The expression of I region controlled antigens on the mouse embryo and placenta has been investigated using immuno-peroxidase labelling and a mixed haemadsorption assay. The results indicate that Ia antigens are absent from selected pre- and post-implantation embryonic and trophoblastic tissues and from the trophoblast of the definitive placenta. In contrast, cells derived from the peri-placental maternal decidual tissues are Ia antigen-positive. The findings are discussed in relation to the allograft status of the conceptus and the identity and functional significance of the Ia-bearing maternal cells at the foeto-maternal interface.

Animals↗

PubMedDate not supplied

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.

Animals↗

PubMedDate not supplied

The ontogeny of T lymphocytes.

We have provided a speculative overview of some of the questions concerning the nature of thymic stem cells and the role of the thymus in their maturation, and have suggested experimental approaches which might provide solutions to some of the questions. Throughout, we have stressed the complexity of the cellular constitution of the thymus and have suggested roles for particular cell types which seem plausible based on present evidence.

Animals↗

PubMedDate not supplied

Successful transplantation across major histocompatibility barrier of deoxyguanosine-treated embryonic thymus expressing class II antigens.

Foreign tissues grafted into healthy recipients are usually rejected by the hosts' immune system largely by means of major histocompatibility complex (MHC) products expressed on donor cells. During ontogeny, developing T lymphocytes acquire tolerance to self-MHC antigens and the thymus has been considered as the most likely site for the abrogation of self-reactive clones. We demonstrate here that embryonic thymus lobes, when organ cultured in the presence of deoxyguanosine, which is toxic to proliferating embryonic thymic lymphocytes but does not affect the epithelial framework, when transplanted to the kidney capsule of normal healthy histoincompatible mice, are not rejected despite their continued expression of both class I and class II donor MHC products but do not induce tolerance. This suggests that immunogenicity is not solely a function of MHC antigen expression but is also influenced by the type of cell upon which the antigens are expressed and, if the thymus is involved in the induction of tolerance to self-MHC products, this is a function of a component other than the epithelium, perhaps thymic dendritic cells.

Animals↗

PubMedDate not supplied

A single stem cell can recolonize an embryonic thymus, producing phenotypically distinct T-cell populations.

There is much interest in early T-cell development, particularly in relation to the diversification of the T-cell receptor repertoire and the elucidation of the lineage relationships between T-cell populations in the thymus and peripheral lymphoid organs. However, the requirements for the growth of the earliest thymic T-cell precursor in 13-14-day mouse embryo thymus in isolation from the thymic environment are unknown. Proliferation and maturation of such cells are not sustained either in the presence of monolayers of thymic stromal cells or by the addition of interleukin-2 (IL-2), despite the expression of receptors for this growth factor on a proportion of thymocytes displaying the immature Thy 1+ Lyt-2-L3T4- phenotype in the embryonic thymus. In contrast, when maintained within the intact thymic environment in organ cultures, 13-14-day thymic stem cells do show a pattern of surface marker and functional development similar to that seen in vivo, suggesting that short-range growth signals, perhaps necessitating direct contact with organized epithelial cells, are required. We have shown, by exploiting the selective toxicity of deoxyguanosine (dGuo) for early T cells, that this organ culture system can be manipulated to produce alymphoid lobes that can be recolonized from a source of precursors in a transfilter system. We now show that recolonization of alymphoid lobes can also be achieved by association with T-cell precursors in hanging drops, allowing recolonization by exposure to defined numbers of precursors, including a single micromanipulated stem cell. Analysis of T-cell marker expression in these cultures shows that a single thymic stem cell can produce progeny of distinct phenotypes, suggesting that these marker-defined populations are not derived from separate prethymic precursors, but arise within the thymus.

Animals↗

PubMedDate not supplied

A single micromanipulated stem cell gives rise to multiple T-cell receptor gene rearrangements in the thymus in vitro.

The extensive range of specificities of T-cell receptors is generated, as for immunoglobulins, by rearrangement of genetic information. Much valuable information about rearrangement processes has been inferred by comparing DNA from (monoclonal) lymphoid lines with germ-line DNA and, for B cells, from rearrangements in some Abelson murine leukaemia virus-transformed cell lines. However, because it is difficult to isolate and grow precursor populations, it has not proved possible to study rearrangements occurring in normal untransformed cells in vitro. Here we show that a single T-cell precursor colonizing an alymphoid thymus lobe in organ culture can generate multiple receptor beta-chain gene rearrangements. These observations provide unequivocal evidence for the intra-thymic diversification of the T-cell repertoire. They also offer the possibility of investigating rearrangement and its control in the clonal progeny of a single normal T-cell precursor without the perturbations involved in the use of viral transformation or the production of T-cell hybridomas.

Animals↗

PubMedDate not supplied

Importance of IL-2 receptors in intra-thymic generation of cells expressing T-cell receptors.

During development, lymphoid stem cells migrate into the thymic rudiment where they proliferate, rearrange their antigen receptor genes and become differentiated into functionally mature T cells. At present, the regulation of these processes is poorly understood, although recent studies have shown that early fetal and adult immature thymocytes express receptors for the T-cell growth factor, interleukin-2 (IL-2). We now present direct evidence that IL-2 receptors have a function in intra-thymic development by demonstrating that proliferation and the generation of cells expressing the T-cell antigen receptor (alpha beta TCR), which is responsible for the recognition of antigens in the context of MHC, are inhibited when antibodies to IL-2 receptors are added to fetal thymus organ cultures. The inhibition is specific in that it does not affect pre-thymic stem cells and can be partially reversed by addition of exogenous recombinant IL-2.

Animals↗