Though treatment with immune checkpoint blockage (ICB) has greatly improved clinical outcomes, not all patients respond to this treatment. Being able to predict which patients are likely to respond would be a significant clinical advance. Thus there remains a need for predictive biomarkers to determine in advance those with the most potential to benefit from immune checkpoint blockade. To date, most biomarkers of response are identified in tumor tissue, whereas biomarkers that can be assessed from peripheral blood are more desirable, due to the ease of access and reproducibility of sampling. To identify biomarkers associated with ICB response from peripheral blood, we apply single cell RNA sequencing to 24 peripheral blood mononuclear cell (PBMC) samples, collected from 12 stage III and IV melanoma patients before and after treatment with anti-PD1 monotherapy. Among these 12 patients, 6 are responders and 6 are non-responders. After quality control, a total of 62,273 single cells remains for further analysis. To define the cell type landscape in an unbiased manner, unsupervised clustering is used. We identify 20 robust cell clusters, including two B cell clusters (B-cell_1; B-cell_2), three myeloid clusters (monocytes, myeloid-derived suppressor cells, and M2 macrophages), twelve clusters enriched for T/NK cells, two clusters of platelets, and Hematopoietic Stem and Progenitor Cells (HSPCs). Across all patients, cell composition differs between pre- and post-treatment samples. Strikingly, responders have significant greater proportions of B-cell_1 cells in their pre-treatment samples than non-responders. At gene level, our analysis identifies an individual overexpressed marker in B-cell_1, i.e. IGLC3, which is highly enriched in responders than non-responders before treatment. In summary, our analysis identifies specific cell type and gene within patients’ PBMC samples that can serve as predictive biomarkers for patient’s response to ICB.

Citation Format: Qianqian Song, Elizabeth Forbes, Lance D. Miller, Pierre L. Triozzi, Liang Liu, Wei Zhang, David R. Soto-Pantoja. Single-cell liquid biopsy reveals circulating heterogeneity and converging subpopulations in relation to immunotherapy response in melanoma [abstract]. In: Proceedings of the AACR Virtual Special Conference on Tumor Heterogeneity: From Single Cells to Clinical Impact; 2020 Sep 17-18. Philadelphia (PA): AACR; Cancer Res 2020;80(21 Suppl):Abstract nr PO-127.