Systemic immunization with an epitope-based vaccine elicits a Th1-biased response and provides protection against Helicobacter pylori in mice
Highlights
► We constructed a vaccine which was composed by predicted CD4+ T cell epitopes. ► We examined the prophylactic potential of the vaccine against Helicobacter pylori. ► Immunization with the epitope-based vaccine significantly reduced bacterial load. ► The epitope-based vaccine elicits systemic and local Th1-biased response. ► The vaccine-mediated Th1-biased response may contribute to the protective effect.
Introduction
Helicobacter pylori is a Gram-negative, microaerophilic bacterium that infects more than half of the human population. Although over 80% of infected people show no symptoms, the bacterium is the main cause of chronic gastritis and peptic ulcers [1], [2]. H. pylori is also associated with cardiovascular, dermatological, pulmonary and hematological diseases [3], [4], [5], [6]. Current first-line clinical therapies, based on the use of antibiotics combined with proton pump inhibitors, are not ideal because of their high cost, low patient compliance and increased antibiotic resistance [7]. A prophylactic vaccine should be cost-effective in controlling H. pylori infection [8], [9].
Different kinds of vaccines have been formulated aiming at protecting against H. pylori infection. Whole cell vaccines share the advantage of eliciting comprehensive protective immunity, yet they may cause side-effects resulting from cross-reaction with host-derived epitopes [10]. In addition, live vector vaccines have also been evaluated for human use, but they failed to elicit an antigen-specific systemic and mucosal protective response [11], [12]. The most common vaccines are prepared using recombinant antigen subunits. Some antigens have been confirmed to be able to trigger immunity against H. pylori infection, such as HpaA, UreB and CagA [13], [14]. A single recombinant antigen induces insufficient immunity and has limited protective effect [15]. Multivalent vaccines, combining different antigens participating in different aspects of colonization and pathogenesis, may be more effective [16]; however, antigen expression and purification remain difficult.
Epitope-based vaccines may be a possible solution to those problems. Antigens are known to display their specificity mainly through epitopes. Consisting of epitopes from versatile antigens, epitope-based vaccines are capable of inducing more specific and potent immune responses than whole antigens [17], [18]. In addition, cross-reactivity to self-antigens can be eliminated to avoid potential side-effects [19], thus providing a safer vaccine.
Current evidence indicates that vaccine-mediated protection against H. pylori relies more on developing a strong antigen-specific CD4+ T cell response than on a humoral response [20]. Though currently the roles that Th1, Th2 and Th17 immune responses play in protection are not completely understood, studies suggest that induction of a Th1 response is necessary for the protective effect [21], [22], [23]. Together with Th1 adjuvants, systemic application of a vaccine consisting of CD4+ T cell epitopes may be a promising approach to protect against H. pylori.
In this study, by means of bioinformatic algorithms, potential immunodominant CD4+ T cell epitopes were screened from HpaA, UreB and CagA. Based on a combination of the predicted CD4+ T cell epitopes, Epivac was constructed, expressed and purified. Together with Th1 adjuvants, the protective effect of Epivac against H. pylori was assessed in BALB/c mice, and both the humoral and cellular immune responses induced by Epivac were evaluated.
Section snippets
Prediction of CD4+ T cell epitopes
Based on the predicted IC50 values for peptides binding to MHC class II molecules, potential immunodominant CD4+ T cell epitopes were screened from UreB, CagA and HpaA by online bioinformatics algorithms (Immune Epitope Database, http://www.immuneepitope.org/).
Construction, expression and purification
Epivac comprises a combination of predicted epitopes. Bi-lysine (KK) spacer sequences were engineered among epitopes of each antigen to optimize epitope processing. Synthetic oligonucleotides encoding Epivac were ligated into the XhoI-Nde
Prediction of CD4+ T cell epitopes with online software
Protein sequences from HpaA, CagA, and UreB were screened for CD4+ T cell epitopes based on their predicted binding affinity to MHC class II molecules. As shown in Table 1, a total of 17 unique peptides were selected to construct the epitope-based vaccine.
Expression and purification of Epivac
Based on SDS-PAGE, the content of Epivac in the inclusion bodies was about 85% of total protein. Epivac was purified by affinity chromatography and ion exchange chromatography. Upon purification, Epivac was more than 90% pure as determined by
Discussion
Consisting of immunodominant epitopes from different protective antigens, epitope-based vaccines are likely to induce strong, comprehensive protective immunity as well as prevent undesired side-effects. Epitope-based vaccines have been developed to protect against certain infections, such as HIV [27], [28], Neisseria meningitides [29] and Mycobacterium tuberculosis [30]. Epitope-based vaccines may also be a potential preventive strategy in H. pylori infection.
Some important protective antigens
Acknowledgements
This work was supported by Chinese National Natural Science Foundation Project (No. 30771992), Major National Science & Technology Specific Projects of China (2009ZX09102-220) and Chongqing Natural Science Foundation (CSTC 2011BB5043).
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