Effectiveness of engineering the nontypeable Haemophilus influenzae antigen Omp26 as an S-layer fusion in bacterial ghosts as a mucosal vaccine delivery

The potential of empty bacterial cell envelopes (ghosts) as a delivery system for mucosal immunization was assessed in a rat model and different routes of immunization were evaluated. Animals were mucosally immunized targeting either gut only or gut and lung mucosal sites with Escherichia coli ghosts harbouring the nontypeable Haemophilus influenzae (NTHi) antigen Omp26. Omp26 was expressed as either a part of an S-layer fusion or as a soluble protein in the periplasm. In the gut/lung regime two initial gut targeted inoculations with the ghosts were followed by an intratracheal (IT) boost with purified Omp26. The gut only immunisation regime showed a moderate enhancement of bacterial clearance following pulmonary challenge whereas the gut/lung immunization regime resulted in significantly enhanced pulmonary clearance of NTHi. Both immunization regimes induced high levels of Omp26 specific antibodies in the serum of immunized rats, with higher levels in the groups that received the IT boost with purified Omp26. Analysis of IgG isotypes present in serum suggest that the immune response was predominantly of a T-helper1 (Th1) type. Additionally, immunization induced a significant cellular immune response with lymphocytes from animals vaccinated using the gut/lung regime responding significantly to Omp26 when compared to control groups. The results of this study show that mucosal immunization with recombinant Omp26 in E.coli ghosts followed by a boost with purified Omp26 can induce a specific and protective immune response in a rodent model of acute lung infection.