Researchers at Case Western Reserve University and Cleveland-based Celloram Inc. have developed an experimental dendritic cell cancer vaccine that harnesses pre-existing CD4+ T-cell immunity against SARS-CoV-2 to strengthen anti-tumor immune responses, including in mouse models resistant to anti-PD-1 checkpoint inhibitors. The findings, published July 27 in Nature Communications, suggest that immune memory generated through COVID-19 vaccination or infection could be repurposed to improve the effectiveness of therapeutic cancer vaccines.
The approach addresses a longstanding challenge in cancer immunotherapy. Although dendritic cell vaccines can activate tumor-specific CD8+ cytotoxic T cells, they have generally failed to generate durable clinical responses because of insufficient CD4+ T-cell help. The FDA-approved prostate cancer vaccine sipuleucel-T (Provenge) demonstrated that dendritic cell vaccination can be clinically effective, but broader success has been limited.
The investigators developed a platform called PROTEXI that loads dendritic cells with two classes of antigens simultaneously: SARS-CoV-2 spike protein epitopes presented on MHC class II molecules to activate pre-existing spike-specific CD4+ memory T cells, and tumor-associated antigens presented on MHC class I molecules to prime tumor-specific CD8+ T cells. Rather than generating new helper T-cell responses from scratch, the strategy exploits the widespread anti-spike immunity now present in much of the global population.
The study showed that both antigen classes must be presented by the same dendritic cell to achieve maximal activity. Vaccines in which spike and tumor antigens were delivered on separate dendritic cells produced substantially weaker anti-tumor responses, indicating that direct co-presentation is required for efficient CD4+ T-cell licensing of dendritic cells and subsequent CD8+ T-cell activation.
In the immune-cold, anti-PD-1-resistant B16F10 melanoma model, PROTEXI significantly reduced tumor growth and prolonged survival compared with conventional dendritic cell vaccines loaded only with tumor antigens. Similar activity was observed in a murine 4T1 breast cancer model. Depletion of CD4+ T cells abolished the therapeutic effect, confirming that helper T cells were essential to the vaccine's activity rather than simply associated with treatment response.
Gene expression analysis demonstrated increased expression of cytotoxic effector genes and T-cell trafficking chemokines following treatment. The investigators also observed evidence of epitope spreading, in which immune responses broadened beyond the original vaccine antigens as tumor cell destruction released additional tumor-associated proteins, potentially expanding the anti-tumor T-cell repertoire.
The platform also enhanced responses when combined with other immunotherapies. In anti-PD-1-resistant melanoma, PROTEXI plus PD-1 blockade produced greater tumor control than either treatment alone. A separate combination with the TGF-β receptor I inhibitor vactosertib also improved tumor control and generated durable complete responses in a subset of animals.