Anchored periplasmic expression (APEx)-based bacterial display for rapid and high-throughput screening of B cell epitopes

We truncated the VP2 protein of infectious bursal disease virus into five fragments: V1-5. All fragments were displayed on the inner membrane of the Escherichia coli periplasm. After disruption of the outer membrane, spheroplasts that had anchored with the VP2 fragment were incubated with an anti-VP2 polyclonal antibody (pAb). Prey pairs were detected and quantitated by flow cytometry with V1, V3, V4 and V5 fragments reacting with the pAb. The antigenicity of all five fragments was analyzed, and our results indicated that epitopes were localized in V1, V3, V4 and V5, consistent with our flow cytometry analysis. Antigenicity analysis of purified VP2 fusion proteins using Western blots confirmed this. Our method provides a rapid, quantitative and simple strategy for identifying linear B cell epitopes.