The Lymphatic Endothelial mCLCA1 Antibody Induces Proliferation and Growth of Lymph Node Lymphatic Sinuses.

Lymphocyte- and leukocyte-mediated lymph node (LN) lymphatic sinus growth (lymphangiogenesis) is involved in immune responses and in diseases including cancer and arthritis. We previously discovered a 10.1.1 Ab that recognizes the lymphatic endothelial cell (LEC) surface protein mCLCA1, which is an interacting partner for LFA1 and Mac-1 that mediates lymphocyte adhesion to LECs. Here, we show that 10.1.1 Ab treatment specifically induces LEC proliferation, and influences migration and adhesion in vitro. Functional testing by injection of mice with 10.1.1 Ab but not control hamster Abs identified rapid induction of LN LEC proliferation and extensive lymphangiogenesis within 23 h. BrdU pulse-chase analysis demonstrated incorporation of proliferating LYVE-1-positive LEC into the growing medullary lymphatic sinuses. The 10.1.1 Ab-induced LN remodeling involved coordinate increases in LECs and also blood endothelial cells, fibroblastic reticular cells, and double negative stroma, as is observed during the LN response to inflammation. 10.1.1 Ab-induced lymphangiogenesis was restricted to LNs, as mCLCA1-expressing lymphatic vessels of the jejunum and dermis were unaffected by 23 h 10.1.1 Ab treatment. These findings demonstrate that 10.1.1 Ab rapidly and specifically induces proliferation and growth of LN lymphatic sinuses and stroma, suggesting a key role of mCLCA1 in coordinating LN remodeling during immune responses.

Lymphatic endothelial murine chloride channel calcium-activated 1 is a ligand for leukocyte LFA-1 and Mac-1.

The lymphatic circulation mediates drainage of fluid and cells from the periphery through lymph nodes, facilitating immune detection of lymph-borne foreign Ags. The 10.1.1 mAb recognizes a lymphatic endothelial Ag, in this study purified by Ab-affinity chromatography. SDS-PAGE and mass spectrometry identified murine chloride channel calcium-activated 1 (mCLCA1) as the 10.1.1 Ag, a 90-kDa cell-surface protein expressed in lymphatic endothelium and stromal cells of spleen and thymus. The 10.1.1 Ab-affinity chromatography also purified LFA-1, an integrin that mediates leukocyte adhesion to endothelium. This mCLCA1-LFA-1 interaction has functional consequences, as lymphocyte adhesion to lymphatic endothelium was blocked by 10.1.1 Ab bound to endotheliumor by LFA-1 Ab bound to lymphocytes. Lymphocyte adhesion was increased by cytokine treatment of lymphatic endothelium in association with increased expression of ICAM-1, an endothelial surface protein that is also a ligand for LFA-1. By contrast, mCLCA1 expression and the relative contribution of mCLCA1 to lymphocyte adhesion were unaffected by cytokine activation, demonstrating that mCLCA1 and ICAM-1 interactions with LFA-1 are differentially regulated. mCLCA1 also bound to the LFA-1-related Mac-1 integrin that is preferentially expressed on leukocytes. mCLCA1-mediated adhesion of Mac-1- or LFA-1-expressing leukocytes to lymphatic vessels and lymph node lymphatic sinuses provides a target for investigation of lymphatic involvement in leukocyte adhesion and trafficking during the immune response.

Lymph node-resident lymphatic endothelial cells mediate peripheral tolerance via Aire-independent direct antigen presentation.

Peripheral immune tolerance is generally thought to result from cross-presentation of tissue-derived proteins by quiescent tissue-resident dendritic cells to self-reactive T cells that have escaped thymic negative selection, leading to anergy or deletion. Recently, we and others have implicated the lymph node (LN) stroma in mediating CD8 T cell peripheral tolerance. We demonstrate that LN-resident lymphatic endothelial cells express multiple peripheral tissue antigens (PTAs) independent of the autoimmune regulator (Aire). They directly present an epitope derived from one of these, the melanocyte-specific protein tyrosinase, to tyrosinase-specific CD8 T cells, leading to their deletion. We also show that other LN stromal subpopulations express distinct PTAs by mechanisms that vary in their Aire dependence. These results establish lymphatic endothelial cells, and potentially other LN-resident cells, as systemic mediators of peripheral immune tolerance.

Atopic dermatitis-like disease and associated lethal myeloproliferative disorder arise from loss of Notch signaling in the murine skin.

BACKGROUND: The Notch pathway is essential for proper epidermal differentiation during embryonic skin development. Moreover, skin specific loss of Notch signaling in the embryo results in skin barrier defects accompanied by a B-lymphoproliferative disease. However, much less is known about the consequences of loss of Notch signaling after birth. METHODOLOGY AND PRINCIPAL FINDINGS: To study the function of Notch signaling in the skin of adult mice, we made use of a series of conditional gene targeted mice that allow inactivation of several components of the Notch signaling pathway specifically in the skin. We demonstrate that skin-specific inactivation of Notch1 and Notch2 simultaneously, or RBP-J, induces the development of a severe form of atopic dermatitis (AD), characterized by acanthosis, spongiosis and hyperkeratosis, as well as a massive dermal infiltration of eosinophils and mast cells. Likewise, patients suffering from AD, but not psoriasis or lichen planus, have a marked reduction of Notch receptor expression in the skin. Loss of Notch in keratinocytes induces the production of thymic stromal lymphopoietin (TSLP), a cytokine deeply implicated in the pathogenesis of AD. The AD-like associated inflammation is accompanied by a myeloproliferative disorder (MPD) characterized by an increase in immature myeloid populations in the bone marrow and spleen. Transplantation studies revealed that the MPD is cell non-autonomous and caused by dramatic microenvironmental alterations. Genetic studies demontrated that G-CSF mediates the MPD as well as changes in the bone marrow microenvironment leading to osteopenia. SIGNIFICANCE: Our data demonstrate a critical role for Notch in repressing TSLP production in keratinocytes, thereby maintaining integrity of the skin and the hematopoietic system.

Visualization and identification of IL-7 producing cells in reporter mice.

Interleukin-7 (IL-7) is required for lymphocyte development and homeostasis although the actual sites of IL-7 production have never been clearly identified. We produced a bacterial artificial chromosome (BAC) transgenic mouse expressing ECFP in the Il7 locus. The construct lacked a signal peptide and ECFP (enhanced cyan fluorescent protein) accumulated inside IL-7-producing stromal cells in thoracic thymus, cervical thymus and bone marrow. In thymus, an extensive reticular network of IL-7-containing processes extended from cortical and medullary epithelial cells, closely contacting thymocytes. Central memory CD8 T cells, which require IL-7 and home to bone marrow, physically associated with IL-7-producing cells as we demonstrate by intravital imaging.

Lessons from thymic epithelial heterogeneity: FoxN1 and tissue-restricted gene expression by extrathymic, endodermally derived epithelium.

Modeling of thymic epithelial differentiation has been guided by several important underlying assumptions. One is that within epithelial tissues derived from pharyngeal endoderm, FoxN1 expression is signature for the thymic epithelial lineage. Another is that expression of tissue-restricted Ag (TRA) is a unique feature of thymic epithelium. In this murine study, we evaluate the thymic expression of a subset of TRA, parathyroid hormone, calcitonin, and thyroglobulin, as part of an effort to better define the heterogeneity of medullary thymic epithelial cells. In this study, we demonstrate that both conventional and cystic epithelial cells display a history of FoxN1 expression using a cre-lox approach. We also document that extrathymic epithelial tissues that originate from pharyngeal endoderm also have a history of FoxN1 expression, indicating that FoxN1 expression per se is not a signature for the thymic lineage and suggesting that FoxN1 expression, whereas necessary for thymic epithelium, development, is not sufficient for this process to occur. Both cystic and conventional medullary thymic epithelial cells express these TRAs, as do extrathymic epithelial tissues that are not usually considered to be sources of these molecules. This finding supports the proposition that promiscuous gene expression is not unique to the thymus. Furthermore, the pattern of promiscuous gene expression in these extrathymic epithelia is consistent with developmental regulation processes and suggests that it is premature to discard the possibility that some promiscuous gene expression in the thymus reflects normal differentiation programs of epithelia.

Alterations of the medullary epithelial compartment in the Aire-deficient thymus: implications for programs of thymic epithelial differentiation.

A widely held model of thymic epithelial differentiation is based on patterns of keratin expression, where a K8(+)K5(+) progenitor gives rise to K8(+)K5/K14(-) cortical thymic epithelium (CTEC), and medullary thymic epithelium (MTEC) are K8(-)K5(+)K14(+). The thymic phenotype of p63-deficient mice indicates that p63 is an important regulator of proximal stages of thymic epithelial differentiation. In this study, we have examined several features of the thymic medullary compartment in wild-type and Aire-deficient thymi in an effort to integrate the proapoptotic activity of Aire with these different perspectives of TE differentiation. Patterns of keratin and p63 expression by MTEC described here are difficult to reconcile with postmitotic MTEC that express a K8(-)K14(+) phenotype and suggest that the patterns of p63 and keratin expression reflecting differentiation programs of other epithelial tissues provide a useful framework for revising models of TE differentiation. Alterations of the Aire(-/-) MTEC compartment included reduced expression of p63, increased frequency of MTEC expressing truncated Aire protein, and shifts in the pattern of keratin expression and epithelial morphology. These data suggest a scenario where cellular targets of Aire-mediated apoptosis are postmitotic MTEC that have not yet completed their terminal differentiation program. According to this view, the minor population of globular K8(+)K14(-/low) MTEC observed in the Aire(+/+) thymus and significantly expanded in the Aire(-/-) thymic medulla represent end-stage, terminally differentiated MTEC. These Aire-dependent alterations of the MTEC compartment suggest that the activity of Aire is not neutral with respect to the program of MTEC differentiation.

Differentiation of regulatory Foxp3+ T cells in the thymic cortex.

Regulatory Foxp3(+) T cells (T(R)) are indispensable for preventing autoimmune pathology in multiple organs and tissues. During thymic differentiation T cell receptor (TCR)-ligand interactions within a certain increased affinity range, in conjunction with gammac-containing cytokine receptor signals, induce Foxp3 expression and thereby commit developing thymocytes to the T(R) lineage. The contribution of distinct MHC class II-expressing accessory cell types to the differentiation process of Foxp3(+) thymocytes remains controversial, because a unique role in this process has been ascribed to either thymic dendritic cells (tDC) or to medullary thymic epithelial cells (mTEC). Furthermore, it was suggested that the thymic medulla, where the bulk of the negative selection of self-reactive thymocytes takes place, provides a specialized microenvironment supporting T(R) differentiation. Here, we report that the cortex, as defined by cortical thymic epithelial cells (cTEC), is sufficient for supporting T(R) differentiation. MHC class II expression restricted to both cTEC and mTEC or to cTEC alone did not significantly affect the numbers of Foxp3(+) thymocytes. Furthermore, genetic or pharmacologic blockade of thymocyte migration resulted in a prominent accumulation of Foxp3(+) thymocytes in the cortex, demonstrating that secondary signals required for Foxp3 up-regulation exist in the cortex. Our results suggest that mTEC or tDC do not serve as a cell type singularly responsible for T(R) differentiation and that neither the cortex nor the medulla exclusively provides an environment suitable for Foxp3 induction. Instead, multiple accessory cell types probably contribute to the thymic generation of regulatory Foxp3(+) T cells.

Notch-deficient skin induces a lethal systemic B-lymphoproliferative disorder by secreting TSLP, a sentinel for epidermal integrity.

Epidermal keratinocytes form a highly organized stratified epithelium and sustain a competent barrier function together with dermal and hematopoietic cells. The Notch signaling pathway is a critical regulator of epidermal integrity. Here, we show that keratinocyte-specific deletion of total Notch signaling triggered a severe systemic B-lymphoproliferative disorder, causing death. RBP-j is the DNA binding partner of Notch, but both RBP-j-dependent and independent Notch signaling were necessary for proper epidermal differentiation and lipid deposition. Loss of both pathways caused a persistent defect in skin differentiation/barrier formation. In response, high levels of thymic stromal lymphopoietin (TSLP) were released into systemic circulation by Notch-deficient keratinocytes that failed to differentiate, starting in utero. Exposure to high TSLP levels during neonatal hematopoiesis resulted in drastic expansion of peripheral pre- and immature B-lymphocytes, causing B-lymphoproliferative disorder associated with major organ infiltration and subsequent death, a previously unappreciated systemic effect of TSLP. These observations demonstrate that local skin perturbations can drive a lethal systemic disease and have important implications for a wide range of humoral and autoimmune diseases with skin manifestations.

A mechanism for the initiation of allergen-induced T helper type 2 responses.

Both metazoan parasites and simple protein allergens induce T helper type 2 (TH2) immune responses, but the mechanisms by which the innate immune system senses these stimuli are unknown. In addition, the cellular source of cytokines that control TH2 differentiation in vivo has not been defined. Here we showed that basophils were activated and recruited to the draining lymph nodes specifically in response to TH2-inducing allergen challenge. Furthermore, we demonstrate that the basophil was the accessory cell type required for TH2 induction in response to protease allergens. Finally, we show that basophils were directly activated by protease allergens and produced TH2-inducing cytokines, including interleukin 4 and thymic stromal lymphopoietin, which are involved in TH2 differentiation in vivo.

FGFR2IIIb signaling regulates thymic epithelial differentiation.

Heterogeneous epithelial populations comprising the thymic environment influence early and late stages of T-cell development. The processes that regulate the differentiation of thymic epithelium and that are responsible for this heterogeneity are not well understood, although mesenchymal/epithelial interactions are clearly involved. Here, we show that targeted expression by thymocytes of an fibroblast growth factor receptor-2IIIb (FGFR2IIIb) ligand, FGF10, profoundly alters the differentiation and function of thymic epithelium (TE). Reconstitution of irradiated lckFGF10 mice with normal bone marrow restores normal thymic organization and function, while wild-type mice reconstituted with lckFGF10 bone marrow recapitulates some of the thymic alterations seen in lckFGF10 mice. We also demonstrate that interference with FGFR2IIIb signaling in the thymus with a soluble FGFR2IIIb dominant-negative fusion protein leads to precocious reductions in thymic size and cellularity that resemble age-related thymic involution. These findings indicate that TE compartments are dynamically maintained and that FGF signals are involved in this process.

Increased TSLP availability restores T- and B-cell compartments in adult IL-7 deficient mice.

Interleukin 7 (IL-7) plays a crucial role in adult lymphopoiesis, while in fetal life its effect can be partially compensated by TSLP. Whether adult hematopoietic progenitor cells are unresponsive to TSLP or whether TSLP is less available in adult microenvironments is still a matter of debate. Here, we show that increased TSLP availability through transgene (Tg) expression fully restored lymphopoiesis in IL-7-deficient mice: it rescued B-cell development, increased thymic and splenic cellularities, and restored double-negative (DN) thymocytes, alphabeta and gammadelta T-cell generation, and all peripheral lymphoid compartments. Analysis of bone marrow chimeras demonstrated that hematopoietic progenitor cells from adult wild-type mice efficiently differentiated toward B- and T-cell lineages in lethally irradiated IL-7 deficient mice provided TSLP Tg was expressed in these mice. In vitro, TSLP promoted the differentiation of uncommitted adult bone marrow progenitors toward B and T lineages and the further differentiation of DN1 and DN2 thymocytes. Altogether, our results show that adult hematopoietic cells are TSLP responsive and that TSLP can sustain long-term adult lymphopoiesis.

Lack of Foxp3 function and expression in the thymic epithelium.

Foxp3 is essential for the commitment of differentiating thymocytes to the regulatory CD4(+) T (T reg) cell lineage. In humans and mice with a genetic Foxp3 deficiency, absence of this critical T reg cell population was suggested to be responsible for the severe autoimmune lesions. Recently, it has been proposed that in addition to T reg cells, Foxp3 is also expressed in thymic epithelial cells where it is involved in regulation of early thymocyte differentiation and is required to prevent autoimmunity. Here, we used genetic tools to demonstrate that the thymic epithelium does not express Foxp3. Furthermore, we formally showed that genetic abatement of Foxp3 in the hematopoietic compartment, i.e. in T cells, is both necessary and sufficient to induce the autoimmune lesions associated with Foxp3 loss. In contrast, deletion of a conditional Foxp3 allele in thymic epithelial cells did not result in detectable changes in thymocyte differentiation or pathology. Therefore, in mice the only known role for Foxp3 remains promotion of T reg cell differentiation within the T cell lineage, whereas there is no role for Foxp3 in thymic epithelial cells.

Thymic stromal lymphopoietin transgenic mice develop cryoglobulinemia and hepatitis with similarities to human hepatitis C liver disease.

Essential mixed cryoglobulinemia in humans is strongly associated with chronic hepatitis C virus infection. It remains controversial whether liver injury in hepatitis C is primarily attributable to direct viral cytopathic effect or to an immune-mediated response. We characterized the role of cryoglobulinemia in the development of liver disease in thymic stromal lymphopoietin (TSLP) transgenic mice that produce mixed cryoglobulinemia and develop hepatitis. The role of immune complexes in this animal model was evaluated using techniques of light, immunofluorescence, and electron microscopy. To assess the role of Fc receptor engagement in mediation of the disease, TSLP transgenic mice were crossbred with mice deficient for immunoglobulin-binding receptor gamma IIb (FcgammaRIIb). Livers from the TSLP transgenic animals showed mild to moderate liver injury, minimal to mild fibrosis, and deposition of immunoglobulin around the portal tracts. TSLP transgenic mice deficient in inhibitory FcgammaRIIb had more severe hepatitis and accelerated mortality. TSLP-associated hepatitis bears strong similarity to hepatitis C virus-related hepatitis as it occurs in humans, making this a valuable model system of chronic hepatitis and fibrosis to study therapies aimed at manipulating immune responses. Periportal immune complex deposition may play an important role in the pathogenesis of hepatitis occurring in the setting of systemic cryoglobulinemia.

Aire-dependent alterations in medullary thymic epithelium indicate a role for Aire in thymic epithelial differentiation.

The prevalent view of thymic epithelial differentiation and Aire activity holds that Aire functions in terminally differentiated medullary thymic epithelial cells (MTECs) to derepress the expression of structural tissue-restricted Ags, including pancreatic endocrine hormones. An alternative view of these processes has proposed that Aire functions to regulate the differentiation of immature thymic epithelial cells, thereby affecting tissue-restricted Ag expression and negative selection. In this study, we demonstrate that Aire impacts several aspects of murine MTECs and provide support for this second model. Expression of transcription factors associated with developmental plasticity of progenitor cells, Nanog, Oct4, and Sox2, by MTECs was Aire dependent. Similarly, the transcription factors that regulate pancreatic development and the expression of pancreatic hormones are also expressed by wild-type MTECs in an Aire-dependent manner. The altered transcriptional profiles in Aire-deficient MTECs were accompanied by changes in the organization and composition of the medullary epithelial compartment, including a reduction in the medullary compartment defined by keratin (K) 14 expression, altered patterns of K5 and K8 expression, and more prominent epithelial cysts. These findings implicate Aire in the regulation of MTEC differentiation and the organization of the medullary thymic compartment and are compatible with a role for Aire in thymic epithelium differentiation.

A VEGF165-induced phenotypic switch from increased vessel density to increased vessel diameter and increased endothelial NOS activity.

Although vascular endothelial growth factor-165 (VEGF(165)) regulates numerous angiogenic cellular activities, its complex effects on vascular morphology are not highly quantified. By fractal-based, multiparametric branching analysis of 2D vascular pattern in the quail chorioallantoic membrane (CAM), we report that vessel density increased maximally at lower VEGF concentrations, but that vessel diameter and activity of endothelial nitric oxide synthase (eNOS) increased maximally at higher VEGF concentrations. Following exogenous application of human VEGF(165) to the CAM at embryonic day 7, vessel density and diameter were measured after 24 h at arterial end points by the fractal dimension (D(f)) and generational branching parameters for vessel area density (A(v)), vessel length density (L(v)) and vessel diameter (D(v)) using the computer code VESGEN. The VEGF-dependent phenotypic switch from normal vessels displaying increased vessel density to abnormal, dilated vessels typical of tumor vasculature and other pathologies resulted from an approximate threefold increase in VEGF concentration (1.25 to 5 microg/CAM) and correlated positively with increased eNOS activity. Relative to control specimens, eNOS activity increased maximally to 60% following VEGF treatment at 5 microg/CAM, compared to 10% at 1.25 microg/CAM, and was accompanied by no significant change in activity of inducible NOS. In summary, VEGF(165) induced a phenotypic switch from increased vessel density associated with low VEGF concentration, to increased vessel diameter and increased eNOS activity at high VEGF concentration.

Features of medullary thymic epithelium implicate postnatal development in maintaining epithelial heterogeneity and tissue-restricted antigen expression.

Although putative thymic epithelial progenitor cells have been identified, the developmental potential of these cells, the extent of medullary thymic epithelium (mTEC) heterogeneity, and the mechanisms that mediate the expression of a wide range of peripheral tissue-restricted Ags (TRAs) by mTECs remain poorly defined. Here we have defined several basic properties of the mTEC population that refine our understanding of these cells and impose important constraints for any model of mTEC differentiation and function. We report here that mTECs from adult mice are mitotically active, implying continual turnover, differentiation, and replacement of mTEC populations in the adult thymus. The mTEC population in adult thymus expresses transcription factors implicated in the maintenance of multipotential progenitor cell populations, suggesting that epithelial progenitors in the adult thymus may not be restricted to a thymic fate. mTECs also express multiple transcription factors required for the specification of multiple epithelial lineages in peripheral tissues. Thus, expression of some TRAs by mTECs may represent coordinated gene expression that reflects alternate programs of epithelial differentiation among mTECs. Analysis of TRA expression in individual and small pools of sorted mTECs show that mTECs are highly heterogeneous; each individual mTEC expresses a limited spectrum of TRAs, and the frequency of mTECs that express any individual TRA is quite low (>0.4-2%). Collectively, these findings suggest that the differentiation of mTECs can involve some of the developmental programs used by other epithelial lineages and that expression of some TRAs by mTECs may reflect this activity.

Cervical thymus in the mouse.

Although thymic ectopy has long been recognized in humans, the functional activity or potential immunological significance of this thymic tissue is unknown. In this study, we describe murine thymic ectopy, cervical thymic tissue that possesses the same general organization as the thoracic thymus, that is able to support T cell differentiation, and that can export T cells to the periphery. Unexpectedly, the pattern of autoantigen expression by ectopic thymic tissue differs from that of the thoracic thymus, raising the possibility that these two thymic environments may project different versions of self.

Developmental regulation of Foxp3 expression during ontogeny.

Thymectomy of neonatal mice can result in the development of autoimmune pathology. It has been proposed that thymic output of regulatory T (T reg) cells is delayed during ontogeny and that the development of autoimmune disease in neonatally thymectomized mice is caused by the escape of self-reactive T cells before thymectomy without accompanying T reg cells. However, the kinetics of T reg cell production within the thymus during ontogeny has not been assessed. We demonstrate that the development of Foxp3-expressing T reg cells is substantially delayed relative to nonregulatory thymocytes during ontogeny. Based on our data, we speculate that induction of Foxp3 in developing thymocytes and, thus, commitment to the T reg cell lineage is facilitated by a signal largely associated with the thymic medulla.

An organized medullary epithelial structure in the normal thymus expresses molecules of respiratory epithelium and resembles the epithelial thymic rudiment of nude mice.

The expression of tissue-specific Ags (TSA) within the thymic environment has emerged as an important contribution to the establishment of self-tolerance. The mechanistic basis for this property is poorly understood. One model has proposed stochastic derepression of gene expression by mature medullary epithelial cells, whereas another model has suggested that this property of thymic epithelial cells reflects transcriptional activity during their differentiation. Most of the analyses of thymic TSA expression have been done with populations of dissociated thymic epithelial cells; therefore, there is little information regarding the spatial pattern of TSA expression within the thymus. We have evaluated a subset of thymic epithelial cells in the murine thymus that display several unique features. First, within the normal thymus, they form cysts that express several TSA of respiratory epithelium and exhibit some morphological features consistent with respiratory epithelium. These cells also display a phenotypic profile that has been proposed for immature thymic epithelium. The cystic epithelia in the normal thymus and in the nude thymic rudiment are phenotypically very similar, suggesting that they may have a similar developmental program. The coordinated expression of respiratory TSA by an organized subset of thymic epithelial cells and the phenotypic resemblance of these cells to progenitor cells seem consistent with a developmental basis for TSA expression by thymic epithelial cells. Finally, epitopes that define thymic epithelial heterogeneity are reciprocally expressed by respiratory epithelium, which raises interesting questions regarding the developmental relationship of different endodermal derivatives.