Revisiting the Road Map of Medullary Thymic Epithelial Cell Differentiation

The basic two-step terminal differentiation model of the medullary thymic epithelial cell (mTEC) lineage from immature MHC class II (MHCII)^lo to mature MHCII^hi mTECs has recently been extended to include a third stage, namely the post-Aire MHCII^lo subset as identified by lineage-tracing models. However, a suitable surface marker distinguishing the phenotypically overlapping pre- from the post-Aire MHCII^lo stage has been lacking. In this study, we introduce the lectin Tetragonolobus purpureas agglutinin (TPA) as a novel cell surface marker that allows for such delineation. Based on our data, we derived the following sequence of mTEC differentiation: TPA^loMHCII^lo → TPA^loMHCII^hi → TPA^hiMHCII^hi → TPA^hiMHCII^lo Surprisingly, in the steady-state postnatal thymus TPA^loMHCII^lo pre-Aire rather than terminally differentiated post-Aire TPA^hiMHCII^lo mTECs were marked for apoptosis at an exceptionally high rate of ∼70%. Hence, only the minor cycling fraction of the MHCII^lo subset (<20%) potentially qualified as mTEC precursors. FoxN1 expression inversely correlated with the fraction of slow cycling and apoptotic cells within the four TPA subsets. TPA also further subdivided human mTECs, although with different subset distribution. Our revised road map emphazises close parallels of terminal mTEC development with that of skin, undergoing an alternative route of cell death, namely cornification rather than apoptosis. The high rate of apoptosis in pre-Aire MHCII^lo mTECs points to a "quality control" step during early mTEC differentiation.