Transforming growth factor-β2 is involved in quantitative genetic variation in thymic involution

The mechanisms regulating thymic involution are unclear. In inbred mouse strains the rate of thymic involution and the function of the hematopoietic stem cell (HSC) compartment are subject to quantitative genetic variation. We have shown previously that transforming growth factor-β2 (TGF-β2) is a genetically determined positive regulator of HSCs. Here, we demonstrate that genetic variation in the rate of thymic involution correlates with genetic variation in the responsiveness of hematopoietic stem and progenitor cells to TGF-β2. Corroborating these correlations, thymic cellularity and peripheral naive T-cell frequency were higher in old Tgfb2^+/- mice than in wild-type littermates. The frequency of early T-cell precursors was increased in Tgfb2^+/- mice, suggesting that TGF-β2 affects the earliest stages of T-cell development in old mice. Reciprocal transplantation experiments indicated that TGF-β2 expressed both in the (micro)environment and in the hematopoietic system can accelerate thymic involution; however, the age of the stem cells appeared irrelevant. Thus, although thymic involution is largely determined by the aged environment, TGF-β2 plays a major modulatory role that is subject to genetic variation and is possibly mediated through its regulatory effects on early hematopoiesis.