Myeloid Landscape of BRAF-Mutant Papillary Thyroid Cancer and Thyroiditis

Abstract

Papillary thyroid carcinoma (PTC) is less aggressive when associated with lymphocytic thyroiditis (LT), even in the presence of oncogenic BRAF, including smaller tumours, less lymph node involvement, and reduced extrathyroidal extension (1). To investigate possible immune mechanisms underlying this association, we compared the tumour microenvironment of PTC-BRAF with and without LT using single-cell RNA sequencing (scRNA-seq). Single-cell libraries were generated from fresh and fixed tumour samples with post-dissociation viability >70% using the 10x Genomics Chromium platform and sequenced on an Illumina NovaSeq 6000. We analysed scRNA-seq data from 11 PTC-BRAF tumours, including four with LT (one publicly available sample) and seven without LT. Downstream analyses included quality control, batch correction, dimensionality reduction, and differential gene expression analysis. We found neutrophils were the predominant myeloid cell type in PTCs without LT. Thyrocytes without LT showed significant expression of the neutrophil recruitment chemokine ECRG4. In the absence of LT, neutrophils expressed oncogenic genes with poor clinical outcomes. In contrast, thyrocytes from tumours with LT showed increased expression of MHC-II antigen presentation, consistent with effective immune surveillance. Thyrocytes and macrophages in the presence of LT showed enrichment of interferon gamma response pathways. Our data suggests LT in thyroid cancer is associated with enhanced antigen presentation and fewer features of pro-tumorigenic innate immune activity. These results identify previously under recognized innate immune cell population and associated transcriptomic features which suggests new mechanisms to target immune treatments in PTC refractory to other therapies.