Parallel immunizations of rabbits using the same antigen yield antibodies with similar, but not identical, epitopes.

A problem for the generation of polyclonal antibodies is the potential difficulties for obtaining a renewable resource due to batch-to-batch variations when the same antigen is immunized into several separate animals. Here, we have investigated this issue by determining the epitopes of antibodies generated from parallel immunizations of rabbits with recombinant antigens corresponding to ten human protein targets. The epitopes were mapped by both a suspension bead array approach using overlapping synthetic 15-mer peptides and a bacterial display approach using expression of random fragments of the antigen on the surface of bacteria. Both methods determined antibody binding with the aid of fluorescent-based analysis. In addition, one polyclonal antibody was fractionated by peptide-specific affinity capture for in-depth comparison of epitopes. The results show that the same antigen immunized in several rabbits yields polyclonal antibodies with similar epitopes, but with larger differences in the relative amounts of antibodies to the different epitopes. In some cases, unique epitopes were observed for one of the immunizations. The results suggest that polyclonal antibodies generated by repeated immunizations do not display an identical epitope pattern, although many of the epitopes are similar.

Generation of monospecific antibodies based on affinity capture of polyclonal antibodies.

A method is described to generate and validate antibodies based on mapping the linear epitopes of a polyclonal antibody followed by sequential epitope-specific capture using synthetic peptides. Polyclonal antibodies directed towards four proteins RBM3, SATB2, ANLN, and CNDP1, potentially involved in human cancers, were selected and antibodies to several non-overlapping epitopes were generated and subsequently validated by Western blot, immunohistochemistry, and immunofluorescence. For all four proteins, a dramatic difference in functionality could be observed for these monospecific antibodies directed to the different epitopes. In each case, at least one antibody was obtained with full functionality across all applications, while other epitope-specific fractions showed no or little functionality. These results present a path forward to use the mapped binding sites of polyclonal antibodies to generate epitope-specific antibodies, providing an attractive approach for large-scale efforts to characterize the human proteome by antibodies.

High nuclear RBM3 expression is associated with an improved prognosis in colorectal cancer.

PURPOSE: In this study, we investigated the prognostic impact of human RBM3 expression in colorectal cancer using tissue microarray-based immunohistochemical analysis. EXPERIMENTAL DESIGN: One polyclonal antibody and four monoclonal anti-RBM3 antibodies were generated and epitope mapped using two different methods. Bacterial display revealed five distinct epitopes for the polyclonal antibody, while the four mouse monoclonal antibodies were found to bind to three of the five epitopes. A peptide suspension bead array assay confirmed the five epitopes of the polyclonal antibody, while only one of the monoclonal antibodies could be mapped using this approach. Antibody specificity was confirmed by Western blotting and immunohistochemistry, including siRNA-mediated knock-down. Two of the antibodies (polyclonal and monoclonal) were subsequently used to analyze RBM3 expression in tumor samples from two independent colorectal cancer cohorts, one consecutive cohort (n=270) and one prospectively collected cohort of patients with cancer of the sigmoid colon (n=305). RBM3-expression was detected, with high correlation between both antibodies (R=0.81, p<0.001). RESULTS: In both cohorts, tumors with high nuclear RBM3 staining had significantly prolonged the overall survival. This was also confirmed in multivariate analysis, adjusted for established prognostic factors. CONCLUSION AND CLINICAL RELEVANCE: These data demonstrate that high tumor-specific nuclear expression of RBM3 is an independent predictor of good prognosis in colorectal cancer.

Staphylococcal surface display in combinatorial protein engineering and epitope mapping of antibodies.

The field of combinatorial protein engineering for generation of new affinity proteins started in the mid 80s by the development of phage display. Although phage display is a prime example of a simple yet highly efficient method, manifested by still being the standard technique 25 years later, new alternative technologies are available today. One of the more successful new display technologies is cell display. Here we review the field of cell display for directed evolution purposes, with focus on a recently developed method employing Gram-positive staphylococci as display host. Patents on the most commonly used cell display systems and on different modifications as well as specific applications of these systems are also included. General strategies for selection of new affinity proteins from cell-displayed libraries are discussed, with detailed examples mainly from studies on the staphylococcal display system. In addition, strategies for characterization of recombinant proteins on the staphylococcal cell surface, with an emphasis on an approach for epitope mapping of antibodies, are included.

Epitope mapping using gram-positive surface display.

Antibodies have proven to be invaluable tools for a vast number of applications during the last decades, including protein purification and characterization, medical diagnosis and imaging, and treatment using therapeutic antibiotics. No matter what the aims of the application are, the antibody's binding characteristics will still be the main features determining the assay's reliability. Here, we describe a protocol for determination of antibody-binding epitopes using an antigen-focused, library-based approach where library members are generated by fragmentation of antigen DNA and presented as cloned peptides on the cell surface of the Gram-positive bacterium Staphylococcus carnosus. The rigid cell structure of this organism allows for multivalent expression and permits rapid library analysis and sorting of antibody-binding cells using flow-sorting devices. Epitopes are determined by DNA sequencing of the sorted cells and alignment back to the antigen sequence. The protocol described here has been shown useful for mapping of both monoclonal and polyclonal binders with varying epitope lengths.

Exploring epitopes of antibodies toward the human tryptophanyl-tRNA synthetase.

There is a need to characterize the epitopes of affinity reagents to develop high quality affinity reagents for research, diagnostics and therapy. Here, we describe the analysis of epitopes of antibodies generated toward human tryptophanyl-tRNA synthetase (WARS) using both combinatorial bacterial display and suspension bead array. The bacterial display revealed that the polyclonal antibody binds to three separate epitopes and peptide scanning using 15-mers revealed binding to a 13 amino acid consensus sequence (ELINRIERATGQR). A mouse monoclonal antibody was generated and the mapping approach revealed binding toward a slightly shifted position of the same epitope. Structural analysis showed that the antibodies bind to alpha-helical regions on the surface of the target protein. An alanine-scanning experiment showed binding to four specific residues. The implications for the systematic analysis of antibody epitopes on the basis of these results are discussed.

Epitope mapping of antibodies using bacterial surface display.

We describe a method for mapping the epitopes recognized by antibodies, based on bacterial surface expression of antigen protein fragments followed by antibody-based flow-cytometric sorting. We analyzed the binding sites of both monoclonal and polyclonal antibodies directed to three human protein targets: (i) the human epidermal growth factor receptor 2 (HER2), (ii) ephrin-B3 and (iii) the transcription factor SATB2. All monoclonal antibodies bound a single epitope, whereas the polyclonal antibodies showed, in each case, a binding pattern with one to five separate epitopes. A comparison of polyclonal and monoclonal antibodies raised to the same antigen showed overlapping binding epitopes. We also demonstrated that bacterial cells with displayed protein fragments can be used as affinity ligands to generate epitope-specific antibodies. Our approach shows a path forward for systematic validation of antibodies for epitope specificity and cross-reactivity on a whole-proteome level.