"Most clinical trials evaluating cancer vaccines administer the experimental therapy along with chemotherapy because chemo is standard of care," said Noblecourt. "But they do so without reference to established evidence on how chemotherapies affect vaccine responses."

Timing is everything

The Ludwig Oxford team tested a panel of chemotherapies alongside a cancer vaccine developed in the Van den Eynde lab. The vaccine targets a cancer antigen identified and characterized by Ludwig researchers and comes in two shots: a "prime" dose followed days later by a "boost" dose.

CarboTaxol improved vaccine efficacy in several preclinical cancer models when given together with either dose. "Chemotherapies enhanced vaccine-induced CD8+ T cell responses against the targeted cancer antigen, expanded the pool of tumor-targeting CD8+ T cells and extended survival in preclinical models," Noblecourt said. "This effect was further enhanced by ICB."

Crucially, the adjuvant effect appeared only when chemotherapy and vaccine were given on the same day. Administer them days apart, and the benefit vanished.

A reservoir of resilient T cells

Mechanistically, the researchers traced the effect to a specific immune population. CarboTaxol expanded a pool of CD8+ T cells carrying a protein called TCF1, a master regulator of gene expression. TCF1 switches on genes that rejuvenate these T cells, extending their lifespan, preserving their function and enabling them to form immunological memory of the antigen they have encountered.

"CD8+ T cells that express TCF1 are of critical importance to initiating and sustaining anticancer immune responses," said Wicki. "The expansion of these cells by chemotherapy occurred independently of vaccination and established a deep pool of T cells for mobilization upon subsequent exposure to the vaccine antigen."

The effect may not be confined to mice. The researchers detected expansion of TCF1+ CD8+ T cells in samples from two independent cohorts of patients with different cancer types who had received CarboTaxol.

Implications for clinical trials

"Our findings have significant implications for clinical trials of cancer vaccines," said Van den Eynde. "Current studies may not be taking advantage of a potential adjuvant effect that might be obtained by simply altering the timing of vaccination relative to chemotherapy. Our study paves the way to defining the optimal interval to induce such synergies."

Because TCF1+ CD8+ T cells also matter for optimal responses to checkpoint blockade, the mechanism could extend to other immunotherapy combinations. Still, the tumor-control and survival results come from preclinical models, and the clinical observations so far cover only immune-cell changes, not patient outcomes. Clinical evaluation of the vaccine–chemotherapy–anti-PD-1 combination will be needed before any change in practice.

The paper is published as Laurine Noblecourt et al., "Chemotherapy enhances cancer vaccine efficacy and expands stem-like TCF1+CD8+ T cells," Cancer Cell (2026).