We have previously demonstrated that metastatic colorectal cancer patients who exhibit immune responses to a dendritic cell (DC) vaccine have superior recurrence-free survival following surgery, compared to patients in whom responses do not occur. We sought to characterize the patterns of T lymphocyte infiltration and somatic mutations in metastases that are associated with and predictive of response to the DC vaccine.

Cytotoxic, memory, and regulatory T cells in resected metastases and surrounding normal liver tissue from 22 patients (11 responders and 11 non-responders) were enumerated by immunohistochemistry prior to vaccine administration. In conjunction with tumor sequencing, the Combined Multivariate and Collapsing method was used to identify gene mutations that are associated with vaccine response. We also derived a response prediction score for each patient using his/her tumor genotype data and variant association effect sizes computed from the other 21 patients; greater weighting was placed on gene products with cell membrane-related functions.

There was no correlation between vaccine response and intra-tumor, peri-tumor, or hepatic densities of T cell subpopulations. Associated genes were found to be enriched in the PI3K/Akt/mTOR signaling axis (P < 0.001). Applying a consistent prediction score cutoff over 22 rounds of leave-one-out cross-validation correctly inferred vaccine response in 21 of 22 patients (95%).

Adjuvant dendritic cell vaccination has shown promise as a form of immunotherapy for patients with metastatic colorectal cancer. Its efficacy may be influenced by somatic mutations that affect pathways involving PI3K, Akt, and mTOR, as well as tumor surface proteins.