Characterization of murine thymic stromal-cell lines immortalized by temperature-sensitive simian virus 40 large T or adenovirus 5 E1a

Microinjection for the ex Vivo Modification of Cells with Artificial Organelles

Microinjection is extensively used across fields to deliver material intracellularly. Here we address the fundamental aspects of introducing exogenous organelles into cells to endow them with artificial functions. Nanocarriers encapsulating biologically active cargo or extreme intraluminal pH were injected directly into the cytosol of cells, where they bypassed subcellular processing pathways and remained intact for several days. Nanocarriers' size was found to dictate their intracellular distribution pattern upon injection, with larger vesicles adopting polarized agglomerated distributions and smaller colloids spreading evenly in the cytosol. This in turn determined the symmetry or asymmetry of their dilution following cell division, ultimately affecting the intracellular dose at a cell population level. As an example of microinjection's applicability, a cell type relevant for cell-based therapies (dendritic cells) was injected with vesicles, and its migratory properties were studied in a co-culture system mimicking lymphatic capillaries.

Use of explant technology in the study of in vitro immune responses

The establishment of in vitro culture systems provides an accessible means to study events within the immune system. In contrast to either dispersed suspension or two-dimensional monolayer culture, the explantation of tissue fragments under organ culture conditions is, to date, the only method which allows essential three-dimensional cellular interactions to be maintained under conditions which permit controlled experimental manipulation in vitro. Recent modifications of explant technology, particularly within the area of fetal thymic organ culture, now allow the controlled reassociation of defined cellular subsets and manipulation of gene expression, under conditions where the functioning of both lymphoid and stromal cell types closely resembles that in vivo.

Studies on thymic epithelial cells in vitro

Thymic epithelial cells are unique in their ability to support positive selection and are essential throughout thymocyte development. Here, we describe a technique for measuring the proliferation of thymic epithelial cells by flow cytometry using a combination of BrdU and pancytokeratin labelling, and we examine the effects of different in vitro culture strategies on thymic epithelial cell function. We find that at d15 gestation, 74% (+/- 0.4%) of thymic epithelial cells are in cycle, which declines to 63% (+/- 1.3%) by d16, and to 34% (+/- 1.9%) by d18. This decline in proliferation is also found in organ cultures and in cultures depleted of lymphoid cells by 2-dGuo, suggesting that the cell cycle status of thymic epithelial cells is independent of the lymphoid population. When cultured in vitro as 3-dimensional aggregates, purified MHC class II+ thymic epithelial cells retain the ability to support thymocyte maturation. In contrast, 2-dimensional monolayer culture abrogates the ability of these cells to support positive selection, causes a reduction in whn gene expression and reduces their ability to re-form coherent reaggregate structures. Intact lobes and 3-dimensional aggregates are therefore the best way of maintaining thymic epithelial cell function and gene expression in vitro.

Discrimination between maintenance- and differentiation-inducing signals during initial and intermediate stages of positive selection

As well as signaling through the alphabeta T cell receptor complex, positive selection of immature CD4+ 8+ thymocytes involves additional ill-defined accessory interactions provided by thymic epithelial cells. Here, we have isolated CD4+ 8+ thymocytes at a pre-positive selection stage of development (TCR- CD69- 4+ 8+ cells), or after initiation of positive selection (CD69+ 4+ 8+ cells), from mice where the normal lifespan of thymocytes is extended by the presence of a bcl-2 transgene, to allow us to discriminate between requirements for maintenance and differentiation signals during positive selection. We find that MHC class II+ thymic epithelial cells drive positive selection of TCR- CD69- 4+ 8+ bcl-2 tg thymocytes to the CD4+ and CD8+ stage, while no such mature subsets are observed when thymocytes are cultured alone or with major histocompatibility complex (MHC) class II+ salivary epithelial cells. However, CD4+ 8+ cells remain in such cultures in considerable numbers, and retain the potential for positive selection if re-cultured with thymic epithelium, suggesting that thymic epithelial cells provide specific differentiation-inducing signals for positive selection. In contrast, intermediate CD69+ 4+ 8+ thymocytes show some capacity for phenotypic conversion in the absence of thymic stromal cells although strikingly the single-positive CD4+ and CD8+ cells generated are not functionally competent. Finally, we show that prior culture of thymic epithelial cells under monolayer conditions abrogates their ability to support the initiation of positive selection, suggesting that the epithelial cell molecules necessary for the provision of differentiation signals during positive selection are down-regulated under such conditions.

The role of the thymus during T-lymphocyte development in vitro

To date, fetal thymic organ culture is the only in-vitro system capable of supporting a complete programme of T-lymphocyte development in a manner comparable to that seen in vivo. In this review, we will summarise recent studies in which thymic organ cultures have been used to investigate the regulatory mechanisms of particular stages of thymocyte development. In addition, the use of other culture systems of T-cell maturation will be discussed in an attempt to define the optimal conditions for T-cell development in vitro.

Expression of human factor IX by microencapsulated recombinant fibroblasts

Deficiency of clotting factor IX (FIX) causes hemophilia B in humans. We propose a novel approach to its treatment by engineering FIX-secreting cell lines suitable for implantation in different allogeneic hosts. To prevent graft rejection following implantation, the recombinant cells can be protected with biocompatible membranes that permit exit of FIX but not entry of cellular immune mediators. To explore the feasibility of this approach, we now report on the creation of mouse Ltk- fibroblast cell lines that can deliver FIX through such immune-protective membranes. Mouse fibroblasts (Ltk-) were transfected with the cDNA for human FIX and clones secreting high levels of FIX were isolated. About 70% of the secreted FIX was biologically active. Over 98% of the recovered biological activity was precipitable by barium citrate, indicating appropriate. gamma-carboxylation of the secreted FIX. The secreted FIX was similar in molecular weight and immunoreactivity to plasma-derived human FIX. Upon enclosure in microcapsules fabricated from the biocompatible polymers, alginate-polylysine-alginate, the cells survived the encapsulation procedure with about 70-90% viability, proliferated within the microcapsules to twice their original number within 2 weeks, and continued to secrete FIX into the culture medium at similar rates as the unencapsulated cells. The biological activity, degree of post-translational gamma-carboxylation, and immunoreactivity of the FIX recovered from the culture media of the encapsulated cells were identical to those of the FIX secreted by the unencapsulated cells. In conclusion, fibroblasts engineered to secrete recombinant human FIX can proliferate and continue to secrete biologically active FIX through the alginate microcapsules. This demonstrates the feasibility of using microencapsulated recombinant cells to deliver human FIX and the potential for allogeneic somatic gene therapy for hemophilia B.

Studies on T cell maturation on defined thymic stromal cell populations in vitro

We describe an in vitro system in which positive selection of developing T cells takes place on defined stromal cell preparations, which include major histocompatibility complex class II+ epithelial cells but exclude cells of bone marrow origin. In this system, maturation of double-positive T cell receptor negative (TCR-), CD4+8+ thymocytes into single-positive TCR+, CD4+ and CD8+ cells takes place together with the development of functional competence. As in vivo, this maturation is associated with the upregulation of TCR levels as cells progress from double-positive to single-positive status. We also show that class II+ epithelial cells in these cultures are less efficient than dendritic cells in mediating the deletion (negative selection) of V beta 8+ cells by the superantigen staphylococcal enterotoxin B. For the first time, this approach provides a model in which the cellular interactions involved in both positive and negative selection can be studied under controlled in vitro conditions.