Multi-epitope schistosome vaccine candidates tested for protective immunogenicity in mice

Abstract

The major challenge in the development of anti-schistosome vaccines is to use defined antigens to stimulate an appropriate immune response that leads to resistance. Several promising candidate vaccine antigens including the glycolytic enzyme triose-phosphate isomerase (SmTPI), a 28 kDa glutathione-S-transferase (Sm28), the myofibrilar protein paramyosin (Sm97), an integral membrane protein (Sm23) and calpain (Smcalpain) have been characterised and their primary sequences derived for Schistosoma mansoni. Furthermore, sequences are available for synthetic peptides mimicking epitopes on these molecules capable of inducing schistosome-specific T- and B-cell responses. These schistosome vaccine candidates have generally been tested with varying degrees of success as single components, with only one report of the use of a multivalent antigen or multi-epitope approach. We describe the assembly of multiple defined and different epitopes of S. mansoni into a variety of single covalent structures; these included a DNA vaccine encoding different epitopes in tandem, the polyprotein itself that is encoded by this DNA and branched synthetic peptide epitope-based polymers in which the individual epitopes are pendant from an inert backbone. Each of the vaccine constructs examined, with the exception of the DNA vaccine, generated antibodies that were capable of binding to a tandem sequence of the epitopes. Although these results were encouraging, none of the constructs protected animals from subsequent challenge infection, indicating that the immune responses elicited were inadequate or inappropriate for parasite killing in vivo.

Introduction

The search for an effective vaccine against schistosomiasis, a parasitic disease currently affecting over 200 million people, remains a challenging and elusive goal. Research demonstrating the ability of humans to acquire natural immunity to schistosome infection, together with the successful use in animals of attenuated vaccines and the encouraging results obtained with defined antigens, suggests that development of a vaccine is achievable [1]. Noteworthy also are the recent findings on immune correlates which shed light on the complex, but putative protective immune response mechanisms which operate in humans [1], [2]. All of these developments have improved the prospects of success [1], [2].

The major challenge in the development of anti-schistosome vaccines is to use defined antigens to stimulate an appropriate immune response that leads to resistance. Concomitant with the improved understanding of the mechanisms involved in protective immunity which indicate that a schistosome vaccine should include antigens that can sensitize both Th1 and Th2 CD4⁺ cells (see, for example, [3]), several promising candidate vaccine antigens have been characterised and their primary sequences derived for S. mansoni. These antigens include the glycolytic enzyme triose-phosphate isomerase (SmTPI) [4], [5], a 28 kDa glutathione-S-transferase (Sm28) [6], [7], the myofibrilar protein paramyosin (Sm97) [8], an integral membrane protein (Sm23) [9] and calpain (Smcalpain) [10]. In addition, sequences are available for synthetic peptides mimicking epitopes on SmTPI [5], Sm28 [7], Sm23 [11], Smcalpain [12] and Sm97 [13], [14] capable of inducing schistosome-specific T- and B-cell responses.

These schistosome vaccine candidates have generally been tested with varying degrees of success as single components. To date, there is only one report of the use of a multivalent antigen or multi-epitope approach, namely that by Ferru et al. [15], who constructed a Multiple Antigen Peptide (MAP) composed of three sequences from Sm28 and SmTPI. To extend this approach, we have used two different methods to assemble multivalent constructs comprising known schistosome B- and T-cell sequences and tested them for protective immunogenicity in mice. The first approach involved the construction of a synthethic gene that coded for a single artificial polyepitope (polytope) protein [16] comprising eight S. mansoni sequences containing T- and B-cell epitopes linked in tandem. The full length DNA fragment was inserted into plasmid VR1012, for use as a nucleic acid vaccine [17], and into pQE31 for Escherichia coli-expression as a recombinant polytope protein vaccine [18]. The second approach utilised a recently developed generic chemical method [19], [20] to synthesise and co-polymerise a number of schistosome epitope-containing peptide sequences to produce two synthetic-based multi-epitope polymers (polytope polymer 1 and polytope polymer 2), which were also tested for immunogenicity and protective efficacy.