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Background: While the human fetal immune system defaults to a program of tolerance, there is concurrent need for protective immunity to meet the antigenic challenges encountered after birth. Activation of T cells in utero is associated with the fetal inflammatory response with broad implications for the health of the fetus and of the pregnancy. However, the characteristics of the fetal effector T cells that contribute to this process are largely unknown., Methods: We analyzed primary human fetal lymphoid and mucosal tissues and performed phenotypic, functional, and transcriptional analysis to identify T cells with pro-inflammatory potential. The frequency and function of fetal-specific effector T cells was assessed in the cord blood of infants with localized and systemic inflammatory pathologies and compared to healthy term controls., Results: We identified a transcriptionally distinct population of CD4+ T cells characterized by expression of the transcription factor Promyelocytic Leukemia Zinc Finger (PLZF). PLZF+ CD4+ T cells were specifically enriched in the fetal intestine, possessed an effector memory phenotype, and rapidly produced pro-inflammatory cytokines. Engagement of the C-type lectin CD161 on these cells inhibited TCR-dependent production of IFNγ in a fetal-specific manner. IFNγ-producing PLZF+ CD4+ T cells were enriched in the cord blood of infants with gastroschisis, a natural model of chronic inflammation originating from the intestine, as well as in preterm birth, suggesting these cells contribute to fetal systemic immune activation., Conclusion: Our work reveals a fetal-specific program of protective immunity whose dysregulation is associated with fetal and neonatal inflammatory pathologies.

The expression pattern and distribution of proteasome immune subunits LMP7 and LMP2 in the developing rat spleen and liver as well as the periarterial lymphoid sheath formation were investigated. LMP7 and LMP2 were detected by immunoblotting in the spleen on the 21st embryonic day and during the first postnatal days in equal amounts. Their levels increased by the 8th and 18th postnatal days. Double immunofluorescent labeling the spleen cells revealed LMP7 and LMP2 in T and B lymphocytes localized in the red pulp in embryogenesis. Few T lymphocytes were discovered in periarterial zones on the 8th postnatal day. T lymphocytes filled these zones and formed lymphoid sheaths by the 18-19th day. In the liver, LMP7 and LMP2 were revealed by the 17-19th postnatal day. Immunofluorescent analysis showed their presence in hepatocytes at this period. The data suggest that T cell-mediated immune response in relation to hepatocytes is possible beginning from 18th to 19th postnatal day.

Intestinal intraepithelial T lymphocytes (IELs) are likely to play a key role in host mucosal immunity and, unlike other T cells, have been proposed to differentiate from local precursors rather than from thymocytes. We show here that IELs expressing the alphabeta T cell receptor are derived from precursors that express RORgammat, an orphan nuclear hormone receptor detected only in immature CD4+CD8+ thymocytes, fetal lymphoid tissue-inducer (LTi) cells, and LTi-like cells in cryptopatches within the adult intestinal lamina propria. Using cell fate mapping, we found that all intestinal alphabeta T cells are progeny of CD4+CD8+ thymocytes, indicating that the adult intestine is not a significant site for alphabeta T cell development. Our results suggest that intestinal RORgammat+ cells are local organizers of mucosal lymphoid tissue.

The lymphoid and immunohaematopoietic tissues of the embryonic and full-term brushtail possums was investigated histologically and immunohistochemically using antibodies to the T- and B-cell markers, CD3, CD5, CD79a and CD79b. No clearly defined thymus, bone marrow, spleen, lymph nodes, gut-associated lymphoid tissues or bronchus-associated lymphoid tissues were observed histologically. The liver was haematopoietic and contained erythrocytic and granulocytic precursors. No mature lymphocytes were observed histologically or detected using antibodies to T- and B-cell markers in any of the tissues. These results are consistent with other studies of the early postnatal tissues of other marsupials and support the proposition that neonatal marsupials are substantially reliant on maternal immunological protection at the time of birth and for a significant period of pouch life.

The thymus is the major primary immune tissue for the production of functional T lymphocytes in vertebrates. However, its evolutionary origin is unknown. It has recently been shown that the generation of local T cells also occurs in gut-associated lymphoid tissues (GALT). This suggests that the thymus and GALT have similar functions and that they might be evolutionarily related. We discuss the possibility that the thymus may have evolved from mucosa-associated lymphoid tissues (MALT) located in the gill region in early vertebrates. Various facts supporting this proposal are summarized.

To identify the physiological functions of the retinoid-related orphan receptor gamma (RORgamma), a member of the nuclear receptor superfamily, mice deficient in RORgamma function were generated by targeted disruption. RORgamma(-/-) mice lack peripheral and mesenteric lymph nodes and Peyer's patches, indicating that RORgamma expression is indispensable for lymph node organogenesis. Although the spleen is enlarged, its architecture is normal. The number of peripheral blood CD3(+) and CD4(+) lymphocytes is reduced 6- and 10-fold, respectively, whereas the number of circulating B cells is normal. The thymus of RORgamma(-/-) mice contains 74.4% +/- 8.9% fewer thymocytes than that of wild-type mice. Flow cytometric analysis showed a decrease in the CD4(+)CD8(+) subpopulation. Terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling (TUNEL) staining demonstrated a 4-fold increase in apoptotic cells in the cortex of the thymus of RORgamma(-/-) mice. The latter was supported by the observed increase in annexin V-positive cells. RORgamma(-/-) thymocytes placed in culture exhibit a dramatic increase in the rate of "spontaneous" apoptosis. This increase is largely associated with CD4(+)CD8(+) thymocytes and may, at least in part, be related to the greatly reduced level of expression of the anti-apoptotic gene Bcl-X(L). Flow cytometric analysis demonstrated a 6-fold rise in the percentage of cells in the S phase of the cell cycle among thymocytes from RORgamma(-/-) mice. Our observations indicate that RORgamma is essential for lymphoid organogenesis and plays an important regulatory role in thymopoiesis. Our findings support a model in which RORgamma negatively controls apoptosis in thymocytes.

We show that the mesenchymal cells that surround the 12-d mouse embryo thymus are necessary for T cell differentiation. Thus, epithelial lobes with attached mesenchyme generate all T cell populations in vitro, whereas lobes from which mesenchyme has been removed show poor lymphopoiesis with few cells progressing beyond the CD4(-)CD8(-) stage of development. Interestingly, thymic mesenchyme is derived from neural crest cells, and extirpation of the region of the neural crest involved results in impaired thymic development and craniofacial abnormalities similar to the group of clinical defects found in the DiGeorge syndrome. Previous studies have suggested an inductive effect of mesenchyme on thymic epithelial morphogenesis. However, we have found that mesenchyme-derived fibroblasts are still required for early T cell development in the presence of mature epithelial cells, and hence mesenchyme might have a direct role in lymphopoiesis. We provide an anatomical basis for the role of mesenchyme by showing that mesenchymal cells migrate into the epithelial thymus to establish a network of fibroblasts and associated extracellular matrix. We propose that the latter might be important for T cell development through integrin and/or cytokine interactions with immature thymocytes.

The thymus is colonized by circulating progenitor cells that differentiate into mature T cells under the influence of the thymic microenvironment. We report here the cloning and function of the avian thymocyte Ag ChT1, a member of the Ig superfamily with one V-like and one C2-like domain. ChT1-positive embryonic bone marrow cells coexpressing c-kit give rise to mature T cells upon intrathymic cell transfer. ChT1-specific Ab inhibits T cell differentiation in embryonic thymic organ cultures and in thymocyte precursor cocultures on stromal cells. Thus, we provide clear evidence that ChT1 is a novel Ag on early T cell progenitors that plays an important role in the early stages of T cell development.

Interleukin-2 receptor alpha chain (IL-2R alpha) expression occurs at specific stages of early T and B lymphocyte development and is induced upon activation of mature lymphocytes. Young mice that lack IL-2R alpha have phenotypically normal development of T and B cells. However, as adults, these mice develop massive enlargement of peripheral lymphoid organs associated with polyclonal T and B cell expansion, which, for T cells, is correlated with impaired activation-induced cell death in vivo. Older IL-2R alpha-deficient mice also develop autoimmune disorders, including hemolytic anemia and inflammatory bowel disease. Thus, IL-2R alpha is essential for regulation of both the size and content of the peripheral lymphoid compartment, probably by influencing the balance between clonal expansion and cell death following lymphocyte activation.

Using immunohistological techniques, we studied the development over time of B- and T-cell compartments in the lymphoid organs of specific-pathogen-free pigs. Tissue samples were collected at various time-points, starting 2 days before the pigs were born until the pigs were 10 months old. The samples were collected from the spleen, thymus, peripheral lymph node, mesenteric lymph node, duodenum, jejunum, ileum, jejunal Peyer's patch and ileal Peyer's patch. Monoclonal antibodies specific to B- and T-cells were used to identify where the following cells were localized: IgM-B cells (cells positive to surface immunoglobulin), IgM-, IgG- and IgA-containing cells (cells positive to cytoplasmic immunoglobulin), and CD2-, CD4- and CD8-positive cells. The development of the B- and T-cell subpopulations in each organ was analysed. Two days before birth, most organs contained quantities of IgM-B cells. The spleen, lymph nodes, Peyer's patches and, notably, the thymus, contained some immunoglobulin-containing cells (Ig-CC); this finding indicates that pigs have cells that secrete immunoglobulins before birth. Just after birth, the incidence of Ig-CC increased in most organs; first IgM-CC increased, then either IgG- or IgA-CC increased, depending on the organ. T-cell development was observed clearly in spleen and in the lamina propria of the small intestine, in contrast to other organs, in which the T-cell compartments containing various T-cell subpopulations were well developed before birth. Comparison of the incidence of CD4+ and CD8+ cells showed that the CD4:CD8 ratio of these cells in the spleen, lymph nodes, Peyer's patches and small intestine is low, especially in adult pigs, compared with the CD4:CD8 ratio in other species. Weaning had little influence on the incidence of B- and T-cells in lymphoid organs. This study is the first immunohistological survey to describe the development of the major B- and T-cell subpopulations in various lymphoid organs of pigs, and it should be useful for future immunopathological and comparative immunological studies in pigs.

In a study of T cell ontogeny using monoclonal antibodies specific for chicken T cell receptors (TcR) and associated cell surface molecules, we found a subset of lymphocytes that express cytoplasmic CD3 epitopes in the absence of cell surface CD3/TcR complexes. Approximately half of these cells, which were present in the spleen, bursa and intestine of young chick embryos, expressed surface CD8. None expressed CD4, TcR 1 (gamma/delta), TcR 2 (alpha/beta) or TcR 3 (a third CD3-associated heterodimer in the chicken). These cytoplasmic CD3+CD8 +/- cells, tentatively named TcR 0 cells to denote their lack of surface TcR, appeared first in the spleen of 8-day embryos, 4 days before TcR expression begins in the thymus, and reached a peak frequency of approximately 10% of the splenic cell in 14-day embryos. The TcR 0 cells were also present in adult birds, where they comprised only about 1% of the CD3+ spleen cells and approximately 40% of the lymphocytes in the intestinal epithelium. We conclude that the avian TcR 0 cells represent a thymus-independent lineage of lymphocytes which, like natural killer cells in mammals, may play an important role in body defense.

In a 51/2-month-old male infant with adenosine deaminase-positive severe combined immunodeficiency disease, who had no suitable bone marrow donor, immunologic reconstitution was attempted with lymphoid cells obtained from the liver of a 4- to 5-week-old-male human embryo. A mild graft-versus-host reaction began three weeks later. T-cells, which were absent prior to infusion of hepatic lymphoid cells, rose to a maximum of 554/mm3 at 16 weeks post transplantation. A normal lymphocyte response to pokeweek mitogen was not present until 25 to 30 weeks and to allogeneic cells until 39 weeks. Postive in vitro lymphocyte responses to Candida albicans were found repeatedly after 52 weeks. Twenty months following transplantation the patient is free of clinical infection, although he requires regular injections of gamma globulin.

The results presented in this paper demonstrate that a mouse IgM monoclonal antibody (T-80) recognizes an antigen on cells of the T-lymphocyte lineage of sheep. However, this antibody does not identify all T cells, as 10-20% of thymocytes and some peripheral-blood T cells are negative. T-80- thymocytes reside in the medulla. The majority of cortical thymocytes are T-80+ and classified as dull cells on the basis of antigen density per cell as measured by flow microfluorometry. In contrast, T-80+ cells in the periphery can be categorized into two populations, i.e., dull cells and bright cells. Suggestive evidence was obtained that bright T-80+ cells are fast recirculating T cells, whereas dull cells are sessile or less easily mobilizable T cells in the periphery. In foetal environment, over 90% of thymocytes and approximately 5% of spleen cells are T-80+ at 54 days of gestation (gestation period = 150 days), which may indicate that T-cell emigration from the thymus commences well before mid-gestation in sheep.