Revisiting the roadmap of medullary thymic epithelial cell differentiation

The basic two-step terminal differentiation model of the medullary thymic epithelial cell (mTEC) lineage from immature MHCIIlo to mature MHCIIhi mTECs has recently been extended to include a third stage namely the post-Aire MHCIIlo subset as identified by lineage-tracing models. Yet, a suitable surface marker distinguishing the phenotypically overlapping pre- from the post-Aire immature MHCIIlo stage has been lacking. Here, 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- MHCIIlo → TPA- MHCIIhi → TPA+ MHCIIhi → TPA+ MHCIIlo. Surprisingly, in the steady-state postnatal thymus most immature TPA- pre-Aire rather than terminally differentiated post-Aire TPA+ MHCIIlo mTECs were marked for apoptosis at an exceptionally high rate of about 70 %. Hence, only the minor cycling fraction of the MHCIIlo 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 further sub-divided human mTECs, though with different subset distribution. Our revised roadmap 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 immature pre-Aire mTECs points to a “quality control” step during early mTEC differentiation. Mouse medullary thymic epithelial cells were sorted with two markers: MHC-II high/low and TPA plus/minus. There are four groups: MHC-II low TPA minus, MHC-II high TPA minus, MHC-II high TPA plus, MHC-II low TPA plus. Each group has four replicates