Measuring of IgG2c isotype instead of IgG2a in immunized C57BL/6 mice with Plasmodium vivax TRAP as a subunit vaccine candidate in order to correct interpretation of Th1 versus Th2 immune response.

Evaluation of the murine isotype antibodies is essential in subunit vaccine development because inbred mouse strains with diverse genetic backgrounds respond different to recombinant proteins. In this regard, the main goal of this study was to measuring and comparing the profile of IgG isotype responses in C57BL/6 mice. For this purpose, the extracellular region of plasmodium vivax thrombospondin-related adhesive protein (PvTRAP) gene was expressed in Escherichia coli Rosetta (DE3)-pET23a. Then, the recombinant PvTRAP alone or emulsified with Freund's complete adjuvant were applied for immunization of the C57BL/6 mice. The role of antibodies and cellular immune responses induced by recombinant PvTRAP were evaluated. The results showed the level of anti-rPvTRAP IgG2c was significantly higher than IgG2a in the groups that received rPvTRAP alone (mean OD490 = 0.798 ± 0.12 and 0.39 ± 0.1, respectively) and emulsified with CFA/IFA (mean OD490 = 1.48 ± 0.07 and 0.605 ± 0.13, respectively; P < 0.05, independent sample t-test). Additionally, the immunized mice with rPvTRAP and rPvTRAP + CFA/IFA had an intermediate-avidity IgG2a antibody but high-avidity IgG2c antibody as well as the mean of serum antibody titers results exhibited that in both rPvTRAP and rPvTRAP + CFA/IFA mouse groups, IgG2a end-point titer (1:3200 and 1:25,600, respectively) was noteworthy lower than IgG2c (1:25,600 and 1:102,400, respectively). Moreover, the results revealed the eliciting significant levels of IFN-γ (P < 0.05, independent sample t-test) and no detectable level of IL-4 in the mouse groups received rPvTRAP alone and emulsified with CFA/IFA as compared to the mouse control groups. In general, our results showed that for correctly interpreting of Th1 immune responses in C57BL/6 mouse strain it is critical to measure IgG2c instead of IgG2a along with IFN-γ.