Review article: thymus organ cultures and T-cell receptor repertoire development

Stem cell-based multi-tissue platforms to model human autoimmune diabetes

BACKGROUND

Type 1 diabetes (T1D) is an autoimmune disease in which pancreatic insulin-producing β cells are specifically destroyed by the immune system. Understanding the initiation and progression of human T1D has been hampered by the lack of appropriate models that can reproduce the complexity and heterogeneity of the disease. The development of platforms combining multiple human pluripotent stem cell (hPSC) derived tissues to model distinct aspects of T1D has the potential to provide critical novel insights into the etiology and pathogenesis of the human disease.

SCOPE OF REVIEW

In this review, we summarize the state of hPSC differentiation approaches to generate cell types and tissues relevant to T1D, with a particular focus on pancreatic islet cells, T cells, and thymic epithelium. We present current applications as well as limitations of using these hPSC-derived cells for disease modeling and discuss efforts to optimize platforms combining multiple cell types to model human T1D. Finally, we outline remaining challenges and emphasize future improvements needed to accelerate progress in this emerging field of research.

MAJOR CONCLUSIONS

Recent advances in reprogramming approaches to create patient-specific induced pluripotent stem cell lines (iPSCs), genome engineering technologies to efficiently modify DNA of hPSCs, and protocols to direct their differentiation into mature cell types have empowered the use of stem cell derivatives to accurately model human disease. While challenges remain before complex interactions occurring in human T1D can be modeled with these derivatives, experiments combining hPSC-derived β cells and immune cells are already providing exciting insight into how these cells interact in the context of T1D, supporting the viability of this approach.

Gfi1-Foxo1 axis controls the fidelity of effector gene expression and developmental maturation of thymocytes

Double-positive (DP) thymocytes respond to intrathymic T-cell receptor (TCR) signals by undergoing positive selection and lineage differentiation into single-positive (SP) mature cells. Concomitant with these well-characterized events is the acquisition of a mature T-cell gene expression program characterized by the induction of the effector molecules IL-7Rα, S1P₁, and CCR7, but the underlying mechanism remains elusive. We report here that transcription repressor Growth factor independent 1 (Gfi1) orchestrates the fidelity of the DP gene expression program and developmental maturation into SP cells. Loss of Gfi1 resulted in premature induction of effector genes and the transcription factors forkhead box protein O1 (Foxo1) and Klf2 in DP thymocytes and the accumulation of postselection intermediate populations and accelerated transition into SP cells. Strikingly, partial loss of Foxo1 function, but not restored survival fitness, rectified the dysregulated gene expression and thymocyte maturation in Gfi1-deficient mice. Our results establish the Gfi1-Foxo1 axis and the transcriptional circuitry that actively maintain DP identity and shape the proper generation of mature T cells.

Preparation and Applications of Organotypic Thymic Slice Cultures

Thymic selection proceeds in a unique and highly organized thymic microenvironment resulting in the generation of a functional, self-tolerant T cell repertoire. In vitro models to study T lineage commitment and development have provided valuable insights into this process. However, these systems lack the complete three-dimensional thymic milieu necessary for T cell development and, therefore, are incomplete approximations of in vivo thymic selection. Some of the challenges related to modeling T cell development can be overcome by using in situ models that provide an intact thymic microenvironment that fully supports thymic selection of developing T cells. Thymic slice organotypic cultures complement existing in situ techniques. Thymic slices preserve the integrity of the thymic cortical and medullary regions and provide a platform to study development of overlaid thymocytes of a defined developmental stage or of endogenous T cells within a mature thymic microenvironment. Given the ability to generate ~20 slices per mouse, thymic slices present a unique advantage in terms of scalability for high throughput experiments. Further, the relative ease in generating thymic slices and potential to overlay different thymic subsets or other cell populations from diverse genetic backgrounds enhances the versatility of this method. Here we describe a protocol for the preparation of thymic slices, isolation and overlay of thymocytes, and dissociation of thymic slices for flow cytometric analysis. This system can also be adapted to study non-conventional T cell development as well as visualize thymocyte migration, thymocyte-stromal cell interactions, and TCR signals associated with thymic selection by two-photon microscopy.

Lethal and nonlethal murine malarial infections differentially affect apoptosis, proliferation, and CD8 expression on thymic T cells

Although thymic atrophy and apoptosis of the double-positive (DP) T cells have been reported in murine malaria, comparative studies investigating the effect of lethal and nonlethal Plasmodium infections on the thymus are lacking. We assessed the effects of P. yoelii lethal (17XL) and nonlethal (17XNL) infections on thymic T cells. Both strains affected the thymus. 17XL infection induced DP T-cell apoptosis and a selective decrease in surface CD8 expression on developing thymocytes. By contrast, more severe but reversible effects were observed during 17XNL infection. DP T cells underwent apoptosis, and proliferation of both DN and DP cells was affected around peak parasitemia. A transient increase in surface CD8 expression on thymic T cells was also observed. Adult thymic organ culture revealed that soluble serum factors, but not IFN-γ or TNF-α, contributed to the observed effects. Thus, lethal and nonlethal malarial infections led to multiple disparate effects on thymus. These parasite-induced thymic changes are expected to impact the naïve T-cell repertoire and the subsequent control of the immune response against the parasite. Further investigations are required to elucidate the mechanism responsible for these disparate effects, especially the reversible involution of the thymus in case of nonlethal infection.

Identification of a novel lymphoid population in the murine epidermis

T cell progenitors are known to arise from the foetal liver in embryos and the bone marrow in adults; however different studies have shown that a pool of T cell progenitors may also exist in the periphery. Here, we identified a lymphoid population resembling peripheral T cell progenitors which transiently seed the epidermis during late embryogenesis in both wild-type and T cell-deficient mice. We named these cells ELCs (Epidermal Lymphoid Cells). ELCs expressed Thy1 and CD2, but lacked CD3 and TCRαβ/γδ at their surface, reminiscent of the phenotype of extra- or intra- thymic T cell progenitors. Similarly to Dendritic Epidermal T Cells (DETCs), ELCs were radioresistant and capable of self-renewal. However, despite their progenitor-like phenotype and expression of T cell lineage markers within the population, ELCs did not differentiate into conventional T cells or DETCs in in vitro, ex vivo or in vivo differentiation assays. Finally, we show that ELC expressed NK markers and secreted IFN-γ upon stimulation. Therefore we report the discovery of a unique population of lymphoid cells within the murine epidermis that appears related to NK cells with as-yet-unidentified functions.

Thymus-deriving natural regulatory T cell generation in vitro: role of the source of activation signals

In this research we have examined different sources of activation signals in order to optimize culture conditions for in vitro generation of thymus-deriving natural regulatory T cells (nTregs). We have established a novel model using JAWS II dendritic cell line of immature phenotype and compared it to commonly used methods for the generation of Tregs from peripheral lymphoid organs or blood T cells. In our model the first activation signal is provided by anti-CD3 monoclonal antibodies while the second is delivered by costimulatory molecules expressed on JAWS II cells. The presence of JAWS II cells co-cultured in vitro with unsorted thymocytes directly isolated from the thymus gland creates environment favoring SP CD4+ differentiation, provides the apoptotic cells clearance, maintains the survival of thymocytes and facilitate nTreg generation. Moreover the usage of immature dendritic cells stimuli enables to conduct research on agents affecting nTreg survival, proliferation and development in conditions of cell-to-cell contact of undifferentiated thymocytes with dendritic cells.

Effects of catecholamines on thymocyte apoptosis and proliferation depend on thymocyte microenvironment

The present study, through quantification of tyrosine hydroxylase (TH) expression and catecholamine (CA) content in the presence and in the absence of α-methyl-p-tyrosine (AMPT), a TH inhibitor, in adult thymic organ (ATOC) and thymocyte culture, demonstrated that thymic cells produce CAs. In addition, in ATOC an increase in β2-adrenoceptor (AR) mRNA expression and β2-AR thymocyte surface density was registered. Furthermore, AMPT (10(-4)M), as propranolol (10(-4)M), augmented thymocyte apoptosis and diminished thymocyte proliferation in ATOC. Propranolol exerted these effects acting on CD3(high) thymocytes. However, in thymocyte cultures, propranolol (10(-6)M) acting on the same thymocyte subset exerted the opposing effect on thymocyte apoptosis and ConA-stimulated proliferation. This suggested that, depending on thymocyte microenvironment, differential effects can be induced through the same type of AR. Additionally, arterenol (10(-8) to 10(-6)M), similar to propranolol, diminished apoptosis, but increased ConA-stimulated thymocyte proliferation in thymocyte culture. However, differently from propranolol, arterenol affected manly CD3- thymocyte subset, which harbors majority of α1-AR+thymocytes. Additionally, arterenol showed a dose-dependent decrease in efficiency of thymocyte apoptosis and proliferation modulation with the rise in its concentration. Considering greater affinity of arterenol for α1-ARs than for β2-ARs, the previous findings could be attributable to increased engagement of β2-ARs with the rise of arterenol concentration. Consistently, in the presence of propranolol (10(-6)M), a β-AR blocker, the arterenol (10(-8)M) effects on thymocytes were augmented. In conclusion, thymic endogenous CAs, acting through distinct AR types and, possible, the same AR type (but in different cell microenvironment) may exert the opposing effects on thymocyte apoptosis/proliferation.

Sonographic thymic measurements in Down syndrome fetuses

OBJECTIVE

Children with Down syndrome (DS) can have hypoplastic thymuses with an impaired capacity to liberate newly generated T cells. We sought to determine if the size of the thymus in DS fetuses is different from control fetuses.

METHODS

We performed comprehensive prenatal sonographic evaluation that included measurements of the thymus in 874 control fetuses and in 12 DS fetuses. We used graphic comparisons between thymic measurements obtained from the two groups of fetuses and a nested case-control validation study in which DS fetuses were matched for gestational age with control fetuses using a 2-to-1 ratio to make group comparisons.

RESULTS

The plot analysis showed that more than two-thirds of DS fetuses had sonographic signs of thymic hypoplasia. The nested case-control study confirmed that the thymus of DS fetuses was statistically smaller than the thymus of control fetuses [DS transverse diameter 12.3 mm, interquartile range (IQR) 6.2 vs. control 18.1 mm, IQR 4.4; p < 0.001 and, DS perimeter 30.9 mm, IQR 20.1 vs. control 48.6 mm, IQR 20.4; p < 0.01].

CONCLUSIONS

Our findings suggest that the majority of DS fetuses have smaller thymuses than control. Prenatal measurements of the thymus may be clinically useful in the management of DS pregnancies.

Sonographic measurements of the thymus in male and female fetuses

OBJECTIVE

The purpose of this study was to determine whether the size of the thymus is different in male and female fetuses.

METHODS

In this prospective study, the transverse diameter and perimeter of the thymus were measured in healthy fetuses between 24 and 37 weeks' gestation. The means of the study variables from male and female fetuses were compared by the Student t test, and the relationships between the transverse diameter and perimeter of the thymus and gestational age and other common fetal biometric parameters were determined by linear regression modeling.

RESULTS

No differences were noted between male and female fetuses for the means of the study variables. After the relationship between the transverse diameter and perimeter of the thymus and gestational age was confirmed (R(2) = 0.8 and 0.75, respectively; both P < .01), the 95% confidence interval-predicted changes were calculated, and the scatterplots of the measurements suggested that sex did not affect the size of the thymus. Bland-Altman plots were used to analyze intraobserver variability and showed good agreement for both of these thymic measurements for male and female fetuses.

CONCLUSIONS

These results suggest that fetal sex does not affect the size of the thymus and, together with previous reports, support the hypothesis that a quantitative reduction in fetal thymus size could serve as an indirect marker of abnormal thymopoiesis and congenital thymic insufficiency.

Asynchronism of Thymocyte Development In Vivo and In Vitro

Although fetal thymus organ culture (FTOC) has become widely used to investigate T-cell development, the differences between thymocyte development in vivo and in vitro (in FTOC) remain largely unknown. In this study, the viability and numbers of thymocytes recovered from embryonic thymus lobes in different gestation days (gd) mice or from 15 day embryonic thymus lobes cultured for different days in FTOC system were evaluated. The expression of CD3, CD4, CD8, CD95 ligand (CD95L), and CD69 on thymocytes were analyzed by FACS. The results showed that thymocytes, either in vivo or in vitro, could differentiate from double negative (DN) cells to double positive (DP) cells and to single positive (SP) cells. But the number of total thymocytes and the percentage of DP cells in vitro were less than that in vivo, and the expression of CD95L and CD69 on thymocytes in vitro was higher than that in vivo. Our results suggested that although thymocyte development in vitro could recapitulate thymic development in vivo, the proliferation of thymocytes in vitro was less intensive than that in vivo; the differentiation of thymocytes in vitro was delayed compared with that in vivo; and the apoptosis and activation of thymocytes in vitro were higher than that in vivo. In conclusion, FTOC is a useful system for the study of T cell differentiation, but it is necessary to interpret the results from in vitro studies carefully since the thymocyte development in vitro is asynchronous from that in vivo.

Molecular mechanisms governing thymocyte migration: combined role of chemokines and extracellular matrix

Cell migration is crucial for thymocyte differentiation, and the cellular interactions involved now begin to be unraveled, with chemokines, extracellular matrix (ECM) proteins, and their corresponding receptors being relevant in such oriented movement of thymocytes. This notion derives from in vitro, ex vivo, and in vivo experimental data, including those obtained in genetically engineered and spontaneous mutant mice. Thymic microenvironmental cells produce both groups of molecules, whereas developing thymocytes express chemokine and ECM receptors. It is important that although chemokines and ECM proteins can drive thymocyte migration per se, a combined role of these molecules likely concurs for the resulting migration patterns of thymocytes in their various differentiation stages. In this respect, among ECM moieties, there are proteins with opposing functions, such as laminin or fibronectin versus galectin-3, which promote, respectively, adhesion and de-adhesion of thymocytes to the thymic microenvironment. How chemokines and ECM are produced and degraded remains to be more clearly defined. Nevertheless, matrix metalloproteinases (MMPs) likely play a role in the intrathymic ECM breakdown. It is interesting that these molecules also degrade chemokines. Thus, the physiological migration of thymocytes should be conceived as a resulting vector of multiple, simultaneous, or sequential stimuli, involving chemokines, adhesive, and de-adhesive ECM proteins. Moreover, these interactions may be physiologically regulated in situ by matrix MMPs and are influenced by hormones. Accordingly, one can predict that pathological changes in any of these loops may result in abnormal thymocyte migration. This actually occurs in the murine infection by the protozoan Trypanosoma cruzi, the causative agent of Chagas disease. In this model, the abnormal release of immature thymocytes to peripheral lymphoid organs is correlated with the higher migratory response to ECM and chemokines. Lastly, the fine dissection of the mechanisms governing thymocyte migration will provide new clues for designing therapeutic strategies targeting developing T cells. The most important function of the thymus is to generate T lymphocytes, which once leaving the organ, are able to colonize specific regions of peripheral lymphoid organs, the T cell zones, where they can mount and regulate cell-mediated, immune responses. This intrathymic T cell differentiation is a complex sequence of biological events, comprising cell proliferation, differential membrane protein expression, gene rearrangements, massive programmed cell death, and cell migration. In this review, we will focus on the mechanisms involved in controlling the migration of thymocytes, from the entrance of cell precursors into the organ to the exit of mature T cells toward peripheral lymphoid organs. Nevertheless, to better comprehend this issue, it appeared worthwhile to briefly comment on some key aspects of thymocyte differentiation and the tissue context in which it takes place, the thymic microenvironment.

Intrathymic T-cell migration: a combinatorial interplay of extracellular matrix and chemokines?

Cell migration is crucial for intrathymic T-cell differentiation. Chemokines and extracellular matrix proteins per se induce thymocyte migration, and recent data suggest a combinatorial role for these molecules in this event. For example, thymocyte migration induced by fibronectin plus CXCL12/SDF1-alpha (stromal cell-derived factor1-alpha) is higher than that elicited by the chemokine alone. If such interactions are relevant in the thymus, abnormal expression of any of these ligands and/or their corresponding receptors will lead to defects in thymocyte migration. At least in the murine model of Chagas disease, this seems to be the case. Therefore a better knowledge of this complex biological circuitry will provide new clues for understanding thymus physiology and designing therapeutic strategies targeting developing T cells.