Discrete limbal epithelial stem cell populations mediate corneal homeostasis and wound healing

Stem cells (SCs) are traditionally viewed as rare, slow-cycling cells that follow deterministic rules dictating their self-renewal or differentiation. It was several decades ago, when limbal epithelial SCs (LSCs) that regenerate the corneal epithelium were among the first sporadic, quiescent SCs ever discovered. However, LSC dynamics, heterogeneity and genetic signature are largely unknown. Moreover, recent accumulating evidence strongly suggested that epithelial SCs are actually abundant, frequently dividing cells that display stochastic behavior. In this work, we combined single-cell RNA sequencing and advanced quantitative lineage tracing for in-depth analysis of the murine limbal epithelium. The generated data provides an atlas of cell states of the entire corneal epithelial lineage and reveales the co-existence of two novel LSC populations that reside in separate and well-defined sub-compartments. In the “outer” limbus, we discovered a primitive widespread population of quiescent LSCs (qLSCs) that uniformly express Krt15/Gpha2/Ifitm3/Cd63 proteins that serve as SC reservoire and in boundary formation. In the “inner” peri-corneal limbus, we identified prevalent active LSCs (aLSCs) that express Krt15-GFP/Atf3/Mt1-2/Socs3 and maintain homeostasis. We propose that these SC populations are abundant, follow stochastic rules and neutral drift dynamics. Notably, we provide evidence that T cells serve as niche cells for qLSCs, regulating quiescence and wound response. Taken together, we propose that divergent regenerative strategies are tailored to properly support tissue specific physiological constraints. Epithelial cells were isolated from the limbus of 5 individual adult mice (2.5 months old, n=10 eyes). The limbus together with marginal conjunctiva and corneal periphery were carefully dissected, a protocol for isolating mainly epithelial cells was applied, and suspended cells were subjected to 10x Chromium Single-Cell RNA sequencing (scRNA-seq).