Macrophage heterogeneity in lymphoid tissues

Macrophages in lymphoid organs exhibit a wide variety of phenotypes and functions. These cells excel in the removal of apoptotic cells that arise during the generation of immune cells and are thereby essential for the prevention of auto-immune responses. In addition to this macrophages in the secondary lymphoid organs form an important barrier for spreading of infections by phagocytosis of pathogens and the activation of both innate and adaptive immune responses. Thus, the remarkable ability of macrophages to phagocytose and handle a wide range of self and non-self material and to produce immunomediators is effectively exploited within lymphoid organs to regulate immune activation.     

Evolution of lymphoid tissues

Lymphoid organs are integral parts of all vertebrate adaptive immune systems. Primary lymphoid tissues exhibit a remarkable functional dichotomy: T cells develop in specialized thymopoietic tissues located in the pharynx, whereas B cells develop in distinct areas of general hematopoietic areas, such as the kidney or bone marrow. Among secondary lymphoid tissues, the spleen is present in all vertebrates, whereas lymph nodes represent an innovation particular to mammals and some birds. A comparative analysis of anatomical, functional and genomic features thus reveals the core components of adaptive immune systems. Such information has guided recent attempts at reconstructing lymphopoietic functions in vivo and in the future might inspire the development of new strategies for medical interventions restoring and modulating immune functions.     

Bronchus- and nasal-associated lymphoid tissues

Summary: The bronchus‐associated lymphoid tissue (BALT) and the nasal‐associated lymphoid tissue (NALT) constitute organized lymphoid aggregates that are capable of T‐ and B‐cell responses to inhaled antigens. BALT, located mostly at bifurcations of the bronchus in animals and humans, is present in the fetus and develops rapidly following birth, especially in the presence of antigens. Humoral immune responses elicited by BALT are primarily immunoglobulin A secretion both locally and by BALT‐derived B cells that have trafficked to distant mucosal sites. Similarly located T‐cell responses have been noted. On the basis of these findings, the BALT can be thought of as functionally analogous to mucosal lymphoid aggregates in the intestine and is deemed a member of the common mucosal immunologic system. NALT has been described principally in the rodent nasal passage as two separate lymphoid aggregates. It develops after birth, likely in response to antigen, and B‐ and T‐cell responses parallel those that occur in BALT. It is not known whether NALT cells traffic to distant mucosal sites, although mucosal responses have been detected after nasal immunization. NALT appears from many studies to be a functionally distinct lymphoid aggregate when compared with BALT and Peyer's patches. It may exist, however, in humans as a diffuse collection of isolated lymphoid follicles.     

HEV-like vessels in lymphoid and non-lymphoid tissues

High endothelial venules (HEV) are vessels characteristic of lymph nodes specialized for promoting lymphocyte migration between blood and the nodal parenchyma. The studies outlined in this article constitute an overview of work done in my laboratory to investigate some functional aspects of HEV and a group of venules that develop in sites in lymphocyte accumulation in non-lymphoid tissues which resemble HEV in many ways and are therefore known as HEV-like vessels. This is followed by a hypothesis to explain the development of HEV-like vessels and its importance for lymphocyte traffic.     

Lymphocyte subsets in normal human lymphoid tissues

A series of B, T, natural killer (NK) cell, and monocyte-specific monoclonal antibodies was used to determine the localization of lymphocyte subpopulations in frozen tissue sections of human lymph node, spleen, tonsil, and thymus by means of an immunohistochemical technic. In thymus, most cortical thymocytes reacted with Leu 1, Leu 2a, Leu 3a, Leu 4, Lyt 3, OKT3, and OKT6 antibodies. Except for OKT6, Leu 2a, and Leu 3a, these antibodies also reacted with medullary thymocytes. The majority (70-80%) of medullary thymocytes reacted with Leu 3a and a smaller fraction (20-30%) with Leu 2a antibody. The staining pattern of thymic medulla approximates the staining pattern of peripheral T cells. In peripheral lymphoid tissues, the majority of cells in the paracortical region of lymph node and in the periarteriolar sheath of spleen stained with Leu 1, Leu 4, OKT3, and Lyt 3 antibodies. Staining with Leu 3a and Leu 2a identified 60-80% and 20-40% of total T cells, respectively, as defined by Lyt 3 positivity. In addition, a substantial number of Leu 3a+ and Leu 7+ cells were found in the germinal centers of secondary follicles. This finding supports the importance of these subsets of lymphocytes in regulation of the human immune response. Leu 2a+ cells were rare in tissues with prominent follicular hyperplasia, but appeared in considerable number in the red pulp of spleen. In the mantle zone of lymphoid follicles, the majority of lymphocytes were positive for IgM, IgD, and B1. Approximately 60-70% of these cells bore kappa chain and 30-40% lambda chain immunoglobulin. The extracellular substance in germinal centers was positive for B1, IgG, IgM, kappa, and lambda. The majority of germinal center cells appeared to contain no surface or cytoplasmic immunoglobulins. Small mononuclear cells bearing OKM1 marker were abundant in the marginal zone of white pulp and in the red pulp of spleen but, rarely were observed in other portions of lymphoid tissues. OKM1 also reacted with granulocytes. Leu 7+ (NK) cells were rare in the thymus, but frequent in the GC of secondary follicles. The distribution of Leu 7+ cells did not correspond to staining with Lyt 3 and Leu 2a.     

肠道相关淋巴组织与经口免疫耐受

肠道相关淋巴组织（GALT）是位于肠道内的淋巴组织,它缔属于黏膜免疫系统。GALT是肠道内外环境之间的屏障,也是诱导经口免疫耐受的重要场所。人类通过GALT对无毒害的食人性抗原产生免疫耐受,维持正常的生理功能。经动物模型和人类组织的研究均表明某些炎症性肠道疾病和自身免疫性疾病是由于打破了免疫耐受而造成的。经口免疫耐受的发生机制可能是由旁观者抑制、免疫无反应性及克隆无能引起,与肠道中特殊的细胞和细胞因子有关。调节性T细胞（Tr）介导的免疫耐受反应属于一种主动抑制。CD4^＋CD25^＋T细胞可通过分泌IL-4、IL-10、TGFβ等抑制性细胞因子,或通过细胞一细胞间的直接接触对免疫反应产生抑制作用。因此,对经口耐受的发生机制及其在临床上应用的研究将有利于人类疾病的预防和治疗。     

Immuno-architecture of human fetal lymphoid tissues

Summary Spleen, thymus and lymph node of human fetuses from the 12th to the 38rd week (spleen from 9 weeks) were investigated in an immunohistological study on B5-fixed paraffin embedded tissues, employing a panel of recently developed monoclonal antibodies, reactive with antigens resistant against fixation and paraffin embedment. The monoclonal antibodies included were MT1, MT2, MB1, MB2, MB3, LN1, LN2, LN3, LeuM1, Leu7, VIE-G4, together with polyclonal antibodies reactive with immunoglobulin heavy and light chains, and with lysozyme and S100-protein. The preservation of morphological detail together with immunoperoxidase staining of cellular subsets, allowed an accurate determination of the ontogenic development of the different cell types in situ, in relation to their micro-environment. The use of paraffin tissue reactive (monoclonal) antibodies gives an extra dimension to the study of fetal lymphoid tissues. This is of particular advantage in studies on very fragile tissues as in early embryonal and fetal ontogeny.     

Histochemical characterization of chicken lymphoid tissues

Thirteen enzyme activities were studied histochemically in the chicken bursa of Fabricius, thymus and spleen. The bursal epithelium contained a high activity of succinate, lactate and glucose-6-phosphate dehydrogenases and of ATPase. Activities of these enzymes were markedly lower in the follicle-associated epithelium than in the other parts of the bursal epithelium. A high activity of α-esterase occurred in the follicle-associated part in contrast to the other parts of the epithelium. The histochemical differentiation of the follicle-associated epithelium started at the time of hatching and was completed within a week. ATPase positive cells were observed in the interfollicular space of the bursa a few days before and after the hatching. The nature of these cells remained unclear. In the thymus, a good activity of lactate and glucose-6-phosphate dehydrogenases was observed in the area between the cortex and medulla, possibly indicating presence of endocrine cells. In the spleen no specific location of enzyme activities was observed.     

Ectopic lymphoid tissues and local immunity

Ectopic or tertiary lymphoid tissues develop at sites of inflammation or infection in peripheral, non-lymphoid organs. These tissues are architecturally similar to conventional secondary lymphoid organs, with separated B and T cell areas, specialized populations of dendritic cells, well-differentiated stromal cells and high endothelial venules. Ectopic lymphoid tissues are often associated with the local pathology that results from chronic infection or chronic inflammation. However, there are also examples in which ectopic lymphoid tissues appear to contribute to local protective immune responses. Here we review how ectopic lymphoid structures develop and function in the context of local immunity and pathology.     

T cell immunity in lymphoid and non-lymphoid tissues

Immune responses to infection or effective vaccination generally result in the development of memory lymphocytes capable of mounting a rapid response to secondary infection. Since most infections initiate in non-lymphoid tissues, defense at these sites may be important for protection. Recent results suggest that a substantial portion of the T cell response to infection is focused in non-lymphoid tissues. Furthermore, anatomic localization appears to define phenotypic and functional heterogeneity among antigen-specific memory T cell populations.     

Cell-mediated lysis of murine target cells by nonimmune salmonid lymphoid preparations

Lymphoid preparations from nonimmune rainbow and brook trout were found to lyse murine tumor cells (EL-4 &; P815Y) $\text{in}$$\text{vitro}$ in an 18 hr ⁵¹Cr-release assay conducted at 16–18°C. Lysis was proportional to the effector: target cell ratio, required direct cell to cell contact, and was not depleted by the removal of nylon wool adherent cells. Lymphoid populations from peripheral blood, the thymus, and the anterior kidney, but not the spleen, were active in the cytotoxicity assay. Individual fish varied considerably in their ability to lyse one or both target cells. These data and the results of unlabelled target cell inhibition studies suggest that the reaction is selective if not specific. The addition of PHA to the reaction mixture resulted in markedly enhanced cytotoxic reactivity. In the presence of PHA lysis was readily detectable at 4 hr. The data demonstrate that nonimmune Salmonids possess a cytolytic effector cell population which has considerable cytotoxic potential and may represent a heterogeneous “natural killer cell” population.     

Distribution of T-lymphocyte subsets in porcine lymphoid tissues.

The distribution of the functional subsets of porcine T cells, the cytolytic/suppressor (Tc/s) and the helper/inducer (Th/i) cells was studied in cryostat sections of thymus, lymph nodes, tonsils, Peyer's patches, spleen and liver using the indirect immunoperoxidase technique. Three murine monoclonal antibodies (mAb) were used. The mAb 8/1 reacts with an antigen present on all T cells and on cells of the myeloid lineage; the antigen has not yet been characterized biochemically. The mAb 295/33 (anti-T8) binds to the porcine T8 antigen and defines the Tc/s subset, while mAb PT-4 (anti-T4) detects the porcine T4 antigen and defines the Th/i subset. Practically all thymocytes were stained by mAb 8/1. The majority of cortical thymocytes apparently co-expressed T8 and T4, whereas distinct fractions of medullary cells were labelled by either anti-T8 or anti-T4. In peripheral lymphoid organs all three mAb reacted with cells in the thymus-dependent areas and with cells scattered in the lymphoid follicles. In lymph nodes, tonsils and Peyer's patches, anti-T8 and anti-T4 each labelled approximately half of the cells stained by mAb 8/1. In the periarteriolar lymphoid sheath of the spleen, anti-T4 labelled more cells than did anti-T8. The reactivity of mAb 8/1 with the Kupffer cells of the liver demonstrated the expression of the 8/1 antigen on cells of the monocyte lineage. The T8 and T4 antigens could not be detected in acetone-fixed and paraffin-embedded sections, while the antigen recognized by mAb 8/1 remained preserved. Altogether, despite an inverted microanatomical structure of porcine lymph nodes, the frequency and distribution of T8+ and T4+ cells in thymus-dependent areas proved to be similar to those found in other species.     

Human natural killer cell development in secondary lymphoid tissues

For nearly a decade it has been appreciated that critical steps in human natural killer (NK) cell development likely occur outside of the bone marrow and potentially necessitate distinct microenvironments within extramedullary tissues. The latter include the liver and gravid uterus as well as secondary lymphoid tissues such as tonsils and lymph nodes. For as yet unknown reasons these tissues are naturally enriched with NK cell developmental intermediates (NKDI) that span a maturation continuum starting from an oligopotent CD34⁺CD45RA⁺ hematopoietic precursor cell to a cytolytic mature NK cell. Indeed despite the detection of NKDI within the aforementioned tissues, relatively little is known about how, why, and when these tissues may be most suited to support NK cell maturation and how this process fits in with other components of the human immune system. With the discovery of other innate lymphoid subsets whose immunophenotypes overlap with those of NKDI, there is also need to revisit and potentially re-characterize the basic immunophenotypes of the stages of the human NK cell developmental pathway in vivo. In this review, we provide an overview of human NK cell development in secondary lymphoid tissues and discuss the many questions that remain to be answered in this exciting field.     

Immunohistochemical heterogeneity of macrophage subpopulations in human lymphoid tissues

The mononuclear phagocytic system is composed of cells which display a marked immunohistological heterogeneity. In the present study we have investigated the immunohistochemical and enzymatic features of macrophages and accessory cells present in human lymph nodes and spleen and, as control tissues, in thymus, liver, skin and heart. Our investigation has demonstrated that macrophages present in germinal centres display an immunophenotype different from that of macrophages populating T-dependent areas. Furthermore, cells lining lymph node sinuses and splenic sinusoids express endothelial and macrophage markers, and are able to modulate their immunophenotype according to different reactive conditions. These data suggest, on immunohistochemical grounds, that macrophages populating B- and T-dependent areas as well as sinuses of human peripheral lymphoid tissues, may modulate their immunophenotype according to environmental and antigenic influences.     

Interleukin-2 in the ontogeny of human lymphoid tissues

Cells from different fetal and adult lymphoid organs were tested for the capacity to (a) react to the T-cell growth factor interleukin-2 (IL-2) and (b) to produce IL-2 under appropriate conditions. IL-2 was determined as growth promoting activity for mouse T blasts. Optimal conditions for IL-2 production were: 5 × 10⁶ cells/ml; 4–6 µg mitogen, 24 h incubation time. Concanavalin A was preferable for fetal thymus, whereas phytohemagglutinin was the appropriate mitogen for lymphocytes from blood and lymph node. Fetal and adult spleen cells produced equal activities of IL-2 irrespective of the mitogen used. Cells from thymus and spleen of 17–21-week-old fetuses produced IL-2 activities like adult cells. Fetal liver cells from the same fetuses produced no IL-2. A comparison of IL-2 activities released from cells of 26 donors showed that within individual variations there was no decrease of the capacity to produce IL-2 with old age. An activity present in culture media of mitogen treated lymphocytes which increased thymidine uptake in fetal liver cells could be distinguished from IL-2 by its different molecular weight and by a heat treatment which abolished IL-2 but not fetal liver cell growth promoting activity. This latter activity is discussed to be colony-stimulating activity. It is concluded that IL-2 is produced by and reacts with T cells only after they have reached or passed the thymus.     

Acid phosphatase in lymphoid tissues of developing chick embryos

Activity of acid phosphatase (ACPase) in bursa and thymus has been determined in developing chick embryos of either normal or treated with bovine serum albumin (BSA). From the study of light microscopical histochemistry, ACPase activity could be detected in cytoplasm. These ACPase activities were detected in both the follicular medulla and cortex in bursa. In thymus, moderate ACPase activities were also obtained in both the cortex and medulla. ACPase activity in the tissue homogenate was increased in the bursa but the thymic ones showed constant level during the incubation days examined in normal embryos. ACPase distribution observed at the distinct developing stage indicates that ACPase is a useful parameter for growth and development of chicken lymphoid tissues.