Recognition of the Thomsen-Friedenreich pancarcinoma carbohydrate antigen by a lamprey variable lymphocyte receptor

Variable lymphocyte receptors (VLRs) are leucine-rich repeat proteins that mediate adaptive immunity in jawless vertebrates. VLRs were recently shown to recognize glycans, such as the tumor-associated Thomsen-Friedenreich antigen (TFα; Galβ1-3GalNAcα), with a selectivity rivaling or exceeding that of lectins and antibodies. To understand the basis for TFα recognition by one such VLR (VLRB.aGPA.23), we measured thermodynamic parameters for the binding interaction and determined the structure of the VLRB.aGPA.23-TFα complex to 2.2 Å resolution. In the structure, four tryptophan residues form a tight hydrophobic cage encasing the TFα disaccharide that completely excludes buried water molecules. This cage together with hydrogen bonding of sugar hydroxyls to polar side chains explains the exquisite selectivity of VLRB.aGPA.23. The topology of the glycan-binding site of VLRB.aGPA.23 differs markedly from those of lectins or antibodies, which typically consist of long, convex grooves for accommodating the oligosaccharide. Instead, the TFα disaccharide is sandwiched between a variable loop and the concave surface of the VLR formed by the β-strands of the leucine-rich repeat modules. Longer oligosaccharides are predicted to extend perpendicularly across the β-strands, requiring them to bend to match the concavity of the VLR solenoid.

Structural insights into the evolution of the adaptive immune system: the variable lymphocyte receptors of jawless vertebrates

Adaptive immunity in jawless vertebrates is mediated by antigen receptors that are fundamentally different from those of jawed vertebrates. Whereas antibodies and T cell receptors (TCRs) are composed of immunoglobulin (Ig) domains, the variable lymphocyte receptors (VLRs) of jawless fish consist of leucine-rich repeat (LRR) modules. As with antibodies and TCRs, VLRs are assembled by DNA recombination in a process that generates a vast repertoire of receptors. VLRs recognize as diverse an array of particulate and soluble antigens as Ig-based antibodies, and do so with similar affinity and specificity. X-ray crystallographic studies of VLRs in complex with protein and carbohydrate antigens have shown that these LRR-based receptors use nearly all their concave surface to bind ligands, in addition to a highly variable loop in their C-terminal LRR capping module. This structural information, combined with a comprehensive analysis of VLR sequences, has revealed an almost perfect match between antigen-contacting positions and positions with highest sequence diversity. The independent evolution approximately 500 million years ago of LRR-based and Ig-based receptors of comparable diversity and antigen-binding properties provides evidence for the survival value of adaptive immunity in vertebrates.

Structure of a lamprey variable lymphocyte receptor in complex with a protein antigen

Variable lymphocyte receptors (VLRs) are leucine-rich repeat (LRR) proteins that mediate adaptive immunity in jawless vertebrates. VLRs are fundamentally different from the antibodies of jawed vertebrates, which consist of immunoglobulin (Ig) domains. We determined the structure of an anti-hen egg white lysozyme (HEL) VLRB, isolated by yeast display, bound to HEL. The VLR, whose affinity resembles that of IgM antibodies, uses nearly all its concave surface to bind the protein, in addition to a loop that penetrates into the enzyme active site. The VLR-HEL structure, combined with sequence analysis, revealed an almost perfect match between ligand-contacting positions and positions with highest sequence diversity. Thus, we have defined the generalized antigen-binding site of VLRs. We further demonstrated that VLRs can be affinity-matured to affinities as high as those of IgG antibodies, making VLRs potential alternatives to antibodies for biotechnology applications.

Molecular basis of the dual functions of 2B4 (CD244)

2B4 belongs to the CD2 family of molecules and is expressed on all NK, gammadelta, and memory CD8(+) (alphabeta) T cells. The murine NK receptor 2B4 exhibits both inhibitory and activating functions, whereas human 2B4 has been reported to be an activating molecule. How murine 2B4 can act both as an activating and inhibitory receptor and what distinguishes its function from human 2B4 have remained largely unknown. We use here a model system that allows the study of human and murine 2B4 under identical and controlled conditions. These studies reveal that both human and mouse 2B4 can activate or inhibit NK cells. We show here that the level of 2B4 expression and the degree of 2B4 cross-linking play a significant role in the regulation of signaling lymphocyte activation molecule-associated protein-mediated activation by 2B4. A high level of 2B4 expression, heavy cross-linking, and relative paucity of signaling lymphocyte activation molecule-associated protein promote inhibitory function. Our studies demonstrate how a single receptor can have opposing function depending on the degree of receptor expression, extent of its ligation, and the relative abundance of certain adaptor molecules. Because the levels of 2B4 and CD48 are dynamically regulated, these findings have implications for the regulation of NK cell function.

Structure of natural killer receptor 2B4 bound to CD48 reveals basis for heterophilic recognition in signaling lymphocyte activation molecule family

Natural killer (NK) cells eliminate virally infected and tumor cells. Among the receptors regulating NK cell function is 2B4 (CD244), a member of the signaling lymphocyte-activation molecule (SLAM) family that binds CD48. 2B4 is the only heterophilic receptor of the SLAM family, whose other members, e.g., NK-T-B-antigen (NTB-A), are self-ligands. We determined the structure of the complex between the N-terminal domains of mouse 2B4 and CD48, as well as the structures of unbound 2B4 and CD48. The complex displayed an association mode related to, yet distinct from, that of the NTB-A dimer. Binding was accompanied by the rigidification of flexible 2B4 regions containing most of the polymorphic residues across different species and receptor isoforms. We propose a model for 2B4-CD48 interactions that permits the intermixing of SLAM receptors with major histocompatibility complex-specific receptors in the NK cell immune synapse. This analysis revealed the basis for heterophilic recognition within the SLAM family.

Structural basis for recognition of the T cell adaptor protein SLP-76 by the SH3 domain of phospholipase Cgamma1

The enzyme phospholipase Cgamma1 (PLCgamma1) is essential for T cell signaling and activation. Following T cell receptor ligation, PLCgamma1 interacts through its SH2 and SH3 domains with the adaptors LAT and SLP-76, respectively, to form a multiprotein signaling complex that leads to activation of PLCgamma1 by Syk tyrosine kinases. To identify the binding site for PLCgamma1 in SLP-76, we used isothermal titration calorimetry to measure affinities for the interaction of PLCgamma1-SH3 with a set of overlapping peptides spanning the central proline-rich region of SLP-76. PLCgamma1-SH3 bound with high specificity to the SLP-76 motif 186PPVPPQRP193, which represents the minimal binding site. To understand the basis for selective recognition, we determined the crystal structures of PLCgamma1-SH3 in free form, and bound to a 10-mer peptide containing this site, to resolutions of 1.60 A and 1.81 A, respectively. The structures reveal that several key contacting residues of the SH3 shift toward the SLP-76 peptide upon complex formation, optimizing the fit and strengthening hydrophobic interactions. Selectivity results mainly from strict shape complementarity between protein and peptide, rather than sequence-specific hydrogen bonding. In addition, Pro193 of SLP-76 assists in positioning Arg192 into the compass pocket of PLCgamma1-SH3, which coordinates the compass residue through an unusual aspartate. The PLCgamma1-SH3/SLP-76 structure provides insights into ligand binding by SH3 domains related to PLCgamma1-SH3, as well as into recognition by PLCgamma1 of signaling partners other than SLP-76.

Structural basis for differential recognition of tyrosine-phosphorylated sites in the linker for activation of T cells (LAT) by the adaptor Gads

The transmembrane protein, linker for activation of T cells (LAT), is essential for T-cell activation and development. Phosphorylation of LAT at multiple tyrosines creates binding sites for the adaptors Gads and Grb2, leading to nucleation of multiprotein signaling complexes. Human LAT contains five potential binding sites for Gads, of which only those at Tyr171 and Tyr191 appear necessary for T-cell function. We asked whether Gads binds preferentially to these sites, as differential recognition could assist in assembling defined LAT-based complexes. Measured calorimetrically, Gads-SH2 binds LAT tyrosine phosphorylation sites 171 and 191 with higher affinities than the other sites, with the differences ranging from only several fold weaker binding to no detectable interaction. Crystal structures of Gads-SH2 complexed with phosphopeptides representing sites 171, 191 and 226 were determined to 1.8-1.9 A resolutions. The structures reveal the basis for preferential recognition of specific LAT sites by Gads, as well as for the relatively greater promiscuity of the related adaptor Grb2, whose binding also requires asparagine at position +2 C-terminal to the phosphorylated tyrosine.

Limited diagnostic usefulness of antibodies to cytoplasmic proteins of Brucella in early-treated human brucellosis

Antibodies to cytoplasmic proteins (CP) of Brucella have been shown to be useful for the diagnosis of human brucellosis; however, some early-diagnosed patients lack such an antibody response while having high titers of antibodies to lipopolysaccharide (LPS). To address which factors determine this serological discrepancy in the early stages of brucellosis we examined the antibody response to CP and LPS of 21 patients involved in an outbreak of B. melitensis infection who had a short duration of clinical illness at diagnosis (3-40 d). At diagnosis, antibodies to LPS (IgM and/or IgG) were found in all patients, while anti-CP antibodies were detected in 16 subjects (76%). At 6 weeks post-diagnosis IgG to CP (with or without IgM) had been detected in 13 patients and IgM alone had been found in 4; however, 4 other patients (19%) had no response to CP. No significant differences were found between these 3 groups in terms of age, gender, antimicrobial agents or factors that could hamper the immune response. Notably, however, the 4 non-responders and 3 of the 4 patients having only IgM to CP had started antibiotic therapy within 14 d post-symptoms, while treatment was started later in 9 of 13 patients who developed anti-CP IgG. In addition, maximum titers of IgG to CP tended to be lower in early-treated patients. These results suggest that very early antibiotic therapy hampers the antibody response to Brucella CP but has little impact on the anti-LPS response. Given the higher specificity of the former and the higher sensitivity of the latter, both reactivities should be measured in order to diagnose human brucellosis.

Single-shot plasmid DNA intrasplenic immunization for the production of monoclonal antibodies. Persistent expression of DNA

Monoclonal antibodies (Mc. Abs.) were generated against a 18-kDa protein from Brucella abortus 48 h and 25 days after a single intrasplenic injection of a DNA plasmid containing the expression vector for the protein. Hybridomas were also obtained from spleens injected 3, 5, and 10 days before fusion. Somatic cell fusion of spleen cells from mice, injected with the plasmid DNA, in saline, with the NS-0 myeloma cell line resulted in Mc. Abs of the IgG and IgM Isotypes. IgG antibodies were of the IgG2b and IgG1 subtype. Hybridoma tissue culture supernatants were strongly positive by ELISA at dilutions of up to 1/1200 and produced intense specific bands in immunoblotting. All these antibodies recognized the native recombinant protein (the screening antigen) and some of them also recognized the heat-denatured recombinant 18-kDa protein. When compared to standard procedures of immunization, as well as to intramuscular or gene gun DNA immunizations, this technique results in very early, time saving, strong Mc Abs. It is common knowledge that in order to generate specific hybridomas; spleen cells from immunized animals have to be fused no later than 5 days after the last boost. The fact that through single-shot intrasplenic immunization (SSI) specific hybridomas are generated 25 days after one single injection indicates that the gene coding the p18 protein is being expressed in the spleen for at least 20 days. We propose that plasmid DNA intrasplenic immunization can be a helpful tool for the production of specific hybridomas. This route of immunization could also be helpful in the further understanding of early events of the immune response to genetic immunization by naked DNA injection.

Structural, functional and immunological studies on a polymeric bacterial protein

The characterization of proteins from Brucella spp, the causative agent of brucellosis, has been the subject of intensive research. We have described an 18-kDa cytoplasmic protein of Brucella abortus and shown the potential usefulness of this protein as an antigen for the serologic diagnosis of brucellosis. The amino acid sequence of the protein showed a low but significant homology with that of lumazine synthases. Lumazine is an intermediate product in bacterial riboflavin biosynthesis. The recombinant form of the 18-kDa protein (expressed in E. coli) folds like the native Brucella protein and has lumazine-synthase enzymatic activity. Three-dimensional analysis by X-ray crystallography of the homolog Bacillus subtilis lumazine synthase has revealed that the enzyme forms an icosahedral capsid. Recombinant lumazine synthase from B. abortus was crystallized, diffracted X rays to 2.7-A resolution at room temperature, and the structure successfully solved by molecular replacement procedures. The macromolecular assembly of the enzyme differs from that of the enzyme from B. subtilis. The Brucella enzyme remains pentameric (90 kDa) in its crystallographic form. Nonetheless, the active sites of the two enzymes are virtually identical at the structural level, indicating that inhibitors of these enzymes could be viable pharmaceuticals across a broad species range. We describe the structural reasons for the differences in their quaternary arrangement and also discuss the potential use of this protein as a target for the development of acellular vaccines.

Divergence in macromolecular assembly: X-ray crystallographic structure analysis of lumazine synthase from Brucella abortus

We have determined the three-dimensional structure of 6, 7-dimethyl-8-ribityllumazine synthase (lumazine synthase) from Brucella abortus, the infectious organism of the disease brucellosis in animals. This enzyme catalyses the formation of 6, 7-dimethyl-8-ribityllumazine, the penultimate product in the synthesis of riboflavin. The three-dimensional X-ray crystal structure of the enzyme from B. abortus has been solved and refined at 2.7 A resolution to a final R-value of 0.18 (R(free)=0.23). The macromolecular assembly of the enzyme differs from that of the enzyme from Bacillus subtilis, the only other lumazine synthase structure known. While the protein from B. subtilis assembles into a 60 subunit icosahedral capsid built from 12 pentameric units, the enzyme from B. abortus is pentameric in its crystalline form. Nonetheless, the active sites of the two enzymes are virtually identical indicating inhibitors to theses enzymes could be effective pharmaceuticals across a broad species range. Furthermore, we compare the structures of the enzyme from B. subtilis and B. abortus and describe the C teminus structure which accounts for the differences in quaternary structure.

Lack of Significant Differences in Association Rates and Affinities of Antibodies from Short-Term and Long-Term Responses to Hen Egg Lysozyme

The affinities (Ka) and association rate constants (kon) of 23 mouse (BALB/c) anti-lysozyme mAbs obtained after short and prolonged immunizations have been measured by plasmon resonance techniques. The affinities for the 23 Abs, measured using their Fab, range from Ka = 1.1 × 107 to 1.4 × 1010 M−1. There is no significant correlation between time or dose of immunization and affinity or association rates, indicating no time- or dose-dependent maturation of the response within the doses and times that were explored. IgMs are produced early and late in the response, with intrinsic affinities <105 M−1. Two independently derived mAbs, D44.1 (short term) and F10.6.6 (from a longer term response), result from identical or nearly identical somatic recombination events of germline gene segments. F10.6.6 has more mutations and a higher affinity constant (Ka = 1.4 × 1010 M−1) than D44.1 (Ka = 1.1 × 107 M−1). Although higher affinities may result from an accumulation of mutations, they do not correlate with the length and dose of immunogenic challenge.

Lack of significant differences in association rates and affinities of antibodies from short-term and long-term responses to hen egg lysozyme

The affinities (Ka) and association rate constants (kon) of 23 mouse (BALB/c) anti-lysozyme mAbs obtained after short and prolonged immunizations have been measured by plasmon resonance techniques. The affinities for the 23 Abs, measured using their Fab, range from Ka = 1.1 x 10(7) to 1.4 x 10(10) M-1. There is no significant correlation between time or dose of immunization and affinity or association rates, indicating no time- or dose-dependent maturation of the response within the doses and times that were explored. IgMs are produced early and late in the response, with intrinsic affinities <10(5) M-1. Two independently derived mAbs, D44.1 (short term) and F10.6.6 (from a longer term response), result from identical or nearly identical somatic recombination events of germline gene segments. F10.6.6 has more mutations and a higher affinity constant (Ka = 1.4 x 10(10) M-1) than D44.1 (Ka = 1.1 x 10(7) M-1). Although higher affinities may result from an accumulation of mutations, they do not correlate with the length and dose of immunogenic challenge.

Crystallization and preliminary x-ray diffraction analysis of the lumazine synthase from Brucella abortus

Lumazine synthase from Brucella abortus was overexpressed in Escherichia coli, refolded, and purified to apparent homogeneity. Crystals of lumazine synthase were grown by the hanging drop vapor diffusion method using polyethylene glycol 8000 or ammonium sulfate as precipitants. They belong to the trigonal space group P321 with cell parameters a = b = 132.00A, c = 167.25 A. A complete diffraction data set to 3.7 A resolution has been collected using synchrotron radiation. Preliminary analysis of the quaternary structure of this protein by means of a self-rotation function calculated with the diffraction data clearly indicates 532 symmetry compatible with the presence of an icosahedral lumazine synthase particle.

Characterization of anti-anti-idiotypic antibodies that bind antigen and an anti-idiotype

Two mouse monoclonal anti-anti-idiotopic antibodies (anti-anti-Id, Ab3), AF14 and AF52, were prepared by immunizing BALB/c mice with rabbit polyclonal anti-idiotypic antibodies (anti-Id, Ab2) raised against antibody D1.3 (Ab1) specific for the antigen hen egg lysozyme. AF14 and AF52 react with an "internal image" monoclonal mouse anti-Id antibody E5.2 (Ab2), previously raised against D1.3, with affinity constants (1.0 x 10(9) M-1 and 2.4 x 10(7) M-1, respectively) usually observed in secondary responses against protein antigens. They also react with the antigen but with lower affinity (1.8 x 10(6) M-1 and 3.8 x 10(6) M-1). This pattern of affinities for the anti-Id and for the antigen also was displayed by the sera of the immunized mice. The amino acid sequences of AF14 and AF52 are very close to that of D1.3. In particular, the amino acid side chains that contribute to contacts with both antigen and anti-Id are largely conserved in AF14 and AF52 compared with D1.3. Therapeutic immunizations against different pathogenic antigens using anti-Id antibodies have been proposed. Our experiments show that a response to an anti-Id immunogen elicits anti-anti-Id antibodies that are optimized for binding the anti-Id antibodies rather than the antigen.

Humoral immune response against lipopolysaccharide and cytoplasmic proteins of Brucella abortus in cattle vaccinated with B. abortus S19 or experimentally infected with Yersinia enterocolitica serotype 0:9

The humoral immune responses against three different antigens of Brucella abortus were monitored by enzyme-linked immunosorbent assay in cattle vaccinated with B. abortus S19 or experimentally infected with Yersinia enterocolitica serotype 0:9. Immunoglobulin G (IgG) and IgM responses against (i) B. abortus lipopolysaccharide (LPS), (ii) total cytoplasmic proteins depleted of LPS (LPS-free CYT), and (iii) B. abortus 18-kDa cytoplasmic protein were measured. Vaccinated animals and Yersinia-infected animals developed high anti-LPS IgM and IgG titers, which overlapped with those obtained with sera from B. abortus 544-infected animals used as positive controls. In contrast, only a slight or negative IgG and IgM response against LPS-free CYT and the 18-kDa protein was detected in vaccinated or Yersinia-infected cattle, although its levels were always significantly lower than those of B. abortus 544-infected animals. These data indicate that cytoplasmic proteins of B. abortus could be useful for the differential diagnosis of bovine brucellosis.