Chronic inflammation caused by lymphotoxin is lymphoid neogenesis

IL-7 receptor alpha+ CD3(-) cells in the embryonic intestine induces the organizing center of Peyer's patches

Peyer's patch (PP) organogenesis proceeds through three histologically distinct steps: formation of organizing centers expressing VCAM-1 and ICAM-1 in segregated regions of the intestine at 15.5 days post-coitus (d.p.c.) (step I), accumulation of blood cells expressing different sets of surface markers to this region at 16.5-17.0 d.p.c. (step II), and entry of CD3+ and B220+ lymphocytes just before birth (step III). PP formation of both Il7ra-/- and Lta-/- mice is impaired from step I, suggesting involvement of the two molecules at the same timing in PP organogenesis. Expression of lymphotoxin (LT) alpha and LTbeta in IL-7 receptor (IL-7R) alpha+ cells in the intestine indicates that defects of Il7ra-/- and Lta-/- mice are due to functional inability of IL-7Ralpha+ cells in the induction of PP anlage. Blocking of IL-7Ralpha function by a single injection of the antagonistic mAb in 15.5 d.p.c. embryos suppressed appearance of VCAM-1(+) spots and expression of LTalpha and LTbeta in the intestine, which eventually resulted in mice without PP but are otherwise normal. Intestinal IL-7Ralpha+ cells are lymphoid in morphology but CD3(-) and functional in both nu/nu and Rag2-/- mice. These results implicate IL-7Ralpha+ CD3(-) cells as the direct inducer of the organizing center of PP.

The effect of graft-versus-host disease on T cell production and homeostasis

The aim of this work was to decipher how graft-versus-host disease (GVHD) affects T cell production and homeostasis. In GVHD+ mice, thymic output was decreased fourfold relative to normal mice, but was sufficient to maintain a T cell repertoire with normal diversity in terms of Vbeta usage. Lymphoid hypoplasia in GVHD+ mice was caused mainly by a lessened expansion of the peripheral postthymic T cell compartment. In 5-bromo-2'-deoxyuridine pulse-chase experiments, resident T cells in the spleen of GVHD+ mice showed a normal turnover rate (proliferation and half-life). When transferred into thymectomized GVHD- secondary hosts, T cells from GVHD+ mice expanded normally. In contrast, normal T cells failed to expand when injected into GVHD+ mice. Thus, the reduced size of the postthymic compartment in GVHD+ mice was not due to an intrinsic lymphocyte defect, but to an extrinsic microenvironment abnormality. We suggest that this extrinsic anomaly is consistent with a reduced number of functional peripheral T cell niches. Therefore, our results show that GVHD-associated T cell hypoplasia is largely caused by a perturbed homeostasis of the peripheral compartment. Furthermore, they suggest that damage to the microenvironment of secondary lymphoid organs may represent an heretofore unrecognized cause of acquired T cell hypoplasia.

Naive Human CD4+ T Cells Are a Major Source of Lymphotoxin α

It is generally accepted that immunologically naive T cells display a very restricted cytokine production profile consisting mainly of IL-2, which is used as an autocrine growth factor. Here we report that activated naive CD4+ T cells, of neonatal or adult origin, express very high levels of soluble lymphotoxin (LT) α (LTα3), as determined by ELISA, RNase protection assay, and intracytoplasmic staining. Besides LTα3 and IL-2, these cells also produce high levels of TNF-α together with significant amounts of IFN-γ and IL-13. Naive cells also express LTβ mRNA and the membrane form of LTα (LTαβ). On average, naive CD4+ T cells secrete four times more LTα3 than Th1-like cells, twice more than naive CD8+ T cells, and ten times more than B cells. Thus, naive T cells express a large spectrum of cytokines, mainly of the Th1 type, and the very high levels of LTα3/TNF-α that they release may play an hitherto unsuspected role in the early stage of T cell-dependent immune responses.

Plasma Cell Development in Synovial Germinal Centers in Patients with Rheumatoid and Reactive Arthritis

Plasma cells are found surrounding the inflammatory infiltrates of macrophages, T, and B cells in the synovial tissue of patients with rheumatoid and reactive arthritis. This characteristic arrangement suggests that in the synovial tissue CD20+ B cells differentiate into plasma cells. To examine clonal relationships, we have used micromanipulation to separately isolate CD20+ B cells and plasma cells from single infiltrates. DNA was extracted, and from both populations the VH/VL gene repertoires was determined. The data show that in the inflamed synovial tissue activated B cells are clonally expanded. During proliferation in the network of follicular dendritic cells, V gene variants are generated by the hypermutation mechanism. Surprisingly, we do not find identical rearrangements between CD20+ B cells and plasma cells. Nevertheless, the finding of clonally related plasma cells within single infiltrates suggests that these cells underwent terminal differentiation in the synovial tissue. These results indicate that B cell differentiation in the synovial tissue is a dynamic process. Whereas CD20+ B cells may turnover rapidly, plasma cells may well be long lived and thus accumulate in the synovial tissue. The analysis of individual B cells recovered from synovial tissue opens a new way to determine the specificity of those cells that take part in the local immune reaction. This will provide new insights into the pathogenesis of chronic inflammatory diseases like rheumatoid or reactive arthritis.

Lymphotoxin αβ Is Expressed on Recently Activated Naive and Th1-Like CD4 Cells but Is Down-Regulated by IL-4 During Th2 Differentiation

Lymphotoxin (LT) is a cytokine that orchestrates lymphoid neogenesis and formation of germinal center reactions. LT exists as a membrane heterotrimer of α and β subunits and is secreted as a homotrimer, LTα3. Using LTβR.Fc, expression of LTαβ on CD4 T cell subsets was investigated in a TCR transgenic model. LTαβ was evident 24–72 h after activation of naive T cells with specific Ag, and declined thereafter. Early expression was independent of IFN-γ and IL-12, however, IL-12 prolonged expression. LTαβ was reinduced within 2–4 h after Ag restimulation, but declined by 24 h regardless of IL-12 or IFN-γ priming. Exposure of naive T cells to IL-4 did not affect early LTαβ expression at 24 h, but resulted in subsequent down-regulation. IL-4-differentiated Th2 effectors did not re-express LTαβ, and LTαβ was transiently found on Th1 clones but not Th2 clones. LTα3 and TNF were immunoprecipitated from supernatants and lysates of IL-12 primed cells but not IL-4 primed cells. These studies demonstrate that LTαβ is expressed by activated naive CD4 cells, unpolarized IL-2-secreting effectors, and Th1 effectors. In contrast, loss of surface LTαβ and a lack of LTα3 and TNF secretion is associated with prior exposure to IL-4 and a Th2 phenotype.

Signalling by CD95 and TNF receptors: not only life and death

Members of the TNF family of receptors play important roles in normal physiology and in defence. The recent rapid progress in the understanding of the mechanisms of apoptosis has been accompanied by assumptions that TNF family receptors such as CD95(Fas/APO-1) only have a role in regulating cell survival. While regulation of cell death is one important function of TNF family receptors, they are capable of activating signal transduction pathways that have many other effects. The present review will focus on signalling of some TNF family receptors in the immune system, not only for apoptosis, but also for survival or activation.

Lymphotoxin alpha/beta and tumor necrosis factor are required for stromal cell expression of homing chemokines in B and T cell areas of the spleen

Mice deficient in the cytokines tumor necrosis factor (TNF) or lymphotoxin (LT) alpha/beta lack polarized B cell follicles in the spleen. Deficiency in CXC chemokine receptor 5 (CXCR5), a receptor for B lymphocyte chemoattractant (BLC), also causes loss of splenic follicles. Here we report that BLC expression by follicular stromal cells is defective in TNF-, TNF receptor 1 (TNFR1)-, LTalpha- and LTbeta-deficient mice. Treatment of adult mice with antagonists of LTalpha1beta2 also leads to decreased BLC expression. These findings indicate that LTalpha1beta2 and TNF have a role upstream of BLC/CXCR5 in the process of follicle formation. In addition to disrupted follicles, LT-deficient animals have disorganized T zones. Expression of the T cell attractant, secondary lymphoid tissue chemokine (SLC), by T zone stromal cells is found to be markedly depressed in LTalpha-, and LTbeta-deficient mice. Expression of the SLC-related chemokine, Epstein Barr virus-induced molecule 1 ligand chemokine (ELC), is also reduced. Exploring the basis for the reduced SLC expression led to identification of further disruptions in T zone stromal cells. Together these findings indicate that LTalpha1beta2 and TNF are required for the development and function of B and T zone stromal cells that make chemokines necessary for lymphocyte compartmentalization in the spleen.

Lymphotoxin α3 Induces Chemokines and Adhesion Molecules: Insight into the Role of LTα in Inflammation and Lymphoid Organ Development

Lymphotoxin (LT) plays an important role in inflammation and lymphoid organ development, though the mechanisms by which it promotes these processes are poorly understood. Toward this end, the biologic activities of a recently generated recombinant murine (m) LTα preparation were evaluated. This cytokine preparation was effective at inducing cytotoxicity of WEHI target cells with 50% maximal killing observed with 1.2 ng/ml. mLTα also induced the expression of inflammatory mediators in the murine endothelial cell line bEnd.3. rmLTα induced expression of the adhesion molecules VCAM, ICAM, E-selectin, and the mucosal addressin cellular adhesion molecule, MAdCAM-1. When mLTα, human (h) LTα, and mTNF-α were compared, mLTα was the most potent inducer of MAdCAM-1. None of these cytokines induced the peripheral node addressin, PNAd. mLTα also induced expression of the chemokines RANTES, IFN-inducible protein 10 (IP-10), and monocyte chemotactic protein 1 (MCP-1). mRNA levels peaked 4 h following treatment with mLTα and declined through the 24-h treatment period. LTα also induced chemokine protein within 8 h of treatment, which increased through the 24-h treatment period. These data demonstrate that the proinflammatory effects of LTα3 may be mediated in part through the induction of adhesion molecule and chemokine expression. Further, LTα3 may promote development of lymphoid tissue through induction of chemokines and the mucosal addressin MAdCAM-1. These data confirm previous observations in transgenic and knockout mice that LTα3 in the absence of LTβ carries out unique biologic activities.

Both the lymphotoxin and tumor necrosis factor pathways are involved in experimental murine models of colitis

BACKGROUND & AIMS

Membrane lymphotoxin (LT) alpha/beta, a member of the tumor necrosis factor (TNF) family of immune regulatory molecules, is involved both in the development of secondary lymphoid tissues and the maintenance of organized lymphoid tissues in the adult. Defects observed in the mucosal immune system in animals with a genetically disrupted LTalpha/beta pathway coupled with the expression of LTalpha/beta in activated T cells motivated an examination of the importance of this pathway in experimental colitis.

METHODS

Soluble LTbeta receptor (LTbetaR) immunoglobulin fusion protein was used to inhibit the LTalpha/beta/light axis in two independent rodent models of colitis: CD45RBhi CD4(+)-reconstituted SCID mice and bone marrow-transplanted tg26 mice (BM --> tg26).

RESULTS

Treatment with LTbetaR immunoglobulin attenuated the development of both the clinical and histological manifestations of the disease in these two murine models of colitis. Given the success of TNF inhibitors in the treatment of human Crohn's disease, the effects of LTbetaR immunoglobulin have been compared with antibody to TNF in the BM --> tg26 model, and both treatments were equally efficacious.

CONCLUSIONS

The LT pathway plays a role in the development of colitis as important as that of the TNF system and, therefore, represents a potential novel intervention point for the treatment of inflammatory bowel disease.

Immune responsiveness of lymphotoxin-alpha-deficient mice: two reconstitution models

The effects of lymphotoxin-alpha (LT-alpha) deficiency on mucosal immune status has not been defined. We utilized severe combined immunodeficiency (scid) mice as recipients of both mutant and wild-type whole splenocytes to determine whether lymphocytes from mutant mice had impaired homing ability. We also utilized irradiated mutant mice as recipients of wild-type whole splenocytes to determine whether lymphoid tissue anlages had, indeed, failed to develop as a consequence of LT-alpha deficiency. Subsequently, all mice were immunized orally with an attenuated strain of Salmonella typhimurium and mucosal IgA responses were monitored. The data presented here demonstrate that scid recipients generate mucosal responses equally well when reconstituted with mutant or wild-type lymphocytes. In contrast, reconstitution of mutant mice with wild-type cells failed to affect the efficiency of their mucosal immunity. The mutant phenotype, therefore, appears to involve neither impaired lymphocyte homing nor function in the generation of mucosal immunity. However, the mutant phenotype and immune responsiveness cannot be transformed merely by the provision of LT-alpha-expressing donor cell populations. The consequence of LT-alpha deficiency on mucosal immune responsiveness appears to be due to the lack of gut-associated lymphoid tissues, which may include the spleen, in mutant mice.

Dendritic cells induce autoimmune diabetes and maintain disease via de novo formation of local lymphoid tissue

Activation of autoreactive T cells can lead to autoimmune diseases such as insulin-dependent diabetes mellitus (IDDM). The initiation and maintenance of IDDM by dendritic cells (DC), the most potent professional antigen-presenting cells, were investigated in transgenic mice expressing the lymphocytic choriomeningitis virus glycoprotein (LCMV-GP) under the control of the rat insulin promoter (RIP-GP mice). We show that after adoptive transfer of DC constitutively expressing the immunodominant cytotoxic T lymphocyte (CTL) epitope of the LCMV-GP, RIP-GP mice developed autoimmune diabetes. Kinetic and functional studies of DC-activated CTL revealed that development of IDDM was dependent on dose and timing of antigenic stimulation. Strikingly, repeated CTL activation by DC led to severe destructive mononuclear infiltration of the pancreatic islets but also to de novo formation of islet-associated organized lymphoid structures in the pancreatic parenchyma. In addition, repetitive DC immunization induced IDDM with lymphoid neogenesis also in perforin-deficient RIP-GP mice, illustrating that CD8(+) T cell-dependent inflammatory mechanisms independent of perforin could induce IDDM. Thus, DC presenting self-antigens not only are potent inducers of autoreactive T cells, but also help to maintain a peripheral immune response locally; therefore, the induction of autoimmunity against previously ignored autoantigens represents a potential hazard, particularly in DC-based antitumor therapies.

Immunohistochemistry of 'tertiary lymphoid follicles' in oral amalgam-associated lichenoid lesions

OBJECTIVES

To characterise lymphoid follicle-like aggregates incidentally found to occur in biopsies of oral lichenoid reactions (OLR) and to correlate the findings to hyperplastic tonsil follicles.

DESIGN

An immunocytochemical analysis of archival material.

SUBJECTS AND METHODS

In an arbitrarily selected period 1992-1994 with a total of 13,924 oral biopsies, 2407 cases were signed out as OLR or lichen planus. These were microscopically reviewed, with the purpose to retrieve all cases showing histologic changes resembling secondary lymphoid follicles.

RESULTS

87 cases showed lymphoid follicle-like changes and 82 of these were in oral regions known to constitutively lack 'organised MALT'. Unexpectedly, all of the 82 were found retrospectively to be in close or direct contact with amalgam fillings. Immunocytochemically, using antibodies to B and T cells, macrophages, follicular dendritic cells and proliferation and apoptosis markers, the mucosal follicles stained similar to tonsillar secondary follicles. In several of an additional 11 OLR cases with histologic changes suggestive of primary follicles, we also found immunocytochemical evidence of such changes.

CONCLUSIONS

'Tertiary lymphoid follicles' may occasionally develop in OLR at sites of the oral mucosa constitutively lacking organised lymphoid tissue. The microenvironment of the OLR T cell infiltrate may occasionally favour such follicle development and amalgam constituents may causally be involved in an unknown way.

Peyer's patch organogenesis as a programmed inflammation: a hypothetical model

Gene-knock-out studies implicate roles of lymphotoxin (LT) alphabeta and LT betaR in the initial phase of Peyer's patch (PP) organogenesis. We recently identified the requirement of IL-7R alpha/gamma c/Jak3 signal in LT alphabeta production of IL-7R alpha+ cells. These observations lead us to a hypothetical model for PP organogenesis with three cellular components. The first is the producer of the ligand for IL-7R alpha, which then stimulate the IL-7R alpha+ cells to produce LT alphabeta activating the LT betaR+ cells to form an organizing center for PP organogenesis. This model is similar to that of inflammation, suggesting that PP organogenesis is a programmed version of inflammation.

Polymorphism in tumor necrosis factor genes associated with myasthenia gravis

The aim of this study was to analyze associations between myasthenia gravis (MG) and polymorphisms in the tumor necrosis factor (TNF) region in 79 Swedish patients and 155 unrelated controls. The frequency of the TNFa2 allele of a microsatellite located 3.5 kb upstream of the lymphotoxin alpha (LT-alpha) gene in the TNF region was found to be increased in overall MG patients compared to controls. The frequency of the short 5.5 kb fragment (TNFB * 1) of a bi-allelic NcoI RFLP polymorphism located at the first intron of the LT-alpha gene was increased in patients with an early onset of disease compared to patients with a later onset.

The lymphotoxin beta receptor controls organogenesis and affinity maturation in peripheral lymphoid tissues

Lymphotoxin beta receptor (LTbetaR)-/- mice were created by gene targeting. LTbetaR-/- mice lacked Peyer's patches, colon-associated lymphoid tissues, and all lymph nodes. Mucosa patrolling alphaEbeta7high integrin+ T cells were virtually absent. Spleens lost marginal zones; T/B cell segregation and follicular dendritic cell networks were absent. Peanut agglutinin+ cells were aberrantly detectable around central arterioles. In contrast to TNF receptor p55-/- mice, antibody affinity maturation was impaired. Since LTbetaR-/- mice exhibit distinct defects when compared to LTalpha-/- and LTbeta-/- mice, it is suggested that the LTbetaR integrates signals from other TNF family members. Thus, the LTbetaR proves pivotal for the ontogeny of the secondary lymphoid tissues. Furthermore, affinity maturation is dependent on LTalpha1beta2 rather than on LTalpha3.

A role for tumor necrosis factor receptor type 1 in gut-associated lymphoid tissue development: genetic evidence of synergism with lymphotoxin beta

Lymphotoxin alpha (LTalpha) signals via tumor necrosis factor receptors (TNFRs) as a homotrimer and via lymphotoxin beta receptor (LTbetaR) as a heterotrimeric LTalpha1beta2 complex. LTalpha-deficient mice lack all lymph nodes (LNs) and Peyer's patches (PPs), and yet LTbeta-deficient mice and TNFR-deficient mice have cervical and mesenteric LN. We now show that mice made deficient in both LTbeta and TNFR type 1 (TNFR1) lack all LNs, revealing redundancy or synergism between TNFR1 and LTbeta, acting presumably via LTbetaR. A complete lack of only PPs in mice heterozygous for both ltalpha and ltbeta, but not ltalpha or ltbeta alone, suggests a similar two-ligand phenomenon in PP development and may explain the incomplete lack of PPs seen in tnfr1-/- mice.

Challenging cytokine redundancy: inflammatory cell movement and clinical course of experimental autoimmune encephalomyelitis are normal in lymphotoxin-deficient, but not tumor necrosis factor-deficient, mice

Lymphotoxin (LT) is widely regarded as a proinflammatory cytokine with activities equivalent to tumor necrosis factor (TNF). The contribution of LT to experimental autoimmune encephalomyelitis (EAE) was examined using TNF/LTalpha-/- mice, TNF-/- mice, and a new LTalpha-/- line described here. All mice were generated directly in the C57BL/6 strain and used for the preparation of radiation bone marrow chimeras to reconstitute peripheral lymphoid organs and restore immunocompetence. This approach overcame the problems related to the lack of lymph nodes that results from LTalpha gene targeting. We show here that when LT is absent but TNF is present, EAE progresses normally. In contrast, when TNF is absent but LT is present, EAE is delayed in onset and inflammatory leukocytes fail to move normally into the central nervous system parenchyma, even at the peak of disease. In the absence of both cytokines, the clinical and histological picture is identical to that seen when TNF alone is deficient, including demyelination. Furthermore, the therapeutic inhibition of TNF and LTalpha with soluble TNF receptor in unmanipulated wild-type or TNF-/- mice exactly reproduces these outcomes. We conclude from these studies that TNF and LT are functionally distinct cytokines in vivo, and despite sharing common receptors, show no redundancy of function nor mutual compensation.

The roles of alpha 4-integrins in the development of insulin-dependent diabetes mellitus

Lymphocyte/endothelial adhesion followed by transendothelial migration is a key event in the development of organ-specific autoimmunity. Selective interactions of cell surface AM regulate lymphocyte migration under normal as well as pathologic inflammatory conditions. NOD mice are an ideal model for investigating the roles of AM in regulation of lymphocyte migration to target organs in autoimmune diseases such as IDDM. Both in vitro and in vivo studies in NOD mice strongly suggest that the mucosal (alpha 4 beta 7/MAdCAM-1) adhesion system and alpha 4-integrin/VCAM-1 appear to be prominent pathways for insulitis development. In contrast, alpha 4-mediated interactions in NOD inflamed salivary and lacrimal gland and in the inflamed CNS of rodents with EAE seem to be dominated by alpha 4-integrins and VCAM-1. The fact that blocking alpha 4-integrin pathways in NOD mice leads to successful interruption of the diabetogenic process suggests that AM provide a potential therapeutic target for human IDDM. Further studies on IDDM patients will prove helpful for understanding IDDM pathogenesis and in providing a basis for designing AM-based therapeutic approaches.

LIGHT, a new member of the TNF superfamily, and lymphotoxin alpha are ligands for herpesvirus entry mediator

Herpes simplex virus (HSV) 1 and 2 infect activated T lymphocytes by attachment of the HSV envelope glycoprotein D (gD) to the cellular herpesvirus entry mediator (HVEM), an orphan member of the tumor necrosis factor receptor superfamily. Here, we demonstrate that HVEM binds two cellular ligands, secreted lymphotoxin alpha (LTalpha) and LIGHT, a new member of the TNF superfamily. LIGHT is a 29 kDa type II transmembrane protein produced by activated T cells that also engages the receptor for the LTalphabeta heterotrimer but does not form complexes with either LTalpha or LTbeta. HSV1 gD inhibits the interaction of HVEM with LIGHT, and LIGHT and gD interfere with HVEM-dependent cell entry by HSV1. This characterizes herpesvirus gD as a membrane-bound viokine and establishes LIGHT-HVEM as integral components of the lymphotoxin cytokine-receptor system.

Differential Activities of Secreted Lymphotoxin-α3 and Membrane Lymphotoxin-α1β2 in Lymphotoxin-Induced Inflammation: Critical Role of TNF Receptor 1 Signaling

Lymphotoxin (LT, LTα, TNFβ) is a member of the immediate TNF family that also includes TNF-α and lymphotoxin-β (LTβ). LT is produced by activated lymphocytes and functions as either a secreted homotrimer or a membrane-associated heterotrimer that includes the transmembrane protein LTβ. Secreted LTα3 can bind to two cell surface receptors, TNFR1 and TNFR2, while the membrane-bound heterotrimer LTα1β2 has been shown to interact with a distinct receptor, LTβR. LTα induces inflammation at the sites of expression of a rat insulin promoter-driven lymphotoxin (RIPLT) transgene in the pancreas and kidney. To determine the role of the various ligands and their receptors in LT-induced inflammation, mice deficient in either TNFR1, TNFR2, or LTβ were crossed to RIPLT-transgenic mice. Our results indicate that LTα-induced inflammation is dependent on the interaction of LTα3 with TNFR1, and there is no obvious role for TNFR2, since in its absence, LTα-induced inflammation is quantitatively and qualitatively similar to that seen in the wild type. However, the absence of LTβ results in accentuated infiltration of the kidney with an increase in the proportion of memory cells in the infiltrate. These data show a crucial role for the secreted LTα3 signaling via TNFR1 in LTα-induced inflammation, and a separate and distinct role for the membrane LTα1β2 form in this inflammatory process.

Distinct roles of lymphotoxin alpha and the type I tumor necrosis factor (TNF) receptor in the establishment of follicular dendritic cells from non-bone marrow-derived cells

In mice deficient in either lymphotoxin alpha (LT-alpha) or type I tumor necrosis factor receptor (TNFR-I), organized clusters of follicular dendritic cells (FDC) and germinal centers (GC) are absent from the spleen. We investigated the role of LT-alpha and TNFR-I in the establishment of spleen FDC and GC structure by using reciprocal bone marrow (BM) transfer. When LT-alpha-deficient mice were reconstituted with wild-type BM, FDC organization and the ability to form GC were restored, indicating that the LT-alpha-expressing cells required to establish organized FDC are derived from BM. The role of LT-alpha in establishing organized FDC structure was further investigated by the transfer of complement receptor 1 and 2 (CR1/2)-deficient BM cells into LT-alpha-deficient mice. Organized FDC were identified with both the FDC-M1 and anti-CR1 monoclonal antibodies in these BM-chimeric mice, indicating that these cells were derived from the LT-alpha-deficient recipient. Thus, expression of LT-alpha in the BM-derived cells, but not in the non-BM-derived cells, is required for the maturation of FDC from non-BM precursor cells. In contrast, when TNFR-I-deficient mice were reconstituted with wild-type BM, they showed no detectable FDC clusters or GC formation. This indicates that TNFR-I expression on non-BM-derived cellular components is necessary for the establishment of these lymphoid structures. TNFR-I-deficient BM was able to restore FDC organization and GC formation in LT-alpha-deficient mice, indicating that formation of these structures does not require TNFR-I expression on BM-derived cells. The data in this study demonstrate that FDC organization and GC formation are controlled by both LT-alpha-expressing BM-derived cells and by TNFR-I-expressing non-BM-derived cells.

Mediators of inflammation

Therapeutic studies and genetically engineered animals have elucidated the inflammatory roles of cytokines and chemokines in autoimmune disease. Most unexpected has been a continuum of recent evidence demonstrating that inflammatory mediators are crucial in lymphoid organ development, thus suggesting that these hitherto unrelated processes have common elements with implications for determinant spreading.

Peyer's patch organogenesis--cytokines rule, OK?

Targeted inactivation of genes in the tumor necrosis factor (TNF)/lymphotoxin (LT) ligand and receptor system has recently revealed essential roles forthese molecules in lymphoid tissue development and organization. Lymphotoxin-alpha beta (LT alpha beta)/lymphotoxin-beta receptor (LT beta-R) signaling is critical for the organogenesis of lymph nodes and Peyer's patches and for the structural compartmentalization of the splenic white pulp into distinct B and T cell areas and marginal zones. Moreover, an essential role has been demonstrated for TNF/p55 tumor necrosis factor receptor (p55TNF-R) signaling in the formation of splenic B lymphocyte follicles, follicular dendritic cell networks, and germinal centers. In contrast to a previously described essential role for the p55TNF-R in Peyer's patch organogenesis, we show in this report that Peyer's patches are present in both TNF and p55TNF-R knockout mice, demonstrating that these molecules are not essential for the organogenesis of this lymphoid organ. Furthermore, we show that in the absence of TNF/p55TNF-R signaling, lymphocytes segregate normally into T and B cell areas and a normal content and localization of dendritic cells is observed in both lymph nodes and Peyer's patches. However, although B cells are found to home normally within Peyer's patches and in the outer cortex area of lymph nodes, organized follicular structures and follicular dendritic cell networks fail to form. These results show that in contrast to LT alpha beta signaling, TNF signaling through the p55TNF-R is not essential for lymphoid organogenesis but rather for interactions that determine the cellular and structural organization of B cell follicles in all secondary lymphoid tissues.

A critical role for lymphotoxin in experimental allergic encephalomyelitis

The lymphotoxin (LT)/tumor necrosis factor (TNF) family has been implicated in the neurologic inflammatory diseases multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE). To determine the role of individual family members in EAE, C57BL/6 mice, LT-alpha-deficient (LT-alpha-/- mice), or LT-beta-deficient (LT-beta-/- mice), and their wild-type (WT) littermates were immunized with rat myelin oligodendrocyte glycoprotein (MOG) peptide 35-55. C57BL/6 and WT mice developed chronic, sustained paralytic disease with average maximum clinical scores of 3.5 and disease indices (a measure of day of onset and sustained disease scores) ranging from 367 to 663 with central nervous system (CNS) inflammation and demyelination. LT-alpha-/- mice were primed so that their splenic lymphocytes proliferated in response to MOG 35-55 and the mice produced anti-MOG antibody. However, LT-alpha-/- mice were quite resistant to EAE with low average clinical scores (<1), an average disease index of 61, and the negligible CNS inflammation and demyelination. WT T cells transferred EAE to LT-alpha-/- recipients. LT-beta-/- mice were susceptible to EAE, though less than WT, with an average maximum clinical score of 1.9 and disease index of 312. These data implicate T cell production of LT-alpha in MOG EAE and support a major role for LT-alpha3, a minor role for the LT-alpha/beta complex, and by inference, no role for TNF-alpha.

Developing lymph nodes collect CD4+CD3- LTbeta+ cells that can differentiate to APC, NK cells, and follicular cells but not T or B cells

For a brief period during fetal lymph node organogenesis in mice, lymph node postcapillary high endothelial venules surprisingly express the Peyer's patch addressin MAdCAM-1. This expression allows initial seeding of this incipient structure by two unusual lymphocyte populations selectively expressing the Peyer's patch homing receptor integrin alpha4beta7: CD4+CD3- oligolineage progenitors and TCR gammadelta+ T cells. We show here that CD4+CD3- cells are lineage-restricted progenitors that express surface lymphotoxin-beta (LTbeta) and the chemokine receptor BLR1 and that can become natural killer cells, dendritic antigen-presenting cells, and follicular cells of unknown outcome, but these cells do not become T or B lymphocytes. Since the necessity of lymphotoxin in lymphoid organ development has been shown, we propose that the novel subset of CD4+CD3-LTbeta+ fetal cells is instrumental in the development of lymphoid tissue architecture.

Immunological tolerance to a pancreatic antigen as a result of local expression of TNFalpha by islet beta cells

Recent experiments have suggested that tumor necrosis factor alpha (TNFalpha) can down-regulate islet-specific T cells and prevent the development of autoimmune diabetes. Here we demonstrate that transgenic mice expressing both TNFalpha and the Leishmania major LACK antigen in the pancreas (RIP-TNFalpha/RIP-LACK) exhibit an impaired ability to mount a CD4+ T cell response against LACK. In addition, peripheral CD4+ T cells from TCR transgenic mice (TCR-LACK/RIP-TNFalpha/RIP-LACK) produced reduced interleukin-2 but elevated levels of T helper 2 cytokines in response to LACK peptide in vitro. Taken together, our data suggest that TNFalpha may act in vivo to modulate a potentially damaging self-reactive T cell response by inducing tolerance to pancreatic antigens.

Abnormal development of secondary lymphoid tissues in lymphotoxin beta-deficient mice

The tumor necrosis factor (TNF) family cytokines lymphotoxin (LT) alpha and LTbeta form heterotrimers that are expressed on the surface of activated lymphocytes and natural killer cells; LTalpha homotrimers can be secreted as well. Mice with a disrupted LTalpha gene lack lymph nodes (LN), Peyer's patches (PP), and follicular dendritic cell (FDC) networks and reveal profound defects of the splenic architecture. However, it is unclear which of these abnormalities is the result of the absence in LTalpha homotrimers or LTalphabeta heterotrimers. To distinguish between these two possibilities, a mouse strain deficient in LTbeta was created employing Cre/loxP-mediated gene targeting. Mice deficient in LTbeta reveal severe defects in organogenesis of the lymphoid system similar to those of LTalpha-/- mice, except that mesenteric and cervical LN are present in most LTbeta-deficient mice. Both LTbeta- and LTalpha-deficient mice show significant lymphocytosis in the circulation and peritoneal cavity and lymphocytic infiltrations in lungs and liver. After immunization, PNA-positive B cell clusters were detected in the splenic white pulp of LTbeta-deficient mice, but FDC networks were severely underdeveloped. Collectively, these results indicate that LTalpha can signal independently from LTbeta in the formation of PNA-positive foci in the spleen, and especially in the development of mesenteric and cervical LN.

Independent signals regulate development of primary and secondary follicle structure in spleen and mesenteric lymph node

Lymphotoxin-alpha-deficient (LT-alpha-/-) mice manifest congenital absence of lymph nodes (LNs) and Peyer's patches and disturbed spleen follicle structure. The splenic white pulp areas show loss of discrete T and B lymphocyte zones, of follicular dendritic cell (FDC) clusters, and of germinal centers (GCs). Tumor necrosis factor receptor I-deficient (TNFR-I-/-) mice show similar absence of FDC clusters and GCs but retain segregation of T and B cell zones. Rarely are mesenteric LNs found in LT-alpha-/- mice. These mesenteric LNs show segregation of T and B cell zones similar to wild-type mice. In contrast, mesenteric LNs in TNFR-I-/- mice manifest grossly disturbed organization of T and B cells. Both LT-alpha-/- and TNFR-I-/- mice lacked FDC clusters in LNs and spleen. Interestingly, although both LT-alpha-/- and TNFR-I-/- mice that had been immunized with sheep red blood cells failed to form GCs in the spleen, they both developed GC-like clusters of peanut agglutinin-positive (PNA+) cells in their LNs. Furthermore, when lethally irradiated recombination activating gene (RAG)-1-deficient (RAG-1(-/-)) mice that had received spleen cells from LT-alpha-/- mice were immunized with sheep red blood cells, they failed to generate PNA+ clusters in the reconstituted spleen but showed robust PNA+ clusters in the reconstituted LNs. These data demonstrate that the signals that regulate the development of distinct T and B cell zones as well as the signals that regulate B cell activation to produce clusters of PNA+ cells differ between the spleen and LNs.

Surface lymphotoxin alpha/beta complex is required for the development of peripheral lymphoid organs

For more than a decade, the biological roles and the apparent redundancy of the cytokines tumor necrosis factor (TNF) and lymphotoxin (LT) have been debated. LT alpha exists in its soluble form as a homotrimer, which like TNF only binds the TNF receptors, TNF-R55 or TNF-R75. The cell surface form of LT exists as a heteromer of LT alpha and LT beta subunits and this complex specifically binds the LT beta receptor (LT beta-R). To discriminate the functions of the LT and TNF systems, soluble LT beta-R-immunoglobulin (Ig) or TNF-R-Ig fusion proteins were introduced into embryonic circulation by injecting pregnant mice. Exposure to LT beta-R-Ig during gestation disrupted lymph node development and splenic architecture in the progeny indicating that both effects are mediated by the surface LT alpha/beta complex. These data are the first to identify a cell surface ligand involved in immune organ morphogenesis. Moreover, they unambiguously discriminate the functions of the various TNF/LT ligands, provide a unique model to study compartmentalization of immune responses and illustrate the generic utility of receptor-Ig fusion proteins for dissecting/ordering ontogenetic events in the absence of genetic modifications.

Tumour necrosis factors in immune regulation: everything that's interesting is...new!

Tumour necrosis factors have been classically studied as molecules central to the pathogenesis of infectious, inflammatory and autoimmune diseases. The recent generation of mice deficient in TNF alpha, LT alpha, or their receptors, has provided exciting new insights into the physiological role of these molecules in the development of secondary lymphoid tissues and in the organisation of the humoral immune response.