Pseudomonas aeruginosa outer membrane protein OprF as an expression vector for foreign epitopes: the effects of positioning and length on the antigenicity of the epitope

OprF, the major outer membrane (OM) protein of Pseudomonas aeruginosa, has been proposed to be comprised of a series of β-strands separated by periplasmic or surface-exposed loop regions. In this study, a simple malarial epitope was used to demonstrate that OprF can be used as an expression vector to present foreign peptide sequences, namely, the 4-amino-acid (aa) repeating epitope (Asn-Ala-Asn-Pro=NANP) of the circumsporozoite protein of the human malarial parasite Plasmodium falciparum. Eight permissive sites, that allowed the expression and surface exposure of the malarial epitope, were identified throughout OprF. Using a monoclonal antibody (mAb) specific for the malarial epitope, we investigated the effects of positioning and length of the epitope on its antigenicity in the OprF expression vector system. It was demonstrated that the malarial epitope inserted at aa²⁶ was significantly more reactive with the epitope-specific mAb (i.e., more antigenic) when assayed in the context of whole cells whereas those at aa²¹³ and aa²⁹⁰ were more antigenic when assayed in the OM. The malarial epitope inserted at aa¹⁸⁸ and aa¹⁹⁶ was moderately antigenic, while this epitope inserted at aa²¹⁵ and aa³¹⁰ showed low antigenicity with the same mAb in both whole cell and OM assays. For two insertion sites, aa²⁶ and aa²¹³, we demonstrated that the insertion of multiple copies of the epitope enhanced reactivity with the malarial epitope-specific mAb. These data are discussed with respect to the local OprF sequences into which the epitope was inserted.