Radiotherapy followed by systemic therapy and tislelizumab in MSS rectal cancer with liver metastases: Using single-cell RNA sequencing to reveal cellular dynamics in primary rectal cancer and paired liver metastases of patients enrolled in the MIRAC

e15521 Background: Liver metastasis remains a major hurdle to long-term survival of patients with rectal cancer. These patients with metastases could benefit from preoperative immunotherapy combined with conventional chemoradiotherapy. However, whether the metastatic baseline cellular microenvironment differs from the primary microenvironment and how they change after neoadjuvant therapy remain largely unexplored. Therefore, we used single-cell RNA sequencing to reveal cellular dynamics in primary rectal cancer and paired liver metastases. Methods: We obtained matched tumor samples from patients enrolled in the MIRACLE-1 (NCT05359393) and MIRACLE-2 (NCT05359406) studies before and after neoadjuvant therapy (n = 15, untreated rectal cancer; n = 4, untreated liver metastasis; n = 2, treated rectal cancer; n = 1, treated liver metastasis). Single-cell RNA sequencing was utilized to characterize the distinct immune microenvironment between primary rectal cancer and paired liver metastases. We also analyzed changes in primary and metastatic tumors between baseline and post-treatment samples. Results: The epithelial cells of primary tumors and liver metastases exhibited markedly different patterns of cell death following treatment. Higher proportions of CLEC10A + conventional type 2 dendritic cells (cDC2) and KLRB1 + mucosal-associated invariant T cells (MAIT) at baseline or increasing proportions during treatment were correlated with a better response to neoadjuvant therapy. PDGFRA + inflammatory cancer-associated fibroblasts (iCAFs), which could modulate T cell function via the GDF pathway, were absent in liver metastases compared to primary tumors prior to treatment (P < 0.01). Increased PDGFRA + iCAFs in the primary lesion after neoadjuvant therapy were associated with worse treatment efficacy. Conclusions: This study presents a single-cell atlas of MSS rectal cancer with liver metastases treated with radiotherapy followed by systemic therapy and Tislelizumab. The microenvironment of primary tumors and liver metastases differed at baseline and underwent distinct changes following treatment. Certain cell subsets, such as CLEC10A + cDC2, exhibited the potential to predict treatment efficacy. We are continuously collecting new patient samples to validate our findings and exploring their implications for clinical practice.