CpG 1018 Enhances mRNA Vaccine–Induced CD8+ T-Cell Responses in Mouse Melanoma
Background: An adjuvant to boost immune responses to mRNA cancer vaccines
When CpG 1018, an adjuvant used in a hepatitis B vaccine, was administered with an mRNA cancer vaccine, CD8+ T-cell responses and tumor growth inhibition increased in mouse melanoma models. Published in Molecular Therapy Oncology, the study highlights the potential of an adjuvant combination strategy that enhances dendritic-cell maturation and local immune stimulation.
Personalized neoantigen mRNA vaccines present the immune system with antigens derived from mutations in cancer cells. Early clinical trials have produced promising results, but strategies to induce strong, durable antigen-specific T-cell responses remain under development. Adjuvants may stimulate the immune system while also potentially interfering with mRNA-driven protein production, so both effects need to be assessed together.
The researchers focused on CpG 1018, which stimulates Toll-like receptor 9 (TLR9). Its established approved use is as an adjuvant in the hepatitis B vaccine HEPLISAV-B; clinical efficacy or approval for this cancer vaccine combination has not been established.
Key findings: Tumor inhibition across models and the contribution of CD8+ T cells
The researchers mixed 5 μg of mRNA encapsulated in lipid nanoparticles (LNPs) with 40 μg of CpG 1018 and administered the mixture intramuscularly. They evaluated an OVA mRNA vaccine in B16F10-OVA melanoma, which expresses the model antigen ovalbumin (OVA), and an mRNA vaccine containing mouse tumor neoantigens in B16F10 melanoma.
In the OVA treatment model, vaccines were given 5 and 10 days after tumor inoculation. The paper reported tumor growth inhibition of 59% in the mRNA-only group and 90% in the CpG 1018 combination group. In the neoantigen treatment model, the corresponding figures were 53% and 75%. Both experiments included five mice per group; these figures do not represent human response rates or survival rates.
In the OVA combination group, CD8+ T cells made up a greater proportion of intratumoral T cells, and the CD8+/CD4+ T-cell ratio was higher. Depletion experiments showed that removing CD8+ T cells weakened tumor inhibition. By contrast, depleting CD4+ T cells or natural killer (NK) cells did not significantly reduce the effect under the same conditions, supporting a key contribution from CD8+ T cells in this model.
Intracellular staining and flow cytometry also showed enhanced immune responses. In the OVA treatment model, the combination group had a higher proportion of granzyme B-positive CD8+ T cells than the other comparison groups. In the neoantigen model, the proportion of granzyme B-positive CD8+ T cells was also significantly higher with the combination than with mRNA alone.
Mechanistic analyses found increased expression of the costimulatory molecules CD40 and CD80 on dendritic cells in the muscle at the injection site. Conventional dendritic cells in the draining lymph nodes showed increased CD40, CD80, and CD86, while local cytokine and chemokine responses in the muscle and lymph nodes were also enhanced. CpG 1018 did not significantly affect expression of the reporter protein enhanced green fluorescent protein (EGFP) in cultured human muscle cells or mouse bone marrow-derived dendritic cells. The researchers interpreted the results as indicating that activation of dendritic cells and local immune responses, rather than increased antigen production, explains the effect.
Implications and outlook: Efficacy and safety in humans remain to be studied
The findings provide preclinical evidence for combining an existing adjuvant with mRNA cancer vaccines. However, antitumor effects were evaluated only in mouse melanoma. OVA is a model antigen, and the neoantigen experiments should also be distinguished from clinical trials evaluating patient-specific vaccines. Local cytokines were measured using tissue samples pooled from multiple animals, making it difficult to assess variation between individual animals.
Transient body-weight reductions of less than 6% in the OVA treatment model and less than 3% in the neoantigen model were reported. Body-weight monitoring is one safety measure and cannot establish safety in humans. Reproducibility in other cancer types, combination doses and administration schedules, and safety with repeated dosing require further evaluation. Combining the vaccine with immune checkpoint inhibitors was proposed by the researchers as a future direction; it was not tested in this study.
The study conditions and results are available in the full paper.
NPJ Vaccines, Published June 2026.
Summary: Bypassing the low immunogenicity velocities and severe loose-response constraints that historically compromise single-agent neoantigen mRNA formulations in aggressive solid tumors, this study constructs a programmable CpG 1018-adjuvanted mRNA vaccine infrastructure. Utilizing a clinically validated TLR9 agonist to shift dendritic cell maturation kinetics, the computing platform optimizes local cytokine and chemokine secretion cascades uncoupled from baseline mRNA translation rates. Longitudinal profiles across B16F10-OVA melanoma models confirmed a non-linear acceleration in tumor-infiltrating Granzyme B, IFN$\gamma$, and TNF$\alpha$-secreting CD8+ T cells, yielding robust anti-tumor indices directly proportional to elevated intratumoral CD8+ to CD4+ ratios. This molecular calibration delivers a validated, non-invasive computational baseline to eliminate systematic off-target cardiotoxicity noise, maintain body weight fluctuations under 6%, and guide prospective adaptive single-cell patient stratification.
In developing mRNA cancer vaccines, researchers can consider not only antigen design and delivery-system optimization but also how the immune system recognizes the antigen and activates T cells after vaccination. This study provides evidence that adding CpG 1018 enhanced dendritic-cell maturation and CD8+ T-cell responses in mice without significantly changing reporter-protein expression in the cells tested.
In follow-up preclinical development, candidate neoantigen mRNA vaccines could be evaluated with and without CpG 1018, comparing antigen-specific T-cell responses, tumor growth, local inflammation, and toxicity with repeated dosing. Experience using CpG 1018 in existing vaccines provides a starting point for considering development, but the appropriate dose, efficacy, and safety of the cancer vaccine combination in humans must be established separately.