The PROTEXI vaccine redirected COVID-19 immune memory against tumors in humanized mice

The PROTEXI vaccine redirected COVID-19 immune memory against tumors in humanized mice

On July 27, Nature Communications published a study of PROTEXI, a dendritic cell vaccine. In mouse models of melanoma and breast cancer, the researchers first established CD4 memory against a model protein fragment and then used that memory against the tumor. In a separate experiment using Spike peptides in humanized mice, the vaccine reduced tumor burden.

A cancer vaccine must accomplish two tasks. CD8 T cells must recognize the tumor target, while CD4 helper T cells must help initiate the response. A dendritic cell takes up protein fragments and presents them to both types of T cell.

CD4 help is usually induced with tumor fragments. However, it is difficult to identify a fragment that will work across different people. The authors of the PROTEXI study tested a different approach. They loaded a single dendritic cell with a tumor peptide for CD8 cells and a peptide already familiar to CD4 cells. In experiments with conventional mice, the researchers first established this memory using a model protein fragment. In the humanized mouse experiment, they used Spike peptides and immune cells from donors who had been vaccinated against COVID-19.

The comparison showed why this design mattered. A single dendritic cell carrying both peptides controlled melanoma more effectively than a mixture in which separate cells carried the helper and tumor signals. In the breast cancer model, suppression of tumor growth depended on previously established CD4 memory. These experiments support the proposed mechanism: a familiar CD4 signal provides help when a dendritic cell presents it alongside the tumor target.

In the humanized mice, the researchers used immune cells from donors who had been vaccinated against COVID-19. A vaccine containing Spike peptides and the human tumor antigens PRAME and MAGE-A3 reduced melanoma tumor burden.

In humanized mice, PROTEXI used memory of Spike as a ready source of CD4 help. The tumor peptide gives CD8 cells a target, while Spike allows the dendritic cell to activate CD4 cells that have already been trained. The study proposes using existing immune memory as a component of a cancer vaccine.