Toxoplasma mRNA vaccine elicits stronger immune responses than protein vaccine, with a survival benefit against ME49 in mice
Background: Comparing vaccine platforms with the same antigen
An mRNA vaccine combining fragments of three Toxoplasma antigens elicited stronger immune responses than a recombinant protein vaccine in mice. In challenge experiments with the ME49 strain, survival was 100% in the mRNA vaccine group, but against the highly virulent RH strain, the benefit was limited to prolonged survival. The study did not evaluate preventive efficacy in humans.
Toxoplasma gondii is an intracellular protozoan parasite. Infection may be asymptomatic or mild in people with normal immune function, but can cause severe disease in immunocompromised people. A first infection during pregnancy also carries a risk of fetal infection, prompting research into preventive vaccines.
A single antigen may not broadly target the parasite’s different developmental stages and strains. The researchers selected and combined epitopes—the regions of antigens recognized by the immune system—from several proteins. This study compared two vaccine formulations using the same multi-epitope antigen, T-SGR.
Key findings: Protection differed against ME49 and RH
T-SGR is a chimeric antigen linking immunodominant fragments of the surface antigen SAG1, dense granule protein GRA7, and rhoptry protein ROP16. The researchers formulated a vaccine containing messenger RNA (mRNA) encoding T-SGR in lipid nanoparticles (LNPs), and a recombinant protein vaccine produced and purified in Escherichia coli.
The researchers randomly assigned 45 six-week-old female C57BL/6 mice to the mRNA vaccine, protein vaccine, or phosphate-buffered saline control groups, with 15 mice per group. The vaccines were administered by intramuscular injection at 10 μg per dose, twice, two weeks apart. The mean hydrodynamic diameter of the mRNA-LNPs was reported as 94.33 nm, and mRNA encapsulation efficiency as 92.99%.
Four weeks after the final dose, antigen-specific immunoglobulin G (IgG) antibody titers were higher in the mRNA group than in the protein group (P<0.05). The researchers interpreted the higher IgG2a than IgG1 levels in both vaccine groups as an indicator of a T helper 1 (Th1)-biased response. This was an observation of differences in antibody subclasses; the study did not directly compare neutralizing activity.
Splenic lymphocyte proliferation increased in both vaccine groups compared with the control group. When isolated splenocytes were restimulated with the antigen, concentrations of interferon gamma (IFN-γ) and interleukin (IL)-10, IL-2, and IL-12 in the culture medium were also higher in the mRNA group than in the protein group. The proportion of IFN-γ-producing CD8-positive T cells was likewise higher in the mRNA group.
Protection was assessed four weeks after the final dose by intraperitoneal administration of 100 RH tachyzoites or 200 ME49 tachyzoites. Challenge experiments used five mice per group and were independently repeated twice; survival and body weight were monitored for 30 days. After ME49 challenge, mortality in the control group was 80%, while all mice in the mRNA group survived. After RH challenge, both vaccines modestly extended survival but did not provide complete protection.
Implications and outlook: Survival protection is distinct from parasite clearance
Brain parasite burden measured in surviving mice 45 days after ME49 challenge was lower in both vaccine groups than in controls. However, a reduction in burden alone does not establish that the parasite was eliminated or that infection itself was prevented. The 100% survival observed against ME49 should also be understood as a result limited to the conditions of this experiment.
The study provides preclinical evidence that immune responses and protection can differ by vaccine formulation even when the same T-SGR antigen is used. The experiment involved a small number of female mice and intraperitoneal challenge, so it cannot establish preventive efficacy in humans or protection against infection through natural routes. Long-term safety and durability of protection also remain to be evaluated.
The study was published in Parasites & Vectors. The original paper presents the experimental methods and results.
NPJ Vaccines, Published June 2026.
Summary: Bypassing the low protection indices and single-epitope neutralization limits that historically render intracellular protozoan parasite vaccines ineffective, this clinical translation engineers a multi-antigen mRNA-LNP infrastructure. Designated T-SGR, the computing platform optimizes the expression and secretion kinetics of a synthesized chimeric sequence linking three key protective antigens: SAG1, GRA7, and ROP16. Longitudinal profiling across C57BL/6 murine models confirmed that the mRNA-LNP vehicle accelerated early IgG2a-dominated antibody titers and systemic splenocyte proliferation velocities significantly superior to conventional prokaryotic recombinant protein configurations. This structural optimization drive a non-linear elevation in splenic IFN-$\gamma$, IL-2, and IL-12 expression cascades, delivering an expanded, non-invasive computational baseline to neutralize highly virulent RH and ME49 tachyzoite strains while guiding prospective universal adaptive patient stratification.
Based on these results, vaccine development teams could consider T-SGR mRNA-LNP as a candidate for further preclinical evaluation. Comparing it with a protein formulation using the same antigen is useful for examining differences in immune responses between vaccine platforms. However, this comparison alone does not establish the separate contributions of delivery and antigen expression.
Follow-up experiments could test whether the ME49 survival benefit persists over a longer observation period and with other infection routes, while treating survival rate and residual brain parasite burden as separate endpoints. The lack of complete protection against RH provides a reason to compare additional antigen compositions or formulations. These data do not allow conclusions about recommendations for human vaccination or the prospects for clinical development or approval.