Normal thymic cortical epithelial cells developmentally regulate the expression of a B-lineage transformation-associated antigen

Three distinct subsets of thymic epithelial cells in rats and mice defined by novel antibodies

AIM

Thymic epithelial cells (TECs) are thought to play an essential role in T cell development and have been detected mainly in mice using lectin binding and antibodies to keratins. Our aim in the present study was to create a precise map of rat TECs using antibodies to putative markers and novel monoclonal antibodies (i.e., ED 18/19/21 and anti-CD205 antibodies) and compare it with a map from mouse counterparts and that of rat thymic dendritic cells.

RESULTS

Rat TECs were subdivided on the basis of phenotype into three subsets; ED18+ED19+/-keratin 5 (K5)+K8+CD205+ class II MHC (MHCII)+ cortical TECs (cTECs), ED18+ED21-K5-K8+Ulex europaeus lectin 1 (UEA-1)+CD205- medullary TECs (mTEC1s), and ED18+ED21+K5+K8dullUEA-1-CD205- medullary TECs (mTEC2s). Thymic nurse cells were defined in cytosmears as an ED18+ED19+/-K5+K8+ subset of cTECs. mTEC1s preferentially expressed MHCII, claudin-3, claudin-4, and autoimmune regulator (AIRE). Use of ED18 and ED21 antibodies revealed three subsets of TECs in mice as well. We also detected two distinct TEC-free areas in the subcapsular cortex and in the medulla. Rat dendritic cells in the cortex were MHCII+CD103+ but negative for TEC markers, including CD205. Those in the medulla were MHCII+CD103+ and CD205+ cells were found only in the TEC-free area.

CONCLUSION

Both rats and mice have three TEC subsets with similar phenotypes that can be identified using known markers and new monoclonal antibodies. These findings will facilitate further analysis of TEC subsets and DCs and help to define their roles in thymic selection and in pathological states such as autoimmune disorders.

Clonal precursor of bone, cartilage, and hematopoietic niche stromal cells

Organs are composites of tissue types with diverse developmental origins, and they rely on distinct stem and progenitor cells to meet physiological demands for cellular production and homeostasis. How diverse stem cell activity is coordinated within organs is not well understood. Here we describe a lineage-restricted, self-renewing common skeletal progenitor (bone, cartilage, stromal progenitor; BCSP) isolated from limb bones and bone marrow tissue of fetal, neonatal, and adult mice. The BCSP clonally produces chondrocytes (cartilage-forming) and osteogenic (bone-forming) cells and at least three subsets of stromal cells that exhibit differential expression of cell surface markers, including CD105 (or endoglin), Thy1 [or CD90 (cluster of differentiation 90)], and 6C3 [ENPEP glutamyl aminopeptidase (aminopeptidase A)]. These three stromal subsets exhibit differential capacities to support hematopoietic (blood-forming) stem and progenitor cells. Although the 6C3-expressing subset demonstrates functional stem cell niche activity by maintaining primitive hematopoietic stem cell (HSC) renewal in vitro, the other stromal populations promote HSC differentiation to more committed lines of hematopoiesis, such as the B-cell lineage. Gene expression analysis and microscopic studies further reveal a microenvironment in which CD105-, Thy1-, and 6C3-expressing marrow stroma collaborate to provide cytokine signaling to HSCs and more committed hematopoietic progenitors. As a result, within the context of bone as a blood-forming organ, the BCSP plays a critical role in supporting hematopoiesis through its generation of diverse osteogenic and hematopoietic-promoting stroma, including HSC supportive 6C3(+) niche cells.

Irving Weissman: Working for regenerative medicine

A force behind both the science and politics of stem cell biology, Weissman now strives to translate his successes to the clinic.

p63 Is essential for the proliferative potential of stem cells in stratified epithelia

The distinguishing feature of adult stem cells is their extraordinary capacity to divide prior to the onset of senescence. While stratified epithelia such as skin, prostate, and breast are highly regenerative and account disproportionately for human cancers, genes essential for the proliferative capacity of their stem cells remain unknown. Here we analyze p63, a gene whose deletion in mice results in the catastrophic loss of all stratified epithelia. We demonstrate that p63 is strongly expressed in epithelial cells with high clonogenic and proliferative capacity and that stem cells lacking p63 undergo a premature proliferative rundown. Additionally, we show that p63 is dispensable for both the commitment and differentiation of these stem cells during tissue morphogenesis. Together, these data identify p63 as a key, lineage-specific determinant of the proliferative capacity in stem cells of stratified epithelia.

The quantitative assessment of MHC II on thymic epithelium: implications in cortical thymocyte development

The dynamics of MHC II expression in various thymic stromal compartments was investigated. By including MHC II in flow cytometry in addition to the cortical CDR1, medullary UEA-1 and pan-epithelial G8.8 markers, thymic stromal compartments were subdivided into at least six different populations. The total level of surface and cytoplasmic MHC II from fresh cortical thymic epithelial cells (cTECs) of normal mouse was as high as MHC II levels in medullary thymic epithelial cells (mTECs). MHC II levels as well as the percentages and cycling status of thymic epithelial cell populations expressing MHC II were not static during post-natal development, suggesting quantitative flexibility in presenting signals to the developing thymocytes. Although there was no evidence found for regulation of surface MHC II levels by TCR or by IFN-gamma, the absence of class II transactivator reduced both the level of MHC II expression and the number of MHC II+ cells. Surprisingly, MHC II molecules were found to form distinct focal aggregates on the surface of cTEC but not mTEC using high-resolution analysis by confocal microscopy. Moreover, these aggregates were formed independent of TCR or TCR-bearing cells in the thymus. These aggregates could potentially generate a functional unit containing a much higher local MHC II concentration to yield a higher avidity interaction. We discuss possible mechanisms for positive selection by weak interactions in the presence of such preformed MHC II aggregate units in cTEC.

Thymic nurse cells: a microenvironment for thymocyte development and selection

Thymic nurse cells (TNCs) represent a unique microenvironment in the thymus for MHC restriction and T cell repertoire selection composed of a cortical epithelial cell surrounding 20-200 immature thymocytes. TNCs have been isolated from many classes of animals from fish to humans. Studies performed using TNC lines showed that TNCs bind viable alphabetaTCRlow CD4(+)CD8(+)CD69(-) thymocytes. A subset of the bound cells is internalized, proliferates within the TNC, and matures to the alphabetaTCRhigh CD4(+)CD8(+)CD69(+) stage, indicative of positive selection. A subset of the internalized population is released while cells that remain internalized undergo apoptosis and are degraded by lysosomes within the TNC. A TNC-specific monoclonal antibody added to fetal thymic organ cultures resulted in an 80% reduction in the number of thymocytes recovered, with a block at the double positive stage of development. Together these data suggest a critical role for TNC internalization in thymocyte selection as well as the removal and degradation of negatively selected thymocytes. Recent studies have shown that in addition to thymocytes, peripheral circulating macrophages are also found within the TNC complex and can present antigens to the developing thymocytes. These circulating macrophages could provide a source of self-antigens used to ensure a self-tolerant mature T cell repertoire. A reduction in TNC numbers is associated with a variety of autoimmune diseases including thyroiditis and systemic lupus erythematosis.

Early defect prethymic in bone marrow T cell progenitors in athymic nu/nu mice

nu/nu mice fail to develop a thymus and mature T cells due to a defect in the whn gene encoding a transcription factor necessary for terminal epithelial cell differentiation. We investigated whether early T cell progenitor development in the nu/nu bone marrow is also defective. We demonstrated a maturation arrest of nu/nu marrow T cell progenitors associated with a lack of pTalpha gene expression and a failure to give rise to mature T cells in adoptive euthymic hosts. Wild-type hemopoietic stem cells rapidly matured into functional T cell progenitors in the marrow of euthymic or thymectomized but not nu/nu hosts. We show that defects in bone marrow prethymic T cell development can also contribute to T cell deficiency in nu/nu mice.

Identification and characterization of thymus LIM protein: targeted disruption reduces thymus cellularity

We have identified a novel LIM gene encoding the thymus LIM protein (TLP), expressed specifically in the thymus in a subset of cortical epithelial cells. TLP was identified as a gene product which is upregulated in a thymus in which selection of T cells is occurring (Rag(-/-) OT-1) compared to its expression in a thymus in which selection is blocked at the CD4+ CD8+ stage of T-cell development (Rag(-/-) Tap(-/-) OT-1). TLP has an apparent molecular mass of 23 kDa and exists as two isomers (TLP-A and TLP-B), which are generated by alternative splicing of the message. The sequences of TLP-A and TLP-B are identical except for the C-terminal 19 or 20 amino acids. Based on protein sequence alignment, TLP is most closely related to the cysteine-rich proteins, a subclass of the family of LIM-only proteins. In both medullary and cortical thymic epithelial cell lines transduced with TLP, the protein localizes to the cytoplasm but does not appear to be strongly associated with actin. In immunohistochemical studies, TLP seems to be localized in a subset of epithelial cells in the cortex and is most abundant near the corticomedullary junction. We generated mice with a targeted disruption of the Tlp locus. In the absence of TLP, thymocyte development and thymus architecture appear to be normal but thymocyte cellularity is reduced by approximately 30%, with a proportional reduction in each subpopulation.

Questionable thymic nurse cell

Since their discovery in 1980, thymic nurse cells (TNCs) have been controversial. Questions pertaining to the existence of the TNC as a "unit" cell with thymocytes completely enclosed within its cytoplasm were the focus of initial debates. Early skeptics proposed the multicellular complex to be an artifact of the procedures used to isolate TNCs from the thymus. Since that time, TNCs have been found in fish, frogs, tadpoles, chickens, sheep, pigs, rats, mice, and humans. Their evolutionary conservation throughout the animal kingdom relieved most speculations about the existence of TNCs and at the same time demonstrated their apparent importance to the thymus and T-cell development. In this review we will discuss and debate reports that describe (i) the organization or structure of TNCs, (ii) the thymocyte subset(s) found within the cytoplasm of TNCs and their uptake and release, and (iii) the function of this fascinating multicellular interaction that occurs during the process of T-cell development. Discussions about the future of the field and experimental approaches that will lead to answers to remaining questions are also presented.

The thymus in the mouse changes its activity during pregnancy: a study of the microenvironment

The mouse thymus changes dramatically during pregnancy. It shrinks in size, and the cortex is extensively reduced from midpregnancy onwards. Despite this, there is surprisingly little evidence for any increase in apoptosis, and considerable evidence that mitosis of thymocytes continues throughout pregnancy. In spite of overall involution the thymic medulla actually expands in midpregnancy due to a combination of mitosis of epithelial cells and an accumulation of lymphocytes. The extent and nature of these changes are examined in this study at the ultrastructural level. The epithelial cells of the subcapsular cortex (type 1 cells) become wrinkled and exhibit powers of phagocytosis, whilst the other cortical epithelial cells are relatively unchanged, although the formation of epithelial/thymocyte rosettes and thymic nurse cells is more clearly seen in midpregnancy than usual. Other changes associated with pregnancy involve the medullary epithelial cells that undergo an increased level of mitosis. Their greater numbers surround accumulations of lymphocytes to form the characteristic medullary epithelial rings. Cell movement through blood vessel walls was clearly observed in midpregnancy, but not at other times. Interdigitating cells in the medulla become more conspicuous as pregnancy proceeds and the cells become phagocytic. The endoplasmic reticulum in plasma cells becomes expanded, indicating increased secretory activity. These results highlight the active nature of the thymus in pregnancy in spite of its involution. This picture contradicts the conventional notion that an involuted thymus is inactive.

Altered aging-related thymic involution in T cell receptor transgenic, MHC-deficient, and CD4-deficient mice

During aging in mice and humans, a gradual decline in thymus integrity and function occurs (thymic involution). To determine whether T cell reactivity or development affects thymic involution, we compared the thymic phenotype in old (12 months) and young (2 months) mice transgenic for rearranged alphabeta or beta 2B4 T cell receptor (TCR) genes, mice made deficient for CD4 by gene targetting (CD4(-/-)), mice made deficient for major histocompatibility complex (MHC) class I (beta2M-/-) or class II genes (A(beta)(b-/-) on C57Bl/6 background) or both. The expected aging-related reductions in thymic weights were observed for all strains except those bearing disruption of both class I and class II MHC genes. Therefore, disruption of MHC class I and class II appeared to reverse or delay aging-related thymic atrophy at 12 months. Immunohistochemical analysis of aging-associated alterations in thymic morphology revealed that TCR alphabeta transgenes, CD4 disruption, and MHC class II disruption all reduced or eliminated these changes. All strains examined at 12 months showed alterations in the distribution of immature thymocyte populations relative to young controls. These results show that aging-associated thymic structural alterations, size reductions, and thymocyte developmental delays can be separated and are therefore causally unrelated. Furthermore, these results suggest that the T cell repertoire and/or its development play a role in aging-related thymic involution.

Cathepsin L: critical role in Ii degradation and CD4 T cell selection in the thymus

Degradation of invariant chain (Ii) is a critical step in major histocompatibility complex class II-restricted antigen presentation. Cathepsin L was found to be necessary for Ii degradation in cortical thymic epithelial cells (cTECs), but not in bone marrow (BM)-derived antigen-presenting cells (APCs). Consequently, positive selection of CD4+ T cells was reduced. Because different cysteine proteinases are responsible for specific Ii degradation steps in cTECs and BM-derived APCs, the proteolytic environment in cells mediating positive and negative selection may be distinct. The identification of a protease involved in class II presentation in a tissue-specific manner suggests a potential means of manipulating CD4+ T cell responsiveness in vivo.

The enigmatic role of glutamyl aminopeptidase (BP-1/6C3 antigen) in immune system development

The murine B-lymphocyte differentiation antigen BP-1/6C3, a homodimeric integral membrane protein composed of M, 140,000 subunits, has been identified as glutamyl aminopeptidase (EAP, EC 3.4.11.7). This ecto-enzyme cleaves acidic amino acid residues from the amino terminal of polypeptide substrates such as angiotensin II and cholecystokinin-8. Although BP-1/6C3/EAP is expressed by cells in many tissues, among hematopoietic cell lineages this ecto-enzyme is restricted to immature B-lineage cells where its expression is upregulated by interleukin-7 and viral transformation. BP-1/6C3/EAP thus serves as a valuable marker of progression along the B-cell differentiation pathway, but a corresponding biological role has not yet been established.

Thymic overexpression of CD40 ligand disrupts normal thymic epithelial organization

We characterized the distribution of CD40 and CD40 ligand (CD40-L) in the adult and developing murine thymus. Before birth, CD40 was almost exclusively localized to scattered foci of medullary cells. By birth there was a dramatic upregulation of CD40 expression by cortical epithelial cells, which was accompanied by a consolidation of medullary epithelial foci. CD40-L+ thymocytes displayed a medullary location. Analysis of mice deficient in CD40-L expression indicated that CD40-L/CD40 interactions were not required for development of the medullary compartment. Overexpression of CD40-L targeted to thymocytes altered thymic architecture, as reflected by a dramatic loss of cortical epithelial cells, expansion of the medullary compartment, and extensive infiltration of the capsule with a mixture of CD3+ cells, B-cells, and macrophages/dendritic cells. Reconstitution of lethally irradiated normal mice with lck CD40-L bone marrow cells also resulted in loss of cortical epithelium and expansion of the medullary compartment. Disruption of the normal pattern of thymic architecture and epithelial differentiation as a consequence of increased intrathymic levels of CD40-L expression points to a role for CD40-L/CD40 interactions in the normal pattern of epithelial compartmentalization/differentiation within the thymic environment.

Mouse thymus dendritic cells: kinetics of development and changes in surface markers during maturation

The early thymus precursor population of adult mice has the capacity to generate T cells, B cells and dendritic cells (DC). These precursors were injected into the thymus of irradiated recipients in order to follow the kinetics of thymic DC development. The resultant cohort of T-lineage cells developing in the thymus was accompanied by a parallel cohort of DC, present at 10(3)-fold lower frequency. The intrathymic lifespan of these DC was as short as that of T-lineage thymocytes. As the thymic DC matured, some markers characteristic of the original precursor population gradually declined (Ly-5, c-kit, Sca-2) whereas markers characteristic of thymic DC appeared and were maintained (major histocompatibility complex class II, CD11c, NLDC-145 and CD8 alpha). Some thymic DC expressed the early B-cell marker BP-1, and BP-1 mRNA, throughout their maturation. The surface markers on thymic DC could be divided into two groups. Some markers, including class I and class II MHC, CD8 alpha and BP-1, appeared to be integral components of the DC surface. In contrast, other markers, including Thy-1, CD4 and CD8 beta, had probably been picked up from associated thymocytes.

Characterization of a 43 kD protein associated to aminopeptidase A from murine kidney

SDS-PAGE of affinity-purified APA under reducing conditions showed in addition to the specific APA band of M(r) 130 kD, a second band of M(r) 43 kD. Internal amino acid sequencing of three tryptic peptides from this second band, that was cut out of the polyacrylamide gel, matched the actin sequence. The identity of the 43 kD band was also confirmed by Western blotting.

Discrimination between thymic epithelial cells and peripheral antigen-presenting cells in the induction of immature T cell differentiation

During their intrathymic migration, immature CD4+ CD8+ thymocytes that express a TCR able to recognize the expressed MHC molecules are positively selected, i.e., complete their differentiation program and become mature T cells. Using the immature CD4+CD8+ T cell line DPK, which can be induced to differentiate in culture, we show here that a subset of isolated thymic epithelial cells, but not peripheral antigen-presenting cells, can induce differentiation, suggesting a unique function of these cells in T cell development. In addition, analysis of activation markers induced by thymic epithelial cells versus specific antigen gives the first direct evidence that positive selection is associated with low level cell activation. In contrast with strict affinity-avidity models of thymic selection, we propose that a specialized antigen-presenting cell environment is an essential contributor to TCR-mediated differentiation in the thymus.

In vitro development of B cells and macrophages from early mouse fetal thymocytes

The developmental potentialities of early mouse fetal thymocytes were analyzed by in vitro culturing cell suspensions obtained from 12-15-gestational day thymus in contact with a bone marrow stroma clonal cell line that supports pre-B and myeloid cell differentiation. B cell and macrophage development from fetal thymocytes was observed at all fetal stages tested, but limiting dilution analysis revealed that the frequency of cells forming colonies on bone marrow stroma is the highest in the fetal thymus at day 12, then dramatically decreases until day 15. These observations suggest that the thymus rudiment is seeded by multipotential precursor cells which are not immediately committed to T cell development in the thymic cellular environment.

Widespread tissue distribution of aminopeptidase A, an evolutionarily conserved ectoenzyme recognized by the BP-1 antibody

Early B-lineage cells in mice express a cell surface glycoprotein, recognized by the BP-1 and 6C3 monoclonal antibodies, that has been identified as aminopeptidase A (APA E.C.3.4.11.7). In the present studies we obtained evidence by DNA "zoo-blot" analysis that the APA gene is highly conserved. This ectoenzyme catalyzes the removal of N-terminal Glu- and Asp-residues to convert angiotensin II to angiotensin III, a degradation step important in local regulation of blood pressure in mammals. To gain further insight into the physiology of this molecule, which is shared between immune and vascular systems, we examined the tissue distribution of BP-1 mRNA using a cDNA probe and of the protein antigen using the BP-1 antibody for immunohistology. APA transcripts were present in all tissues examined. Abundant BP-1/APA was found in the intestinal brush border of the small intestine, renal glomeruli, proximal renal tubules, pulmonary alveolar walls and vascular endothelium in many organs. Other tissues containing the BP-1 antigen included stromal cells in the thymus cortex, bile canaliculi in liver, gall bladder epithelium, interlobular ducts in pancreas, the ovarian theca interna, basement membrane of the epididymis and the splanchnopleure in placenta. APA enzyme activities have been identified in most of these locations, in keeping with identification of the BP-1/6C3 antigen as APA. The data suggest this ectopeptidase may serve diverse physiologic roles in a broad spectrum of tissues.

Phenotypically diverse mouse thymic stromal cell lines which induce proliferation and differentiation of hematopoietic cells

The heterogeneity of the thymic stroma has made careful characterization of particular thymic stromal cell types difficult. To this end, we have derived a panel of cloned thymic stromal cell lines from simian virus 40 T antigen (SV40-T antigen) transgenic mice. Based on their analysis with monoclonal antibodies that distinguish among subsets of thymic stroma cells, and on the morphology and ultrastructural features of the different clones, we suggest that our panel includes representatives of the thymic subcapsular cortex or thymic nurse cells (427.1), the deep cortex or cortical reticular cells (1308.1) and the medulla including medullary interdigitating (IDC)-like cells (6.1.1) and medullary epithelial cells (6.1.7). A fifth cell type of undesignated but apparent medullary origin (6.1.11) was also isolated. All of the cell lines constitutively express the SV40 T antigen transgene and the class I antigens of the major histocompatibility complex (MHC), and they can be induced to express MHC class II antigens upon stimulation with recombinant interferon-gamma (IFN-gamma). These cell lines elaborate a factor(s) that induces the proliferation of cells from the fetal liver and bone marrow, but not from the neonatal thymus. A factor(s) elaborated by the 1308.1 cell line also induces the proliferation of fetal thymocytes in the absence of mitogens, phorbol esters or calcium ionophore which is augmented with the addition of recombinant interleukin-2 (IL-2). Analysis by reverse transcription polymerase chain reaction with primers for some mouse cytokines reveals that each of these cell lines contain granulocyte-macrophage colony-stimulating factor (GM-CSF) transcripts and that 1308.1, 6.1.1 and 6.1.7 produce IL-6 mRNA. Cell lines 1308.1 and 6.1.1 also produce IL-7; 6.1.1 produces IL-1 beta and tumor necrosis factor (TNF)-alpha while the 427.1 cell line produces IL-5 and IFN-gamma mRNA. None of the cell lines tested express the IL-2 receptor, IL-2, IL-3, IL-4, TNF-beta or macrophage inflammatory proteins mRNA. Conditioned medium (CM) from 1308.1 and 6.1.11 induced differentiation of cells purified from the mouse fetal liver into granulocytes; 1308.1 CM also induced differentiation of the mouse hematopoietic stem cell line 32DCl3(G) suggesting that the CM contains granulocyte (G)-CSF activity. Each cell line produces GM-CSF but the greatest activity is associated with 1308.1 and 6.1.11 CM. The availability of these well-characterized, functional, cloned thymic stromal cells will allow a more detailed analysis of the role of each cell type in both myeloid and T cell development.

Histidine residue in the zinc-binding motif of aminopeptidase A is critical for enzymatic activity

The murine BP-1 antigen (also called 6C3) is a homodimeric, phosphorylated cell surface glycoprotein that is expressed on immature B-lineage cells, bone marrow stromal cell lines, thymic cortical epithelial cells, endothelial cells, enterocytes, and renal proximal tubular cells. The amino acid sequence deduced from a BP-1 cDNA predicted a type II integral membrane protein with a zinc-binding motif (His-Glu-Xaa-Xaa-His) found in zinc-dependent metallopeptidases, and functional analysis suggested that BP-1 is aminopeptidase A [APA; L-alpha-aspartyl(L-alpha-glutamyl)-peptide hydrolase, EC 3.4.11.7]. Here we constructed an expression vector in which the BP-1 cDNA was placed downstream from the SR alpha promoter and used this construct to transfect COS-7 and Ltk- cells. Both transfectants expressed the BP-1 antigen on the cell surface and APA activity. The enzymatic activity of recombinant APA was increased by Ca2+ and inhibited by Zn2+, consistent with previous reports with purified APA. Point mutation of one of the histidine residues in the zinc-binding motif to phenylalanine completely abolished APA enzymatic activity, suggesting the structure of the zinc-binding motif of APA is critical for catalytic activity. Both wild-type and mutant BP-1 were glycosylated, transported to the cell surface, and possessed molecular weights similar to native BP-1 molecules on the murine 18.81 pre-B-cell line. The successful expression of both wild-type and mutant APA should allow more precise analysis of the diverse physiological roles of this ectoenzyme.

Two subsets of epithelial cells in the thymic medulla

Information was sought on the features of epithelial cells in the murine thymic medulla. The expression of major histocompatibility complex (MHC) molecules on medullary epithelium was defined by light microscopy with the aid of bone marrow chimeras and MHC-transgenic mice. A proportion of medullary epithelial cells was found to show conspicuously high expression of conventional MHC (H-2) class I (K, D, L) and class II (I-A, I-E) molecules. These cells express a high density of the Y-Ae epitope, a complex of an E alpha peptide and I-Ab molecules found on typical bone marrow-derived cells. MHC+ medullary epithelial cells show limited expression of I-O molecules, a class of atypical nonpolymorphic MHC-encoded class II molecules present on B cells. Other medullary epithelial cells express a high density of I-O molecules but show little or no expression of typical MHC class I or II molecules. MHC and I-O expression thus appear to subdivide medullary epithelial cells into two phenotypically distinct subsets. This applies in adults. In the embryonic thymus most medullary epithelial cells express both types of molecules.

Growth of epithelial cells in the thymic medulla is under the control of mature T cells

Epithelial cells in the thymic medulla are conspicuous in normal adult mice, but sparse in the early fetal thymus and the thymus of adult T cell-deficient SCID mice. To examine whether growth of medullary epithelial cells (MEC) depends upon local contact with mature T cells, we used the finding that the SCID thymus is unusually permeable to mature T cells entering from the bloodstream. When SCID mice received multiple injections of mature lymph node T cells from birth, the thymus accumulated large numbers of mature TCR+ T cells of resting phenotype, but contained virtually no immature (CD4+8+) cells. The injected T cells localized in the medullary region of the thymus and led to marked regeneration of MEC. These and other data suggest that the growth of MEC is under the control of mature T cells. Placing MEC under T cell control might be a device for regulating the size and integrity of the medulla, especially during the phase of rapid thymic growth. Maintaining the cellular components of the medulla in proper balance could be critical for ensuring efficient self tolerance induction.

Thymic hyperplasia in transgenic mice caused by immortal epithelial cells expressing c-kit ligand

To dissect mechanisms that co-ordinate specific events in thymopoiesis we have characterized alterations in thymic structure and function caused by expression of a transgene. This gene encodes SV40Tag and is specifically expressed in a subset of thymic epithelial (TE) cells around birth. As a result the number of immortal TE cells increases, thymic mass increases (up to 3 g), and thymopoiesis is expanded. The latter is reflected by a approximately 100-fold increase of the major thymocyte subsets and increased peripheral T cell counts. Grossly hyperplastic thymi retain many but not all morphological features of a normal thymus. Also in grafts, SV40Tag+ TE cells steer expansion (up to 8 g) and organize a tissue with mainly cortex-like features that includes mainly SV40Tag+ TE cells, thymocytes, and macrophages. To investigate expression of specialized gene functions in the immortal TE cells, a cell line was derived. The Epi-A1 cell line expresses the genes for major histocompatibility complex class I and II, Thy-1, interleukin (IL)-6, IL-7, macrophage-colony-stimulating factor, and transforming growth factor-beta 3. Most importantly, Epi-A1 cells also express the IL-4 receptor and the c-kit ligand (KL), a factor that, in concert with commitment factors, channels progenitors into hemopoietic lineages. The expression of low constitutive levels of KL mRNA does not require IL-4, but KL mRNA levels are increased dramatically in response to IL-4. Since constitutive expression of KL mRNA in vivo is restricted to a small subset of TE cells in the thymus, our findings reveal a novel specific interaction between thymocytes and a specialized subset of TE cells.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphological analysis of the cellular interaction between thymocytes and a thymic stromal cell line (TEL-2)

In a previous study (Nakashima et al., Eur. J. Immunol., 20: 47-53, 1990), a cloned stromal cell line TEL-2 was established from Balb/c mouse thymus. Incubation of thymocytes with TEL-2 cells resulted in the selective elimination of CD4 and CD8 double-positive thymocytes from the culture. In the present report, both phase-contrast and scanning electron microscopes were used to examine, at various time intervals, TEL-2 cells cocultivated with thymocytes in order to elucidate the kinetic sequence of their cellular interaction. The thymocytes attached to the TEL-2 cell surface were more numerous at early times (30 min to 1 h), and their number decreased gradually with time. In contrast, the thymocytes that migrated into the TEL-2 cell layers were less abundant at early times, their number increasing with time thereafter. Destruction of the regular arrangement of TEL-2 cells was found at later than 1 h, suggesting active cellular interaction. The thymocytes adherent to the TEL-2 cell surface were found to be of various shapes and often showed variable profiles, e.g., extending small cytoplasmic processes along the surface of TEL-2 cells or appearing ameboidal. A remarkable feature of the TEL-2 cells was that they formed numerous "round spaces" at the surface of the TEL-2 cell layers. The thymocytes were often located around "round spaces," and some were seen migrating into TEL-2 cell layers through these round spaces. In addition, complementary examinations by transmission electron microscopy revealed that the internalization of thymocytes into TEL-2 cells occurs inside the TEL-2 cell layers after migration.(ABSTRACT TRUNCATED AT 250 WORDS)

Aminopeptidase A activity of the murine B-lymphocyte differentiation antigen BP-1/6C3

The predicted amino acid sequence of the cDNA encoding the murine B-lymphocyte differentiation antigen BP-1/6C3 suggested that it is a member of the zinc-dependent metalloprotease family, possibly an aminopeptidase related to aminopeptidase N [microsomal aminopeptidase; alpha-aminoacyl-peptide hydrolase (microsomal), EC 3.4.11.2]. In the present studies, we examined the enzymatic activity of this antigen. From brush border preparations of the small intestine, a rich source of many endopeptidases and exopeptidases, the BP-1 antibody selectively removed aminopeptidase A [APA; L-alpha-aspartyl(L-alpha-glutamyl)-peptide hydrolase, EC 3.4.11.7] activity. The APA activity of a panel of cell lines correlated in linear fashion with cell-surface levels of the BP-1/6C3 antigen. APA activity was demonstrated for the BP-1/6C3 antigen immunopurified from the pre-B-cell membrane. This activity was enhanced by alkaline earth metals such as Ca2+ and was abrogated by amastatin and angiotensin, which are known competitive inhibitors of APA. The data indicate that the murine BP-1/6C3 antigen is active APA, an enzyme that catalyzes specifically the removal of unsubstituted, N-terminal glutamic acid and aspartic acid residues from peptides.

A developmental switch in thymic lymphocyte maturation potential occurs at the level of hematopoietic stem cells

Hematopoietic stem cells (HSCs) isolated from mouse fetal liver, like adult HSCs, are Thy-1lo Lin- Sca-1+. Donor-derived V gamma 3+ T cells were detected in fetal thymic lobes repopulated in vitro with fetal liver HSCs, but not in those with adult bone marrow HSCs. Single clonogenic fetal HSCs gave rise to thymic progeny that include V gamma 3+, other gamma delta+, and alpha beta+ T cells. No V gamma 3+ T cells were detected in adult thymus injected intrathymically with either fetal or adult HSCs. These results support the hypothesis that only fetal HSCs have the capacity to differentiate into V gamma 3+ T cells in the fetal thymic microenvironment and that the developmental potential of HSCs may change during ontogeny.

Molecular cloning of the murine BP-1/6C3 antigen: a member of the zinc-dependent metallopeptidase family

The BP-1/6C3 antigen is a phosphorylated cell surface glycoprotein that can be identified by monoclonal antibodies on mouse pre-B cells, immature B cells, and certain stromal cell lines from bone marrow. Expression of this antigen is increased in stromal-dependent pre-B cell lines and retrovirally transformed pre-B cells. Expression of the BP-1/6C3 antigen thus correlates with proliferation and transformation of immature B-lineage cells. In this study, we report the isolation and characterization of cDNAs encoding the BP-1/6C3 antigen. Northern blot analysis revealed a major 4.1-kilobase mRNA in all BP-1/6C3+ mouse pre-B lines and in a human pre-B cell line. BP-1/6C3 mRNA was either absent or truncated in BP-1/6C3- cell lines. The cDNA sequence predicts a type II integral membrane protein of 945 amino acids with an intracytoplasmic amino terminus of only 17 amino acids and a typical zinc-binding motif in its extracellular domain. BP-1/6C3 has significant homology to aminopeptidase N and is the second member of the zinc-dependent metallopeptidase gene family to be found on the surface of early B-lineage cells.

Selective elimination of double-positive immature thymocytes by a thymic epithelial cell line

A cloned epithelial cell line, TEL-2, has been established from the stroma tissues of normal mouse thymus. Incubation of mouse thymocytes on TEL-2 cells resulted in the selective elimination of double-positive (CD4+CD8+) cells from the culture, whereas single-positive (CD4+CD8- or CD4-CD8+) thymocytes remaining in the culture were concentrated in non-integrated cell population. The CD3- or CD3 low-positive thymocytes were also eliminated by the TEL-2 cells from the culture, followed by the concentration of CD3 high-positive cells in the culture. Only intact viable thymocytes were integrated into TEL-2 cells. Electron microscopic examination showed that the integrated cells into TEL-2 cytoplasm were gradually degenerated. Mature single-positive T cells, mature B cells or double-negative thymocytes were not integrated into TEL-2 cells. The TEL-2 cell may provide information on the mechanism of selective disappearance of double-positive immature cells from the thymus.