CD27 Agonism Enhances Long-Lived CD4? T Cell Vaccine Responses Critical for 2 Anti-Tumor Immunity

Patients with metastatic breast cancer were vaccinated with dendritic cell (DC) vaccine targeting HER2, and all seven survived >18 years. PBMC analysis revealed HER2-specific CD27? memory CD4? and CD8? T cells, suggesting that CD27 signaling supports durable immune memory. We tested this by combining an anti-CD27 agonist antibody (Varlilumab) with a HER2 vaccine, which enhanced HER2-specific responses, particularly long-lived CD4? memory T cells detectable up to 300 days post-vaccination. CD27 agonism improved tumor control (~40% regression) compared to vaccine alone (~6%), and synergy with PD-1 blockade led to complete tumor rejection in ~90% of mice. CD4? T cells were essential for this effect, as shown by depletion and adoptive transfer experiments, while CD8? T cells played a less critical role. We utilized combined scRNA-seq and TCR sequencing to identify expanded clonotypes and increased CD8+ and CD4+ functional subsets with combined CD27 agonism and vaccination as well as vaccination alone. These findings demonstrate that antigen-specific huCD27? CD4? T cells are key effectors of vaccine-induced immunity and support CD27 agonism as a promising strategy to enhance therapeutic cancer vaccination. Overall design: MM3MG-HER2del16 cells were orthotopically implanted into the mammary fat pad of female 8-12 weeks old F1 offspring mice, by crossing between HuCD27-Tg with regular Balb/C or Jh-Balb/C (1x106 cells/animal). Four arms of these tumors were harvested for combined TCR and scRNA-seq sequencing by 10X: control IgG treated mice, anti-human CD27 antibotdy treated alone, vaccination with AdHER2, and combined vaccination with anti-human CD27 antibody treatment. Mice were sacrificed and tumor collected at 14 days post vaccination.