The staphylococcal superantigen-like proteins (SSLs) are close relatives of the superantigens but are coded for by a separate gene cluster within a 19-kb region of the pathogenicity island SaPIn2. rSSL7 (formally known as SET1) bound with high affinity (KD, 1.1 nM) to the monomeric form of human IgA1 and IgA2 plus serum IgA from primate, pig, rat, and horse. SSL7 also bound the secretory form of IgA found in milk from human, cow, and sheep, and inhibited IgA binding to cell surface FcαRI (CD89) and to a soluble form of the FcαRI protein. In addition to IgA, SSL7 bound complement factor C5 from human (KD, 18 nM), primate, sheep, pig, and rabbit serum, and inhibited complement-mediated hemolysis and serum killing of a Gram-negative organism Escherichia coli. SSL7 is a superantigen-like protein secreted from Staphylococcus aureus that blocks IgA-FcR interactions and inhibits complement, leading to increased survival of a sensitive bacterium in blood.

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 Å resolution. Two SSL7 molecules interact with the Fc (one per heavy chain) primarily at the junction between the C␣2 and C␣3 domains. The binding site on each IgA chain is extensive, with SSL7 shielding most of the lateral surface of the C␣3 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, Fc␣RI (CD89), thereby explaining how SSL7 potently inhibits IgAdependent cellular effector functions mediated by Fc␣RI, 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.Staphylococcus aureus is an important human pathogen causing conditions ranging from minor superficial skin infections to life-threatening syndromes, including sepsis, toxic shock syndrome, osteomyelitis, pneumonitis, and endocarditis. It is carried without symptoms in at least 20% of individuals (1, 2). The emergence in the 1960s of pandemic penicillin-resistant S. aureus has been followed by a variety of hospital-associated and community-associated methicillin-resistant strains (HA-and CA-MRSA) (2, 3). The increased prevalence of MRSA infections and corresponding rise in life-threatening syndromes have made it imperative to elucidate the mechanisms of pathogenesis for S. aureus. The interaction between S. aureus and the host is complex and is mediated by an array of bacterial proteins that both mediate the various pathologies and modify the immune system of the host (4-10).SSL7 (formerly named SET1) is the first described member of a new family of putative S. aureus toxins, the staphylococcal superantigen-like (SSL) proteins (11, 12), related to the staphylococcal enterotoxins (SEs) or superantigens (13). The SSL proteins have Ϸ30% sequence identity with toxic shock syndrome 1 (TSST-1) and 25% or less identity with other SEs. Despite the sequence differences, the SSL proteins have a typical SE tertiary structure consisting of a distinct oligonucleotide/oligosaccharide binding (OB-fold) linked to a ␤-grasp domain (14 -16).Similar to the se genes, the ssl genes are located in a pathogenicity island (SaPIn2) and ...

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...

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