Identification of fibroblast progenitors in the developing thymus

The thymus stroma constitutes a fundamental microenvironment for T cell generation. Despite the chief contribution of thymic epithelial cells, recent studies emphasize the regulatory role of mesenchymal cells in thymic function. Mesenchymal progenitors are suggested to exist in the postnatal thymus, nonetheless our understanding of their nature and the mechanism controlling their homeostasis in vivo remain elusive. Here, we resolved two new thymic fibroblast subsets with distinct developmental and genetic features. While CD140??+GP38+SCA-1- cells prevailed in the embryonic thymus and declined thereafter, CD140??+GP38+SCA-1+ cells emerged in the late embryonic period and predominated in the postnatal life. The fibroblastic-associated transcriptional program was upregulated in CD140??+GP38+SCA-1+ cells, suggestive of a more mature subset. Concordantly, lineage analysis showed that CD140??+GP38+SCA-1+ maintained their phenotype in thymic organotypic cultures. Strikingly, CD140??+GP38+SCA-1- generated CD140??+GP38+SCA-1+, inferring that this subset harboured progenitor cell activity. Moreover, CD140??+GP38+SCA-1+ fibroblasts were profoundly reduced in alymphoid Rag2-/-Il2rg-/- thymi, indicating that fibroblast development depends on thymic crosstalk. Our findings identify CD140??+GP38+SCA-1- as a source of fibroblast progenitors and define SCA-1 as a molecular marker to map a developmental stage in thymic fibroblast differentiation.