negative selection

Negative selection occurs after positive selection during T lymphocyte maturation in the thymus. Following positive selection in the cortex (which selects for thymocytes with functional TCRs capable of weak-to-moderate binding to self-MHC), thymocytes migrate to the corticomedullary junction and medulla, where they undergo negative selection. Here, medullary thymic epithelial cells (mTECs) and dendritic cells present self-antigens—including tissue-restricted antigens expressed via the transcription factor AIRE (autoimmune regulator)—on MHC class I and II molecules. Thymocytes whose T cell receptors bind with high affinity to these self-peptide-MHC complexes receive strong apoptotic signals and are deleted (clonal deletion), preventing the release of self-reactive T cells into the periphery. Some self-reactive thymocytes with intermediate affinity may instead differentiate into regulatory T cells (Tregs) rather than being deleted—a process sometimes called agonist selection.

Clinical significance: Defects in negative selection contribute to autoimmune disease. AIRE gene mutations cause Autoimmune Polyendocrine Syndrome Type 1 (APS-1/APECED), characterized by chronic mucocutaneous candidiasis, hypoparathyroidism, and adrenal insufficiency (the classic triad), due to failure to express tissue-specific antigens in the thymus, allowing autoreactive T cells to escape deletion. This concept is frequently tested alongside positive selection (occurring in the cortex, mediated by cortical thymic epithelial cells, selecting thymocytes with weak self-MHC affinity) to contrast the two checkpoints in T cell education. Negative selection is a cornerstone of central tolerance, distinct from peripheral tolerance mechanisms (e.g., anergy, Treg suppression, and immune privilege) that further prevent autoimmunity outside the thymus.