Background: Cytotoxic CD8+ T cells are the dominant immune cell population involved in eliminating malignant cells. Peripheral tolerance and exhaustion restrict CD8+ T cell function leading to inadequate tumour control and progression. Peripheral tolerance limits initial T cell activation in lymphoid organs preventing subsequent tissue infiltration, while exhaustion restrains established effector cell functions. Despite extensive preclinical evidence that tumours can exploit peripheral tolerance, most cancer immunotherapy studies have focused on exhausted CD8+ T cells within the tumour. In fact, tolerant tumour-specific CD8+ T cells have never been captured in human studies. Excluding MSI-H tumours, colorectal cancers (CRCs) are usually ‘cold’ tumours with little immune infiltration and respond poorly to checkpoint inhibitor immunotherapy. We hypothesise that a subset of CRCs induces peripheral tolerance in the tumour draining lymph node (TDLN), posing a major roadblock to the induction of an anti-tumour immune response, leading to immunotherapy resistance and poor overall outcomes.
Objectives: To identify the human tolerant CD8+ T cell gene signature and explore the role of CD8+ T cell tolerance in CRC.
Methods: We developed the TIPTOE study (ACTRN:12622001037796) to collect fresh tissue samples (primary tumour, TDLN, metastasis, normal tissue, blood) from patients with resectable stage IV CRC undergoing surgery at Peter MacCallum Cancer Centre. CD8+ T cells were isolated by fluorescence-activated cell sorting (FACS) and analysed by scRNAseq.
Results: 10 patients were recruited between August 2022 – October 2023. 6 patients were ineligible for analysis (3x no surgery, 2x unable to identify TDLNs, 1x insufficient yield post FACS). Preliminary analysis of the scRNAseq data to date revealed two CD8+ T cell clusters with enrichment of tolerance associated genes. These populations were present in TDLNs and blood but largely absent in other sites.
Conclusion: Preliminary analysis of our scRNAseq data revealed two CD8+ T cell clusters enriched for tolerance gene expression. Further bioinformatic analysis, patient recruitment, functional assessment and clinical correlation will help clarify the relevance of peripheral tolerance in CRC. These early data suggest that CD8+ T cell tolerance may contribute to immunotherapy resistance in a subset of CRC patients, thereby representing a novel immuno-therapeutic target.