Foxn1 Protein Expression in the Developing, Aging, and Regenerating Thymus

Abstract

The forkhead box N1 (Foxn1) protein is the key regulator of thymic epithelial cell (TEC) development, yet how Foxn1 functions remains largely unknown. All mature TECs arise from Foxn1-expressing progenitors/immature TECs and it is widely assumed that TECs as a whole are defined by Foxn1 expression. However, data on the Foxn1 protein are virtually lacking. In this study, we developed novel tools to visualize Foxn1 protein expression at single-cell resolution. We generated Foxn1 knock-in mice expressing a C-terminal hemagglutinin-tagged Foxn1 protein, and a cytometry-grade monoclonal anti-Foxn1 Ab. We evaluated Foxn1 expression patterns in TEC subsets and its dynamics during normal thymus development, aging, injury, and regeneration. Upon challenges, upregulation of Foxn1 was a common feature of thymus regeneration, but the timing of Foxn1 expression changed and the responding TEC subsets depended on the type of treatment. Whereas dexamethasone and recombinant human fibroblast growth factor 7 promoted expansion of Foxn1+Ly51+CD80− TECs, castration led to expansion of Foxn1+Ly51−CD80+ TECs. Collectively, Foxn1 expression is highly heterogeneous in the normal thymus, with large fractions of Foxn1low or Foxn1− TECs accumulating with age. Furthermore, Foxn1 expression is responsive to perturbations.

A novel Foxn1eGFP/+ mouse model identifies Bmp4‐induced maintenance of Foxn1 expression and thymic epithelial progenitor populations

“…Given that eGFP has a stable half-life of approximately 26 h [37], it might linger in our mouse model. Therefore, downregulation of Foxn1 in some cells with a lag in eGFP degradation is plausible, especially in postnatal life where Foxn1 − TECs have been identified [17,31]. Despite these findings, we demonstrated clear co-localization between eGFP and Foxn1 by using a recently developed antibody [17].…”

Thymus: the next (re)generation

“…Additionally, a subsequent study correlated the increased expression of IL-22 after damage with increased expression of Foxn1 (71). Foxn1 is a molecule critically involved with thymus ontogeny and is also clearly important for thymic maintenance and regeneration (72, 73). In fact, forced expression of Foxn1 leads to regeneration of the aged thymus (74, 75) and thus activation of Foxn1 after damage could represent a potent pathway of endogenous or exogenous thymic regeneration.…”

Restoration of Thymus Function with Bioengineered Thymus Organoids

“…Foxn1 is another key regulator of adult TEC regeneration. In normal thymus, the expression of Foxn1 is highly heterogeneous, and the numbers of TEC with low levels of Foxn1 increase with age[89]. Mice with increased levels of Foxn1 expression in TECs are resistant from thymic involution[90].…”