Leukocyte migration is a key event both in host defense against invading pathogens as well as in inflammation. Bacteria generate chemoattractants primarily by excretion (formylated peptides), complement activation (C5a), and subsequently through activation of leukocytes (e.g., leukotriene B4, platelet-activating factor, and interleukin 8). Here we describe a new protein secreted by Staphylococcus aureus that specifically impairs the response of neutrophils and monocytes to formylated peptides and C5a. This chemotaxis inhibitory protein of S. aureus (CHIPS) is a 14.1-kD protein encoded on a bacteriophage and is found in >60% of clinical isolates. CHIPS reduces the neutrophil recruitment toward C5a in a mouse peritonitis model, even though its activity is much more potent on human than on mouse cells. These findings suggest a new immune escape mechanism of S. aureus and put forward CHIPS as a potential new antiinflammatory therapeutic compound.

IntroductionStaphylococcus aureus is a common human pathogen that induces both community-acquired and nosocomial infections. This Grampositive bacterium is well known for its suppurative diseases such as skin-limited abscesses and boils and more seriously endocarditis, sepsis, and toxic shock syndrome. 1,2 Its invasiveness is ascribed to the production of a wide repertoire of cell surface-expressed as well as secreted virulence factors that interfere with host defense. 2,3 Superantigens constitute a large portion of the secreted arsenal of staphylococci and modulate immune responses. They trigger nonspecific activation of T lymphocytes by binding to major histocompatibility complex (MHC) class II molecules on antigenpresenting cells outside the antigen-binding cleft and V ␤ domains of T-cell receptors (TCRs). 4 We have described chemotaxis inhibitory protein of S aureus (CHIPS), an excreted virulence factor of S aureus. 5,6 CHIPS is known to inhibit formylated peptide-and complement factor C5a-induced responses in neutrophils through direct binding to the formyl peptide receptor and C5a receptor (C5aR), respectively. 7 Thereby, CHIPS inhibits the initial activation and migration of neutrophils to the site of infection; thus, it hampers clearance of S aureus by innate immune cells. Recently, the structure of CHIPS consisting of residues 31 to 121 (CHIPS ) was resolved. 8 CHIPS is composed of an ␣-helix packed onto a 4-stranded antiparallel ␤-sheet, a domain also present in the C-terminal domain of superantigens. This protein also revealed to be homologous to the C-terminal domain of staphylococcal superantigen-like 5 (SSL5) and SSL7.SSLs are a family of secreted proteins identified through sequence homology to staphylococcal and streptococcal superantigens. 9 Eleven different SSLs exist that are encoded on staphylococcal pathogenicity island 2 in a conserved order. Staphylococci contain 7 to 11 different SSLs, and their homology varies between 36% and 67%. Allelic variants show 85% to 100% homology. 10,11 Determination of the crystal structures of SSL5 12 and SSL7 13 also revealed their high structural homology to superantigens; the N-terminal oligonucleotide/oligosaccharide-binding fold and the C-terminal ␤-grasp domain characteristic for superantigens are also observed in SSLs. However, residues important for MHC class II and TCR binding of superantigens are not conserved in SSLs, which may explain their inability to display superantigenic activities. 9,10,12 Recently, Langley et al 14 described binding of complement component 5 and immunoglobulin A (IgA) by SSL7, suggesting a role for SSLs in staphylococcal defense against host immune responses. SSL7 was subsequently found to bind the C␣2/C␣3 interface of IgA Fc, which is the adhesion site for the Fc␣RI. 15 So far, no other functions have been linked to the SSLs.Neutrophil recruitment to sites of infection is a multistep process. 16 The initial tethering and rolling of neutrophils on the endothelium of vessel walls during inflammation are mediated by P-selec...

Staphylococcus aureus excretes a factor that specifically and simultaneously acts on the C5aR and the formylated peptide receptor (FPR). This chemotaxis inhibitory protein of S. aureus (CHIPS) blocks C5a- and fMLP-induced phagocyte activation and chemotaxis. Monoclonal anti-CHIPS Abs inhibit CHIPS activity against one receptor completely without affecting the other receptor, indicating that two distinct sites are responsible for both actions. A CHIPS-derived N-terminal 6 aa peptide is capable of mimicking the anti-FPR properties of CHIPS but has no effect on the C5aR. Synthetic peptides in which the first 6 aa are substituted individually for all other naturally occurring amino acids show that the first and third residue play an important role in blocking the FPR. Using an Escherichia coli expression system, we created mutant CHIPS proteins in which these amino acids are substituted. These mutant proteins have impaired or absent FPR- but still an intact C5aR-blocking activity, indicating that the loss of the FPR-blocking activity is not caused by any structural impairment. This identifies the first and third amino acid, both a phenylalanine, to be essential for CHIPS blocking the fMLP-induced activation of phagocytes. The unique properties of CHIPS to specifically inhibit the FPR with high affinity (kd = 35.4 ± 7.7 nM) could be an important new tool to further stimulate the fundamental research on the mechanisms underlying the FPR and its role in disease processes.

Chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) is excreted by the majority of S. aureus strains and is a potent inhibitor of C5a-and formylated peptide-mediated chemotaxis of neutrophils and monocytes. Recently, we reported that CHIPS binds to the C5a receptor (C5aR) and the formylated peptide receptor, thereby blocking activation by C5a and formylated peptides, respectively. The anaphylatoxin C5a plays an important role in host immunity and pathological inflammatory processes. For C5a a two-site binding model is proposed in which C5a initially binds the C5aR N terminus, followed by interaction of the C5a C-terminal tail with an effector domain on the receptor. We have shown here that CHIPS does not affect activation of the C5aR by a peptide mimic of the C5a C terminus. Moreover, CHIPS was found to bind human embryonic kidney 293 cells expressing only the C5aR N terminus. Deletion and mutation experiments within this C5aR N-terminal expression system revealed that the binding site of CHIPS is contained in a short stretch of 9 amino acids (amino acids 10 -18