Analysis of class II MHC structure in thymic nurse cells

Thymic nurse cells provide microenvironment for secondary T cell receptor α rearrangement in cortical thymocytes

Distinct subsets of thymic epithelial cells (TECs) support T-cell development and selection. Isolated TECs contain multicellular complexes that enclose many viable thymocytes. However, the functions of those TECs, termed thymic nurse cells (TNCs), are unclear and the idea that TNCs are present in vivo is questioned. Here, we show that TNCs represent a fraction of cortical (c)TECs that are defined by the expression of thymoproteasomes. Intravital imaging revealed TNCs in the thymic cortex in situ, whereas TNCs were detected neither during embryogenesis nor in the postnatal thymuses of various "positive-selector" T-cell receptor (TCR)-transgenic mice, indicating that TNCs are not essential for T-cell differentiation, including positive selection. Rather, cells within TNCs were enriched for long-lived CD4(+)CD8(+) thymocytes that underwent secondary TCR-Vα rearrangement. Thus, TNC complexes are formed in vivo by persistent cTEC-thymocyte interactions that then provide a microenvironment that optimizes T-cell selection through secondary TCR rearrangement.

Cellular and nerve fibre catecholaminergic thymic network: steroid hormone dependent activity

The thymus plays a critical role in establishing and maintaining the peripheral T-cell pool. It does so by providing a microenvironment within which T-cell precursors differentiate and undergo selection processes to create a functional population of major histocompatibility complex-restricted, self-tolerant T cells. These cells are central to adaptive immunity. Thymic T-cell development is influenced by locally produced soluble factors and cell-to-cell interactions, as well as by sympathetic noradrenergic and endocrine system signalling. Thymic lymphoid and non-lymphoid cells have been shown not only to express beta- and alpha(1)- adrenoceptors (ARs), but also to synthesize catecholamines (CAs). Thus, it is suggested that CAs influence T-cell development via both neurocrine/endocrine and autocrine/paracrine action, and that they serve as immunotransmitters between thymocytes and nerves. CAs acting at multiple sites along the thymocyte developmental route affect T-cell generation not only numerically, but also qualitatively. Thymic CA level and synthesis, as well as AR expression exhibit sex steroid-mediated sexual dimorphism. Moreover, the influence of CAs on T-cell development exhibits glucocorticoid-dependent plasticity. This review summarizes recent findings in this field and our current understanding of complex and multifaceted neuroendocrine-immune communications at thymic level.

Questionable thymic nurse cell

Since their discovery in 1980, thymic nurse cells (TNCs) have been controversial. Questions pertaining to the existence of the TNC as a "unit" cell with thymocytes completely enclosed within its cytoplasm were the focus of initial debates. Early skeptics proposed the multicellular complex to be an artifact of the procedures used to isolate TNCs from the thymus. Since that time, TNCs have been found in fish, frogs, tadpoles, chickens, sheep, pigs, rats, mice, and humans. Their evolutionary conservation throughout the animal kingdom relieved most speculations about the existence of TNCs and at the same time demonstrated their apparent importance to the thymus and T-cell development. In this review we will discuss and debate reports that describe (i) the organization or structure of TNCs, (ii) the thymocyte subset(s) found within the cytoplasm of TNCs and their uptake and release, and (iii) the function of this fascinating multicellular interaction that occurs during the process of T-cell development. Discussions about the future of the field and experimental approaches that will lead to answers to remaining questions are also presented.

Thymic heterotypic cellular complexes in gene-targeted mice with defined blocks in T cell development and adhesion molecule expression

Thymocytes form unique multicellular complexes with epithelial cells (thymic nurse cells, TNC) and rosettes (ROS) with macrophages, epithelial cells and dendritic cells. To investigate the role of differentiation checkpoints in the formation of the thymic heterotypic complexes in vivo, we used mutant mice which have genetically defined blocks at early and late stages of T cell development. We show that RAG-1-/-, TCRbeta-/- , and p56lck-/- mice lack thymocyte ROS formation with epithelial cells, macrophages, or dendritic cells. TNC formation was not affected by TCRbeta and p56lck gene mutations but partially decreased in RAG-1-/- mice, indicating that TNC are the earliest thymocyte-stromal cell complexes formed in development, whereas ROS only appear after thymocytes have rearranged and expressed a functional TCRbeta chain. Genetic blocks in CD8 lineage commitment (CD8-/- and IFN regulatory factor-1-/- mice) and positive and negative T cell selection (CD45-/-, TCRalpha-/-, and CD30-/- mice) did not affect thymocyte-stromal cell complexes. Surprisingly, CD4-/- mice, but not MHC class II-/- mice, had significantly reduced numbers of TNC and ROS, in particular, a severe defect in ROS formation with thymic dendritic cells. The CD4-/- block in ROS and TNC formation was rescued by the introduction of a human CD4 transgene. Moreover, we show that the adhesion receptors CD44 and LFA-1 cooperate in the formation of the thymic microenvironment. These results provide genetic evidence on the role of defined stages in T cell development and adhesion molecules on thymocyte/stromal cell interactions in vitro.

Thymic nurse cells: their functional ultrastructure

Thymic nurse cells are defined in vitro as multicellular complexes of epithelial cells and thymocytes. Although these structures have been implicated in the intrathymic differentiation of thymocytes, little is known about the biology of this cell complex and about the occurrence of the cells in the thymus in situ. Therefore, to clarify the matter, in this review we have presented characteristics of epithelial cells capable of forming complexes with thymocytes, in light of the literature data and the experience of the authors. The structure of cells within the complexes allowed us to distinguish three types of thymic nurse cells. Three-dimensional reconstruction of the thymus and observations employing TEM and SEM demonstrated the presence of distinct types of complexes in various topographic regions of the thymus. Where possible, the functional relevance of the morphological data was analyzed.