Microenvironments of T and B lymphocytes : a light- and electromicroscopic study

Peripheral blood cells- erythrocytes, granulocytes, monocytes, thrombocytes and lymphocytes-are the end products of a differentiation process which occurs in the bone marrow and, in rodents, also in the spleen. Normal haemopoietic tissue is a cell renewal system with an accurate balance between cell production originating from pluripotent haemopoietic stem cells and continuous cell loss. The important function of haemopoietic stem cells was emphasized by Till and McCulloch (1961) in bone marrow transplantation studies in mice. They noted that intravenous injection of small numbers of bone marrow cells into lethally irradiated syngeneic recipient mice caused the appearance of haemopoietic colonies in the spleen of the recipient mice. These colonies consisted either of erythroid, gran uloid, megakaryocytic or mixed cell populations (Curry and Trentln, 1967). The technique used by Till and McCulloch is known as the "spleen colony assay" and has established two major qualities of haemopoietic stem cells: (I) they have the capacity of self replication (Trentin and Fahlberg, 1963; Curry et a!., 1967) and (2) they are pluripotent since they give rise to clones of different cell types of which the differentiated "end" cells recirculate in the blood (Till and McCulloch, 1961; Becker eta!., 1963; Till, 1976). In contrast to erythroid and myeloid colonies, lymphoid colonies were not detectable with the spleen colony assay; however, Ford et al. (1966), Micklem et a!. (1966), and Wu et a!. (1968) demonstrated with chromosome marker techniques that lymphoid cells were also derived from pluripotent haemopoietic stem cells. One of the major questions in cell biological investigations of haemopoiesis concerns the factors which determine the commitment and differentiation of pluripotent haemopoietic stem cells. At present it is generally accepted that two types of factors are involved in the regulation of haemopoiesis: (I) microenvironmental factors (see 1.2), and (2) humoral factors (see 1.4)

Crosstalk in the mouse thymus

The development of mature T cells within the thymus is dependent upon intact cortical and medullary microenvironments. In turn, thymic microenvironment themselves are dependent on lymphoid cells to maintain their integrity. Here, Willem van Ewijk and colleagues discuss experiments that have established the phenomenon of ‘crosstalk’ within the mouse thymus and suggest a mechanism whereby lymphoid and stromal cells influence each other in a consecutive manner during T-cell development

A Common Stem Cell for Murine Cortical and Medullary Thymic Epithelial Cells?

We have addressed the question whether the epithelial stroma in the thymus is derived from a common stem cell or whether cortical and medullary epithelial cells are derived from different embryonic stem cells emerging, for example, from endoderm and ectoderm. By the use of rapidly expanding cultures of thymic epithelial cells (TEC) from 14 to 16 day-old murine fetuses and by specific antibodies against cortical and medullary epithelium, respectively, we were able to demonstrate a small subpopulation of double-labeled TEC in the cultures. These cells were not present in TEC cultures initiated from thymuses of neonatal mice. Double-labeled TEC were also found in tissue sections from fetal thymuses. These findings may indicate that TEC populations of the cortex and the medulla are derived from a common stem cell, with potential for differentiation toward both cortical and medullary TEC

A Common Stem Cell for Murine Cortical and Medullary Thymic Epithelial Cells?

We have addressed the question whether the epithelial stroma in the thymus is derived from a common stem cell or whether cortical and medullary epithelial cells are derived from different embryonic stem cells emerging, for example, from endoderm and ectoderm. By the use of rapidly expanding cultures of thymic epithelial cells (TEC) from 14 to 16 day-old murine fetuses and by specific antibodies against cortical and medullary epithelium, respectively, we were able to demonstrate a small subpopulation of double-labeled TEC in the cultures. These cells were not present in TEC cultures initiated from thymuses of neonatal mice. Double-labeled TEC were also found in tissue sections from fetal thymuses. These findings may indicate that TEC populations of the cortex and the medulla are derived from a common stem cell, with potential for differentiation toward both cortical and medullary TEC

Stepwise development of thymic microenvironments in vivo is regulated by thymocyte subsets

T-cell development is under the tight control of thymic microenvironments. Conversely, the integrity of thymic microenvironments depends on the physical presence of developing thymocytes, a phenomenon designated as 'thymic crosstalk'. We now show, using three types of immunodeficient mice, i.e. CD3(epsilon) transgenic mice, RAG(null) mice and RAG(null)-bone-marrow-transplanted CD3(epsilon) transgenic mice, that the control point in lymphoid development where triple negative (CD3(-),CD4(-),CD8(-)) thymocytes progress from CD44(+)CD25(-) towards CD44(-)CD25(+), influences the development of epithelial cells, critically inducing the extra, third dimension in the organization of the epithelial cells in the cortex. This tertiary configuration of the thymic epithelium is a typical feature for the thymus, enabling lymphostromal interaction during T-cell development. Crosstalk signals at this control point also induce the formation of thymic nurse cells. Moreover, our data indicate that establishment of a thymic cortex is a prerequisite for the development of the thymic medulla. Thus, differentiating thymocytes regulate the morphogenesis of thymic microenvironments in a stepwise fashion

Inhibition of proliferation and differentiation during early T cell development by anti-transferrin receptor antibody

Proliferating cells require iron and, therefore, express the transferrin receptor (CD71) that mediates cellular iron uptake. Cycling thymocytes, which have the CD4−8−3−, CD4−8+3−, or CD4+8+3− phenotypes, also express CD71. The importance of CD71-mediated iron uptake for proliferation and maturation of thymocytes was studied using fetal thymus organ cultures at day 14 of gestation and treating them for 7 days with a CD71 monoclonal antibody (mAb). The intracellular iron deficiency caused by this treatment, inhibits both proliferation and maturation of the thymocytes. Cell recovery was reduced by 60%, but cells still expanded tenfold during the culture. Remarkably, the final maturation of αβ T cells was completely blocked as no thymocytes with low or high CD3/αβTcR expression developed. Moreover, only few cells reached the CD4+8+3− stage of T cell development. CD4−8−3− thymocytes, however, as well as its CD44−25+ subset developed in normal numbers, suggesting that CD44−25+ CD4−8−3− cells, or their immediate progeny, were most vulnerable to CD71 mAb treatment. The development of γδ T cells, which also express CD71, was not affected in these cultures. This suggests that γδ T cells are either less iron-dependent or possess alternative iron-uptake mechanisms. Thus, our observation

The monoclonal antibody ER-BMDM1 recognizes a macrophage and dendritic cell differentiation antigen with aminopeptidase activity

Abstract Here we describe the reactivity of monoclonal antibody (mAb) ER-BMDM1, directed against a 160-kDa cell membrane-associated antigen (Ag) with aminopeptidase activity. The aminopeptidase recognized by ER-BMDM1 is present on various mouse macrophage (MΦ) and dendritic cell (DC) subpopulations as well as on microvillous epithelia. Analysis of ER-BMDM1 Ag expression in in vitro models of MΦ maturation revealed that the Ag is expressed at increasing levels upon maturation of MΦ. In vivo, high level expression of the ER-BMDM1 Ag occurs after thmonocytic stage of maturation, since bone marrow cells and peripheral blood monocytes are essentially ER-BMDM1 negative. Analysis of isolated-resident and elicited MΦ populations showed that ER-BMDM1 recognizes a specific subpopulation of mature MΦ: only some resident peritoneal and alveolar MΦ are ER-BMDM1 positive, whereas virtually all thioglycollate-elicited peritoneal exudate MΦ bind the mAb. In lymphoid organs, a subpopulation of MΦ is recognized as well as interdigitating cells (IDC) located in T cell areas. Phenotypic analysis of isolated DC- the in vitro equivalents of IDC - from spleen and lymph nodes confirmed that the majority of this important antigen-presenting cell population expresses the ER-BMDM1 aminopeptidase. The molecular characteristics of the ER-BMDM1 Ag suggest that it may represent the mouse homolog of human CD13