Derivation of transplantable human thyroid follicular epithelial cells from induced pluripotent stem cells

The production of mature functioning thyroid follicular cells (TFCs) from human induced pluripotent stem cells (iPSCs) is critical for potential novel therapeutic approaches to post-surgical and congenital hypothyroidism. To accomplish this, we developed a novel human iPSC line that expresses fluorophores targeted to the NKX2-1 and PAX8 loci, allowing for the identification and purification of cells destined to become TFCs. Optimizing a sequence of defined, serum-free media to promote stepwise developmental directed differentiation, we found that bone morphogenic protein 4 (BMP4) and fibroblast growth factor 2 (FGF2) stimulated lineage specification into TFCs from multiple iPSC lines. Single-cell RNA sequencing demonstrated that BMP4 withdrawal after lineage specification promoted TFC maturation, with mature TFCs representing the majority of cells present within 1 month. After xenotransplantation into athyreotic immunodeficient mice, engrafted cells exhibited thyroid follicular organization with thyroglobulin protein detected in the lumens of NKX2-1-positive follicles. While our iPSC-derived TFCs presented durable expression of thyroid-specific proteins, they were unable to rescue hypothyroidism in vivo.