Non-toxic derivatives of LT as potent adjuvants

Abstract

The heat-labile enterotoxin of Escherichia coli (LT) consists of an enzymatically active A subunit (LTA) and a pentameric B subunit (LTB). LT has been extensively studied as a potent modulator of immune responses but wild-type LT is toxic and therefore unsuitable for clinical use. Approaches pursued to avoid the toxicity associated with the use of the native toxin while retaining its adjuvant properties have included isolation of subunit B (LTB) and construction of non-toxic LT AB complex mutants, such as LTK63 mutant. Here we review the immunomodulatory characteristics of LTB and LTK63 and their potential as mucosal and parenteral vaccine adjuvants.

Introduction

Vaccination is undoubtedly the intervention with the greatest impact on global health [1]. New generation vaccines, particularly subunit vaccines, based on recombinant or purified proteins and synthetic peptides, are less reactogenic but poorly immunogenic in comparison with whole-organism vaccines which contain many immunostimulatory components. Thus, subunit vaccines are typically administered with adjuvants to amplify and direct vaccine-specific immunity [2], [3].

Adjuvants are defined as pharmacological or immunological agents that can stimulate the immune system and increase, modulate and/or prolong the intrinsic immunogenicity of co-administered antigens, thereby enhancing vaccine efficacy. The word “adjuvant” comes from the Latin word adjuvare, meaning to help or aid [4]. Adjuvants are a heterogeneous group of compounds and can be divided in two groups according to their dominant mechanisms of action. They can operate via activation and potentiation of innate immunity either directly or via pattern-recognition receptors (PRRs) to generate robust and long-lasting adaptive immune response. Alternatively, delivery systems may concentrate and display antigens in repetitive patterns, target vaccine antigens to antigen-presenting cells (APCs) and help localize antigens and immune potentiators to ensure that the vaccine is delivered to the right place at the right time [5], [6]. These activities are not mutually exclusive and some adjuvants exhibit both properties [7]. In addition, immune polarization of the vaccine-induced response is an important consideration [8].

A range of compounds including emulsions, saponins, bacterial products, Toll-like receptor (TLR) agonists, nucleic acids, virosomes, liposomes and a combination of some of them have been shown to display potent adjuvant activity in animal models [2], [3], [5], [7]. However, only a few vaccine adjuvants are licensed for use in humans. For more than 70 years, aluminum mineral salts was the only vaccine adjuvant approved worldwide for clinical use and remained the only one licensed for human use by the U.S. Food and Drug Administration (FDA) until 2009. In the past decade, two oil-in water emulsions (MF59 and AS03), one recombinant toxin (cholera toxin B subunit, CTB), a virosome (immunopotentiating reconstituted influenza virosomes, IRIV) and one TLR-4 agonist (monophosphoryl lipid A formulated in aluminum hydroxide, AS04) were licensed by the European Medicines Agency (EMA) as vaccine adjuvants for human use. Recently, the USA FDA approved the adjuvant AS04 for clinical use [2], [3], [9]. The need for new adjuvants has been led mainly by the shortcoming of the currently approved ones in eliciting the desired immune response against different target pathogens.

A number of novel adjuvants are in development and evaluation and some have demonstrated profound effects on vaccine potency in preclinical models. These include synthetic TLR agonists, such as unmethylated viral or bacterial CpG DNA and oligonucleotides [10], lipopeptides such as tripalmitoyl-S-glyceryl cysteine (Pam3Cys) [11], water-in-oil emulsions such as Montanide ISA 720 [12], saponins such as QS21 [7], and bacterial enterotoxins such as the heat-labile enterotoxin from Vibrio cholerae (cholera toxin, CT) and Escherichia coli (LT) [13], [14], [15].

Of particular interest are products of the heat-labile enterotoxin of E. coli (LT) including subunit B (LTB) and non-toxic LT AB complex mutants such as LTK63. These enterotoxins have demonstrated potent immunomodulatory activity with a range of antigens in different animal models, with enhanced immunogenicity as well as protective efficacy, and are considered potent mucosal and parenteral adjuvants [13]. Herein, we review the immunomodulatory characteristics of LTB and LTK63 and provide examples of how these properties have been exploited for vaccine development.