Design and screening of M13 phage display cDNA libraries.
Vol. 35, No. 4 (2003) BioTechniques 673 Immunoglobulin E (IgE)-mediated allergic diseases have become a serious health problem in industrialized countries (1,2). Routine diagnosis of allergic conditions is based on clinical history, skin test reactivity, and serum IgE determinations against the offending agents (3). Both skin test reactivity and determinations of serum IgE levels reflect directly on the quality of the allergen extracts used (4). Depending on the exact content of a single allergen in the extract and on the sensitization pattern of the patient, discrepant diagnostic results can be obtained using commercial allergen preparations. Unfortunately, most of the extracts used for diagnosis are not standardized (5,6). The biochemical nature and content of the allergenic components are unknown; therefore, the allergenicity of different extracts cannot be compared. The application of modern molecular biology techniques in allergology allowed for the cloning and molecular characterization of major and minor allergens (7) shown to be powerful reagents for the in vitro and in vivo diagnosis of allergic conditions (4). However, in the majority of cases, reagents allowing for the quantification of individual allergens in commercial allergenic products are still lacking. Monospecific antibodies raised against allergens would substantially facilitate improved quality control and standardization of allergen extracts and lead to the development of more consistent products for clinical use (6,8) . Recombinant immunoglobulin gene libraries cloned in phage or phagemid vectors are an in vitro simulation of antibody repertoires, allowing for the production of antibodies in prokaryotic hosts without immunization or the use of animals (9). The fast-increasing number of recombinant allergens and the need for sensitive reagents for their detection prompted us to implement high-throughput technologies (10) for the rapid selection of allergen-specific antibodies. Starting from “single pot” naive human antibody libraries that have been previously described (11), we performed parallel selections of phage carrying functional, binding single-chain fragment variables (scFvs) against different recombinant allergens. Phage antibody libraries were processed and enriched using a prototype pin-based magnetic particle processor (ThermoLabsystems, Helsinki, Finland) combined with liquid-handling robots (KingFisher®; ThermoLabsystems) (10). The magnetic particle processor is an automated device that enables the parallel handling of up to 96 individual screenings using 96 magnetic pins that correspond to the position of a 96-well microplate. The magnetic particle processor has a head that consists of two slide-in holders in standard microplate format that are equipped with up to 96 pin-based permanent magnets in rows of eight and the corresponding number of polypropylene caps. The head can be moved along the x-, yand z-axis (up and down). Moving the whole head along the y-axis, the magnets covered with the plastic caps can be inserted into a prefilled plate containing magnetic particles that are attracted and hold in position until the magnets are pulled from the caps. By moving the head along the x-axis, the particles can be transferred to new plates for washing steps and finally resuspended by pulling the magnets from the caps. Every movement of the head and slide-in holders is software-driven and can be controlled with respect to speed, position, and time by user-defined protocols. Different recombinant allergens (Table 1) were expressed in Escherichia coli and affinity-purified by immobilized metal affinity chromatography (IMAC) (12). Prior to loading, magnetic particles [10 μL/250 μg nickel-nitrilotriacetic acid (Ni-NTA) Silica Beads; Qiagen, Hilden, Germany] were washed twice for 10 min in PBST [phosphate-buffered saline (PBS), 0.1% Tween® 20 (Fluka, Buchs, Switzerland)] and subsequently loaded by incubation for 1 h at room temperature in 200 μL purified protein (100 μg/mL in PBS). Allergen-loaded magnetic particles were washed twice in PBST, and the remaining free binding sites were blocked for 1 h at room temperature with PTM (PBS, 1% Tween 20, and 2% skimmed milk powder). Blocked, protein-loaded magnetic particles and scFv-carrying phage were co-incubated for 1 h at room temperature and subjected to 20 automated washing steps using the robotic device to decrease unspecific phage. After washing, the particles carrying bound phage were released into microplate wells containing 200 μL log phase E. coli TG1 cells and incubated for 30 min at room temperature for infection. Phage titration was performed as previously described (13), and the plates were shaken overnight at 37°C to generate phage for further rounds of affinity selection. After 4–5 rounds, affinity-enriched phage were subjected to further investigation. Phage ELISAs were performed in MaxiSorp® ImmunoPlates (Nalge Nunc International, Roskilde, Denmark). Recombinant allergens (400 ng in 150 μL PBS/well, pH 7.4) were coated overnight at 4°C or 2 h at room temperature. Blocking was performed with PTM for at least 3 h at room temperature or at 4°C overnight. After washing with PBST, 1010 scFv-carrying phagemid particles [colony-forming unit (cfu)] in 150 μL PTM were applied to the first well and serially diluted at a 1:1 ratio throughout the plate. Alternatively, the allergens were diluted in the same way, starting from 1 μg protein in 150 μL PBS applied in the first well. In these experiments, the amount of scFv phage particles was kept constant (approximately 5 × 109 cfu/ well). After washing, bound phage was detected with a horseradish peroxidase High-throughput isolation of recombinant antibodies against recombinant allergens