Staphylococcal SSL5 inhibits leukocyte activation by chemokines and anaphylatoxins

Abstract

Staphylococcus aureus secretes several virulence factors modulating immune responses. Staphylococcal superantigenlike (SSL) proteins are a family of 14 exotoxins with homology to superantigens, but with generally unknown function. Recently, we showed that SSL5 binds to P-selectin glycoprotein ligand 1 dependently of sialyl Lewis X and inhibits Pselectin-dependent neutrophil rolling. Here, we show that SSL5 potently and specifically inhibits leukocyte activation by anaphylatoxins and all classes of chemokines. SSL5 inhibited calcium mobilization, actin polymerization, and chemotaxis induced by chemokines and anaphylatoxins but not by other chemoattractants. Antibody competition experiments showed that SSL5 targets several chemokine and anaphylatoxin receptors. IntroductionStaphylococcus aureus is a common human pathogen that causes severe community-acquired and nosocomial infections. This Grampositive bacterium produces a wide variety of virulence proteins that aid in its infection, colonization, and disease progress. These surface-associated and secreted proteins often target the immune system. 1 Chemotaxis inhibitory protein of S aureus (CHIPS) is such an excreted virulence factor of S aureus. CHIPS binds the formyl peptide receptor (FPR) and the C5a receptor (C5aR) and thereby inhibits cell activation by formylated peptides and C5a, respectively. 2,3 Consequently, it attenuates the initial activation and migration of neutrophils to the site of infection. Structurally, CHIPS shows homology to the C-terminal domain of staphylococcal superantigen-like (SSL) proteins. 4 SSLs are a family of 14 exoproteins. Eleven of them are encoded on staphylococcal pathogenicity island 2 present in all S aureus strains, and the remaining 3 are found on the immune evasion cluster 2. 5 They were identified through sequence and structural homology with superantigens, but they do not show superantigenic activity. 6 Latest evidence suggests that SSLs play a role in immune evasion by other means. SSL7 was described to bind C5 and immunoglobulin A (IgA) blocking the complement system and Fc␣RI binding, respectively. 7 Recently, we described that SSL5 binds to P-selectin glycoprotein ligand 1 (PSGL-1). 8 Thereby, SSL5 inhibits neutrophil rolling on endothelial cells by disrupting the interaction of PSGL-1 with its natural ligand P-selectin. Sugar moieties were shown to be crucial in this effect. Recently, the crystal structure of SSL5 in complex with sialyl Lewis X (sLex) was determined. 9 SSL11 was also shown to interact with sLex, 10 and both proteins bind sLex through their C-terminal domain. Therefore, at least a subset of SSLs affects the immune system by targeting glycoproteins.The immune response rapidly reacts to invading pathogens. The response is initiated by extravasation of leukocytes from the blood stream to the site of infection. It is regulated by various chemotactic factors that are produced endogenously by endothelial cells and leukocytes as well as exogenously by the invading pathogen. These comprise the chemoattracta...

Targeting fundamental pathways to disrupt Staphylococcus aureus survival: clinical implications of recent discoveries

“…Similar chemotactic inhibitory functions have since been reported for inhibitors of formyl peptide receptor-like 1, another strong mediator of phagocyte homing (81,82). SSL5 potently inhibits in vitro chemokine-induced activation of human leukocytes and selectin-dependent neutrophil rolling, thereby adding to the broad armamentarium of antiphagocyte functions of S. aureus (83).…”

Staphylococcal Superantigen-Like Protein 3 Binds to the Toll-Like Receptor 2 Extracellular Domain and Inhibits Cytokine Production Induced by Staphylococcus aureus, Cell Wall Component, or Lipopeptides in Murine Macrophages

“…SSL7 binds to IgA and C5 (21); SSL5 binds to PSGL-1 (4), chemoattractant receptors (5), and MMP-9 (15), and SSL10 binds to chemokine receptor (32) and human IgG (16,23). Therefore, the SSL family is able to interfere with multiple points of host immunity, especially humoral immunity, opsonization, and trafficking of leukocytes.…”

Structural basis for inhibition of TLR2 by staphylococcal superantigen-like protein 3 (SSL3)

“…Interestingly, the functions that have been discovered for SSLs so far have all been linked to immune evasion. SSL5 inhibits neutrophil extravasation (17,18) and phagocyte function (19,20), SSL7 binds IgA and inhibits complement (21), and SSL10 inhibits IgG1-mediated phagocytosis (22,23), blood coagulation (24), and the chemokine receptor CXCR4 (25). In addition to SSL3, also weak TLR2 inhibitory activity was observed for SSL4 (5), but it remains unknown whether that is its dominant function.…”