Evaluation of CpG-ODN-Adjuvanted Toxoplasma gondii Virus-Like Particle Vaccine upon One, Two, and Three Immunizations

Abstract

Successful vaccines against specific pathogens often require multiple immunizations and adjuvant usage. Yet, assessing the protective efficacy of different immunization regimens with adjuvanted Toxoplasma gondii vaccines remains elusive. In this study, we investigated the vaccine efficacy induced by CpG-ODN-adjuvanted T. gondii virus-like particles (VLPs) after challenge infection with T. gondii (ME49) in mice (BALB/c) upon one, two, and three immunizations. Immunization with adjuvanted T. gondii VLPs induced higher levels of T. gondii-specific IgG and/or IgA antibody responses, germinal center (GC) B cells, total B cells, and CD4+ and CD8+ T cells compared with unadjuvanted VLPs. Increasing the number of immunizations was strongly correlated with enhanced protective immunity against T. gondii in mice, with the highest protection being demonstrated in mice thrice-immunized with either adjuvanted T. gondii VLPs or VLPs alone. Notably, lesser bodyweight reductions and cerebral cyst counts were observed in mice receiving multiple immunizations with the adjuvanted VLPs, thereby confirming the effectiveness of adjuvanted boost immunizations. These results demonstrated that multiple immunizations with T. gondii VLPs is an effective approach, and the CpG-ODN can be developed as an effective adjuvant for T. gondii VLP vaccines.

Passive Immunity and Antibody Response Induced by Toxoplasma gondii VLP Immunization

“…In order to prepare the immune sera for passive immunization, VLPs containing the T. gondii inner membrane complex (TG1), T. gondii rhoptry protein 18 (TG4) and T. gondii microneme protein 8 (TG6) were produced and characterized as described previously [ 22 ]. The mice were immunized with VLPs, and parasite-specific IgG, IgG1, IgG2a, IgG2b and IgM antibody responses at weeks 1 and 4 post-immunization and post-challenge were determined as scheduled.…”

“…The Toxoplasma gondii RH and ME49 strains were maintained in seven-week-old female BALB/c mice purchased from NARA Biotech (Seoul, Korea), as previously described [ 22 ]. The Spodoptera frugiperda Sf9 insect cells (ATCC, CRL-1711) were cultured in serum-free SF900-II medium (Invitrogen, Carlsbad, CA, USA), and were subsequently used to generate recombinant baculovirus (rBV) and virus-like particles (VLPs).…”

“…T. gondii RH antigens were prepared by sonicating tachyzoites in PBS, as described previously [ 22 ]. The T. gondii RH antigen-specific IgG antibody responses were determined by ELISA, as described [ 21 ].…”

“…The mice were monitored daily for 30 days in order to evaluate their changes in bodyweight and survival. At 30 days post-infection (dpi), the mouse brain tissues were isolated for cyst counting, as previously described [ 22 ]. Mice that lost 20% of initial bodyweight were humanely euthanized, as described [ 21 , 23 ].…”

“…To date, VLP vaccine efficacies against T. gondii infections were mostly evaluated following a prime-boost immunization regimen [ 19 , 20 , 21 ]. Recently, in our previous study, the protective efficacies of CpG-ODN adjuvanted T. gondii VLPs from mice receiving a differing number of immunizations were assessed [ 22 , 23 ]. Although limited information has been presented for a single immunization, the results of our previous study revealed that VLP-immunization induces potent levels of parasite-specific antibody responses, which signified its potential application in passive immunization studies.…”