New strategies for combination vaccines based on the extended recombinant bacterial ghost system

Controlled expression of cloned PhiX174 gene E in Gram-negative bacteria re sults in lysis of the bacteria by formation of an E-specific transmembrane tunnel structure built through the cell envelope complex. Bacterial ghosts have been produced from a great variety of bacteria and are used as non-liv ing candidate vaccines. In the recombinant ghost system, foreign proteins a re attached on the inside of the inner membrane as fusions with specific an chor sequences. Ghosts have a sealed periplasmic space and the export of pr oteins into this space vastly extents the capacity of ghosts or recombinant ghosts to function as carriers of foreign antigens, immunomodulators or ot her substances. In addition, S-layer proteins forming shell-like self assem bly structures can be expressed in bacterial candidate vaccine strains prio r to E;mediated lysis. Such recombinant S-layer proteins carrying inserts o f foreign epitopes of up to 600 amino acids within the flexible surface loo p areas of the S-layer further extend the possibilities of ghosts as carrie rs of foreign epitopes. As ghosts do not need the addition of adjuvants to induce immunity in experimental animals they can also be used as carriers o r targeting vehicles or as adjuvants in combination with subunit vaccines. Matrixes like dextran which can be used to fill the internal lumen of ghost s can be substituted with various ligands to bind the subunit or other mate rials of interest. Oral, aerogenic or parenteral immunization of experiment al animals with recombinant ghosts induced specific humoral and cellular im mune responses against bacterial and target components including protective mucosal immunity. The most relevant advantage of ghosts and recombinant ba cterial ghosts as immunogens is that no inactivation procedures that denatu re relevant immunogenic determinants are employed in the production of ghos ts. This fact explains the superior quality of ghosts when compared to othe r inactivated vaccines. As carriers of foreign antigens there is no limitat ion in the size of foreign antigens to be inserted and the capacity of all spaces including the membranes, periplasma and internal lumen of the ghosts can be fully utilized. Using the different building blocks and combining t hem into the recombinant ghost system represents a new strategy for adjuvan t free combination vaccines. (C) 1999 Elsevier Science Ltd. All rights rese rved.