Regulation of peripheral lymph node genesis by the tumor necrosis factor family member TRANCE

The importance of regional lymph nodes for mucosal tolerance

The immune system is organized as a number of distinct lymphoid organs interconnected by recirculating lymphocytes. These organs, such as lymph nodes, spleen, and gut-associated Peyer's patches, are compartmentalized, providing separate niches for T and B cells. In addition, regional compartmentalization of lymphoid organs themselves exists, leading to the distinction between the mucosal and the systemic immune systems. This distinction not only reflects the anatomical localization but also is based on functional differences, with predominant tolerance induction via mucosal routes and immunity seen after systemic antigen exposure. These differences are associated with regional differences in the lymphoid organs and with environmental conditions of the tissues in which the immune system functions. Recirculation patterns of lymphocytes differ between mucosal and systemic lymphoid organs, and more insight into the mechanisms that imprint this behavior has been generated recently. Differences in dendritic cells have been observed between mucosal and systemic sites, and knowledge on how local factors contribute to the immune system is emerging. From our studies on mucosal tolerance in mouse models, it has become evident that regional lymph nodes draining the mucosa are important sites to direct immune responses. Here, we discuss the way regional lymph nodes contribute to the direction of immune responses and what is known about the local factors and cell behavior that form the basis for these differences.

Neonatal and adult CD4+ CD3- cells share similar gene expression profile, and neonatal cells up-regulate OX40 ligand in response to TL1A (TNFSF15)

We report here the quantitative expression of a set of immunity-related genes, including TNF family members, chemokine receptors, and transcription factors, in a CD4+ CD3- accessory cell. By correlating gene expression between cell-sorted populations of defined phenotype, we show that the genetic fingerprint of these CD4+ CD3- cells is distinct from dendritic cells, plasmacytoid dendritic cells, T cells, B cells, and NK cells. In contrast, it is highly similar to CD4+ CD3- cells isolated from embryonic and neonatal tissues, with the exception that only adult populations express OX40L and CD30L. We have previously reported that IL-7 signals regulate CD30L expression. In the present study, we show that both neonatal and adult CD4+ CD3- cells express the TNF family member, death receptor 3 (TNFRSF25), and that addition of TL1A (TNFSF15), the ligand for death receptor 3, up-regulates OX40L on neonatal CD4+ CD3- cells. Finally, we demonstrate that this differentiation occurs in vivo: neonatal CD4+ CD3- cells up-regulate both CD30L and OX40L after adoptive transfer into an adult recipient.

Molecular Networks Orchestrating GALT Development

During evolution, the development of secondary lymphoid organs has evolved as a strategy to promote adaptive immune responses at sites of antigen sequestration. Mesenteric lymph nodes (LNs) and Peyer's patches (PPs) are localized in proximity to mucosal surfaces, and their development is coordinated by a series of temporally and spatially regulated molecular events involving the collaboration between hematopoietic, mesenchymal, and, for PPs, epithelial cells. Transcriptional control of cellular differentiation, production of cytokines as well as adhesion molecules are mandatory for organogenesis, recruitment of mature leukocytes, and lymphoid tissue organization. Similar to fetal and neonatal organogenesis, lymphoid tissue neoformation can occur in adult individuals at sites of chronic stimulation via cytokines and TNF-family member molecules. These molecules represent new therapeutic targets to manipulate the microenvironment during autoimmune diseases.

Interaction of mature CD3+CD4+ T cells with dendritic cells triggers the development of tertiary lymphoid structures in the thyroid

Ectopic expression of CC chemokine ligand 21 (CCL21) in the thyroid leads to development of lymphoid structures that resemble those observed in Hashimoto thyroiditis. Deletion of the inhibitor of differentiation 2 (Id2) gene, essential for generation of CD3-CD4+ lymphoid tissue-inducer (LTi) cells and development of secondary lymphoid organs, did not affect formation of tertiary lymphoid structures. Rather, mature CD3+CD4+ T cells were critical for the development of tertiary lymphoid structures. The initial stages of this process involved interaction of CD3+CD4+ T cells with DCs, the appearance of peripheral-node addressin-positive (PNAd+) vessels, and production of chemokines that recruit lymphocytes and DCs. These findings indicate that the formation of tertiary lymphoid structures does not require Id2-dependent conventional LTis but depends on a program initiated by mature CD3+CD4+ T cells.

Abnormal lymphoid organ development in immunodeficient mutant mice

Development of the primary and secondary lymphoid organs is a tightly controlled process. These tissues are highly organized to maximize efficiency of the immune response. Spontaneous and targeted mutations in laboratory mice have led to better understanding of the molecular interactions and signaling pathways essential to the development and organization of lymphoid tissues, and the functional consequences of loss or disruption of the normal structures. On the basis of studies of mutations in mice and other species, it has been determined that a wild-type allele of the Foxn1 gene is required for normal thymic development and function. The Tlx1, Bapx1, Tcf21, Wt1 and Dh genes are essential for development of the spleen, while mutations of Nkx2-3, Lta, Ltb, Ltbr, Map3k14, Relb, Tnf, Tnfrsf1a, Cxcl13, Blr1 (Cxcr5), or cpdm genes result in disruption of normal splenic microarchitecture. The requirements for organized lymph nodes vary according to anatomic location, but most rely on Id2 (Idb2) and Rorc, in addition to lymphotoxins and Tnfrsf11a, Tnfsf11, Relb, Map3k14, Cxcl13, and Blr1 genes. Development of Peyer's patches is dependent on Id2 and Rorc genes, lymphotoxins, and Relb, Map3k14, Il7r, and cpdm genes. Less is known about the requirements for nasal-associated lymphoid tissues (NALT), but Id2 is a requirement. Here we review abnormalities of lymphoid organ development in immunodeficient mutant mice, including spontaneous and targeted mutations of Id2, Rorc, Tnf, Tnfrsf1a, Lta, Ltb, Ltbr, Tnfrsf11a, Tnfsf11, Relb, Map3k14, IL7r, Blr1, and Cxcl13 genes.

The alternative NF-kappaB pathway from biochemistry to biology: pitfalls and promises for future drug development

The past two decades have led to a tremendous work on the transcription factor NF-kappaB and its molecular mechanisms of activation. The nuclear translocation of NF-kappaB is controlled by two main pathways: the classical and the alternative NF-kappaB pathways. The classical NF-kappaB pathway activates the IKK complex that controls the inducible degradation of most IkappaB family members that are IkappaBalpha, IkappaBbeta, IkappaBvarepsilon and p105. The alternative NF-kappaB pathway induces p100 processing and p52 generation through the activation of at least two kinases, which are NIK and IKKalpha. Genetic studies have shown that IKKgamma is dispensable for the alternative pathway, which suggests the existence of an alternative IKKalpha-containing complex. It is noteworthy that activation of particular p52 heterodimers like p52/RelB requires solely the alternative pathway while activation of p52/p65 or p52/c-Rel involves a "hybrid pathway". Among others, LTbetaR, BAFF-R, CD40 and RANK have the ability to induce the alternative pathway. The latter plays some roles in biological functions controlled by these receptors, which are the development of secondary lymphoid organs, the proliferation, survival and maturation of B cell, and the osteoclastogenesis. Exacerbated activation of the alternative pathway is potentially associated to a wide range of disorders like rheumatoid arthritis, ulcerative colitis or B cell lymphomas. Therefore, inhibitors of the alternative pathway could be valuable tools for the treatment of inflammatory disorders and cancers.

Role of T and NK cells and IL7/IL7r interactions during neonatal maturation of lymph nodes

Lymph node (LN) development depends on prenatal interactions occurring between LN inducer and LN organizer cells. We have distinguished defects in LN formation due to failure in embryonic development (aly/aly) from defects in postnatal maturation (Il2rgamma(-/-)Rag2(-/-)). Both mutant strains form normal primordial LNs with differing fate. In aly/aly mice, the LN primordium dissipates irreversibly late in gestation; in contrast, Il2rgamma(-/-)Rag2(-/-) LN anlage persists for a week after birth but disperses subsequently, a process reversible by neonatal transfer of WT IL7r(+) TCR(+) T or natural killer (NK) cells, suggesting a role for IL7/IL7r interactions. Thus, we reveal a unique stage of postnatal LN development during which mature lymphocytes and IL7/IL7r interactions may play an important role.

OSTEOIMMUNOLOGY: Interplay Between the Immune System and Bone Metabolism

Studies of bone and the immune system have converged in recent years under the banner of osteoimmunology. The immune system is spawned in the bone marrow reservoir, and investigators now recognize that important niches also exist there for memory lymphocytes. At the same time, various factors produced during immune responses are capable of profoundly affecting regulation of bone. Mechanisms have evolved to prevent excessive interference by the immune system with bone homeostasis, yet pathologic bone loss is a common sequela associated with autoimmunity and cancer. There are also developmental links, or parallels, between bone and the immune system. Cells that regulate bone turnover share a common precursor with inflammatory immune cells and may restrict themselves anatomically, in part by utilizing a signaling network analogous to lymphocyte costimulation. Efforts are currently under way to further characterize how these two organ systems overlap and to develop therapeutic strategies that benefit from this understanding.

Tumor-infiltrating T lymphocytes: friends or foes?

The prognostic significance of tumor-infiltrating lymphocytes (TILs) has been a longstanding topic of debate. In cases where TILs have improved patient outcome, T lymphocytes are recognized as the main effectors of antitumor immune responses. However, recent studies have revealed that a subset of CD4(+) T cells, referred to as CD4(+)CD25(+) regulatory T cells (Treg), may accumulate in the tumor environment and suppress tumor-specific T-cell responses, thereby hindering tumor rejection. Hence, predicting tumor behavior on the basis of an indiscriminate evaluation of tumor-infiltrating T cells may result in inconsistent prognostic accuracy. The presence of infiltrating CD4(+)CD25(+) Treg may be detrimental to the host defense against the tumor, while the presence of effector T lymphocytes, including CD8(+) T cells and non-regulatory CD4(+) helper T cells may be beneficial. Enhanced recruitment of antitumor effector T lymphocytes to tumor tissue in addition to inhibition of local Treg, may therefore be an ideal target for improving cancer immunotherapy. This article reviews the antitumor functions of T-lymphocytes, with special attention given to CD4(+) regulatory T-cells within the tumor environment.

Osteoclast differentiation independent of the TRANCE-RANK-TRAF6 axis

Osteoclasts are derived from myeloid lineage cells, and their differentiation is supported by various osteotropic factors, including the tumor necrosis factor (TNF) family member TNF-related activation-induced cytokine (TRANCE). Genetic deletion of TRANCE or its receptor, receptor activator of nuclear factor κB (RANK), results in severely osteopetrotic mice with no osteoclasts in their bones. TNF receptor-associated factor (TRAF) 6 is a key signaling adaptor for RANK, and its deficiency leads to similar osteopetrosis. Hence, the current paradigm holds that TRANCE–RANK interaction and subsequent signaling via TRAF6 are essential for the generation of functional osteoclasts. Surprisingly, we show that hematopoietic precursors from TRANCE-, RANK-, or TRAF6-null mice can become osteoclasts in vitro when they are stimulated with TNF-α in the presence of cofactors such as TGF-β. We provide direct evidence against the current paradigm that the TRANCE–RANK–TRAF6 pathway is essential for osteoclast differentiation and suggest the potential existence of alternative routes for osteoclast differentiation.

Protection against osteoporosis by active immunization with TRANCE/RANKL displayed on virus-like particles

TNF-related activation-induced cytokine (TRANCE), also known as receptor activator of NF-kappaB ligand (RANKL), is the key molecule responsible for the bone loss observed in osteoporosis. Passive administration of osteoprotegerin, the soluble decoy receptor of TRANCE/RANKL, is efficient in blocking disease progression, but may not find widespread clinical use due to patient compliance problems and the expected high costs. In this study, we describe an efficient, safe, and potentially cost-effective active immunization strategy against TRANCE/RANKL. We show in mice that immunization with TRANCE/RANKL covalently linked to virus-like particles can overcome the natural tolerance of the immune system toward self proteins and produce high levels of specific Abs without the addition of any adjuvant. Serum Abs of immunized mice neutralized TRANCE/RANKL activity in vitro and were highly active in preventing bone loss in a mouse model of osteoporosis. Active immunization against TRANCE/RANKL was essentially reversible and did not produce any measurable immunosuppressive side effects, underscoring its potential as a new therapeutic approach to the treatment of human bone-degenerative disorders.

OX40 ligand and CD30 ligand are expressed on adult but not neonatal CD4+CD3- inducer cells: evidence that IL-7 signals regulate CD30 ligand but not OX40 ligand expression

In this report, we have examined the expression of the T cell survival signals, OX40 ligand (OX40L) and CD30 ligand (CD30L) on CD4(+)CD3(-)CD11c(-)B220(-)IL-7Ralpha(+) inducer cells from birth to adulthood in mice. We found that adult but not neonatal inducer cells expressed high levels of OX40L and CD30L, whereas their expression of TNF-related activation-induced cytokine (TRANCE) and receptor activator of NF-kappaB (RANK) was comparable. The failure of neonatal inducer cells to express the ligands that rescue T cells helps to explain why exposure to Ag in neonatal life induces tolerance rather than immunity. The expression of OX40L and CD30L on inducer cells increased gradually in the first few weeks of life achieving essentially normal levels around the time mice were weaned. We found that IL-7 signaling through the common cytokine receptor gamma-chain was critical for the optimal expression of both TNF-related activation-induced cytokine and CD30L but not OX40L. Furthermore, glucocorticoids, which potently suppress T effector function, did not influence the expression of OX40L and CD30L in the presence of IL-7.

Organizing a mucosal defense

Gastrointestinal associated lymphoid tissue can be divided into loosely organized effector sites, which include the lamina propria and intraepithelial lymphocytes, and more organized structures, such as mesenteric lymph nodes (LNs), Peyer's patches (PPs), isolated lymphoid follicles, and cryptopatches (CPs). These organized structures in the gastrointestinal tract have been hypothesized to play the role of primary lymphoid organ, supporting the extrathymic development of T lymphocytes (CPs), secondary lymphoid organs involved in the induction of the mucosal immune response (PPs), and tertiary lymphoid structures whose function is still under debate (isolated lymphoid follicles). The most widely studied lymphoid structure found in the small intestine is the PP. PPs are secondary lymphoid structures, and their development and function have been extensively investigated. However, single lymphoid aggregates resembling PPs have been also described in humans and in the murine small intestines. These isolated lymphoid follicles have both germinal centers and an overlying follicle-associated epithelium, suggesting that they also can function as inductive sites for the mucosal immune response. This review compares and contrasts the development and function of the four main organized gastrointestinal lymphoid tissues: CPs, isolated lymphoid follicles, PPs, and mesenteric LNs.

Fate and function of lymphoid tissue inducer cells

The discovery that Peyer's patch and lymph node development is regulated by the collaboration between fetal hematopoietic cells and mesenchymal cells has thrown new light on our understanding of the mechanisms underlying the formation of lymphoid organs. Lymphoid tissue inducer cells trigger a coordinated series of events leading to cell clustering and changes in gene expression and differentiation. Nevertheless, many questions regarding the origin, recruitment and fate of the inducer cells and cellular crosstalk with neighboring cells remain unanswered.

Adipose tissue and the immune system

Adipocytes anatomically associated with lymph nodes (and omental milky spots) have many special properties including fatty acid composition and the control of lipolysis that equip them to interact locally with lymphoid cells. Lymph node lymphocytes and tissue dendritic cells acquire their fatty acids from the contiguous adipocytes. Lymph node-derived dendritic cells suppress lipolysis in perinodal adipocytes but those that permeate the adipose tissue stimulate lipolysis, especially after minor, local immune stimulation. Inflammation alters the composition of fatty acids incorporated into dendritic cells, and that of node-containing adipose tissue, counteracting the effects of dietary lipids. Thus these specialised adipocytes partially emancipate the immune system from fluctuations in the abundance and composition of dietary lipids. Prolonged, low-level immune stimulation induces the local formation of more adipocytes, especially adjacent to the inflamed lymph node. This mechanism may contribute to hypertrophy of the mesentery and omentum in chronic inflammatory diseases such as HIV-infection, and in smokers. Paracrine interactions between adipose and lymphoid tissues are enhanced by diets rich in n-6 fatty acids and attentuated by fish oils. The latter improve immune function and body conformation in animals and people. The partitioning of adipose tissue in many depots, some specialised for local, paracrine interactions with other tissues, is a fundamental feature of mammals.

Network communications: lymphotoxins, LIGHT, and TNF

Lymphotoxins (LT) provide essential communication links between lymphocytes and the surrounding stromal and parenchymal cells and together with the two related cytokines, tumor necrosis factor (TNF) and LIGHT (LT-related inducible ligand that competes for glycoprotein D binding to herpesvirus entry mediator on T cells), form an integrated signaling network necessary for efficient innate and adaptive immune responses. Recent studies have identified signaling pathways that regulate several genes, including chemokines and interferons, which participate in the development and function of microenvironments in lymphoid tissue and host defense. Disruption of the LT/TNF/LIGHT network alleviates inflammation in certain autoimmune disease models, but decreases resistance to selected pathogens. Pharmacological disruption of this network in human autoimmune diseases such as rheumatoid arthritis alleviates inflammation in a significant number of patients, but not in other diseases, a finding that challenges our molecular paradigms of autoimmunity and perhaps will reveal novel roles for this network in pathogenesis.

Upregulation of receptor activator of nuclear factor-kappaB ligand expression in the thymic subcapsular, paraseptal, perivascular, and medullary epithelial cells during thymus regeneration

The receptor activator of nuclear factor (NF)-kappaB ligand (RANKL; also termed TRANCE/OPGL/ODF/TNFSF11), a new member of the tumor-necrosis factor (TNF) superfamily, was identified as a key cytokine involved in the differentiation of the immune system and the regulation of immunity as well as in bone metabolism. In particular, RANKL-deficient mice showed defects in the early differentiation of T lymphocytes, suggesting that RANKL is a novel regulator of early thymocyte development. Here, we describe the expression of RANKL during regeneration following acute involution induced by cyclophosphamide in the rat thymus. The present study demonstrates the presence and upregulated expression of the RANKL in thymic subcapsular, paraseptal, perivascular, and medullary epithelial cells during thymus regeneration. Our results suggest that the RANKL expressed in these thymic epithelial cells plays a role in the development of T cells during thymic regeneration.

Cellular Interactions in Lymph Node Development

The organized accumulation of lymphocytes is a biological phenomenon used to optimize both homeostatic immune surveillance, as well as chronic responses to pathogenic stimuli. During embryonic development, circulating hemopoietic cells gather at predestined sites throughout the body, where they are subsequently arranged in T and B cell-specific areas characteristic of secondary lymphoid organs. In contrast, the body seems to harbor a limited second set of selected sites that support formation of organized lymphoid aggregates. However, these are only revealed at times of local, chronic inflammation, when so-called tertiary lymphoid structures appear. Once thought of as two distinct phenomena, recent insights suggest that highly similar networks of paracrine interactions regulate the formation of both secondary and tertiary lymphoid structures. This review will focus on these cellular interactions between organizing and inducing cell populations leading to the formation of lymph nodes or organized inflammatory infiltrates.

Induction of secondary and tertiary lymphoid structures in the skin

During embryogenesis a developmental program leading to the formation of lymph nodes and Peyer's patches is initiated. We now show that lymph node-like structures as well as tertiary lymphoid structures can ectopically be induced by intradermal injection of newborn lymph node-derived cells. ICAM-1/VCAM-1-expressing stromal organizers, follicular dendritic cells, lymphatic endothelium, and HEVs in these structures are of donor origin, while all hematopoietic cells are host derived. Formation depends on lymphotoxin-expressing donor cells, whereas further organization requires lymphotoxin-expressing recipient cells. While induced secondary lymphoid structures develop a normal cellular architecture, the degree of organization in tertiary structures is correlated to the immune activation status of the host. These results indicate that the cellular and molecular requirements for the establishment of lymph nodes and tertiary structures are remarkably similar and that hyperactivated lymphocytes can fulfill the role of lymphoid tissue inducer cells during inflammatory responses.

Initiation of cellular organization in lymph nodes is regulated by non-B cell-derived signals and is not dependent on CXC chemokine ligand 13

The molecular and cellular events that initiate the formation of T and B cell areas in developing lymph nodes are poorly understood. In this study we show that formation of the lymphoid architecture in murine neonatal lymph nodes evolves through a series of distinct stages. The initial segregation of T and B cells is regulated in a CXCL13-independent manner, characterized by the localization of B cells in a ring-like pattern in the outer cortex on day 4. However, during this CXCL13-independent phase of lymph node modeling, CXCL13 is expressed and regulated in a lymphotoxin-alpha1beta2 (LTalpha1beta2)-dependent manner. Surprisingly, neonatal B cells are unable to respond to this chemokine and also lack surface LTalpha1beta2 expression. At this time, CD45+CD4+CD3- cells are the predominant LTalpha1beta2-expressing cells and are also capable of responding to CXCL13. From day 4 on, architectural changes become CXCL13 dependent, and B cells become fully CXCL13 responsive, express LTalpha1beta2, and cluster in anatomically distinct follicles. Because the initial induction of CXCL13 is dependent on LTalpha1beta2, a role for CD45+CD4+CD3- cells in inducing chemokine expression in the developing lymph nodes is proposed and, as such, a role in initiation of the shaping of the microenvironment.

Osteoclasts enhance myeloma cell growth and survival via cell-cell contact: a vicious cycle between bone destruction and myeloma expansion

Multiple myeloma (MM) expands in the bone marrow and causes devastating bone destruction by enhancing osteoclastic bone resorption in its vicinity, suggesting a close interaction between MM cells and osteoclasts (OCs). Here, we show that peripheral blood mononuclear cell-derived OCs enhanced growth and survival of primary MM cells as well as MM cell lines more potently than stromal cells, and that OCs protected MM cells from apoptosis induced by serum depletion or doxorubicin. OCs produced osteopontin (OPN) and interleukin 6 (IL-6), and adhesion of MM cells to OCs increased IL-6 production from OCs. In addition, IL-6 and OPN in combination enhanced MM cell growth and survival. However, the effects of OCs on MM cell growth and survival were only partially suppressed by a simultaneous addition of anti–IL-6 and anti-OPN antibodies and were completely abrogated by inhibition of cellular contact between MM cells and OCs. These results demonstrate that OCs enhance MM cell growth and survival through a cell-cell contact-mediated mechanism that is partially dependent on IL-6 and OPN. It is suggested that interactions of MM cells with OCs augment MM growth and survival and, thereby, form a vicious cycle, leading to extensive bone destruction and MM cell expansion.

Presumptive lymph node organizers are differentially represented in developing mesenteric and peripheral nodes

During murine embryogenesis, the formation of Peyer's patches (PPs) is initiated by CD45(+)CD4(+)CD3(-) lymphoid tissue inducers that trigger adhesion molecule expression and specific chemokine production from an organizing stromal cell population through ligation of the lymphotoxin-beta receptor. However, the steps involved in the development of lymph nodes (LNs) are less clear than those of PPs, and the characteristics of the organizing cells within the LN anlagen have yet to be documented. In this study, we show for the first time that the early anlage is bordered by an endothelial layer that retains a mixed lymphatic and blood vascular phenotype up to embryonic day 16.5. This in turn encompasses CD45(+)CD4(+)CD3(-) cells interspersed with ICAM-1/VCAM-1/mucosal addressin cell adhesion molecule-1, lymphotoxin-beta receptor-positive, chemokine-producing cells analogous to the organizing population previously observed in PPs. Moreover, these LN organizers also express the TNF family member, TRANCE. Lastly, we show that the ICAM-1/VCAM-1/mucosal addressin cell adhesion molecule-1 cells present in peripheral and mesenteric LN form two discrete populations expressing either intermediate or high levels of these adhesion molecules but that the former population is specifically reduced in PLN. These findings provide a possible explanation for the well-known differences in developmental requirements for nodes at peripheral or mesenteric locations.

Role of chemokines in the development of secondary and tertiary lymphoid tissues

Organogenesis of lymph nodes and Peyer's patches is initiated during embryonic development and depends on the correct expression of a wide variety of molecules. Essential for this process is the paracrine triggering of stromal cells by CD45+CD4+CD3- cells. Hereto CD45+CD4+CD3- cells and stromal cells need to be spatially positioned in close proximity to each other. Expression of adhesion molecules and chemokines is thought to be essential for this process. During adult life, similar processes might also be at the basis of development of organized tertiary lymphoid structures often seen in inflammatory lesions.

Lymphotoxin and TNF produced by B cells are dispensable for maintenance of the follicle-associated epithelium but are required for development of lymphoid follicles in the Peyer's patches

Organogenesis of Peyer's patches (PP), follicle-associated epithelium, and M cells is impaired in mice lacking B cells. At the same time, lymphotoxin (LT) and TNF are known to be critical for the development of PP. To directly address the function of LT and TNF expressed by B cells in the maintenance of PP structure, we studied the de novo formation of PP in B cell-deficient mice after the transfer of bone marrow from mice with targeted mutations in LT, TNF, or their combinations. We found that although the compartmentalization of T and B cell zones and development of follicular dendritic cells were affected by the lack of B cell-derived LT and TNF, the development of follicle-associated epithelium and M cells in PP was completely independent of LT/TNF production by B cells.

A stroma-derived defect in NF-kappaB2-/- mice causes impaired lymph node development and lymphocyte recruitment

The NF-kappaB family of transcription factors is vital to all aspects of immune function and regulation in both the hemopoietic and stromal compartments of immune environments. Recent studies of mouse models deficient for specific members of the NF-kappaB family have revealed critical roles for these proteins in the process of secondary lymphoid tissue organogenesis. In this study, we investigate the role of NF-kappaB family member NF-kappaB2 in lymph node development and lymphocyte recruitment. Inguinal lymph nodes in nfkappab2(-/-) mice are reduced in size and cellularity, most notably in the B cell compartment. Using in vitro and in vivo lymph node grafting assays, we show that the defect resides in the stromal compartment. Further examination of the nfkappab2(-/-) inguinal lymph nodes revealed that expression of peripheral node addressin components CD34 and glycosylation-dependent cell adhesion molecule-1 along with the high endothelial venule-restricted sulfotransferase HEC-GlcNAc6ST was markedly reduced. Furthermore, expression of the lymphocyte homing chemokines CCL19, CCL21, and CXCL13 was down-regulated. These data highlight the role of NF-kappaB2 in inguinal lymph node organogenesis and recruitment of lymphocytes to these organs due to its role in up-regulation of essential cell adhesion molecules and chemokines, while suggesting a potential role for NF-kappaB2 in organization of lymph node endothelium.

The role of TNF-related activation-induced cytokine-receptor activating NF-kappa B interaction in acute allograft rejection and CD40L-independent chronic allograft rejection

We analyzed the role of TNF-related activation-induced cytokine (TRANCE), a member of the TNF family expressed on activated T cells that shares functional properties with CD40L, and its receptor-activating NF-kappaB (RANK) which is mostly expressed on mature dendritic cells, during allogenic responses in vivo using a rodent heart allograft model. TRANCE mRNA was strongly up-regulated in acutely rejected allografts on days 4 and 5 posttransplantation whereas RANK was detected as early as day 1 but did not show further up-regulation during the first week. Immunofluoresence analyses of heart allografts showed that 80 and 100% of TRANCE and RANK-expressing cells were T cells and APCs, respectively. We show for the first time that short-term TRANCE blockade using a mouse RANKIg fusion molecule can significantly prolong heart allograft survival in both rat and mouse models. Similarly, rat heart allografts transduced with a RANKIg encoding recombinant adenovirus exhibited a significant prolongation of survival (14.3 vs 7.6 days, p < 0.0001). However, TRANCE blockade using RANKIg did not appear to inhibit allogeneic T and B cell priming humoral responses against RANKIg. Interestingly, TRANCE blockade induced strong up-regulation of CD40 ligand (CD40L) mRNA in allografts. Combined CD40L and TRANCE blockade resulted in significantly decreased chronic allograft rejection lesions as well as allogeneic humoral responses compared with CD40L blockade alone. We conclude that TRANCE-RANK interactions play an important role during acute allograft rejection and that CD40L-independent allogeneic immune responses can be, at least in part, dependent on the TRANCE pathway of costimulation.

Modulating the intestinal immune system: the role of lymphotoxin and GALT organs

The gut associated immune system fences off potentially harmful intestinal antigens from the systemic circulation and induces systemic tolerance against luminal antigens. Intestinal immune responses against luminal antigens include IgA secretion and induction of regulatory cells. Unlike few other cytokines, lymphotoxin alpha/beta regulates the development of intestinal lymphoid organs. The embryonic development of Peyer's patches, postnatal lamina propria B cell development, and isolated lymphoid follicle development all depend on lymphotoxin beta receptor interactions. Lymphotoxin alpha/beta signalling also contributes to the development of mesenteric lymph nodes. In addition, intestinal inflammation is suppressed by inhibition of lymphotoxin beta signalling, an observation which has initiated clinical studies using this treatment principal. Intestinal follicular lymphoid organs are sites of antigen presentation. Antigen presenting cells tune the delicate balance between intestinal immune tolerance and inflammation. Therefore, gut associated lymphatic organs and factors regulating their development are critical for the prevention of adverse immune reactions to intestinal antigens. This review provides an overview on the role of lymphotoxin and the gut associated lymphatic organs in the regulation of oral tolerance and intestinal inflammation.

An essential function for the nuclear receptor RORgamma(t) in the generation of fetal lymphoid tissue inducer cells

Lymphoid tissue inducer (LTi) cells are associated with early development of lymph nodes and Peyer's patches. We show here that during fetal life the nuclear hormone receptor RORgamma(t) is expressed exclusively in and is required for the generation of LTi cells. RORgamma(t+) LTi cells provide essential factors, among which lymphotoxin-alpha1beta2 is necessary but not sufficient for activation of the mesenchyma in lymph node and Peyer's patch anlagen. This early LTi cell-mediated activation of lymph node and Peyer's patch mesenchyma forms the necessary platform for the subsequent development of mature lymphoid tissues.

Organogenesis of peripheral lymphoid organs

The discovery that lymphotoxin alpha (LTalpha) knockout mice lack peripheral lymphoid tissues reformed the study of organogenesis of peripheral lymphoid tissues from a research field that was solely descriptive and dependent on histological methods to one requiring all modern technologies. The concepts of inducer cells for organogenesis of peripheral lymphoid tissues as a separate hematopoietic lineage and of mesenchymal organizer cells have been established through this progress. These discoveries led to the comprehension of the basic framework of the events during organogenesis of peripheral lymphoid tissues. However, many important questions remain unanswered. This review discusses those questions which have arisen from our studies on the organogenesis of Peyer's patches.

Regulation of secondary lymphoid organ development by the nuclear factor-kappaB signal transduction pathway

In primary lymphoid organs, such as thymus and bone marrow, B and T lymphocytes differentiate from lymphoid stem cells into mature albeit naïve effector cells. In contrast, secondary lymphoid organs, such as the spleen, lymph nodes, and Peyer's patches (PPs), provide an environment that enable lymphocytes to interact with each other, with accessory cells, and with antigens, resulting in the initiation of antigen-specific primary immune responses. Recently, the analysis of gene-knockout mice has shed light on the signaling pathways, cellular requirements, and molecular mechanisms involved in secondary lymphoid organ development. In particular, signals that converge on the nuclear factor-kappaB (NF-kappaB) pathway have been demonstrated to play an important role in both early developmental steps as well as maintenance of secondary lymphoid organ structures. Analysis of the histopathological changes in secondary lymphoid tissues of mice lacking individual Rel/NF-kappaB family members, upstream kinases, and receptors strongly indicates that activation of the recently described alternative NF-kappaB pathway by membrane-bound lymphotoxin, via p52-RelB heterodimers, plays a major role during initiation steps of secondary lymphoid organ development. Induction of the classical p50-RelA NF-kappaB activity, as exemplified by tumor necrosis factor receptor signaling, clearly also contributes, but seems to be involved primarily in later developmental step, such as the proper cellular and structural organization of B-cell follicles.

The impact of CCR7 and CXCR5 on lymphoid organ development and systemic immunity

The development of secondary lymphoid organs is a complex process dependent on a coordinated interaction of cells of hematopoietic and non-hematopoietic origin. In this context, chemokines and cytokines belonging to the tumor necrosis factor (TNF)/lymphotoxin (LT) family are critical signaling molecules during the initial steps of lymph node and Peyer's patch organogenesis. Homeostatic chemokines, such as CXCL13, CCL21, and CCL19, as well as their corresponding receptors, CXCR5 and CCR7, have now been shown to closely cooperate in the development of lymphoid organs and the maintenance of lymphoid tissue microarchitecture. We summarize recent data on the function of CXCR5 and CCR7 and their intricate connection to the TNF/LT system in order to refine the current model of lymphoid organ development.

The receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) is a new chemotactic factor for human monocytes

Bone resorption is regulated by the immune system, where receptor activator of nuclear factor (NF)kappaB ligand (RANKL), a new member of the tumor-necrosis factor family, may contribute to pathological conditions. Due to the role of RANKL in the maturation of monocyte-derived osteoclasts, we hypothesized that RANKL could exert chemotactic properties toward monocytic cells. Our results demonstrate that RANKL induces the migration of MonoMac-6 monocytic cells as well as human freshly isolated total peripheral blood mononuclear cells (PBMC) and CD14+ purified PBMC. RANKL induces the migration of MonoMac-6 cells in a dose-dependent manner and with an efficacy similar to MCP-1. After an 8-h incubation, the soluble form of RANKL (sRANKL) started to exhibit a chemoattractive effect on MonoMac-6 cells, with an increased effect observed up to 24 h. RANKL elicits an additive chemotactic effect to MCP-1. Furthermore, addition of the RANKL decoy receptor osteoprotegerin in the lower well or RANKL in the upper well abrogates the RANKL-induced migration of MonoMac-6 cells, hallmarking a true specific activity. RNase protection assay experiments indicate that exposure of MonoMac-6 cells to RANKL had no significant effect on the expression of a variety of chemokines, known to attract monocytes. This study provides evidence that RANKL behaves as a chemotactic factor for monocytic cells, emphazing the cross-talk between bone and immune systems.

TNF-receptor-associated factors as targets for drug development

TNF-receptor-associated factors (TRAFs) are the bottleneck of the TNF-receptor (TNF-R) family signal transduction. They integrate the signalling from many members of the TNF-R family and initiate intracellular signalling cascades aimed at the activation of NF-kappaB and c-jun, the reprogramming of gene expression and the control of cell death. Deregulation of these pathways is the cause of several autoimmune and inflammatory diseases. The specificity and interaction of the members of the TRAF family with the TNF-R entails the recognition of just a 4 - 6 amino acid motif in the cytosolic region of the receptor, suitable as an attractive target for drug discovery. This review summarises the current knowledge on TRAFs and discusses the pros and cons of their application as targets for drug discovery.

Biology of the TRANCE axis

As the TNF and TNFR superfamilies have grown to more than two dozen combined members over the past 30 years, their involvement in interactions between immune cells, with regard to the events governing cellular differentiation, activation, and survival have been well established. The recently identified TNF superfamily cytokine, TRANCE (RANKL/OPGL/ODF/TNFSF11), which interacts with two receptors-one functional, TRANCE-R (RANK/TNFRSF11A), and one decoy, OPG (TNFRSF11B)-is a survival factor for activated dendritic cells, and may also be important for the maintenance of immune tolerance. TRANCE is also the key cytokine involved in osteoclast differentiation and activation, making TRANCE signaling crucial for proper bone homeostasis, and a potential therapeutic target in diseases such as osteoporosis, osteolytic metastatic cancer, arthritis, and periodontitis. Importantly, the positive role that TRANCE has in activating the immune system, appears to significantly contribute to pathologic bone loss. These observations have spurred intense study of the various ways in which the immune system can influence bone. Furthermore, TRANCE has also been demonstrated to play essential roles in the developmental processes leading to both lymph node formation, and the expansion and function of mammary glands during pregnancy and lactation. Thus, TRANCE is quickly emerging as a cytokine of significant importance to further understanding unique aspects of mammalian biology.

The role of lymphotoxin in development and maintenance of secondary lymphoid tissues

Secondary lymphoid organs provide the necessary microenvironment for the cooperation of antigen-specific T- and B-lymphocytes and antigen-presenting cells in order to initiate an efficient immune response. Remarkable progress in understanding of the mechanisms of lymphoid organogenesis was achieved due to the analysis of various gene-targeted mice. This review primarily focuses on the role of lymphotoxin (LT) in development, maturation and maintenance of secondary lymphoid organs.

Overlapping roles of CXCL13, interleukin 7 receptor alpha, and CCR7 ligands in lymph node development

Lymphoid tissue development is associated with local accumulation of CD4+ CD3- IL-7R alpha hi hematopoietic cells that deliver lymphotoxin (LT)alpha 1 beta 2 signals to resident stromal cells. Previous studies have established an important role for CXCL13 (BLC) in the development of Peyer's patches (PP) and some peripheral lymph nodes (LNs), but the chemokine requirements for several LN types, including mesenteric LNs, remain undefined. Using CXCL13-/- mice that additionally carry the paucity of LN T cell mutation (plt/plt), we discovered that CCR7 ligands function in peripheral LN development. We also tested for a genetic interaction during LN development between CXCL13 and a cytokine receptor required in PP development, IL-7R alpha. Mice deficient for both CXCL13 and IL-7R alpha displayed a striking absence of LNs, including mesenteric LNs. These data extend the role of CXCL13 to the development of all LNs and establish a previously unappreciated role for IL-7R alpha in this process. Both circulating and LN CD4+ CD3- IL-7R alpha hi cells are shown to express LT alpha 1 beta 2 in an IL-7R alpha-dependent manner. Furthermore, CXCL13 was found to be sufficient to mediate CD4+ CD3- IL-7R alpha hi cell recruitment in vivo to an ectopic site. These findings indicate that CXCL13 and CCR7 ligands promote accumulation of CD4+ CD3- IL-7R alpha hi cells, delivering IL-7R alpha-dependent LT alpha 1 beta 2 signals critical for LN development.

Organogenesis of lymphoid tissues

The development of lymphoid organs depends on the correct expression of several molecules within a defined timeframe during ontogeny. Although this is an extremely complex process, with each secondary lymphoid tissue requiring subtly different signals, a common framework for lymphoid development is beginning to emerge. Drawing on studies of lymph nodes, Peyer's patches and nasal-associated lymphoid tissue, an integrative model of lymphoid-tissue development, involving adhesion molecules, cytokines and chemokines, which emphasizes the role of interactions between CD3-CD4+CD45+ 'inducer' cells and VCAM1+ICAM1+ stromal 'organizer' cells is presented.

Concerted action of the chemokine and lymphotoxin system in secondary lymphoid-organ development

Chemokines are essential regulators of lymphocyte migration throughout the body. The chemokine system controls lymphocyte recirculation in immune-system homeostasis, as well as the activation-dependent and tissue-selective trafficking of effector and memory lymphocytes during immune responses. In addition, there is now substantial evidence that chemokines are critical factors for the development and organization of secondary lymphoid organs and that they are involved in all stages of lymphoid organogenesis.

RANK-Fc: a therapeutic antagonist for RANK-L in myeloma

BACKGROUND

Severe bone destruction due to inappropriate osteoclastogenesis is a prominent feature of multiple myeloma (MM). MM increases bone loss by disrupting the checks that normally control signaling by receptor activator of nuclear factor kappaB ligand (RANK-L, also called TRANCE [tumor necrosis factor-related, activation-induced cytokine], osteoprotegerin ligand [OPG-L], osteoclast differentiation factor [ODF], and tumor necrosis factor superfamily member 11 [TNFSF11]), a TNF-family cytokine required for osteoclast differentiation and activation. RANK-L binds to its functional receptor RANK (TNF receptor superfamily member 11a [TNF RSF11a]) to stimulate osteoclastogenesis. Osteotropic cytokines regulate this process by controlling bone marrow stromal expression of RANK-L. Further control over osteoclastogenesis is maintained by regulated expression of osteoprotegerin (OPG, also called osteoclastogenesis inhibitory factor and TNFRSF11b), a soluble decoy receptor for RANK-L. In normal bone marrow, abundant stores of OPG in stroma, megakaryocytes, and myeloid cells provide a natural buffer against increased RANK-L. MM disrupts these controls by increasing expression of RANK-L and decreasing expression of OPG. Concurrent deregulation of RANK-L and OPG expression is found in bone marrow biopsies from patients with MM but not in specimens from patients with non-MM hematologic malignancies.

METHODS

RANK-Fc is a recombinant RANK-L antagonist that is formed by fusing the extracellular domain of RANK to the Fc portion of human immunoglobulin G(1) (hIgG(1)). In vitro, addition of RANK-Fc virtually eliminates the formation of osteoclasts in cocultures of MM with bone marrow and osteoblast/stromal cells. The severe combined immunodeficiency (SCID)/ARH77 mouse model and the SCID-hu-MM mouse model of human MM were used to assess the ability of RANK-Fc to block the development of MM-induced bone disease in vivo. Mice received either RANK-Fc or hIgG(1) 200 microg intravenously three times per week.

RESULTS

RANK-Fc limited bone destruction in both the SCID/ARH-77 model and the SCID-hu-MM model. Administration of RANK-Fc also caused a marked reduction in tumor burden and serum paraprotein in SCID-hu-MM mice that was associated with the restoration of OPG and a reduction in RANK-L expression in the xenograft.

CONCLUSIONS

MM-induced bone destruction requires increased RANK-L expression and is facilitated by a concurrent reduction in OPG, a natural decoy receptor for RANK-L. Administration of the RANK-L antagonist RANK-Fc limits MM-induced osteoclastogenesis, development of bone disease, and MM tumor progression.

Different cytokines induce surface lymphotoxin-alphabeta on IL-7 receptor-alpha cells that differentially engender lymph nodes and Peyer's patches

The formation of lymph nodes (LN) and Peyer's patches (PP) can be distinguished by the requirement of RANK for LN but not IL-7R(alpha), which is essential for PP development. However, lymphotoxin-alphabeta (LT(alpha)beta) signaling is required for both organs. The cellular basis underlying this dichotomy was revealed by the finding that the fetal IL-7R(alpha)(+) population responded equally well to IL-7 and RANKL to express LT(alpha)beta. IL-7R(alpha)(+) cells harvested from TRAF6(-/-) embryos expressed LTalphabeta in response to IL-7 but not RANKL, demonstrating that the RANK-TRAF6 signaling pathway regulates LT(alpha)beta expression in LN but not in PP. Soluble IL-7 administered to TRAF6(-/-) embryos was sufficient to restore LN genesis indicating the functional similarities of the IL-7R(alpha)(+) inducer cells for LN and PP genesis.

Long-lived immature dendritic cells mediated by TRANCE-RANK interaction

Immature dendritic cells (DCs) reside in interstitial tissues (int-DC) or in the epidermis, where they capture antigen and, thereafter, mature and migrate to draining lymph nodes (LNs), where they present processed antigen to T cells. We have identified int-DCs that express both TRANCE (tumor necrosis factor-related activation-induced cytokine) and RANK (receptor activator of NF-kappaB) and have generated these cells from CD34(+) human progenitor cells using macrophage colony-stimulating factor (M-CSF). These CD34(+)-derived int-DCs, which are related to macrophages, are long-lived, but addition of soluble RANK leads to significant reduction of cell viability and Bcl-2 expression. This suggests that constitutive TRANCE-RANK interaction is responsible for CD34(+)-derived int-DC longevity. Conversely, CD1a(+) DCs express only RANK and are short-lived. However, they can be rescued from cell death either by recombinant soluble TRANCE or by CD34(+)-derived int-DCs. CD34(+)-derived int-DCs mature in response to lipopolysaccharide (LPS) plus CD40 ligand (L) and become capable of CCL21/CCL19-mediated chemotaxis and naive T-cell activation. Upon maturation, they lose TRANCE, making them, like CD1a(+) DCs, dependent on exogenous TRANCE for survival. These findings provide evidence that TRANCE and RANK play important roles in the homeostasis of DCs.

Role of RANKL and RANK in bone loss and arthritis

The tumour necrosis factor family molecule RANKL (RANKL, TRANCE, ODF) and its receptor RANK are key regulators of bone remodelling and regulate T cell/dendritic cell communications, and lymph node formation. Moreover, RANKL and RANK are expressed in mammary gland epithelial cells and control the development of a lactating mammary gland during pregnancy and the propagation of mammalian species. Importantly, RANKL and RANK are essential for the development and activation of osteoclasts and bone loss in response to virtually all triggers tested. Therapeutically, inhibition of RANKL function via the decoy receptor osteoprotegerin completely prevents bone loss at inflammed joints and has partially beneficial effects on cartilage destruction in all arthritis models studied. Modulation of these systems provides a unique opportunity to design novel treatments to inhibit bone loss and crippling in arthritis.

The role of CD45+CD4+CD3- cells in lymphoid organ development

In the last 10 years the continuing search for gene function has yielded many mutant mice that unexpectedly showed a complete lack of lymph nodes and/or Peyer's patches. With the realization that all these functionally highly diverse genes are involved at some point in the development of lymphoid organs, the challenge now is to assign a function to the molecules involved in lymphoid organ development. It will be important to determine the sequence of molecular events and assign this to the cellular events that lead to an accumulation of hematopoietic cells in one location, ultimately forming an organized lymphoid organ. Here we will focus on CD45+CD4+CD3- cells that are the early colonizing cells in lymph nodes and Peyer's patches and develop a hypothetical model of their contribution to the creation of organized lymphoid structures.

CD4+CD3- cells induce Peyer's patch development: role of alpha4beta1 integrin activation by CXCR5

CD4+CD3- cells are the predominant hematopoietic cells found in mouse fetal intestine. We prove their role as Peyer's patch (PP)-inducing cells by transfer into neonatal PP-deficient mice. To test the requirement of chemokines and adhesion molecules in induction of PP, we studied mice deficient in CXCR5 and/or alpha4beta1 integrin-mediated adhesion. CXCR5-/- mice have CD4+CD3- cells, which are inefficient in inducing PP formation. We show here that CXCR5/CXCL13 signaling activates alpha4beta1 integrin on CD4+CD3- cells. Blocking of beta1 integrin or VCAM-1, the ligand of alpha4beta1 integrin, inhibits PP formation. This study demonstrates the link between chemokine receptors and adhesion molecules that regulates stromal/hematopoietic cell interaction leading to PP formation.

Role of zeta PKC in B-cell signaling and function

The atypical protein kinase C isoform, zeta PKC, has been implicated in the control of extracellular signal-regulated kinase (ERK) and nuclear factor (NF)-kappa B pathways. Recent evidence from zeta PKC knock-out mice demonstrates that this kinase is important for NF-kappa B transcriptional activity but not for ERK activation in embryonic fibroblasts. The lack of zeta PKC produces in mice a number of alterations in the development of secondary lymphoid tissues that could be accounted for, at least in part, by defects in B-cell function. Here, we present evidence that the loss of zeta PKC selectively impairs signaling through the B-cell receptor, resulting in inhibition of cell proliferation and survival, as well as defects in the activation of ERK and the transcription of NF-kappa B-dependent genes. Furthermore, zeta PKC-/- mice are unable to mount an optimal T-cell-dependent immune response. Collectively, these results genetically establish a critical role for zeta PKC in B-cell function in vitro and in vivo.

Initiation of NALT organogenesis is independent of the IL-7R, LTbetaR, and NIK signaling pathways but requires the Id2 gene and CD3(-)CD4(+)CD45(+) cells

Initiation of nasopharyngeal-associated lymphoid tissue (NALT) development is independent of the programmed cytokine cascade necessary for the formation of Peyer's patches (PP) and peripheral lymph nodes (PLN), a cytokine cascade which consists of IL-7R, LTalpha1beta2/LTbetaR, and NIK. However, the subsequent organization of NALT seems to be controlled by these cytokine signaling cascades since the maturation of NALT structure is generally incomplete in those cytokine cascade-deficient mice. NALT as well as PP and PLN are completely absent in Id2(-/-) mice. NALT organogenesis is initiated following the adoptive transfer of CD3(-)CD4(+)CD45(+) cells into Id2(-/-) mice, constituting direct evidence that CD3(-)CD4(+)CD45(+) inducer cells can provide an IL-7R-, LTalpha1beta2/LTbetaR-, and NIK-independent tissue organogenesis pathway for secondary lymphoid tissue development.

RANK-L and RANK: T cells, bone loss, and mammalian evolution

TNF and TNFR family proteins play important roles in the control of cell death, proliferation, autoimmunity, the function of immune cells, or the organogenesis of lymphoid organs. Recently, novel members of this large family have been identified that have critical functions in immunity and that couple lymphoid cells with other organ systems such as bone morphogenesis and mammary gland formation in pregnancy. The TNF-family molecule RANK-L (RANK-L, TRANCE, ODF) and its receptor RANK are key regulators of bone remodeling, and they are essential for the development and activation of osteoclasts. Intriguingly, RANK-L/RANK interactions also regulate T cell/dendritic cell communications, dendritic cell survival, and lymph node formation; T cell-derived RANK-L can mediate bone loss in arthritis and periodontal disease. Moreover, RANK-L and RANK are expressed in mammary gland epithelial cells, and they control the development of a lactating mammary gland during pregnancy and the propagation of mammalian species. Modulation of these systems provides us with a unique opportunity to design novel therapeutics to inhibit bone loss in arthritis, periodontal disease, and osteoporosis.

Visualization of lymphotoxin-beta and lymphotoxin-beta receptor expression in mouse embryos

The heteromeric lymphotoxin alphabeta ligand (LT) binds to the LTbeta receptor (LTbetaR) and provides an essential trigger for lymph node (LN) development. LTbetaR signaling is also critical for the emergence of pathological ectopic lymph node-like structures and the maintenance of an organized splenic white pulp. To better understand the role of LT in development, the expression patterns of LTbeta and LTbetaR mRNA were examined by in situ hybridization in the developing mouse embryo. Images of LTbeta ligand expression in developing peripheral LN in the E18.5 embryo revealed a relatively early phase structure and allowed for comparative staging with LN development in rat and humans. The LTbetaR is expressed from E16.5 onward in respiratory, salivary, bronchial, and gastric epithelium, which may be consistent with early communication events between lymphoid elements and epithelial specialization over emerging mucosal LN. Direct comparison of mouse fetal and adult tissues by FACS analysis confirmed the elevated expression of LTBR in some embryonic epithelial layers. Therefore, surface LTBR expression may be elevated during fetal development in some epithelial layers.