Hassall's corpuscles with cellular-senescence features maintain IFN production through neutrophils and pDC activation in the thymus

Hassall's corpuscles (HCs) are composed of cornifying, terminally differentiated medullary thymic epithelial cells (mTECs) that are developed under the control of Aire. Here, we demonstrated that HC-mTECs show features of cellular senescence and produce inflammatory cytokines and chemokines including CXCL5, thereby recruiting and activating neutrophils to produce IL-23 in the thymic medulla. We further indicated that thymic plasmacytoid dendritic cells (pDCs) expressing IL-23 receptors constitutively produced Ifna, which plays a role in single positive (SP) cell maturation, in an Il23a-dependent manner. Neutrophil depletion with anti-Ly6G antibody injection resulted in a significant decrease of Ifna expression in the thymic pDCs, suggesting that thymic neutrophil activation underlies the Ifna expression in thymic pDCs in steady state conditions. A New Zealand White mouse strain showing HC hyperplasia exhibited greater numbers and activation of thymic neutrophils and pDCs than B6 mice, whereas Aire-deficient B6 mice with defective HC development and SP thymocyte maturation showed significantly compromised numbers and activation of these cells. These results collectively suggested that HC-mTECs with cell-senescence features initiate a unique cell activation cascade including neutrophils and pDCs leading to the constitutive IFN expression required for SP T-cell maturation in the thymic medulla.