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.

Structural studies of complexes of T cell receptor (TCR) and peptide-major histocompatibility complex (MHC) have focused on TCRs specific for foreign antigens or native self. An unexplored category of TCRs includes those specific for self determinants bearing alterations resulting from disease, notably cancer. We determined here the structure of a human melanoma-specific TCR (E8) bound to the MHC molecule HLA-DR1 and an epitope from mutant triosephosphate isomerase. The structure had features intermediate between 'anti-foreign' and autoimmune TCR-peptide-MHC class II complexes that may reflect the hybrid nature of altered self. E8 manifested very low affinity for mutant triosephosphate isomerase-HLA-DR1 despite the highly tumor-reactive properties of E8 cells. A second TCR (G4) had even lower affinity but underwent peptide-specific formation of dimers, suggesting this as a mechanism for enhancing low-affinity TCR-peptide-MHC interactions for T cell activation.

The staphylococcal enterotoxin-like toxins (SETs) are a family of proteins encoded within the Staphylococcus aureus genome that were identified by their similarity to the well described bacterial superantigens. The first crystal structure of a member of the SET family, SET3, has been determined to 1.9 Å (R ‫؍‬ 0.205, R free ‫؍‬ 0.240) and reveals a fold characteristic of the superantigen family but with significant differences. The SET proteins are secreted at varying levels by staphylococcal isolates, and seroconversion studies of normal individuals indicate that they are strongly antigenic to humans. Recombinant SETs do not exhibit any of the properties expected of superantigens such as major histocompatibility complex class II binding or broad Tcell activation, suggesting they have an entirely different function. The fact that the whole gene family is clustered within the pathogenicity island SaIn2 of the S. aureus genome suggests that they are involved in host/ pathogen interactions.The bacterial superantigen (SAg) 1 family is a large protein family exclusive to three pathogenic species: Staphylococcus aureus, Streptococcus pyogenes, and Streptococcus equi. The former two organisms are opportunistic human pathogens, and the latter is a pathogen of horses. Members of this family of proteins have been implicated in a range of human diseases, including staphylococcal food poisoning, scarlet fever, toxic shock, rheumatoid arthritis, and secondary HIV infection (1). S. equi is the causative agent of strangles in horses (2). Superantigens function by immune modulation. They cross-link major histocompatibility complex class II (MHC-II) and T-cell receptor (TCR) molecules, causing nonspecific and disproportionate T-cell proliferation and cytokine release (3). This gives rise to symptoms characteristic of fever and toxic shock. As a result of their close association with serious human pathologies, superantigens have been the subject of intense research over the last decade during which several reviews have been published (3-5).The identification of a staphylococcal gene cluster (6) and the determination of the complete genome sequence of S. aureus (7) have revealed a family of genes with similarity to superantigens from S. aureus. These were first described by Williams and colleagues (6) as a cluster of five related genes in which the protein products were reported to stimulate the production of interleukin-1␤, interleukin-6, and tumor necrosis factor ␣ from human peripheral blood mononuclear cells. The complete genome sequences for two strains of S. aureus subsequently showed that this cluster contains a total of 10 genes in one strain and 9 in the second strain, with these genes related by sequence identities of 36 -67%. The SET gene cluster represents half of a pathogenicity island, SaPIn2, shown in Fig. 1. Downstream (3Ј) to the SET gene cluster is a set of nine lpl genes that contain lipoprotein attachment sites and are thought to code for pathogenic proteins (7). The pathogenicity island is flanked at the 5Ј en...

SummaryStaphylococcus aureus is a major pathogen that produces a family of 14 staphylococcal superantigen-like (SSL) proteins, which are structurally similar to superantigens but do not stimulate T cells. SSL11 is one member of the family that is found in all staphylococcal strains. Recombinant SSL11 bound to granulocytes and monocytes through a sialic aciddependent mechanism and was rapidly internalized. SSL11 also bound to sialic acid-containing glycoproteins, such as the Fc receptor for IgA (FcaRI) and P-selectin glycoprotein ligand-1 (PSGL-1), and inhibited neutrophil attachment to a P-selectin-coated surface. Biosensor analysis of two SSL11 alleles binding to sialyl Lewis X [sLe x -Neu5Aca2-3Galb1-4(Fuc1-3)GlcNAc] coupled to bovine serum albumin gave dissociation constants of 0.7 and 7 mm respectively. Binding of SSL11 to a glycan array revealed specificity for glycans containing the trisaccharide sialyllactosamine (sLacNac -Neu5Aca2-3Galb1-4GlcNAc). A 1.6 Å resolution crystal structure of SSL11 complexed with sLe x revealed a discrete binding site in the C-terminal b-grasp domain, with predominant interactions with the sialic acid and galactose residues. A single amino acid mutation in the carbohydrate binding site abolished all SSL11 binding. Thus, SSL11 is a staphylococcal protein that targets myeloid cells by binding sialyllactosaminecontaining glycoproteins.

Human growth hormone (GH) is a classical pituitary endocrine hormone that is essential for normal postnatal growth and has pleiotropic effects across multiple physiological systems. GH is also expressed in extrapituitary tissues and has localized autocrine/paracrine effects at these sites. In adults, hypersecretion of GH causes acromegaly, and strategies that block the release of GH or that inhibit GH receptor (GHR) activation are the primary forms of medical therapy for this disease. Overproduction of GH has also been linked to cancer and the microvascular complications that are associated with diabetes. However, studies to investigate the therapeutic potential of GHR antagonism in these diseases have been limited, most likely due to difficulty in accessing therapeutic tools to study the pharmacology of the receptor in vivo. This review will discuss current and emerging strategies for antagonizing GH function and the potential disease indications.

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Background: 5Ј-Nucleotidases are important virulence factors found in several bacterial pathogens. Results: Streptococcal 5Ј-nucleotidase A (S5nA) generated immunomodulatory molecules adenosine and deoxyadenosine and rescued Lactococcus lactis in a blood killing assay. Conclusion: S5nA is a novel Streptococcus pyogenes virulence factor that facilitates immune evasion from the host. Significance: S5nA might be a target for developing new therapeutics or vaccines.

The conserved diagonal docking mode observed in structures of Tcell receptors (TCRs) bound to peptide-MHC ligands is believed to reflect coevolution of TCR and MHC genes. This coevolution is supported by the conservation of certain interactions between the germ-line-encoded complementarity-determining region (CDR)1 and CDR2 loops of TCR and MHC. However, the rules governing these interactions are not straightforward, even when the same variable (V) region recognizes the same MHC molecule. Here, we demonstrate that the somatically generated CDR3 loops can markedly alter evolutionarily selected contacts between TCR and MHC ("CDR3 editing"). To understand CDR3 editing at the atomic level, we determined the structure of a human melanoma-specific TCR (G4) bound to the MHC class II molecule HLA-DR1 and an epitope from mutant triose phosphate isomerase (mutTPI). A comparison of the G4-mutTPI-DR1 complex with a complex involving a TCR (E8) that uses the same Vα region to recognize the same mutTPI-DR1 ligand as G4 revealed that CDR1α adopts markedly different conformations in the two TCRs, resulting in an almost entirely different set of contacts with MHC. Based on the structures of unbound G4 and E8, the distinct conformations of CDR1α in these TCRs are not induced by binding to mutTPI-DR1 but result from differences in the length and sequence of CDR3α that are transmitted to CDR1α. The editing of germ-line-encoded TCR-MHC interactions by CDR3 demonstrates that these interactions possess sufficient intrinsic flexibility to accommodate large structural variations in CDR3 and, consequently, in the TCR-binding site.T -cell receptors (TCRs) bind peptide-MHC (pMHC) via their six complementarity-determining region (CDR) loops, three from the variable alpha (Vα) domain and three from Vβ. The first and second CDRs (CDR1 and CDR2) are encoded within the TCR Vα and Vβ gene segments; the third CDR (CDR3) is formed by DNA recombination involving juxtaposition of Vα and Jα segments for the α chain genes, and of Vβ, D, and Jβ segments for the β chain genes. As a result, the somatically generated Vα and Vβ CDR3 loops account for most of the variability of TCRbinding sites, whereas variability contributed by CDR1 and CDR2 is restricted to that already existing in the germ line.Structural studies of numerous (>25) TCR-pMHC complexes have demonstrated remarkable similarities in the overall topology of TCR binding, irrespective of MHC class I or class II restriction (1-3). Typically, the TCR is positioned diagonally over the center of the composite surface created by the peptide and the MHC α-helices that flank the peptide-binding groove, with the Vα domain situated over the N-terminal half of the peptide and the Vβ domain over the C-terminal half, although the exact angle and pitch of TCR engagement vary (1). In addition, these studies revealed the conservation of specific TCR-MHC interactions in complexes involving common V segments and MHC alleles (2, 4-6). For example, crystal structures of six mouse TCRs expressing Vβ8.2 bound to I-A...