stration, colonizes and penetrates the mucosal inductive tissues via the M cell and presumably delivers recombinant proteins to elicit mucosal immunity (see below). Current attenuation mutations involve deletions of two or more genes and are designed to avoid complementation by the host or other indigenous flora. A major advantage of this approach is that oral immunization with recombinant Salmonella can elicit protection from S.typhi, and at present several vaccines relying on recombinant Salmonella strains are in human phase I and phase II trials.

Major breakthroughs have also been made in the development and use of former viral pathogens for the delivery of foreign antigens. Vaccinia virus has been used successfully for several antigens and cytokines. However, since most viruses impinge on the epithelium, which lines the mucosal immune system, more attention has been paid to the development of mucosal virus delivery. Two examples will suffice: recombinant poliovirus, since this has been a successful oral vaccine; and recombinant adenovirus (also a successful respiratory pathogen) for delivery to the mucosal tissues of the gastrointestinal tract as well as to the upper respiratory tract and lungs.

The success and efficacy of OPV make it an attractive vector for the delivery of mucosal vaccines, particularly when immunity to enteric pathogens is desired. This vaccine induces both mucosal and systemic immune responses and offers protection from infection. Polio-virus-specific MHC class II-restricted CD4⁺ T cells in peripheral blood mononuclear cells from orally vaccinated individuals have also been detected. This finding suggests that poliovirus can be used as an antigen delivery vehicle to induce CD4⁺ T-helper cells that can regulate mucosal IgA B-cell responses, in addition to the typical virus-induced CTL type of immunity.

Recently, investigators have used chimeric poliovirus genomes in which gag and pol genes of human immunodeficiency virus type 1 were substituted for the VP2 and VP3 outer capsid genes of poliovirus (Porter et al., 1993; 1995). Transfection of the minireplicon genomes containing the gag or pol gene into cells produced the appropriate fusion protein. These minireplicons were then encapsulated and amplified by transfecting them into cells previously infected with a recombinant vaccinia virus that expresses poliovirus capsid precursor protein P1. For immunization studies, the encapsulated replicons were passaged in the presence of poliovirus type 2 Lansing, which again resulted in encapsidation of the replicons by the capsid proteins provided by poliovirus. These replicons were then given to mice by the intramuscular, intrarectal, or oral routes, and the mice were later boosted by the same route. Results from these studies indicated an increased production of antipoliovirus antibodies in serum and increased virus-specific IgA antibodies in saliva and in gastrointestinal tract secretions. In addition, the detection of anti-Gag and anti-Env antibodies in serum after intramuscular immunization and in external secretions (Moldoveanu et al., 1995) following mucosal delivery has clearly established the immunogenicities of the minireplicons.