Elsevier

Vaccine

Volume 19, Issues 25–26, 14 May 2001, Pages 3509-3517
Vaccine

Immunization of mice with lipopeptide antigens encapsulated in novel liposomes prepared from the polar lipids of various Archaeobacteria elicits rapid and prolonged specific protective immunity against infection with the facultative intracellular pathogen, Listeria monocytogenes

https://doi.org/10.1016/S0264-410X(01)00041-XGet rights and content

Abstract

Protective immunity to intracellular bacterial pathogens usually requires the participation of specific CD8+ T cells. Natural exposure of the host to sublethal infection, or vaccination with attenuated live vaccines are the most effective means of eliciting prolonged protective cell-mediated immunity against this class of pathogens. The ability to replace these immunization strategies with defined sub-unit vaccines would represent a major advance for clinical vaccinology. The present study examines the ability of novel liposomes, termed archaeosomes, made from the polar lipids of various Archaeobacteria to act as self-adjuvanting vaccine delivery vehicles for such defined acellular antigens. Using infection of mice with Listeria monocytogenes as a model system, this study clearly demonstrates the ability of defined, archaeosome-entrapped antigens to elicit rapid and prolonged specific immunity against a prototypical intracellular pathogen. In this regard, all of the tested archaeosomes were superior to conventional liposomes.

Introduction

The facultative intracellular bacterial pathogen, Listeria monocytogenes, is capable of parasitizing both host phagocytes [1], and parenchymal cells such as enterocytes [2] and hepatocytes [3]. Systemically initiated infection of mice with L. monocytogenes has long served as a model for studying adaptive immunity to intracellular pathogens in general [4], [5]. Mice that recover from a primary sublethal infection with L. monocytogenes acquire an enhanced resistance to re-infection which is considered to be a classical example of antigen-specific, CD8+ T-cell-mediated, macrophage-expressed immunity [4], [5]. Likewise, specific immunity against other intracellular bacterial pathogens appears to require the participation of this defense mechanism [6], [7], [8], [9]. Naturally, this immunity is acquired following exposure to sublethal doses of the specific virulent organism. Artificially, it can be generated by vaccination. Generally, live attenuated vaccines have proven most effective against this class of pathogen [10]. A major goal of modern vaccinology is to emulate the efficacy of such live vaccines with suitably adjuvanted, defined acellular vaccines.

In the case of L. monocytogenes, several experimental viable and non-viable vaccines have been reported. Many of these are based on an H-2Kd-restricted immunodominant epitope of the virulence factor, listeriolysin [11]. This epitope has been formulated co-entrapped with Quil-A in conventional liposomes [12], expressed with anthrax toxin as a fusion protein [13], or encoded in plasmid DNA [14]. It has also been expressed in recombinant vaccinia virus [15] or recombinant Salmonella typhimurium [16]. All of these vaccines have been tested in the murine model of systemic listeriosis. In all cases, vaccination elicited varying degrees of protection against subsequent exposure to L. monocytogenes. However, regulatory concerns, including safety concerns, still surround many of these vaccination strategies [17].

Our group has been interested in the utility of archaeosomes, which we [18] have defined as liposomes prepared from the polar lipids of various Archaeobacteria, as vaccine and drug delivery systems. Compared with the natural and synthetic ester phospholipids used to make conventional liposomes, archaeobacterial polar lipids possess distinct chemical features [19]. Consequently, archaeosomes display several unique properties, including enhanced stability against extremes of pH, oxidation, elevated temperatures, and the actions of lipases, that might be expected to impart superior properties to them for certain biotechnology applications compared with conventional liposomes [20], [21]. Indeed, it was recently demonstrated [22] that humoral and cell mediated immune (CMI) responses to model protein antigens entrapped alone in various archaeosomes were superior to those generated by the same antigens entrapped in conventional liposomes, or adsorbed to alum, and were equal to those achieved with Freund's adjuvant. The subsequent finding [23] that soluble antigens entrapped in archaeosomes induced antigen specific CD8+ T-cell responses was particularly interesting. To further explore the latter ability in a more biologically-relevant system, the present study was undertaken. It uses a mouse infection model to examine the ability of synthetic antigens encompassing an H-2Kd-restricted immunodominant epitope of listeriolysin encapsulated in various archaeosomes to elicit protective immunity against systemic challenge with L. monocytogenes. The results show that immunization with archaeosome-based vaccines can rapidly generate protective immunity, which subsequently persists for at least several months.

Section snippets

Mice

Specific-pathogen-free female BALB/c and C57BL/6 mice were purchased from Charles Rivers Laboratories (Montreal, Que.), and entered experiments when they were 8–12 weeks old. Mice were maintained and used in accordance with the recommendations of the Canadian Council on Animal Care Guide to the Care and Use of Experimental Animals.

Preparation of antigen

The known [11] H-2Kd MHC-class-I-restricted, immunodominant nonamer epitope (GYKDGNEYI) of listeriolysin was synthesised either as the free nonapeptide, or as a

Archaeosomes prepared from the total polar lipids of Methanobrevibacter smithii as self-adjuvanting vaccine vehicles for generating anti-Listeria immunity

For the present studies, archaeosomes made from the total polar lipids of M. smithii were initially employed because in earlier studies [22], [23] these particular archaeosomes generated a strong CMI response to other model antigens. In an initial study, 8-week-old female BALB/c mice were vaccinated with 1 mg of M. smithii archaeosomes containing 27 μg of the 20-mer lipopeptide on days 0 and 28. This antigen was chosen for initial studies because lipidation has been shown to enhance the

Discussion

The present study used a murine model of systemic listeriosis to assess the potential of novel liposomes, termed archaeosomes, prepared from the polar lipids of various Archaeobacteria to act as self-adjuvanting antigen presenting vehicles for sub-unit vaccines capable of eliciting protective cell-mediated immunity against an intracellular pathogen. To this end, mice were immunized with listeriolysin-derived lipopeptide vaccines encapsulated in various archaeosome types. The results were

Acknowledgments

The authors thank Chantal Dicaire, Lise Deschatelets, Perry Fleming, Ann Webb, Rhonda KuoLee, Jean-René Barbier, and Hua Shen for their technical expertise and assistance.

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