Evaluation of the immune response to human metapneumovirus multi-epitope antigen in an mouse model

Abstract

Objective To evaluate the immune response triggered by an in-house constructed human metapneumovirus multi-epitope antigen (MEA) in a mouse model. Methods Female SPF BALB/c mice at age 4-6 weeks were used in the study and divided into 7 groups. Mice in the five groups including MEA+ oligodeoxynucleotides containing CpG motifs (CpG ODN) intraperitoneal injection (i.p.) treatment group, MEA+ Alum i. p. treatment group, MEA+ Alum+ CpG ODN i. p. treatment group, MEA+ CpG ODN intranasal (i.n.) treatment group and MEA+ Alum+ CpG ODN i. n. treatment group were immunized three times on days 0, 14 and 21, and those in the other experimental group were immunized intramuscularly with MEA+ Quickantibody5W on days 0 and 21. A control group without treatment was set up accordingly. All mice were sacrificed two weeks after the last immunization. Antibodies including IgG, IgG1, IgG2a and IgA in serum samples were detected by ELISA. MTS assay was performed to analyze the proliferation of lymphocytes. The cytotoxicity of cytotoxic T lymphocytes (CTL) was measured by LDH assay. Flow cytometry was used to detect T lymphocyte subsets. The cytokines secreted by T helper cells (Th1 and Th2) were analyzed with Bio-Rad Liquid Chips. Results High titers of IgG, IgG1 and IgG2a antibodies were produced in MEA treated mice except for those in intranasal treatment groups. Serum samples from three groups including the MEA+ Alum i. p., MEA+ Alum+ CpG ODN i. p. and MEA+ Quickantibody5W i. m. treatment groups were positive for IgA antibody. The highest titer of IgA antibody was detected in mice from the MEA+ Alum+ CpG ODN i. p. treatment group, which was 2.15×103. Compared with the control group, significantly enhanced proliferation of lymphocytes was observed in the MEA+ Alum i. p., MEA+ Alum+ CpG ODN i. p. and MEA+ Quickantibody5W i. m. treatment groups (P<0.05). Enhanced cytotoxic activities of CTL were observed in mice with ip. and i. m. treatments as compared with those in control group (P<0.05). The levels of CD4+ /CD8+ T cells were slightly increased in mice from the MEA+ CpG ODN i. p., MEA+ Alum+ CpG ODN i. p. and MEA+ Quickantibody5W i. m. treatment groups as compared with those in control group (P<0.05). Increased secretion of IL-2, IFN-γ and Th2-type cytokines including IL-4, IL-5 and IL-10 were detected in mice from the MEA+ CpG ODN i. p. treatment group. The MEA+ Alum i. p. treated mice showed a slightly increased secretion of IFN-γ and significantly increased secretions of IL-4, IL-5 and IL-10. Significantly increased secretions of IFN-γ, IL-4, IL-5 and IL-10 were detected in mice from the MEA+ Alum+ CpG ODN i. p. treatment group. Significantly increased secretions of IFN-γ, IL-5, IL-10 and granulocyte-macrophage colony-stimulating factor (GM-CSF) were detected in mice from the MEA+ Quickantibody5W i. m. treatment group. Conclusion MEA together with different adjuvants could stimulate high titers of specific antibodies, increase the proliferation of lymphocytes and enhance the cytotoxic activities of CTL. CpG ODN could balance the Th1/Th2-mediated immune responses, and the balance could be enhanced when using CpG ODN in combination with Alum. A similar effect could be achieved by using the commercial adjuvant Quickantibody5w. This study has paved the way for further investigation on the development of hMPV epitope vaccines and diagnostic reagents for hMPV as well as the epidemiological study of hMPV. Key words: Human metapneumovirus; Multi-epitope antigen; Immune response