Construction of a functional thymic microenvironment from pluripotent stem cells for the induction of central tolerance.

The thymus is required for generation of a self-tolerant, self-restricted T-cell repertoire. The capacity to manipulate or replace thymus function therapeutically would be beneficial in a variety of clinical settings, including for improving recovery following bone marrow transplantation, restoring immune system function in the elderly and promoting tolerance to transplanted organs or cells. An attractive strategy would be transplantation of thymus organoids generated from cells produced in vitro, for instance from pluripotent stem cells. Here, we review recent progress toward this goal, focusing on advances in directing differentiation of pluripotent stem cells to thymic epithelial cells, a key cell type of the thymic stroma, and related direct reprogramming strategies.

An organized and functional thymus generated from FOXN1-reprogrammed fibroblasts.

A central goal of regenerative medicine is to generate transplantable organs from cells derived or expanded in vitro. Although numerous studies have demonstrated the production of defined cell types in vitro, the creation of a fully intact organ has not been reported. The transcription factor forkhead box N1 (FOXN1) is critically required for development of thymic epithelial cells (TECs), a key cell type of the thymic stroma. Here, we show that enforced Foxn1 expression is sufficient to reprogramme fibroblasts into functional TECs, an unrelated cell type across a germ-layer boundary. These FOXN1-induced TECs (iTECs) supported efficient development of both CD4(+) and CD8(+) T cells in vitro. On transplantation, iTECs established a complete, fully organized and functional thymus, that contained all of the TEC subtypes required to support T-cell differentiation and populated the recipient immune system with T cells. iTECs thus demonstrate that cellular reprogramming approaches can be used to generate an entire organ, and open the possibility of widespread use of thymus transplantation to boost immune function in patients.

Structure and function of the thymic microenvironment.

Organs are more than the sum of their component parts--functional competence requires that these parts not only be present in the appropriate proportions, but also be arranged and function together in specific ways. The thymus is an excellent example of the connection between cellular organization and organ function. Unlike more familiar organs, such as lung or kidney, the thymus is not organized into easily identifiable structures such as tubes and ordered cell layers, but instead is a complex meshwork of microenvironments through which T cell progenitors migrate, receiving signals that instruct them to differentiate, proliferate, or die. Proper thymic organization is essential to the optimal production of a functional T cell repertoire. During aging, the thymus undergoes involution, largely due to degradation of the TEC microenvironmental compartment, which then fails to support optimal thymocyte development resulting in reduced output of naive T cells. This review will summarize the current state of understanding of the composition and organization of thymic microenvironments and the mechanisms that promote their proper development and function.