Ultrastructural analysis of thymic nurse cell epithelium

[Thymic nurse cells--a specialized thymic microenvironment]

<zakljucak> Vise od dve decenije nakon prvog opisa, TNC su i dalje velika nepoznanica i potrebno je jos mnogo istrazivanja pre nego sto budemo bili u mogucnosti da definisemo preciznu ulogu ovih celija u razvoju T-limfocita. Mnoga od dosadasnjih saznanja ukazuju da se timociti u kontaktu sa TNC nalaze na prekretnici u svom razvojnom putu: ili ce biti uklonjeni indukcijom apoptoze ili ce nastaviti svoj razvoj i dalje sazrevanje. Brojna pitanja su za sada bez odgovora, a medju njima su dva posebno intrigantna. Koja je razlika izmedju timocita koji se vezuju za TNC i onih koji to ne cine? Koja je razlika izmedju populacije adherentnih timocita koji su selektivno internalizovani i onih koji su iskljuceni iz procesa internalizacije? Buduca istrazivanja kretanja timocita ka, unutar i van TNC ce verujemo pruziti dragocene informacije o ovoj fazi u razvoju T-limfocita. Nezavisno od toga sta ce buducnost pokazati o pravoj ulozi TNC, jedinstven kompleks koji ove celije formiraju sa timocitima je vrlo neobican, uzbudljiv i zagonetan bioloski fenomen.

Thymic nurse cells: a microenvironment for thymocyte development and selection

Thymic nurse cells (TNCs) represent a unique microenvironment in the thymus for MHC restriction and T cell repertoire selection composed of a cortical epithelial cell surrounding 20-200 immature thymocytes. TNCs have been isolated from many classes of animals from fish to humans. Studies performed using TNC lines showed that TNCs bind viable alphabetaTCRlow CD4(+)CD8(+)CD69(-) thymocytes. A subset of the bound cells is internalized, proliferates within the TNC, and matures to the alphabetaTCRhigh CD4(+)CD8(+)CD69(+) stage, indicative of positive selection. A subset of the internalized population is released while cells that remain internalized undergo apoptosis and are degraded by lysosomes within the TNC. A TNC-specific monoclonal antibody added to fetal thymic organ cultures resulted in an 80% reduction in the number of thymocytes recovered, with a block at the double positive stage of development. Together these data suggest a critical role for TNC internalization in thymocyte selection as well as the removal and degradation of negatively selected thymocytes. Recent studies have shown that in addition to thymocytes, peripheral circulating macrophages are also found within the TNC complex and can present antigens to the developing thymocytes. These circulating macrophages could provide a source of self-antigens used to ensure a self-tolerant mature T cell repertoire. A reduction in TNC numbers is associated with a variety of autoimmune diseases including thyroiditis and systemic lupus erythematosis.

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.

Murine thymic nurse cells and rosettes: analysis of adhesion molecule expression using confocal microscopy and a simplified enrichment method

Thymic nurse cells (TNC) and T-cell stromal rosettes (ROS) are two in vivo models for stromal cell-thymocyte interactions. We describe a simplified enrichment method for TNC and ROS that overcomes the necessity for large amounts of tissue. The complexes were further analyzed with confocal microscopy, and three subunits of ROS were defined on the basis of their central cell phenotype, i.e., macrophage, dendritic, or epithelial cell rosettes. Because adhesion molecules are proposed to play a crucial role in T-cell development, we investigated CD44, LFA-1, and ICAM-1 expression in such complexes. The epithelial component of TNC expresses CD44 and ICAM-1, whereas intra-TNC thymocytes are LFA-negative. With regard to ROS, all subsets expressed CD44, and macrophage and dendritic cell ROS were also ICAM-1-positive and LFA-1-positive. The current protocol opens the possibility for further in vivo analysis of stromal cell-thymocyte interactions, e.g., for studies of scarce gene mutant mice.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

Chicken thymic nurse cells: an overview

Thymic nurse cells are multicellular complexes located in the subcortical area of the thymus of all avian, mammalian and amphibian species investigated so far. Since their first description in 1980 many studies have been carried out to characterize their morphological and functional properties. The purpose of this review is to summarize recent morphological as well a functional analyses of chicken thymic nurse cells which suggest a role of these cell complexes in T cell selection.