Ltbetar signaling does not regulate Aire-dependent transcripts in medullary thymic epithelial cells

Thymic medullary epithelial cells (mTECs) play a major role in central tolerance induction by expressing tissue-specific Ags (TSAs). The expression of a subset of TSAs in mTECs is under the control of Aire (autoimmune regulator). Humans defective for AIRE develop a syndrome characterized by autoimmune disease in several endocrine glands. Aire has been proposed to be regulated by lymphotoxin beta receptor (Ltbetar) signaling and there is evidence that, additionally, Aire-independent transcripts may be regulated by this pathway. Given the potential clinical importance of Aire regulation in mTECs for the control of autoimmunity, we investigated the relation between Ltbetar signaling and TSA expression by whole genome transcriptome analysis. In this study, we show that the absence of Ltbetar has no effect on the expression of Aire and Aire-dependent TSAs. Also, the lack of Ltbetar signaling does not disturb regulatory T cells or the distribution of dendritic cells in the thymus. However, mTECs in Ltbetar-deficient mice show an aberrant distribution within the thymic medulla with disruption of their three-dimensional architecture. This is predicted to impair the interaction between mTECs and thymocytes as shown by the reduced surface uptake of MHCII by mature thymocytes in Ltbetar-deficient mice. We propose that the physiological medullary architecture ensures negative-selection by supporting lympho-epithelial interaction through a large epithelial cell surface distributed evenly across the medulla.