Accelerated thymic atrophy as a result of elevated homeostatic expression of the genes encoded by the TNF/lymphotoxin cytokine locus

TNF, lymphotoxin (LT)-α, LT-β and LIGHT are members of a larger superfamily of TNF-related cytokines that can cross-utilize several receptors. Although LIGHT has been implicated in thymic development and function, the role of TNF and LT remains incompletely defined. To address this, we created a model of modest homeostatic overexpression of TNF/LT cytokines using the genomic human TNF/LT locus as a low copy number Tg. Strikingly, expression of Tg TNF/LT gene products led to profound early thymic atrophy characterized by decreased numbers of thymocytes and cortical thymic epithelial cells, partial block of thymocyte proliferation at double negative (DN) 1 stage, increased apoptosis of DN2 thymocytes and severe decline of T-cell numbers in the periphery. Results of backcrossing to TNFR1-, LTbR- or TNF/LT-deficient backgrounds and of reciprocal bone marrow transfers implicated both LT-α/LT-β to LTβR and TNF/LT-α to TNFR1 signaling in accelerated thymus degeneration. We hypothesize that chronic infections can promote thymic atrophy by upregulating LT and TNF production.