, Germany), and 40 mg/ml BCIP (5-Bromo-4-chloro-3′-indolyphosphate, Sigma-Aldrich,

Inc., Germany). The phosphate in the media inhibits the expression of the constitutive DH5α AP gene, while the IPTG induces the tac promoter, allowing the expression of AP fusion proteins that hydrolyses the BCIP substrate resulting in blue colonies ( Boulain and Ducancel, 2004). Three independently positive ampicillin-resistant blue colonies containing sag1 gene fragment were selected and analyzed by sequencing using the ABI PRISM Cycle Sequencing kit (ABI, Applied Biosystems). A single clone pLIP6-sag1–AP was used to transform fresh competent E. coli XL-Blue and W3110 strains ( Sambrook and Russel, 2001). Colonies were grown in LB medium supplemented with 100 μg/ml ampicillin,

at 37 °C until OD600 reaches approximately 0.6 NU7441 order to 0.8. The tac promoter was then induced with IPTG and the culture temperature was shifted to 28 °C for 16 h with shaking at 200 rpm. To optimize the production of SAG1–AP fusion protein, a range of IPTG concentrations was used (0.1–1 mM) and cultures were incubated under the same conditions. Extracts containing periplasmic fusion protein were prepared from pelleted bacteria after a modified cold osmotic shock as previously described (Skerra and Pluckthun, HSP mutation 1988). Briefly, the biomass obtained from 1 l bacterial culture was suspended in 100 ml TES hypertonic solution (300 mM Tris–HCl, pH 8, 1 mM EDTA, 20% sucrose) containing 10 μg/mL lysosyme. After 10 min incubation on ice, the soluble periplasmic fraction was collected by centrifugation at 7000 g for 30 min at 4 °C. Fractions containing the recombinant SAG1–AP were dialyzed against PBS (Phosphate Buffered Saline) and a protease inhibitor cocktail (Roche Applied Science) was added before storing at − 20 °C till use. The presence and the integrity of the recombinant fusion protein were checked using 10% SDS-PAGE gel and silver-staining. Two Western blots were performed to reveal the fusion protein bands,

the first under reducing condition with anti-bacterial AP MAb (Product No. B-6804, Sigma-Aldrich, Inc., Germany) diluted at 1/5000 in PBS containing 0.1% Tween-20 (PBS-T), the second under non-reducing conditions using the anti-T. gondii SAG1 Mab (Product Progesterone No. 11-132, Argene SA, Verniolle; France) diluted at 1/1000 in PBS-T. Both blots were then incubated with alkaline phosphatase-conjugated anti-mouse Fc specific antibody produced in goat (Product No. A7434 Sigma-Aldrich, Inc., Germany). The immune complex was detected by BCIP/NBT AP substrate buffer (100 mM Tris–HCl pH 9.5, 100 mM NaCl, 5 mM MgCl2, containing 0.3 g/l NBT and 0.15 g/l BCIP) and the reaction was stopped by washing with distilled water. For estimating the amounts of the recombinant fusion protein contained in periplasmic extract, SAG1–AP protein levels were quantified on silver-staining SDS-PAGE using Quantity-One Software (version 4.6.