Role of Paracrine Regulation of Mammalian Target of Rapamycin Activity in Thymus Atrophy and Regeneration

摘要

The thymus is a primary lymphoid organ responsible for the development and maturation of na?ve, self-tolerant T lymphocytes which are critical mediators of the adaptive immune system. As important as T lymphocytes are to the adaptive immune system they are consistently lost and therefore need to be consistently replenished to ensure proper immune surveillance. It is therefore surprising that the thymus is the fastest organ to undergo atrophy with only half the original output by middle age. As thymic output of new, na?ve T cells diminishes the homeostatic proliferation of memory T cells in the periphery increases which results in the once polyclonal repertoire to reduce in diversity to an oligoclonal population. This reduction in the T cell receptor (TCR) repertoire diversity reduces the number of possible antigens that the T cell pool can recognize which leads to diminished immune responses in the elderly especially to novel antigens. It is possible to regenerate the thymus to the original peak size with surgical or chemical androgen ablation however, the resulting regeneration is incomplete in restoring thymic architecture and is transient. Effective and long-lasting thymic regeneration is not yet possible because we don't completely understand the mechanisms regulating thymic size. Understanding the mechanisms behind thymic atrophy and regeneration is becoming more relevant as medicine is continuing to prolong average lifespans. Many labs have shown that atrophy is not a matter of loss of thymic epithelial cells (TECs) over time but rather a shrinkage of the TECs. Specifically, it seems that shrinkage of the cortical volume due to age-related changes in cortical thymic epithelial cells (cTECs) is the main result of atrophy. The same thing is true for thymic regeneration which does not require cell proliferation, but instead growth of cTECs and their projections. Venables and colleagues showed that the main pathway identified to be upregulated during regeneration was mammalian target of rapamycin (mTOR). They also noted that medullary thymic epithelial cells (mTECs) and not cTECs dynamically regulated different mTOR stimulating ligands like IGF1 and FGF21 during atrophy and regeneration. This led us to the hypothesis that the growth factors required for promoting thymus growth and maintaining its size are dynamically expressed by mTECs, and act in a paracrine manner upon cTECs through activation of mTOR.Here, we find that mTORC1 activity is similar in cTECs and mTECs and declines significantly as the thymus atrophies between 1 and 3 months of age mTORC2 activity remains relatively stable in mTECs as thymus growth declines, but seems to be dynamically regulated in cTECs Those results seem consistent with mTORC1-dependent cell proliferation early in mTEC and cTEC growth phases that may later decline and perhaps with a role for mTORC2-dependent effects on cytoskeleton/morphology in cTECs. Through microscopy and flow cytometry we also verified LPO as a viable candidate as an mTEC-specific pan mTEC gene that can be used in future experiments.