Gluten-sensitive enteropathy in recessive dystrophic epidermolysis bullosa

Recessive dystrophic epidermolysis bullosa (RDEB) is a blistering genodermatosis due to biallelic loss-of-function variants in the type VII collagen (C7) gene (COL7A1). We report the impact of inflammation/autoimmunity on the gut (and other organs) in the nine children with RDEB recruited to an early-phase clinical trial of systemic cell therapy (NCT04153630). This pilot study provides evidence that autoimmunity may play an important role in sustaining chronic inflammation and the coexistence of coeliac disease, which, in turn, could exacerbate anaemia/malnutrition and progression in RDEB. Testing this hypothesis in a larger cohort including children and adults with RDEB and other epidermolysis bullosa (EB) subtypes is warranted so that targeted interventions may improve outcomes.

High BMP4 expression in low/intermediate risk BCP-ALL identifies children with poor outcomes

Pediatric B-cell precursor acute lymphoblastic leukemia (BCP-ALL) outcome has improved in the last decades, but leukemic relapses are still one of the main problems of this disease. Bone morphogenetic protein 4 (BMP4) was investigated as a new candidate biomarker with potential prognostic relevance, and its pathogenic role was assessed in the development of disease. A retrospective study was performed with 115 pediatric patients with BCP-ALL, and BMP4 expression was analyzed by quantitative reverse transcription polymerase chain reaction in leukemic blasts at the time of diagnosis. BMP4 mRNA expression levels in the third (upper) quartile were associated with a higher cumulative incidence of relapse as well as a worse 5-year event-free survival and central nervous system (CNS) involvement. Importantly, this association was also evident among children classified as having a nonhigh risk of relapse. A validation cohort of 236 patients with BCP-ALL supported these data. Furthermore, high BMP4 expression promoted engraftment and rapid disease progression in an NSG mouse xenograft model with CNS involvement. Pharmacological blockade of the canonical BMP signaling pathway significantly decreased CNS infiltration and consistently resulted in amelioration of clinical parameters, including neurological score. Mechanistically, BMP4 favored chemoresistance, enhanced adhesion and migration through brain vascular endothelial cells, and promoted a proinflammatory microenvironment and CNS angiogenesis. These data provide evidence that BMP4 expression levels in leukemic cells could be a useful biomarker to identify children with poor outcomes in the low-/intermediate-risk groups of BCP-ALL and that BMP4 could be a new therapeutic target to blockade leukemic CNS disease.

Beneficial Effect of Systemic Allogeneic Adipose Derived Mesenchymal Cells on the Clinical, Inflammatory and Immunologic Status of a Patient With Recessive Dystrophic Epidermolysis Bullosa: A Case Report

Recessive dystrophic epidermolysis bullosa (RDEB) is an incurable inherited mucocutaneous fragility disorder characterized by recurrent blisters, erosions, and wounds. Continuous blistering triggers overlapping cycles of never-ending healing and scarring commonly evolving to chronic systemic inflammation and fibrosis. The systemic treatment with allogeneic mesenchymal cells (MSC) from bone marrow has previously shown benefits in RDEB. MSC from adipose tissue (ADMSC) are easier to isolate. This is the first report on the use of systemic allogeneic ADMSC, correlating the clinical, inflammatory, and immunologic outcomes in RDEB indicating long-lasting benefits. We present the case of an RDEB patient harboring heterozygous biallelic COL7A1 gene mutations and with a diminished expression of C7. The patient presented with long-lasting refractory and painful oral ulcers distressing her quality of life. Histamine receptor antagonists, opioid analgesics, proton-pump inhibitors, and low-dose tricyclic antidepressants barely improved gastric symptoms, pain, and pruritus. Concomitantly, allogeneic ADMSC were provided as three separate intravenous injections of 106 cells/kg every 21 days. ADMSC treatment was well-tolerated. Improvements in wound healing, itch, pain and quality of life were observed, maximally at 6-9 months post-treatment, with the relief of symptoms still noticeable for up to 2 years. Remarkably, significant modifications in PBL participating in both the innate and adaptive responses, alongside regulation of levels of profibrotic factors, MCP-1/CCL2 and TGF-β, correlated with the health improvement. This treatment might represent an alternative for non-responding patients to conventional management. It seems critical to elucidate the paracrine modulation of the immune system by MSC for their rational use in regenerative/immunoregulatory therapies.

The choroid plexus stroma constitutes a sanctuary for paediatric B-cell precursor acute lymphoblastic leukaemia in the central nervous system

Despite current central nervous system-directed therapies for childhood B-cell precursor acute lymphoblastic leukaemia, relapse at this anatomical site still remains a challenging issue. Few reports have addressed the study of the specific cellular microenvironments which can promote the survival, quiescence, and therefore chemoresistance of B-cell precursor acute lymphoblastic leukaemia cells in the central nervous system. Herein, we showed by immunofluorescence and electron microscopy that in xenotransplanted mice, leukaemic cells infiltrate the connective tissue stroma of the choroid plexus, the brain structure responsible for the production of cerebrospinal fluid. The ultrastructural study also showed that leukaemia cells are able to migrate through blood vessels located in the choroid plexus stroma. In short-term co-cultures, leukaemic cells established strong interactions with human choroid plexus fibroblasts, mediated by an increased expression of ITGA4 (VLA-4)/ITGAL (LFA-1) and their ligands VCAM1/ICAM1. Upon contact with leukaemia cells, human choroid plexus fibroblasts acquired a cancer-associated fibroblast phenotype, with an increased expression of α-SMA and vimentin as well as pro-inflammatory factors. Human choroid plexus fibroblasts also have the capacity to reduce the proliferative index of leukaemic blasts and promote their survival and chemoresistance to methotrexate and cytarabine. The inhibition of VLA-4/VCAM-1 interactions using anti-VLA-4 antibodies, and the blockade of Notch signalling pathway by using a γ-secretase inhibitor partially restored chemotherapy sensitivity of leukaemia cells. We propose that the choroid plexus stroma constitutes a sanctuary for B-cell precursor acute lymphoblastic leukaemia cells in the central nervous system. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Characterization of human fibroblastic reticular cells as potential immunotherapeutic tools

Fibroblastic reticular cells (FRCs) are essential players during adaptive immune responses not only as a structural support for the encounter of antigen-presenting cells and naive T lymphocytes but also as a source of modulatory signals. However, little is known about this cell population in humans. To address the phenotypical and functional analysis of human FRCs here we established splenic (SP) and mesenteric lymph node (LN) CD45^-CD31^-CD90⁺podoplanin⁺ myofibroblastic cell cultures. They shared the phenotypical characteristics distinctive of FRCs, including the expression of immunomodulatory factors and peripheral tissue antigens. Nevertheless, human FRCs also showed particular features, some differing from mouse FRCs, like the lack of nitric oxide synthase (NOS2) expression after interferon (IFN)γstimulation. Interestingly, SP-FRCs expressed higher levels of interleukin (IL)-6, BMP4, CCL2, CXCL12 and Notch molecules, and strongly adapted their functional profile to lipopolysaccharide (LPS), polyinosinic:polycytidylic acid (Poly I:C) and IFNγ stimulation. In contrast, we found higher expression of transforming growth factor (TGF)β and Activin A in LN-FRCs that barely responded via Toll-Like Receptor (TLR)3 and constitutively expressed retinaldehyde dehydrogenase 1 enzyme, absent in SP-FRCs. This study reveals human FRCs can be valuable models to increase our knowledge about the physiology of human secondary lymphoid organs in health and disease and to explore the therapeutic options of FRCs.

A discrete population of IFN λ-expressing BDCA3hi dendritic cells is present in human thymus

Human thymus contains two major subpopulations of dendritic cells (DCs), conventional DCs (cDCs) and plasmacytoid DCs (pDCs), which are mainly involved in central tolerance and also in protecting the thymus against infections. In blood and peripheral organs cDCs include the subpopulation of BDCA3(hi) DCs, considered as equivalents to mouse CD8α(+) DCs. In this study we describe in human thymus the presence of a discrete population of BDCA3(hi) DCs that, like their peripheral counterparts, express CD13, low-intermediate levels of CD11c, CLEC9A, high levels of XCR1, IRF8 and TLR3, and mostly lack the expression of CD11b, CD14 and TLR7. Thymic BDCA3(hi) DCs display immature features with a low expression of costimulatory molecules and HLA-DR, and a low allostimulatory capacity. Also, BDCA3(hi) DCs exhibit a strong response to TLR3 stimulation, producing high levels of interferon (IFN)-λ1 and CXCL10, which indicates that, similarly to thymic pDCs, BDCA3(hi) DCs can have an important role in thymus protection against viral infections.

Blockade of bone morphogenetic protein signaling potentiates the pro-inflammatory phenotype induced by interleukin-17 and tumor necrosis factor-α combination in rheumatoid synoviocytes

Introduction

Bone morphogenetic proteins (BMPs) are multifunctional secreted growth factors regulating a broad spectrum of functions in numerous systems. An increased expression and production of specific BMPs have been described in the rheumatoid arthritis (RA) synovium. The aim of this study was to analyze the involvement of the BMP signaling pathway in RA synoviocytes in response to interleukin-17 (IL-17) and tumor necrosis factor-alpha (TNF-α).

Methods

The expression of components of the BMP signaling pathway (BMP receptors, BMP ligands, BMP signal transducers, and BMP antagonists) was analyzed by quantitative polymerase chain reaction before and after treatment of RA synoviocytes with TNF-α or IL-17 or both. Regulation was studied in the presence of the specific BMP inhibitor DMH1 (dorsomorphin homologue 1) or an exogenous BMP ligand, BMP6. Expression and production of pro-inflammatory cytokines (IL-6 and granulocyte-macrophage colony-stimulating factor), chemokines (IL-8, CCL2, CCL5, and CXCL10), and matrix metalloproteinases (MMP-1, −2, −3, −9, and −13) were analyzed.

Results

RA synoviocytes express BMP receptors (mainly BMPRIA, ACTRIA, and BMPRII), signal transducers of the Smad family (Smad1 and 5 and co-Smad4), and different BMP antagonists. The modulation of the expression of the BMP target genes—Id (inhibitor of DNA-binding/differentiation) proteins and Runx (Runt-related transcription factor) transcription factors—after the addition of exogenous BMP shows that the BMP signaling pathway is active. RA synoviocytes also express BMP ligands (BMP2, BMP6, and BMP7) which are highly upregulated after activation with TNF-α and IL-17. Autocrine BMP signaling pathway can be blocked by treatment with the inhibitor DMH1, leading to an increase in the upregulated expression of pro-inflammatory cytokines, chemokines, and MMPs induced by the activation of RA synoviocytes with TNF-α and IL-17. Conversely, the additional stimulation of the BMP pathway with the exogenous addition of the BMP6 ligand decreases the expression of those pro-inflammatory and pro-destructive factors.

Conclusion

The results indicate that the canonical BMP pathway is functionally active in human RA synoviocytes and that the inhibition of autocrine BMP signaling exacerbates the pro-inflammatory phenotype induced in RA synoviocytes by the stimulation with IL-17 and TNF-α.

Optimal effector functions in human natural killer cells rely upon autocrine bone morphogenetic protein signaling

Natural killer (NK) cells are critical for innate tumor immunity due to their specialized ability to recognize and kill neoplastically transformed cells. However, NK cells require a specific set of cytokine-mediated signals to achieve optimal effector function. Th1-associated cytokines promote effector functions that are inhibited by the prototypic Th2 cytokine IL4 and the TGFβ superfamily members TGFβ1 and activin-A. Interestingly, the largest subgroup of the TGFβ superfamily are the bone morphogenetic proteins (BMP), but the effects of BMP signaling on NK cell effector functions have not been evaluated. Here, we demonstrate that blood-circulating NK cells express type I and II BMP receptors, BMP-2 and BMP-6 ligands, and phosphorylated isoforms of Smad-1/-5/-8, which mediate BMP family member signaling. In opposition to the inhibitory effects of TGFβ1 or activin-A, autocrine BMP signaling was supportive to NK cell function. Mechanistic investigations in cytokine and TLR-L-activated NK cells revealed that BMP signaling optimized IFNγ and global cytokine and chemokine production, phenotypic activation and proliferation, and autologous dendritic cell activation and target cytotoxicity. Collectively, our findings identify a novel auto-activatory pathway that is essential for optimal NK cell effector function, one that might be therapeutically manipulated to help eradicate tumors. Cancer Res; 74(18); 5019-31. ©2014 AACR.

Autocrine activation of canonical BMP signaling regulates PD-L1 and PD-L2 expression in human dendritic cells

Bone morphogenetic proteins (BMPs) are multifunctional growth factors regulating differentiation and proliferation in numerous systems including the immune system. Previously, we described that the BMP signaling pathway is functional in human monocyte-derived dendritic cells (MoDCs), which were found to express both the specific receptors and the Smad proteins required for signal transduction. In this study, we provide evidence that human MoDCs produce BMP-4 and that this production is increased over the maturation process as is BMP signal transduction. When DCs are matured in the presence of an inhibitor of the BMP pathway, the expression of the maturation markers PD-L1 and PD-L2 is reduced, while cytokine production is not affected. As a result, these mature DCs present an augmented ability to stimulate both T cells and NK cells. Eventually, the inhibition of BMP signaling during maturation causes a reduced expression of IRF-1, a transcription factor that positively regulates the expression of PD-L1 and PD-L2. The present study indicates that the BMP signaling pathway regulates PD-L1 and PD-L2 expression in human MoDCs during the maturation process, probably through the IRF-1 transcription factor, and also points out that the manipulation of BMP signaling might considerably improve the immunogenicity of MoDCs used in immunotherapy.

Wnt5a skews dendritic cell differentiation to an unconventional phenotype with tolerogenic features

Dendritic cells (DCs) are critical regulators of immune responses that integrate signals from the innate and adaptive immune system and orchestrate T cell responses toward either immunity or tolerance. Growing evidence points to the Wnt signaling pathway as a pivotal piece in the immune balance and focuses on DCs as a direct target for their immunoregulatory role. Our results show that the increase in Wnt5a signaling during the differentiation of human DCs from monocytes alters their phenotype and compromises their subsequent capacity to mature in response to TLR-dependent stimuli. These Wnt5a-DCs produce scant amounts of IL-12p70 and TNF-α but increased levels of IL-10. Consequently, these Wnt5a-DCs have a reduced capacity to induce Th1 responses that promote IL-10 secretion by CD4 T cells. Changes in the transcriptional profile of Wnt5a-DCs correlate with their unconventional phenotype caused presumably by increased IL-6/IL-10 signaling during the process of DC differentiation. The effect of Wnt5a is not a consequence of β-catenin accumulation but is dependent on noncanonical Ca(2+)/calmodulin-dependent protein kinase II/NF-κB signaling. Our results therefore suggest that under high levels of Wnt5a, typical of the inflammatory state and sepsis, monocytes could differentiate into unconventional DCs with tolerogenic features.

The CXCL12/CXCR4 pair in aged human thymus

CXCL12 is an important CXC chemokine involved in numerous biological processes. We had previously demonstrated the synergistic participation of CXCL12 and IL-7 in the control of both survival and proliferation of CD34(+) human thymic lymphoid progenitors. On this basis, we hypothesize a presumptive role for CXCL12 and its receptor, CXCR4, in the thymus involution. In this respect, in the current report we describe the expression of both molecules in the human thymus during aging. Our results demonstrate that, despite the profound alterations observed in the thymic epithelial microenvironment of aged thymuses, the proportions of different CD4/CD8 thymocyte subsets do not undergo significant variations. Remarkably, a strong CXCL12 expression was found in older thymuses, which appeared in the same locations as in younger thymuses: the subcapsulary and medullary areas. The proportions of CXCR4(+) cells, most of them belonging to the CD3(-) compartment, showed no important variations in the older thymuses. However, within the CD34(+) cell population, a significant reduction in the expression of CXCR4 molecules was observed.

Interplay between BMP4 and IL-7 in human intrathymic precursor cells

Bone morphogenetic proteins (BMPs) play a pivotal role during vertebrate embryogenesis and organogenesis, and have also been described to function in regulating cell fate and determination in self-renewing tissues in adults. Recent results have demonstrated that the different components of the BMP2/4 signaling pathway are expressed in the human thymus. In this study, we provide evidence that BMP4 and IL-7 interplay is important in the maintenance of the human thymic progenitor population. Intrathymic CD34(+) cells express BMP receptors (BMPRIA, BMPRIB, ActRIA, BMPRII), signal transduction molecules (Smad1, 5, 8 and 4), and produce BMP4. Neutralization of endogenous BMP4 by treatment with the antagonist Noggin reduces thymic precursor cell survival, and the addition of exogenous BMP4 decreases their proliferation. The treatment of chimeric human-mouse fetal thymus organ cultures with BMP4 inhibits cell expansion, arrests thymocyte differentiation, and leads to the accumulation of human CD34(+) precursor cells. This effect is mainly attributed to the ability of BMP4 to counteract the IL-7-induced proliferation and differentiation of CD34(+) cells. BMP4 downregulates in the precursor cell population the expression of CD127 and inhibits the IL-7-dependent STAT5 phosphorylation. In addition, BMP signaling is promoted by IL-7. Our results also demonstrate that in thymic progenitors BMPs act downstream of Sonic Hedgehog, previously described to function as a maintenance factor for human intrathymic CD34(+) precursor cells.

Transient beta-catenin stabilization modifies lineage output from human thymic CD34+CD1a- progenitors

Increasing evidence includes Wnt proteins inside the group of master-signaling pathways that govern immune and nonimmune differentiation systems, fundamental for normal development and homeostasis. Although their precise functions in bone marrow and thymus are still controversial, numerous studies have shown that Wnt signaling is able to control the proliferation of hematopoietic stem cells and thymic progenitors and might also affect their cell-fate decisions and subsequent maturation. In the present work, we analyze the effect of transient stimulation of the canonical Wnt pathway in the differentiation potential of Lin(-)CD34(+) CD1a(-) human thymic progenitors, a multipotent and heterogeneous cell population that has the capacity to develop into T cells, NK cells, monocytes, cDC, and pDC. Our results demonstrate that giving a boost to canonical Wnt signaling, triggered by transient exposure to Wnt3a or LiCl, the differentiation capacity of thymic progenitors changes, enhancing NK cell production. On the contrary, Wnt3a- or LiCl-pretreated thymic progenitors generate a significantly lower number of myeloid lineage cells, monocytes, and cDC and exhibit a reduced capacity to differentiate into pDC lineage. As a possible mechanism for this effect, we show that Wnt3a- and LiCl-pretreated progenitors change their membrane levels of receptors for cytokines pivotal for their expansion and differentiation, such as Flt3L. Moreover, canonical Wnt pathway stimulation modifies the transcription factor profile of CD34(+)CD1(-) thymocytes, increasing Hes-1 and ID3 expression levels.

CXCL12/CXCR4 signaling promotes human thymic dendritic cell survival regulating the Bcl-2/Bax ratio

CXCL12, a member of the chemokine CXC subfamily, and its physiologic receptor CXCR4 are essential for the development of various organs during embryonic development and are also involved in the control of cell survival, proliferation and migration in adult tissues. In the human thymus, CXCL12 is produced by epithelial cells located in the subcapsular and medullary regions and CXCR4 is expressed in different thymocyte subpopulations. Several results have demonstrated that CXCL12/CXCR4 signaling participates in different intrathymic processes including the control of human precursor cell survival and proliferation, and the exit of mature thymocytes to the periphery. In this study, we show that CXCL12 is also produced by human thymic dendritic cells (DCs), most of which express CXCR4 receptor. The addition of exogenous CXCL12 significantly inhibited the serum depletion-induced apoptosis in thymic DCs, and the treatment with neutralizing antibodies against CXCL12 or CXCR4 decreased their survival. The survival-promoting effect of CXCL12 was mediated by the up-regulation of Bcl-2 protein expression and the concomitant down-regulation of Bax protein expression. The higher viability of thymic DCs also enhanced their allostimulatory capacity. Taken together, the results suggest a new function of CXCL12 in the human thymus controlling the survival and functionality of thymic DCs.

Survival and function of human thymic dendritic cells are dependent on autocrine Hedgehog signaling

The Hedgehog (Hh) family of signaling molecules functions in the development of numerous tissues during embryogenesis and has also been involved in adult self-renewing tissues. Recent results have demonstrated that the different components of the Hh signaling pathway are expressed in the human thymus. In this study, we investigate whether thymic dendritic cells (DCs) are cell targets for Hh signaling. Both components of the Hh receptor, Patched and Smoothened, as well as other Hh-binding proteins with modulating functions, are expressed by human thymic DCs. The expression of Gli1, Gli2, and Gli3 transcription factors suggests that the Hh signaling pathway is active in thymic DCs, and approximately one-half of thymic DCs produces Sonic Hh (Shh). The culture of thymic DCs with Shh protects them from apoptosis [similarly to CD40 ligand (CD40L)], and these antiapoptotic effects are related to an up-regulation of Bcl-2 and Bcl-X(L) protein expression. The addition of the Hh pathway inhibitor, cyclopamine, decreases DC viability and impairs their allostimulatory function in vitro. In addition, the blockade of the Hh signaling pathway by cyclopamine treatment abrogates the up-regulation of HLA-DR, CD86, CD80, and CD83 expression induced by CD40L on thymic DCs. Finally, we also show that after activation with CD40L thymic DCs down-regulate the expression of Hh receptor components as well as Shh production. Taken together, these results suggest that the survival and function of thymic DCs are regulated by an autocrine Hh signaling.

Bone morphogenetic protein-2/4 signalling pathway components are expressed in the human thymus and inhibit early T-cell development

T-cell differentiation is driven by a complex network of signals mainly derived from the thymic epithelium. In this study we demonstrate in the human thymus that cortical epithelial cells produce bone morphogenetic protein 2 (BMP2) and BMP4 and that both thymocytes and thymic epithelium express all the molecular machinery required for a response to these proteins. BMP receptors, BMPRIA and BMPRII, are mainly expressed by cortical thymocytes while BMPRIB is expressed in the majority of the human thymocytes. Some thymic epithelial cells from cortical and medullary areas express BMP receptors, being also cell targets for in vivo BMP2/4 signalling. The treatment with BMP4 of chimeric human-mouse fetal thymic organ cultures seeded with CD34+ human thymic progenitors results in reduced cell recovery and inhibition of the differentiation of human thymocytes from CD4- CD8- to CD4+ CD8+ cell stages. These results support a role for BMP2/4 signalling in human T-cell differentiation.

Sonic hedgehog is produced by follicular dendritic cells and protects germinal center B cells from apoptosis

The Hedgehog (Hh) signaling pathway is involved in the development of many tissues during embryogenesis, but has also been described to function in adult self-renewing tissues. In the immune system, Sonic Hedgehog (Shh) regulates intrathymic T cell development and modulates the effector functions of peripheral CD4(+) T cells. In this study we investigate whether Shh signaling is involved in peripheral B cell differentiation in mice. Shh is produced by follicular dendritic cells, mainly in germinal centers (GCs), and GC B cells express both components of the Hh receptor, Patched and Smoothened. Blockade of the Hh signaling pathway reduces the survival, and consequently the proliferation and Ab secretion, of GC B cells. Furthermore, Shh rescues GC B cells from apoptosis induced by Fas ligation. Taken together, our data suggest that Shh is one of the survival signals provided by follicular dendritic cells to prevent apoptosis in GC B cells.

Prolactin affects both survival and differentiation of T-cell progenitors

We have analysed the in vitro effects of prolactin on thymocyte development concluding that PRL favours the survival and differentiation of T-cell progenitors. Fetal, adult thymocytes and CD45(+) fetal liver lymphoid progenitors express PRL-R. PRL induces survival, proliferation and differentiation of lymphoid progenitors whereas both an anti-PRL antiserum and an anti-PRL-R mAb block T-cell development accumulating CD25(+)DN (CD4(-)CD8(-)) cells. Furthermore, IL2 rescues the blockade of T-cell development in FTOC treated with anti-PRL antiserum but PRL does not recover cultures treated with an anti-IL2R alpha chain mAb, which drastically blocks the T-cell development. These results support IL2/IL2R mediation of PRL effects on developing thymocytes.

Sonic hedgehog regulates early human thymocyte differentiation by counteracting the IL-7-induced development of CD34+ precursor cells

The Hedgehog (Hh) family of signaling molecules normally functions in the development of numerous tissues by regulating cellular differentiation and proliferation. Recent results have demonstrated that the different components of the Hh signaling pathway are expressed in the human thymus. In this study, we investigate the potential role of Sonic hedgehog (Shh) in human intrathymic T cell maturation. Results show that the expression of the two components of the Hh receptor, Patched and Smoothened, is mostly restricted to CD34+ precursor cells that are committing to the T cell lineage. Shh significantly increased the viability of CD34+ T cell precursors modulating bcl-2 and bax protein expression, and also inhibited their proliferation. The treatment of chimeric human-mouse fetal thymus organ cultures with Shh resulted in an arrested thymocyte differentiation and an accumulation of CD34+ progenitor cells. This effect was mainly attributed to the ability of Shh to counteract the IL-7-induced proliferation and differentiation of CD34+ cells. Shh down-regulated in the precursor cell population the expression of IL-7R as well as stromal-derived factor-1 chemokine receptor, CXCR4, and inhibited IL-7-dependent STAT5 phosphorylation. Therefore, Shh may function as a maintenance factor for intrathymic CD34+ precursor cells.

Prolactin stimulates maturation and function of rat thymic dendritic cells

The current study analyses the effect of PRL, a hormone involved in numerous physiological processes, on dendritic cells (DC) of rat thymus. Most thymic DC express prolactin receptors (PRL-R) as demonstrated by both immunohistochemistry and flow cytometry. PRL administration during 2 or 6 days to fetal thymus organ cultures (FTOC) does not increase the proportions of DC in cultures but stimulates their differentiation. Furthermore, PRL-treated thymic DC exhibit increased allostimulatory capacity in mixed leukocyte reaction (MLR) assays in association with increased surface expression of both MHC antigens and the co-stimulatory molecule CD80. PRL-treated DC also produce increased amounts of pro-inflammatory cytokines, such as IL-12, TNFalpha and IL-1beta, but not of IL6 or IL-10. Our data suggest a key role for IL-12 in the observed changes in the allostimulatory capacity of PRL-treated DC. Also, they permit us to hypothesize about the physiological role played by PRL in thymus ontogeny.

Expression of hedgehog proteins in the human thymus

The Hedgehog (Hh) family of secreted proteins includes intercellular signaling molecules that specify cell fate and patterning during the development of many tissues. In this study we show that the different components of the Hh signaling pathway are expressed in human thymus. The three mammalian Hh proteins, Sonic (Shh), Indian (Ihh), and Desert (Dhh) hedgehog, are produced by thymic epithelial cells. Shh-expressing epithelial cells are restricted to the thymic subcapsula and medulla, whereas Ihh- and Dhh-producing epithelial cells are distributed throughout the thymus. The requisite Hh receptors, Patched 1(Ptc1) and Smoothened (Smo), and the Gli transcription factors are expressed by thymocytes and also by epithelial cells. Ptc1 is expressed in most thymocyte subsets, whereas Smo expression is mainly associated with immature thymocytes. The isoform of the Ptc receptor, Ptc2, is expressed only by intrathymic progenitor cells and epithelial cells. Other Hh-binding proteins with modulating functions, such as Hedgehog-interacting protein (Hip) and growth arrest-specific gene-1 (Gas-1), are also expressed in human thymus. Our study shows that the intrathymic expression pattern of the Hh signaling pathway components is complex and suggests that Hh proteins may regulate human thymocyte differentiation from the earliest developmental stages, as well as thymic epithelial cell function.

Age-dependent changes in thymic macrophages and dendritic cells

Aging is characterized by the decline and deregulation of several physiological systems, especially the immune system. The involution of the thymus gland has been identified as one of the key events that precedes the age-related decline in immune function. Whereas the decrease in thymocyte numbers and in the thymic output during thymus atrophy has been analyzed by various authors, very little information is available about the age-associated modifications in thymic macrophages and dendritic cells. Here we present evidence that these thymic stromal cell components are only slightly affected by age.

The role of morphogens in T-cell development

The Hedgehog (Hh) and Wnt family proteins, and the bone morphogenetic proteins (BMPs) 2 and 4, act as morphogens during vertebrate embryogenesis and organogenesis by regulating patterning and cell fate. They have recently been found to have a role in regulating cell fate and determination in self-renewing tissues in adults, such as the immune system and haematopoietic system. This Review presents studies on the role of Sonic Hh (Shh), Wnts and BMP2/4 in the regulation of thymocyte development. Shh and BMP2/4 act as negative regulators of thymocyte development. By contrast, Wnt signalling, through beta-catenin, has a positive role in the control of T-cell development, such that an absence or reduction in the Wnt signal leads to a reduction in cell number and cell proliferation rate and differentiation to the CD4+CD8+ double-positive stage.

Expression and function of the Eph A receptors and their ligands ephrins A in the rat thymus

Thymus development and function are dependent on the definition of different and graded microenvironments that provide the maturing T cell with the different signals that drive its maturation to a functional T lymphocyte. In these processes, cell-cell interactions, cell migration, and positioning are clues for the correct functioning of the organ. The Eph family of receptor tyrosine kinases and their ligands, the ephrins, has been implicated in all these processes by regulating cytoskeleton and adhesion functioning, but a systemic analysis of their presence and possible functional role in thymus has not yet been conducted. In this regard, the current study combines different experimental approaches for analyzing the expression of four members of the Eph A family and their ligands, ephrins A, in the embryonic and adult rat thymus. The patterns of Eph and ephrin expression in the distinct thymic regions were different but overlapping. In general, the studied Eph A were expressed on thymic epithelial cells, whereas ephrins A seem to be more restricted to thymocytes, although Eph A1 and ephrin A1 are expressed on both cell types. Furthermore, the supply of either Eph A-Fc or ephrin A-Fc fusion proteins to fetal thymus organ cultures interferes with T cell development, suggesting an important role for this family of proteins in the cell mechanisms that drive intrathymic T cell development.

Rat peripheral CD4+CD8+ T lymphocytes are partially immunocompetent thymus-derived cells that undergo post-thymic maturation to become functionally mature CD4+ T lymphocytes

CD4+CD8+ double-positive (DP) T cells represent a minor subpopulation of T lymphocytes found in the periphery of adult rats. In this study, we show that peripheral DP T cells appear among the first T cells that colonize the peripheral lymphoid organs during fetal life, and represent approximately 40% of peripheral T cells during the perinatal period. Later their proportion decreases to reach the low values seen in adulthood. Most DP T cells are small size lymphocytes that do not exhibit an activated phenotype, and their proliferative rate is similar to that of the other peripheral T cell subpopulations. Only 30-40% of DP T cells expresses CD8beta chain, the remaining cells expressing CD8alphaalpha homodimers. However, both DP T cell subsets have an intrathymic origin since they appear in the recent thymic emigrant population after injection of FITC intrathymically. Functionally, although DP T cells are resistant to undergo apoptosis in response to glucocorticoids, they show poor proliferative responses upon CD3/TCR stimulation due to their inability to produce IL-2. A fraction of DP T cells are not actively synthesizing the CD8 coreceptor, and they gradually differentiate to the CD4 cell lineage in reaggregation cultures. Transfer of DP T lymphocytes into thymectomized SCID mice demonstrates that these cells undergo post-thymic maturation in the peripheral lymphoid organs and that their CD4 cell progeny is fully immunocompetent, as judged by its ability to survive and expand in peripheral lymphoid organs, to proliferate in response to CD3 ligation, and to produce IL-2 upon stimulation.

Stromal cell-derived factor 1/CXCR4 signaling is critical for early human T-cell development

The present study investigated the potential role of stromal cell-derived factor 1 (SDF-1) in human intrathymic T-cell differentiation. Results show that SDF-1 is produced by human thymic epithelial cells from the subcapsular and medullary areas, and its receptor, CXCR4, is up-regulated on CD34(+) precursor cells committed to the T-cell lineage. Chimeric human-mouse fetal thymus organ culture (FTOC) seeded with purified CD34(+) thymic progenitors and treated with neutralizing antibodies against SDF-1 or CXCR4 showed a significant reduction of the number of human thymocytes and an arrested thymocyte differentiation in the transition between CD34(+) precursor cells and CD4(+) immature thymocytes. SDF-1-treated FTOC showed an increase of human thymocyte numbers, mainly affecting the most immature subpopulations. Moreover, these results suggest that CXCR4/SDF-1 signaling is not critical for the CD34(+) cell precursor recruitment to the thymus. On the other hand, SDF-1 significantly increased the viability of CD34(+) T-cell precursors modulating the expression of BCL-2 and BAX genes, and stimulated the proliferation of CD34(+) thymic precursor cells, particularly in synergy with interleukin 7 (IL-7), but not with other cytokines, such as stem cell factor or flt3-ligand. Accordingly, only IL-7 was able to up-regulate CXCR4 expression on CD34(+) thymic progenitors. In addition, deprivation of SDF-1 partially inhibited human thymocyte expansion induced by IL-7 in human-mouse FTOC. This study indicates that SDF-1/CXCR4 signaling is required for the survival, expansion, and subsequent differentiation of human early thymocytes and identifies a new mechanism by which IL-7 mediates its effects on human thymopoiesis.

Distinct mechanisms contribute to generate and change the CD4:CD8 cell ratio during thymus development: a role for the Notch ligand, Jagged1

In adult life, the high CD4:CD8 cell ratio observed in peripheral lymphoid organs originates in the thymus. Our results show that the low peripheral CD4:CD8 cell ratio seen during fetal life also has an intrathymic origin. This distinct production of CD4(+)CD8(-) and CD4(-)CD8(+) thymocytes is regulated by the developmental age of the thymic stroma. The differential expression of Notch receptors and their ligands, especially Jagged1, throughout thymus development plays a key role in the generation of the different CD4:CD8 cell ratios. We also show that the intrathymic CD4:CD8 cell ratio sharply changes from fetal to adult values around birth. Differences in the proliferation and emigration rates of the mature thymocyte subsets contribute to this change.

Role of glucocorticoids in early T-cell differentiation

The results of the T-cell differentiation in the progeny of adrenalectomized pregnant rats (Adx fetuses), an experimental model that ensures the absence of glucocorticoids (GCs) during the first stages of development, are summarized. In Adx thymuses there is an accelerated maturation of thymocytes that is reversed by in vivo GC replacement. In addition, Adx thymuses show decreased cell content, which correlates with both the increased numbers of apoptotic cells and an early migration of DP (CD4+CD8+) and SP (both CD4+CD8- and CD4-CD8+) thymocytes to the spleen. As shown by in vitro recolonization assays, accelerated T-cell differentiation is a consequence of changes in the biology of lymphoid precursors occurring in the fetal liver of Adx fetuses. They arrive at the thymic primordium earlier and mature faster than the fetal liver lymphoid progenitors from Sham control fetuses. After the establishment of a fetal hypothalamus-pituitary-gland-adrenal-gland (HPA) axis, there is a gradual normalization of the T-cell development Adx fetuses.

Analysis of the human neonatal thymus: evidence for a transient thymic involution

The neonatal period is marked by the impairment of the major components of both innate and adaptive immunity. We report a severe depletion of cortical CD4+CD8+ double-positive thymocytes in the human neonatal thymus. This drastic reduction in immature double-positive cells, largely provoked by an increased rate of cell death, could be observed as early as 1 day after birth, delaying the recovery of the normal proportion of this thymocyte subset until the end of the first month of postnatal life. Serum cortisol levels were not increased in newborn donors, indicating that the neonatal thymic involution is a physiological rather than a stress-associated pathological event occurring in the perinatal period. Newborn thymuses also showed increased proportions of both primitive CD34+CD1- precursor cells and mature TCRalphabetahighCD69-CD1-CD45RO+/RAdull and CD45ROdull/RA+ cells, which presumably correspond to recirculating T lymphocytes into the thymus. A notable reinforcement of the subcapsular epithelial cell layer as well as an increase in the intralobular extracellular matrix network accompanied modifications in the thymocyte population. Additionally neonatal thymic dendritic cells were found to be more effective than dendritic cells isolated from children's thymuses at stimulating proliferative responses in allogeneic T cells. All these findings can account for several alterations affecting the peripheral pool of T lymphocytes in the perinatal period.

Early maturation of T-cell progenitors in the absence of glucocorticoids

In the present work, we demonstrated that both fetal liver and thymic T-cell precursors express glucocorticoid receptors (GRs) indirectly suggesting a role for glucocorticoids (GCs) in the earliest events of T-cell differentiation. To evaluate this issue, we analyzed the thymic ontogeny in the progeny of adrenalectomized pregnant rats (Adx fetuses), an in vivo experimental model, which ensures the absence of circulating GCs until the establishment of the fetal hypothalamus-pituitary-adrenal (HPA) axis. In the absence of maternal GCs, T-cell development was significantly accelerated, the process being reversed by in vivo GC replacement. Mature single positive thymocytes (both CD4 and CD8) appeared in 16-day old fetal Adx thymus when in the control fetuses, most thymocytes still remained in the double-negative (DN) CD4(-)CD8(-) cell compartment. In addition, emigration of T-cell receptor (TcR)alphabeta positive cells to the spleen also occurred earlier in Adx fetuses than in control ones. In vitro recolonization of cultured deoxiguanosine-treated mouse fetal thymus lobes with 13-day-old fetal liver cell suspensions from both Adx and control fetuses demonstrated changes in the developmental capabilities of fetal liver T-cell precursors from embryos grown in the absence of GCs. Furthermore, a precocious lymphoid colonization of the thymic primordium from Adx fetuses was evidenced by ultrastructural analysis of both Adx and Sham early thymus. Both findings accounted for the accelerated T-cell differentiation observed in Adx fetuses. Together, these results support a role for GCs not only in the thymic cell death, but also in the early steps of T-cell differentiation.

Partial blockade of T-cell differentiation during ontogeny and marked alterations of the thymic microenvironment in transgenic mice with impaired glucocorticoid receptor function

Glucocorticoids (GCs) are widely known to be potent modulators of the immune system. The role of GCs in thymopoiesis as well as the integration of the thymus with the neuroendocrine system is, however, poorly understood. In the present work, we have studied, in transgenic mice with an impaired GC function, the alterations which occur in both T-cell differentiation and thymic stroma maturation, throughout ontogeny as well as in adult condition, analyzing their possible rebounding on the status of adult splenic T lymphocyte populations. These transgenic mice have been described to present a significant decrease (60-70%) of thymic and splenic GC receptor binding capacity but maintain normal their basal plasma ACTH and corticosterone levels. The animals showed a partial blockade of T-cell differentiation and decreased percentages of apoptotic cells during fetal development but not in adult life, when thymic cellularity was significantly increased although thymocyte apoptosis response was not affected. In contrast, thymic stroma was profoundly altered from early fetal stages and large epithelium-free areas appeared in adult thymus. On the other hand, our study revealed a reduction of the splenic TcRalphabeta population accompanied by an increase in the CD4/CD8 ratio. The analysis of different adhesion molecules as well as activation markers demonstrated that most of them (CD5, CD11a, CD11b, CD69 and MHC Class II) were normally expressed in transgenic lymphocytes, whereas CD44 and CD62L expression was altered indicating the existence of an increased proportion of primed T-cells in these animals. In view of the mutual interdependence of thymic stroma and thymocyte maturation, the partial blockade of T-cell differentiation during ontogeny and the profound alterations of the stromal cell compartment in transgenic mice with impaired GR function suggest a key role for GCs in coordinating the physiological dialogue between the developing thymocytes and their microenvironment.

Glucocorticoid-mediated regulation of thymic dendritic cell function

The possible effects of glucocorticoids (GC) on the biology of thymic dendritic cells (DC) have been analyzed. Both DC and GC seem to be involved in intrathymic T cell selection but possible relationships, if any, between them remain currently unknown. For the first time, we have proved the expression of GC receptors in thymic DC. Moreover, our data demonstrate that in vitro dexamethasone (Dex) treatment barely affects the viability of mature thymic DC, which are largely resistant to its apoptotic effect. Dex-treated thymic DC also show a slightly reduced surface expression of some adhesion and co-stimulatory molecules in correlation with diminished allostimulatory properties. Furthermore, the production of both IL-1beta and tumor necrosis factor (TNF)-alpha, but not that of IL-6 and IL-10, diminished in the mixed leukocyte reaction established with Dex-treated thymic DC. However, the addition of recombinant rat IL-1beta and TNF-alpha, alone or in combination, did not recover the allostimulatory capacity. Taken together, these results support certain GC-mediated regulation of the activity of thymic DC which could be relevant for the biology of the thymus gland.

Early differentiation of thymic dendritic cells in the absence of glucocorticoids

The possible role of glucocorticoids (GCs) in the maturation of thymic dendritic cells (DCs) during early ontogeny was analyzed in the progeny of adrenalectomized pregnant rats (Adx foetuses). This experimental model ensured the lack of GCs until establishment of foetal hypothalamus-pituitary gland-adrenal (HPA) axis, and showed profound modifications of the development of thymus gland. In the absence of maternal GCs, there was a high percentage of DCs, many of them exhibiting a mature phenotype, in the 15-16 day-old Adx foetal thymus, which sharply decreased to reach control values on foetal day 17. On the other hand, the absolute number of DCs of Sham foetal rats increased throughout ontogeny, whereas the high numbers found in 15-16 day-old Adx foetuses significantly diminished in the following days. This process was closely correlated with the thymocyte life span, previously demonstrated, and the early appearance of DCs in the spleen. Our results demonstrate that like for other cell components of rat thymus, DC maturation is accelerated in an early foetal microenvironment devoid of glucocorticoids.

Accelerated maturation of the thymic stroma in the progeny of adrenalectomized pregnant rats

The possible role played by glucocorticoids (GCs) in the development of thymic stromal cell components has been studied in the progeny of adrenalectomized pregnant rats (FAdx), an experimental model which ensures the absence of GCs until the establishment of the fetal hypothalamus-pituitary gland-adrenal gland axis. As previously demonstrated for thymocytes, the lack of GCs early in ontogeny results in an accelerated maturation of the thymic stromal elements. Early expression of specific cell markers for thymic epithelial cell subsets and appearance of a well-established cytokeratin-positive epithelial cytoreticulum confirmed the ultrastructural evidence of a faster maturation of the thymic epithelium in FAdx than in FSham. A similar faster and stronger pattern of both class I and class II molecule expression on the epithelial cells occurred in the former fetuses than in control ones. Changes in the pattern of expression of laminin, but not that of fibronectin, throughout thymic maturation also reflected accelerated maturation. Immunohistochemically identified thymic macrophages appeared late in both FSham and FAdx but in higher numbers in these latter indirectly demonstrating their faster development. Finally, the maturation and turnover of thymic dendritic cells showed a remarkable acceleration in the FAdx. In 15- to 16-day-old FAdx thymuses there was a high number of dendritic cells which sharply decreased in the following days suggesting a massive migration to the periphery and/or in situ cell death. In parallel a new wave of dendritic cell progenitors began to differentiate in the FAdx thymuses but not in the FSham ones. The results are discussed from the view of close relationships known to occur between thymocytes and the stromal components, although a direct effect of GCs cannot be discarded.

Interleukin-7 influences the development of thymic dendritic cells

Interleukin-7 (IL-7) has been shown to be a critical factor in B and T lymphopoiesis, and to influence the differentiation of myeloid cell lineages. In the present study we extend these results demonstrating that IL-7 also plays an important role in the development of thymic dendritic cells (DC). The addition of IL-7 to rat fetal thymus organ cultures (FTOC) resulted in a drastic increase in the number of CD3(-)CD4(-)CD8(-) cells, which mostly expressed typical DC markers, including major histocompatibility complex class II, OX-62, CD11b, CD68, and CD54. These cells exhibited morphological and ultrastructural features of DC, and were potent stimulators of the allogeneic mixed leukocyte reaction. Although increased numbers of DC were continuously generated throughout the culture period in the presence of IL-7, they were not actively dividing, indicating that DC in IL-7-treated cultures did not arise by expansion of pre-existing cells. Reduced DC numbers obtained after the addition of neutralizing anti-IL-7 antibodies to mouse FTOC confirmed the relevance of endogenously produced IL-7 on thymic DC development. Furthermore, the addition of IL-7 to FTOC derived from severe combined immunodeficient mice also generated large numbers of DC in the absence of thymocyte maturation.

Expression of functionally active alpha 4 beta 1 integrin by thymic epithelial cells

We have investigated the expression and function of the VLA-4 heterodimer alpha 4 beta 1, a member of the beta 1 integrin subfamily, on human thymic epithelial cells (TEC) derived from cortical epithelium. The expression of the alpha 4 integrin chain was studied in four different cloned TEC lines derived from either fetal or post-natal human thymus by both flow cytometry and immunoprecipitation techniques with anti-alpha 4 MoAbs. All different cell lines assayed expressed significant levels of alpha 4, as revealed by their reactivity with MoAbs specific for distinct alpha 4 epitopes. The alpha 4 subunit expressed by TEC was associated to beta 1 but not to beta 1 chain, and displayed the characteristic 80/ 70 kD pattern of proteolytic cleavage. The VLA-4 integrin in these cells was constitutively active in terms of adhesiveness to both fibronectin and vascular cell adhesion molecule-1 (VCAM-1). In addition, this heterodimer localized to punctate regions of the cell in the area of contact with the substratum, named point contacts assessed by staining with the anti-beta 1 activation epitope 15/7 MoAb. According to the cortical origin of the TEC lines expressing VLA-4, human thymus sections stained with different anti-alpha 4 antibodies revealed the presence of cortical, and in smaller numbers medullary epithelial cells bearing alpha 4 integrin. The expression of alpha 4 in the thymus was also found in both adult and fetal rats, in which epithelial cells were also specifically stained. Altogether, our data show that VLA-4 is an additional component of the integrin repertoire of TEC, and suggest that it could have an important role in thymus epithelial cell-thymocyte interactions.

Histogenesis of the epithelial component of rat thymus: an ultrastructural and immunohistological analysis

BACKGROUND

Despite the assumed importance of thymic cell microenvironments for governing T-cell maturation, little is known about the ontogeny of their cell components. A few studies have analyzed previously the ontogenetical development of rat thymic epithelium (Bogojevic et al. 1990. Period. Biol., 92:126; Kampinga and Aspinall 1990 Harwood Acad. Pub., London, pp. 149-186; Micic et al., 1991 Dev. Comp. Immunol., 15:443-450) and recently we have reported the development of both interdigitating/dendritic cells and macrophages (Vicente et al., 1994 Immunology, 82:75-81, 1995 Immunology, 85:99-105).

METHODS

In the present work we analyze in situ ultrastructural, immunohistochemical, and histoenzymatically the appearance and development of the thymic epithelial cell component in both embryonic and neonatal Wistar rats with special emphasis on the origin of the different epithelial cell types, the occurrence or absence of a common precursor for these, and the expression of MHC molecules.

RESULTS

The thymic primordium of 13-day-old embryos is formed by a homogeneous population of primitive epithelial cells differentiating gradually into various epithelial cell subtypes of both the cortex and the medulla. In the cortex, subcapsular and stroma-supporting epithelial cells appear at days 14-15 as two structurally different cell entities. At the same time, stroma-supporting, keratinized, and vacuolated epithelial cells occur in the thymic medulla. These last two cell types differentiate subsequently into Hassall's bodies and hypertrophied cells. Lympho-epithelial cell complexes are identified in the deep cortex around birth, when the cortical parenchyma houses a transitional erythropoiesis. mAbs (His-39, RMC-20) which recognize medullary epithelial cells in the adult thymus stain positively cells of the thymic primordium as early as day 16 of embryonic life. Cortical epithelial cell markers (His-37, RMC-17) appear, however, slightly later and the subcapsulary region is not established until postnatal life. MHC class I and class II molecules can be identified on epithelial cells in the thymus of 15-day-old embryonic rats although they reach the highest expression around birth.

CONCLUSIONS

Our results confirm the heterogeneity of the thymic epithelial component, the persistence of primitive, non-differentiated epithelial cells morphologically similar to those occurring in the early thymic primordium in adult thymus, and the mutual relevance of epithelial cells and thymocytes for an adequate development of rat thymus gland.

Macrophage-lymphocyte cell clusters in the hypothalamic ventricle of some elasmobranch fish: ultrastructural analysis and possible functional significance

BACKGROUND

Previous studies have demonstrated the existence of lympho-haemopoietic tissue in the meninges and choroid plexuses of various primitive vertebrates, including the stingray Dasyatis akajei and in early human embryos. In the present study, we extend these results analyzing macrophage-lymphocyte cell clusters found in the floor of the hypothalamic ventricle of several specimens of elasmobranchs.

METHODS

After aseptical isolation of the brain from several specimens of smooth dogfish Triakis scyllia, cloudy dogfish Scyliorhinus torazame, gummy shark Mustelus manazo, and stingray Dasyatis akajei their hypothalamic regions were processed routinely by light, scanning, and transmission electron microscopy.

RESULTS

The study of serial histological sections demonstrated that the macrophage-lymphocyte cell clusters proceeded from the meningeal lymphohaemopoietic tissue, reaching the ventricular lumen along large blood vessels. In this tissue, macrophages, different sized lymphocytes, lymphoblasts, granulocytes, monocytes, and developing and mature plasma cells were closely packed among a meshwork of fibroblastic reticular cell processes. It never invaded the brain parenchyma. A cell layer of glial elements and a continuous basement membrane interposed between the lymphoid tissue and the neural elements although some macrophages had migrated across the ependymal cell layer. In the ventricular lumen very irregular macrophages with long cell processes and containing abundant engulfed material of unknown origin formed big cell clusters with neighboring lymphocytes, lymphoblasts, and plasma cells, similar to those described during the immune response. Moreover, electron lucent cells which resembled the antigen-presenting cells of higher vertebrates established intimate surface cell contacts with the surrounding lymphocytes. In the third ventricle of several specimens of gummy shark, Mustelus manazo, morphologically similar cell clusters appeared but these were not connected to the meningeal lympho-haemopoietic tissue. No intraventricular cell aggregates were found in the stingray brain.

CONCLUSIONS

Although we cannot rule out that these macrophage-lymphocyte cell clusters represent a permanent structure in the elasmobranch brain they rather seem to be only established after specific stimulation for preventing the entrance of noxious, foreign materials into the elasmobranch brain parenchyma.

The relevance of cell microenvironments for the appearance of lympho-haemopoietic tissues in primitive vertebrates

In higher vertebrates, mainly in mammals, a role for the non-lymphoid components of lymphoid organs in governing the maturation and functioning of immune system has been largely demonstrated. In contrast, such a role in the evolution of the vertebrate immune system has only been evidenced indirectly. In the present review we summarize histophysiological results which emphasize the relevance of lympho-haemopoietic stromal elements in the emergence and evolution of vertebrate lymphoid organs. The most primitive vertebrates, the Agnatha, have no true lymphoid organs and, accordingly, their immune responses seem more related to the non-anticipatory defence mechanisms of invertebrates than to the immune responses of vertebrates. So, the appearance and evolution of vertebrate lymphoid organs seems closely related with the emergence of immune capacities. Thymus, spleen and gut-associated lymphoid organs appear early in phylogeny whereas lymph nodes and bone marrow are late phylogenetical adquisitions. However, bone marrowless vertebrates contain numerous organs (i.e., gonads, kidney, brain, etc...), the cell microenvironments of which support lympho-haemopoiesis mimicking the condition of higher vertebrate bone marrow. On the other hand, the lack of germinal centres, another feature of the lymphoid organs of ectothermic vertebrates which impedes the selection of B cells raised after somatic hypermutation, presumably reflects the absence of some of the elements necessary for this organization.

Ontogeny of rat thymic macrophages. Phenotypic characterization and possible relationships between different cell subsets

In the present study we combined electron microscopy, immunohistology and primary stromal cell cultures to analyse the ontogeny of rat thymic macrophages (M phi) in an attempt to clarify the relationships between the different macrophage cell subsets described in adult rat thymus. Although phagocytic cells were observed in 15-day-old fetal thymus, monoclonal antibodies (mAb) which recognize different adult macrophage types were unable to identify positive cells until the end of embryonic life. However, our in vitro results from primary thymic stromal cell cultures of 16-day-old fetal rats, and the phenotyping of enriched thymic CD2- cell suspensions, demonstrated that monocyte-like cells which strongly expressed major histocompatibility complex (MHC) class II molecules colonized the embryonic thymus early, giving rise later to distinct macrophage subsets. During the process of maturation, macrophage precursors gradually lost their MHC class II expression, acquired other surface markers (CD45, Thy-1, CD25, CD4, etc.) and increased the acid phosphatase activity. In this respect, ED1+ macrophages, which appeared for the first time in the last stages of embryonic life, consisted of a MHC class II molecule-expressing phagocytic cell population, presumably involved in the elimination of non-selected cortical thymocytes, and of non-phagocytic cells which, in the thymic cortex, might differentiate to ED2+ macrophages throughout ED1+ED2lo/med and ED1+ ED2high intermediate cell stages, observed in vitro in 16-day-old fetal thymic stromal cell cultures. At the end of embryonic life and during the postnatal period the numbers of thymic macrophages increased, particularly in the medulla and corticomedullary border (CMZ), and more slowly in the thymic cortex. This increase was presumably due to the arrival, through perivascular spaces, of new macrophage progenitors, rather than in situ proliferation of pre-existent mature macrophages. The possible function of different thymic macrophage subsets, as well as the relationships between themselves and with their presumptive monocyte-like precursors, are discussed.