Tissue interaction in the development of thymus lymphocytes

Embryonic mouse thymus development: stem cell entry and differentiation

The entry and subsequent division and differentiation of blood-borne stem cells within the embryonic thymus has been studied in a system where the inflowing cells were either chromosomally or antigenically marked. H-2 positive stem cells enter the thymus within 5 days of grafting but may wait for up to 14 days before proliferating to give rise to Thy-1 positive thymocytes.

An in vitro morphological study of the mouse visceral yolk sac and possible yolk sac immunocyte precursors

Mouse visceral yolk sac has been organ cultured from 9 days of gestation, a time prior to the thymus being lymphoid, until 12 days of gestation, a time after which the thymus is lymphoid. During the culture period the endodermal epithelial cells survived well, erythropoiesis diminished, endothelial-lined cavities formed in the mesodermal mass, and cells developed which have been classified as large, medium and small immunocyte precursors. The cytoplasm of the immunocyte precursors contains polysomes, spherical mitochondria, a few profiles of rough endoplasmic reticulum, occasional granules and a large Golgi complex. This study offers morphological support for the yolk sac origin of immunocyte precursors in the mouse which may seed the thymus and liver.

Organ culture studies of nude mouse thymus

There is evidence that peripheral lymphoid organs of nude mice, born from homozygous matings, contain a small proportion of theta-positive lymphocytes indicating that nude mice may not be totally devoid of T cell function. It has been suggested that such lymphocytes may develop within the dysplastic nude thymus itself. While this suggestion receives no support from morphological studies, it has been claimed that on explanation to organ culture the developing nude thymus becomes lymphoid. In this present study we confirm the presence of theta-positive lymphocytes in peripheral lymphoid tissues of homozygous nude mice born of nude parents. However, when we have organ-cultured nude thymus, explained from homozygous nude embryos at days 13, 14, 16 and 18 of gestation, we have found no histological sign of lymphopoiesis nor have we detected any theta-positive cells in such cultured material. On the contrary, the nude thymus in vitro develops into the polycystic structure characteristic of the adult nude thymus. We conclude that the small number of theta-positive cells present in the periphery result from extrathymic differentiation.

Circulating thymic-hormone activity in congenital immunodeficiency

Circulating thymic-hormone activity was assayed by measuring Thy 1-2 antigen induction on null lymphocytes from athymic mice incubated with human plasma or serum. Plasma from 19 normal children aged under 10 had inductive activity equivalent to 10-6-16-2 ng thymopoitin/ml. Plasma from 15 infants were severe combined immuno-deficiency, 2 of whom had appreciable immunoglobulin synthesis, and from 2 infants with DiGeorge syndrome had little or no inductive activity. Successful reconstitution with thymus or bone-marrow grafts and with red-cell infusions (if adenosine-deaminase deficiency is present) was followed by a rise in circulating thymic-hormone activity.

Embryonic mouse thymocyte development. Enhancing effect of corticosterone at physiological levels

The action of corticosterone on embryonic mouse thymus development has been studied in vitro. Effects of the hormone on lymphoid differentiation varied according to the dose. 0.2-1.0 microgram/ml had an enhancing effect, resulting in an increase in the proportion of Thy-1.2 positive cells as compared with control cultures. Two to fifty micrograms per millilitre had an inhibitory effect, appearing to selectively kill small lymphocytes while leaving large and medium lymphocytes intact. At higher does these immature cells were also killed.

Embryonic bursa development in vitro

An in vitro culture system for embryonic chicken bursa is described, in which development, as measured by follicle formation (site of B lymphocyte development), position of large basophilic cells (stem cells), and presence of lymphocytes and cells containing cytoplasmic immunoglobulin (B cells), was comparable to that of the bursa in vivo. Using this system, preliminary studies on B lymphocyte differentiation were made. These support the view that bursal lymphocytes develop from precursors that migrate through the surrounding mesenchyme and into the follicles, but indicate that at least some of these cells may be "pre-B cells" rather than pluripotent stem cells.

Electron microscopic study on the early histogenesis of thymus in the toad, Xenopus laevis

Sequential electron microscopic observations of thymic histogenesis in the toad, Xenopus laevis, reveal that the thymus arises as epithelial buddings of the visceral pouches at Nieuwkoop-Faber stage 40, and acquires its basic histological features at stages 48-49. In the rudiments and the surrounding mesenchyme at stages 43-45, there are non-epithelial cells with pseudopodia, abundant ribosomes, and marginated heterochromatin. These cells, possible precursor cells of thymic lymphocytes, are frequently observed to attach and pass through the basal lamina which coats the thymic rudiment. The proliferation and differentiation of large lymphocytes are evident at stage 47. During stages 48-49 the small lymphocytes, lymphoid cortex and epithelial medulla including the thymic cysts, differentiate, and vascularization occurs. The results provide an ultrastructural basis for recent experimental evidence that the thymus exerts its essential function at stages 47-48. The possibility of non-epithelial derivation of thymic lymphocytes is discussed.

Generation of T-cell function in organ culture of foetal mouse thymus. II. Mixed lymphocyte culture reactivity

The generation of MLC reactive cells in organ cultures of embryonic mouse thymus is described. At least 4 days in culture are required before MLC reactive thymocytes are detectable in this system, which is in marked contrast to the short incubation period required for the expression of Thy-1 on T-cell precursors in other systems. Our data demonstrates the generation of T-cell subsets independent of influences from peripheral lymphoid organs, and in the absence of B cells or their products. One very important application of this technique is in the investigation of the mechanism of T-cell tolerance to self.

Development of sheep thymus in relation to in utero thymectomy experiments

The developing thymus of sheep embryos has been examined from 20 days of gestation onwards. The initial relationships of the thymus to the parathyroid III and the pericardium are maintained so that the thymus develops as an extensive and complex structure consisting of paired cords of tissues on either side of the neck continuous with a single, fused structure in the anterior mediastinum. Histological examination of the developing thymus revealed that thymic lymphopoiesis commences at 36 days of gestation (term = 150 days). The results indicate that conclusions drawn from sheep thymectomy, usually performed after 60 days of gestation, must be interpreted with caution.

Morphologic and antigenic maturation of lymphocytes in the mouse thymus in vitro

A simple technique for organ culture of the thymus of 14-day-old mouse embryos is decribed. It allowed a characteristic time-dependent development of the immature thymus into a lymphoid thymus with large numbers of nondividing small lymphocytes. Most of these cells carried the T-lymphocyte antigenic markers Thy-1 and TL, and all expressed H-2 antigens as determined in cytotoxicity assays. They probably developed from precursor cells without detectable Thy-1 and TL. This development appeared to be dependent on the thymic microenvironment, since it also occurred in serum-free organ cultures and in cultures with medium supplemented with serum from nude mice.

Studies on the maturation of thymus stem cells. The effects of catecholamines, histamine and peptide hormones on the expression of T cell alloantigens

Studies have been performed in order to investigate the effects of catecholamines. histamine, peptide hormones and various other agents known to increase cellular adenosine 3':5'-cyclic monophosphate (cAMP) levels on the expression of Thy-1 and TL antigens on 14-day fetal thymic stem cells. It has been shown that the proportions of Thy-1 and TL positive cells, as detected by dye exclusion cytotoxicity tests, can be significantly increased by some of these agents and that these effects may be inhibited by beta adrenergic and H2 antagonists. The value of these tests as a means for studying normal thymic stem cell maturation and for investigating T cell deficiency is discussed.

The development of the thymus in the nude mouse

The developing thymus of nude mouse embryos (derived from homozygous, nu/nu x nu/nu, matings) has been examined from 13 days' gestation onwards for the presence of large basophilic stem cells or their lymphoid progeny. No trace of either stem cells or lymphocytes has been found at any stage, indicating that from the earliest stages of thymic development lymphopoiesis is defective. However, histological evidence alone is not sufficient to rule out the possibility that small numbers of lymphocytes may be present in the nude thymus. Recent evidence that nude mice born of heterozygous (nu/+) females possess some T lymphocytes and "T lineage" cells is discussed in relation to these findings. A series of morphological changes have been defined within the epithelial component of the developing nude thymus, further analysis of which may help to determine the nature of the thymic defect.

Tracing of cells of the avian thymus through embryonic life in interspecific chimeras

Differences in the structure of the interphase nucleus between two species of birds, the Japanese quail (Coturnix coturnix japonica) and the chick (Gallus gallus) has been used to distinguish cells from different origins in interspecies combinations. This biological cell marking technique was applied to thymus histogenesis. Using various combinations between components of quail and chick thymic rudiments, the respective contribution of endodermal epithelium, mesenchyme, and blood-borne extrinsic elements to the histogenesis of thymus was analyzed. It was demonstrated that the whole lymphoid population of the thymus is derived from immigrant blood-borne stem cells which are chemically attracted by the endoderm of the 3rd and 4th pharyngeal pouch. The latter is determined to differentiate into thymic epithelial reticulum as soon as the 15-somite stage, and is able to attract blood stem cells even when transplanted in an heterotopic position such as the ventral body wall of the embryo. It was shown that the thymic mesenchyme originates from the neural crest mesectoderm which colonizes early the 3rd and 4th branchial arches. It participates in the formation of perivascular mesenchyme, but does not give rise to lymphocytes. From heterospecific transplantations of quail thymuses into chick embryo (and inversely) at various stages of development is appeared that the thymic rudiment becomes attractive for lymphoid stem cells at a precise stage of its evolution for each species. The attractivity period lasts about 24 h for the quail and 36 h for the chick. Then, the inflow of stem cells becomes very low until the end of the incubation period. At this time, a second wave of lymphocytoblasts invades the thymus and the primitive embryonic lymphoid population is completely renewed around the hatching time. Competent thymic stem cells are present in the blood before and after the period of physiological thymic attractivity. The identity of basophilic cells appearing in the thymus during its histogenesis and lymphoid stem cells has been demonstrated from the analysis of quail-chick chimeric thymuses.

Lymphoid cell necrosis, thymic atrophy, and growth retardation in newborn mice inoculated with murine cytomegalovirus

During studies on the effect of murine cytomegalovirus on the developing retina, virus was inoculated into the eyes of newborn Swiss mice, and the animals were sacrificed at various times thereafter. Controls consisted of mice inoculated with ultraviolet-inactivated murine cytomegalovirus and uninjected mice. Marked lymphoid cell necrosis, thymic atrophy, pronounced growth retardation, bacteremia, and death occurred in the animals inoculated with live virus. this virus-induced injury resulted in a marked depletion of lymphocytes in the subcapsular and cortical areas of the thymus as well as in the spleen, lymph nodes, and Peyer's patches. Areas of necrosis with viral inclusions were present at the site of inoculation and in various other organs including the spleen and bone marrow. Since growth retardation has been associated with thymic atrophy due to other causes, the observed abnormal physical development in the present study was interpreted as a sequel to the thymic injury. An implication of this study is that some human infants with concomitant immune deficiency and viral infection may have a primary viral disease with resultant secondary lymphoid tissue alterations, rather than a thymic disorder with a subsequent viral infection.

Studies on the effect of various agents on the maturation of thymus stem cells

Experiments were carried out on 14-day fetal mouse thymic stem cells which were treated with agents known to increase cellular cAMP levels for a period of 90 min at 37 degrees C. When compared to controls, a high proportion of treated stem cells were found to develop Thy-1 (theta) antigen as indicated by dye exclusion tests using anti-Thy-1 serum and complement.

Induction of T-cells differentiation in vitro by thymus epithelial cells

Thymus-reticular epithelial cells (TE-cells) were grown in a cell culture devoid of any lymphocytic elements. These cells were able to induce T-cell differentiation in spleen cells from T-dificient mice as expressed by con-A responsiveness and GvH reactivity. It was also shown that xenogeneic rat TE cells were as effective in the induction of T-cell differentiation in vitro as syngeneic TE cells. This system is therefore ideal for the study of T-cell development.

Ontogeny and peripheralization of thymic lymphocytes

Reciprocal transplantation of undifferentiated thymic primordia between diploid and triploid chromosomally marked frog embryos has revealed that thymic lymphocytes are ontogenically derived from elements of the thymic primordium rather than from blood-borne stem cells that migrated into the developing organ. Virtually all the lymphocytes in the spleen, kidneys, and bone marrow of adult frogs are descendants of these original thymic stem lymphocytes.

Haemopoietic stem cells and progenitors of functional T-lymphocytes in the bone marrow of 'nude' mice

The stem cell potential of thymusless nu/nu mice measured in terms of CFU ratio and the capacity to reprieve lethally irradiated mice was similar to that of nu/+ and CBA mice. The thymus of nu/nu→T6T6 rediation chimaeras was repopulated by donor cells. Proliferation of donor cells, presumed to be T-lymphocytes, was observed in regional lymph nodes 3 days after sensitization with oxazolone.

T6T6 thymus grafts implanted subcutaneously in nu/nu mice restored the response to C57BL skin grafts. The mitotic figures in the thymus implant 30 and more days after grafting were almost exclusively of host karyotype. The high proportion of host cells dividing in the lymph nodes of thymus-grafted nu/nu mice suggested that T-cells derived originally from nu/nu bone marrow were present. Allogeneic thymus grafts partially restored the response of nu/nu mice to oxazolone.

It is concluded that the failure of thymic development in nu/nu mice is not due to an absence of thymocyte precursors in the bone marrow or to anything in the general physiological environment which prevents their differentiation to thymocytes and T-lymphocytes. It is presumably the result of defective embryonic development of the epithelial rudiment.

Functional heterogeneity of murine lymphoid cells. V. Lymphocytes lacking detectable surface theta or immunoglobulin determinants

An appreciable percent (3-14%) of the lymphocyte-like cells of the mouse spleen lack both the theta-isoantigen and sufficient surface immunoglobulin to be detected by conventional immunofluorescence or autoradiographic procedures. These theta(-),Ig(-) cells are increased in frequency after treatment of mice with antithymocyte serum or in mice that have been thymectomized, irradiated (850 R), and reconstituted with bone marrow cells. Moreover, in chimeric C57BL/6 mice in which the T cells are derived from (BALB/c x C57BL/6)F(1) donors, theta(-),Ig(-) cells also lack BALB/c histocompatibility antigens. These experiments indicate that theta(-),Ig(-) cells are not theta(-) T lymphocytes. Removal of complement receptor lymphocytes from spleen cell populations increases the frequency of theta(-),Ig(-) cells, indicating that such cells lack the complement receptor. Partially purified populations of theta(-),Ig(-) cells have been obtained by cytolysis by anti-theta- and anti-kappa-antibody and complement and by density gradient ultracentrifugation. These cells closely resemble lymphocytes in morphology. The only exceptional feature is the existence of prominent nucleoli. The theta(-),Ig(-) cells lack hemoglobin and endogenous peroxidases, are not actively phagocytic, and do not adhere to glass. This suggests they are not of the erythroid, myeloid, or monocytoid lines. [(3)H]Thymidine labeling studies indicate that theta(-),Ig(-) cells are members of a relatively slowly dividing cell pool. Whether theta(-),Ig(-) cells are members of the "classical" B lymphocyte line or belong to another, as yet undescribed, lineage is not yet certain.

Immune responses in congenitally thymus-less mice. I. Absence of response to oxazolone

Cell proliferation in regional lymph nodes of mice sensitized with oxazolone was measured by uptake of ¹²⁵IUdR. Experimental factors affecting incorporation of IUdR into lymph nodes are described. The delayed hypersensitivity response was assessed by measuring the increase in ear thickness after subsequent challenge with oxazolone. Unlike normal mice, thymus-less `nude' mice showed no detectable response to oxazolone. Implantation of an allogeneic neonatal thymus, either subcutaneously or under the kidney capsule, adoptively conferred on `nude' mice the ability to mount a lymphoproliferative response.