IgA. Thus the SSL7 and Fc␣RI binding sites are likely to be the same or at least closely overlapping. Furthermore the interaction of SSL7 was independent of the IgA Asn-263 li...

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

“…We previously identified the C␣2/C␣3 junction, and in particular the C␣3 domain FG loop sequence PLAF (Fc residues 440-443) were critical for binding to SSL7 and Fc␣RI (23). This finding is now confirmed in the three-dimensional structure of SSL7 and Fc, where both L441 and F443 are prominent interface residues (Table 2 and Figs.…”

“…WT and mutant IgA1 Fc regions were expressed on CHOP cells as fusion proteins with the transmembrane region of the transferrin receptor and assayed for SSL7 and Fc␣RI-Ig-binding activities as described previously (23).…”

“…Both SSL5 (21) and SSL11 (M. C. Chung, B.D.W., H. Baker, R. J. Langley, E. N. Baker, and J.D.F., unpublished data) interact with sialyl-Le x and related oligosaccharides, thereby inhibiting key cellular adhesion processes such as neutrophil transmigration. Notably, SSL7 exhibits multiple activities, including the binding of complement component C5 and serum and mucosal forms of IgA, thereby inhibiting both C5 and fragment crystalline (Fc) receptor for IgA (Fc␣RI)-mediated immunity (22,23). Furthermore, SSL7 has been observed to alter cytokine secretion (12) and to bind, and be rapidly internalized by, monocytes and dendritic cells, but the ligands involved are yet to be identified (16,19).…”

Specificity of Staphylococcal Superantigen-Like Protein 10 toward the Human IgG1 Fc Domain

“…The SSLs are not T cell mitogens like sAgs (15,17,18) but do target components of immunity, including complement (18), the IgA FcR-mediated Ab interaction (19), and glycoproteins expressing the trisaccharide sialyllactosamine (20). A subgroup of SSLs consisting of SSL2, 3,4,5,6, and 11 share a conserved sialyllactosamine binding site (21) allowing targeted binding to a number of different glycoproteins, both on the cell membrane and in plasma.…”

“…sAgs bind the MHC class II and TCR. SSL7 binds to the Ca2/Ca3 region of IgA Fc through its N-terminal OB-fold domain (19,22) and complement factor C5 via the Cterminal b-grasp domain, making it a potent inhibitor of end-stage complement and IgA-FcaRI-mediated activities (18,23).…”