Conversion of the Thymus into a Bipotent Lymphoid Organ by Replacement of Foxn1 with Its Paralog, Foxn4

Abstract

The thymus is a lymphoid organ unique to vertebrates, and it provides a unique microenvironment that facilitates the differentiation of immature hematopoietic precursors into mature T cells. We subjected the evolutionary trajectory of the thymic microenvironment to experimental analysis. A hypothetical primordial form of the thymus was established in mice by replacing FOXN1, the vertebrate-specific master regulator of thymic epithelial cell function, with its metazoan ancestor, FOXN4, thereby resetting the regulatory and coding changes that have occurred since the divergence of these two paralogs. FOXN4 exhibited substantial thymopoietic activity. Unexpectedly, histological changes and a functional imbalance between the lymphopoietic cytokine IL7 and the T cell specification factor DLL4 within the reconstructed thymus resulted in coincident but spatially segregated T and B cell development. Our results identify an evolutionary mechanism underlying the conversion of a general lymphopoietic organ to a site of exclusive T cell generation.

Artificial Thymus: Recreating Microenvironmental Cues to Direct T Cell Differentiation and Thymic Regeneration

“…In contrast, expression of FoxN4 resulted in generation of both T and B cells, though segregated into different microenvironments within the thymus. B cell generation occurred even when FoxN1 and FoxN4 were co-expressed (Swann et al 2014). This recapitulated what is observed in the fish, where a primary lymphoid organ generates both B and T cells.…”

“…One was to overexpress the closely related FoxN1 paralog, FoxN4 and determine if it could rescue the SCID phenotype in nude mice. This could also determine the role and importance of FoxN1 in thymus formation (Swann et al 2014). The second approach was to express genes downstream of FoxN1, only in the thymic anlage, to determine which combination was sufficient to reconstruct the thymic function (Calderon and Boehm 2012).…”

FOXN1 in thymus organogenesis and development

“…Indeed in evolution, the emergence of FOXN4L and thymus‐like function preceded the pinching off of pharyngeal pouches into a distinct organ, as observed in the lamprey gill basket. Both Foxn1 and Foxn4 are expressed in the thymi of catshark, zebrafish, and medaka; however Foxn4 is either not expressed or expressed at very low levels in higher order organisms . Indeed, Foxn1 appears to have a unique role in the thymus in jawed vertebrates, which cannot be completely substituted by Foxn4 .…”

L’épithélium thymique, un passé dans la dualité et un présent unifié

“…Une étude de Swann et al [2] montre que l'introduction de FoxN1 chez la souris nude (porteuse d'une mutation spontanée du gène Foxn1) permet de restaurer le développement T, alors que celle de FoxN4 aboutit au développement de cellules B dans le thymus. Dans ces conditions, c'est la diminution de DL4 (dont l'expression dépend de FoxN1) et le maintien de la production d'IL-7 qui promeuvent le développement B à l'identique de ce qui est connu dans la moelle osseuse.…”