The Average IFN-γ Secreting Capacity of Specific CD8+ T Cells Is Compromised While Increasing Copies of a Single T Cell Epitope Encoded by DNA Vaccine

Previous studies suggested that both the frequency and the mean fluorescence intensity (MFI) of cytokine secreting T cells could be of great value for immunogenicity evaluation of a vaccine. In this study, by constructing epitope-based DNA vaccines encoding a previously identified CD8+ T cell epitope, we investigated the influence of multiplying epitope copies on both the frequency and the MFI of specific IFN-γ secreting CD8+ T cells. We found that frequencies of specific CD8+ T cell could be improved by multiplying epitope copies, while the MFI of IFN-γ secreted by epitope-specific CD8+ T cells decreased synchronously. And further analysis showed that the decrease of MFI was not caused by the functional avidity variation of CD8+ T cell receptor. 1. Introduction Traditional vaccines have dramatically diminished morbidity and mortality of a large number of infectious diseases, while their success cannot be easily translated into developing of vaccines against HIV, malaria, and cancer [1]. Novel approaches are urgently needed. In formality of either recombinant vectored vaccines or synthetic peptides, epitope-based vaccine represents one of these emerging approaches. Taking benefits of well-defined epitopes with a minimal structure influence, this epitope-based approach can focus immune responses on conserved epitopes and also increase the potency and breadth of specific immune responses [2, 3]. Although it has been widely employed in vaccine development against HIV, HCV, HBV, HPV, cancer, and Helicobacter pylori [4, 5], its relatively weak immunogenicity still remains a major restraint for the practical application of epitope-based vaccines [6]. Previous studies suggested that the immunogenicity of epitope-based DNA vaccines could be enhanced by introducing intracellular targeting signals to direct the encoded gene product to the endoplasmic reticulum (ER) [7, 8], by including Pan HLA-DR epitope (PADRE) to support the development of specific immune responses [7, 9] and by incorporating spacer sequences between epitopes to optimize epitope processing [10]. Most recently, another study also suggested that increasing the copy number of epitope coding gene could augment the magnitude of specific T cell responses against a carcinoembryonic-antigen-(CEA-) derived epitope [11]. However, in most of these studies, the frequency of specific cytokine (IFN- ) secreting T cells was typically used as the parameter to measure the specific T cell responses. But this might not be the best indicator of protective T cell responses, because several lines of evidence have