Crystal structure and thermodynamic analysis of diagnostic mAb 106.3 complexed with BNP 5-13 (C10A).

B-type natriuretic peptide (BNP) is a naturally secreted regulatory hormone that influences blood pressure and vascular water retention in human physiology. The plasma BNP concentration is a clinically recognized biomarker for various cardiovascular diseases. Quantitative detection of BNP can be achieved in immunoassays using the high-affinity monoclonal IgG1 antibody 106.3, which binds an epitope spanning residues 5-13 of the mature bioactive peptide. To understand the structural basis of this molecular recognition, we crystallized the Fab fragment complexed with the peptide epitope and determined the three-dimensional structure by X-ray diffraction to 2.1 A resolution. The structure reveals the detailed interactions that five of the complementarity-determining regions make with the partially folded peptide. Thermodynamic measurements using fluorescence spectroscopy suggest that the interaction is enthalpy driven, with an overall change in free energy of binding, DeltaG = -54 kJ/mol, at room temperature. The parameters are interpreted on the basis of the structural information. The kinetics of binding suggest a diffusion-limited mechanism, whereby the peptide easily adopts a bound conformation upon interaction with the antibody. Moreover, comparative analysis with alanine-scanning results of the epitope explains the basis of selectivity for BNP over other related natriuretic peptides.

Cellular uptake followed by class I MHC presentation of some exogenous peptides contributes to T cell stimulatory capacity.

The T cell stimulatory activity of peptides is known to be associated with the cell surface stability and lifetime of the peptide-MHC (pepMHC) complex. In this report, soluble high-affinity T cell receptors (TCRs) that are specific for pepMHC complexes recognized by the mouse CD8+ clone 2C were used to monitor the cell surface lifetimes of synthetic agonist peptides. In the 2C system, L(d)-binding peptide p2Ca (LSPFPFDL) has up to 10,000-fold lower activity than peptide QL9 (QLSPFPFDL) even though the 2C TCR binds to p2Ca-L(d) and QL9-L(d) complexes with similar affinities. Unexpectedly, p2Ca-L(d) complexes were found to have a longer cell surface lifetime than QL9-L(d) complexes. However, the strong agonist activity of QL9 correlated with its ability to participate in efficient intracellular delivery followed by cell surface expression of the peptide, resulting in high and persistent surface levels of QL9-L(d). The ability of target cells to take up and present QL9 was observed with TAP-deficient cells and TAP-positive cells, including dendritic cells. The process was brefeldin A-sensitive, indicating a requirement for transport of the pepMHC through the ER and/or golgi. Thus, strong T cell stimulatory activity of some pepMHC complexes can be accomplished not only through long cell surface lifetimes of the ligand, but through a mechanism that leads to delayed presentation of the exogenous antigen after intracellular uptake.

Development of a new ARCHITECT automated periostin immunoassay.

BACKGROUND: Periostin is being investigated as a potential biomarker for T-helper-2 (Th2)-driven asthma or eosinophilic inflammation and may help to identify patients more likely to benefit from interleukin-13-targeted treatments. We report the development and analytic performance of the investigational use only ARCHITECT Periostin Immunoassay, a new automated assay developed to detect serum periostin concentrations. METHODS: We assessed assay performance in terms of precision, sensitivity, linearity, interference from classical immunoassay interferents and representatives of common asthma medications, specimen handling, and isoform reactivity. The assay was also used to assess the biological variability of serum periostin concentrations in samples from healthy volunteers and from subjects with uncontrolled asthma (the intended use population). RESULTS: The percentage CVs for 5-day total precision, assessed using two instruments, was <6% across 2 controls and one serum-based panel. Limit of quantitation was 4ng/mL (dilution adjusted concentration), suiting the needs for this application. Dilution analysis yielded linear results and no endogenous sample or drug interferences were observed. All known periostin isoforms expressed in the mature human lung were detected by the assay. CONCLUSION: Our studies provide support that the ARCHITECT Periostin Immunoassay is a reliable and robust test for measuring serum periostin concentrations.

Development and analytical performance of a new ARCHITECT automated dipeptidyl peptidase-4 immunoassay.

BACKGROUND: Dipeptidyl peptidase-4 (DPP-4) may be a suitable biomarker to identify people with severe asthma who have greater activation of the interleukin-13 (IL-13) pathway and may therefore benefit from IL-13-targeted treatments. We report the analytical performance of an Investigational Use Only immunoassay and provide data on the biological range of DPP-4 concentrations. METHODS: We assessed assay performance, utilising analyses of precision, linearity and sensitivity; interference from common endogenous assay interferents, and from asthma and anti-diabetic medications, were also assessed. The assay was used to measure the range of serum DPP-4 concentrations in healthy volunteers and subjects with diabetes and severe, uncontrolled asthma. RESULTS: The total precision of DPP-4 concentration measurement (determined using percentage coefficient of variation) was ≤5% over 20 days. Dilution analysis yielded linear results from 30 to 1305 ng/mL; the limit of quantitation was 19.2 ng/mL. No notable endogenous or drug interferences were observed at the expected therapeutic concentration. Median DPP-4 concentrations in healthy volunteers and subjects with asthma or Type 1 diabetes were assessed, with concentrations remaining similar in subjects with diabetes and asthma across different demographics. CONCLUSION: These analyses indicate that the ARCHITECT DPP-4 Immunoassay is a reliable and robust method for measuring serum DPP-4 concentration.

A yeast display system for engineering functional peptide-MHC complexes.

In a cellular immune response, antigenic peptides derived by intracellular processing of foreign pathogens are bound to the class I major histocompatability complex (MHC I) and presented to CD8(+) cytotoxic T cells. Although the crystal structures of several different MHC products have been solved, many MHC molecules, including some associated with diseases, have not been amenable to biochemical and structural studies. The variability in this success is based largely on the fact that peptide-MHC complexes vary extensively in their stability. These properties also are intimately tied to the biological activity of the complexes. The ability to apply the techniques of directed evolution to this system in order to engineer stable complexes has been complicated by the trimeric structure of peptide-MHC complexes, requiring association of three polypeptides: the heavy chain, beta2-microglubulin (beta2m), and a short peptide. We show here that single-chain forms of peptide-MHC complexes can be expressed as Aga-2 fusions on the surface of yeast. Three different complexes, SIYRYYGL-K(b)-beta2m (SIYR-K(b)), EQYKFYSV-K(b)-beta2m (dEV8-K(b)), and SIINFEKL-K(b)-beta2m (OVA-K(b)), were expressed on yeast and detected by flow cytometry with a conformation-specific anti-K(b) antibody (B.8.24.3). In addition, yeast displaying K(b) loaded with exogenous SIYR and OVA peptides were recognized by a high-affinity T cell receptor that is specific for SIYR-K(b) and by an antibody (25.D1-16) that is specific for OVA-K(b), respectively. Finally, yeast that display the SIYRYYGL-K(b) also directly stimulated CD69 up-regulation on naive 2C T cells. Hence, yeast display represents a technology that can be used for directed evolution of any of the components of the trimeric pep-MHC complex.

Generation and characterization of chimeric antibodies against NS3, NS4, NS5, and core antigens of hepatitis C virus.

Mouse-human chimeric antibodies (cAbs) against hepatitis C virus (HCV) core, NS3 (nonstructural), NS4, and NS5 antigens were developed as quality control (QC) reagents to replace the use of human sera/plasma for Abbott HCV immunoassays. The cAb retains the mouse monoclonal antibody (MAb) specificity and affinity but still reacts in the existing HCV assay format, which measures human anti-HCV immunoglobulin. Mouse heavy-chain (V(H)) and light-chain (V(L)) variable regions of anti-HCV core, NS3, NS4, and NS5 antigens were PCR amplified from hybridoma lines and then cloned with human IgG1 heavy-chain (C(H)) and light-chain (C(L)) constant regions, respectively. A single mammalian expression plasmid containing both heavy-chain and light-chain immunoglobulin genes was constructed and transfected into dihydrofolate reductase (DHFR)-deficient Chinese hamster ovary (CHO) cells. The transfected CHO cells were selected using hypoxanthine- and thymidine-free medium and screened by an enzyme immunoassay (EIA). The clone secreting the highest level of antibody was isolated from the CHO transfectants and further subcloned. Each cAb-expressing CHO cell line was weaned into serum-free medium, and the cAb was purified by protein A affinity chromatography. The levels of cAb production for the various CHO cell lines varied from 10 to 20 mg/liter. Purified anti-HCV cAbs were tested with Abbott HCV immunoassays and showed reactivity. Moreover, yeast surface display combined with alanine-scanning mutagenesis was used to map the epitope at the individual amino acid level. Our results suggest that these HCV cAbs are ideal controls, calibrators, and/or QC reagents for HCV assay standardization.

Engineering and characterization of a stabilized alpha1/alpha2 module of the class I major histocompatibility complex product Ld.

The major histocompatibility complex (MHC) is the most polymorphic locus known, with thousands of allelic variants. There is considerable interest in understanding the diversity of structures and peptide-binding features represented by this class of proteins. Although many MHC proteins have been crystallized, others have not been amenable to structural or biochemical studies due to problems with expression or stability. In the present study, yeast display was used to engineer stabilizing mutations into the class I MHC molecule, Ld. The approach was based on previous studies that showed surface levels of yeast-displayed fusion proteins are directly correlated with protein stability. To engineer a more stable Ld, we selected Ld mutants with increased surface expression from randomly mutated yeast display libraries using anti-Ld antibodies or high affinity, soluble T-cell receptors (TCRs). The most stable Ld mutant, Ld-m31, consisted of a single-chain MHC module containing only the alpha1 and alpha2 domains. The enhanced stability was in part due to a single mutation (Trp-97 --> Arg), shown previously to be present in the allele Lq. Mutant Ld-m31 could bind to Ld peptides, and the specific peptide.Ld-m31 complex (QL9.Ld-m31) was recognized by alloreactive TCR 2C. A soluble form of the Ld-m31 protein was expressed in Escherichia coli and refolded from inclusion bodies at high yields. Surface plasmon resonance showed that TCRs bound to peptide.Ld-m31 complexes with affinities similar to those of native full-length Ld. The TCR and QL9.Ld-m31 formed complexes that could be resolved by native gel electrophoresis, suggesting that stabilized alpha1/alpha2 class I platforms may enable various structural studies.