Expression of lymphotoxin beta governs immunity at two distinct levels

Reconstitution of a functional human thymus by postnatal stromal progenitor cells and natural whole-organ scaffolds

The thymus is a primary lymphoid organ, essential for T cell maturation and selection. There has been long-standing interest in processes underpinning thymus generation and the potential to manipulate it clinically, because alterations of thymus development or function can result in severe immunodeficiency and autoimmunity. Here, we identify epithelial-mesenchymal hybrid cells, capable of long-term expansion in vitro, and able to reconstitute an anatomic phenocopy of the native thymus, when combined with thymic interstitial cells and a natural decellularised extracellular matrix (ECM) obtained by whole thymus perfusion. This anatomical human thymus reconstruction is functional, as judged by its capacity to support mature T cell development in vivo after transplantation into humanised immunodeficient mice. These findings establish a basis for dissecting the cellular and molecular crosstalk between stroma, ECM and thymocytes, and offer practical prospects for treating congenital and acquired immunological diseases.

Innate lymphoid cell and adaptive immune cell cross-talk: A talk meant not to forget

Innate lymphoid cells (ILCs) are a relatively new class of innate immune cells with phenotypical characters of lymphocytes but genotypically or functionally behave as typical innate immune cells. They have been classically divided into 3 groups (group 1 ILCs or ILC1s, group 2 ILCs or ILC2s, and group 3 ILCs or ILC3s). They serve as the first line of defense against invading pathogens and allergens at mucosal surfaces. The adaptive immune response works effectively in association with innate immunity as innate immune cells serve as APCs to directly stimulate the adaptive immune cells (various sets of T and B cells). Additionally, innate immune cells also secrete various effector molecules, including cytokines or chemokines impacting the function, differentiation, proliferation, and reprogramming among adaptive immune cells to maintain immune homeostasis. Only superantigens do not require their processing by innate immune cells as they are recognized directly by T cells and B cells. Thus, a major emphasis of the current article is to describe the cross-talk between different ILCs and adaptive immune cells during different conditions varying from normal physiological situations to different infectious diseases to allergic asthma.

Mechanisms of lymphatic system-specific viral replication and its potential role in autoimmune disease

Viral infections can be fatal because of the direct cytopathic effects of the virus or the induction of a strong, uncontrolled inflammatory response. Virus and host intrinsic characteristics strongly modulate the outcome of viral infections. Recently we determined the circumstances under which enhanced replication of virus within the lymphoid tissue is beneficial for the outcome of a disease. This enforced viral replication promotes anti-viral immune activation and, counterintuitively, accelerates virus control. In this review we summarize the mechanisms that contribute to enforced viral replication. Antigen-presenting cells and CD169⁺ macrophages exhibit enforced viral replication after infection with the model viruses lymphocytic choriomeningitis virus (LCMV) and vesicular stomatitis virus (VSV). Ubiquitin-specific peptidase 18 (Usp18), an endogenous type I interferon blocker in CD169⁺ macrophages, has been identified as a proviral gene, as are B cell activating factor (BAFF) and carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1). Lymphotoxins (LT) strongly enhance viral replication in the spleen and lymph nodes. All these factors modulate splenic architecture and thereby promote the development of CD169⁺ macrophages. Tumor necrosis factor alpha (TNF-α) and nuclear factor kappa-light-chain-enhancer of activated B cell signaling (NF-κB) have been found to promote the survival of infected CD169⁺ macrophages, thereby similarly promoting enforced viral replication. Association of autoimmune disease with infections is evident from (1) autoimmune phenomena described during a chronic virus infection; (2) onset of autoimmune disease simultaneous to viral infections; and (3) experimental evidence. Involvement of virus infection during onset of type I diabetes is strongly evident. Epstein-Bar virus (EBV) infection was discussed to be involved in the pathogenesis of systemic lupus erythematosus. In conclusion, several mechanisms promote viral replication in secondary lymphatic organs. Identifying such factors in humans is a challenge for future studies.

Clinical aspects and polymorphisms in the LTA, TNFA, LTB genes and association with dental implant loss

BACKGROUND

This study shows the relationship between host factors and environmental factors in the influence of susceptibility to loss of dental implants.

PURPOSE

The aim of this study was to investigate the association of clinical aspects and tag SNPs of the genes LTA, TNFA, and LTB with dental implant loss.

MATERIALS AND METHODS

The subjects consisted of 244 patients, divided into two groups: control group (C)-163 individuals who did not lose any implants, being in function for at least 6 months; and study group (S)-81 individuals who had lost at least one implant. DNA was collected from saliva, and the genotypes were determined by real time PCR. Univariate and multivariate analysis were employed p < .05.

RESULTS

After multivariate analysis, dental implant loss remained associated with the presence of teeth (p = .011), a larger amount of placed implants (p = .001), and allelle C of rs2009658 of the LTA gene (p = .006). For the other tag SNPs of these studied genes, there was no association between the groups C and S with dental implants loss.

CONCLUSION

Presence of teeth, number of placed implants and allele C of rs2009658 of LTA gene were associated with implant loss.

mTORC1 activation in B cells confers impairment of marginal zone microarchitecture by exaggerating cathepsin activity

Mammalian target of rapamycin complex 1 (mTORC1) is a key regulator of cell metabolism and lymphocyte proliferation. It is inhibited by the tuberous sclerosis complex (TSC), a heterodimer of TSC1 and TSC2. Deletion of either gene results in robust activation of mTORC1. Mature B cells reside in the spleen at two major anatomical locations, the marginal zone (MZ) and follicles. The MZ constitutes the first line of humoral response against blood-borne pathogens and undergoes atrophy in chronic inflammation. In previous work, we showed that mice deleted for TSC1 in their B cells (TSC1^BKO ) have almost no MZ B cells, whereas follicular B cells are minimally affected. To explore potential underlying mechanisms for MZ B-cell loss, we have analysed the spleen MZ architecture of TSC1^BKO mice and found it to be severely impaired. Examination of lymphotoxins (LTα and LTβ) and lymphotoxin receptor (LTβR) expression indicated that LTβR levels in spleen stroma were reduced by TSC1 deletion in the B cells. Furthermore, LTα transcripts in B cells were reduced. Because LTβR is sensitive to proteolysis, we analysed cathepsin activity in TSC1^BKO . A higher cathepsin activity, particularly of cathepsin B, was observed, which was reduced by mTORC1 inhibition with rapamycin in vivo. Remarkably, in vivo administration of a pan-cathepsin inhibitor restored LTβR expression, LTα mRNA levels and the MZ architecture. Our data identify a novel connection, although not elucidated at the molecular level, between mTORC1 and cathepsin activity in a manner relevant to MZ dynamics.

Receptor Activator of NF-κB Orchestrates Activation of Antiviral Memory CD8 T Cells in the Spleen Marginal Zone

The spleen plays an important role in protective immunity to bloodborne pathogens. Macrophages and dendritic cells (DCs) in the spleen marginal zone capture microbial antigens to trigger adaptive immune responses. Marginal zone macrophages (MZMs) can also act as a replicative niche for intracellular pathogens, providing a platform for mounting the immune response. Here, we describe a role for RANK in the coordinated function of antigen-presenting cells in the spleen marginal zone and triggering anti-viral immunity. Targeted deletion of RANK results in the selective loss of CD169⁺ MZMs, which provide a niche for viral replication, while RANK signaling in DCs promotes the recruitment and activation of anti-viral memory CD8 T cells. These studies reveal a role for the RANKL/RANK signaling axis in the orchestration of protective immune responses in the spleen marginal zone that has important implications for the host response to viral infection and induction of acquired immunity.

Reduced lymphotoxin-beta production by tumour cells is associated with loss of follicular dendritic cell phenotype and diffuse growth in follicular lymphoma

Cytokine production is essential for follicular dendritic cell (FDC) maintenance and organization of germinal centres. In follicular lymphoma, FDCs are often disarrayed and may lack antigens indicative of terminal differentiation. We investigated the in situ distribution of cells producing lymphotoxin‐beta (LTB), lymphotoxin‐alpha (LTA), and tumour necrosis factor‐alpha (TNFA) transcripts in human reactive lymph nodes and in follicular lymphomas with follicular or diffuse growth pattern. LTB was the cytokine most abundantly produced in germinal centres. LTB was present in nearly 90% of germinal centre cells whereas LTA and TNFA were detected in 30 and 50%, respectively. Moreover, the amount of LTB expressed in reactive germinal centre cells was 80‐fold higher than that of LTA and 20‐fold higher than that of TNFA. LTB‐positive cells were more numerous in the germinal centre dark zone, whereas expression of the FDC proteins CD21, CD23, VCAM, and CXCL13 was more intense in the light zone. Tumour cells of follicular lymphomas produced less LTB than reactive germinal centre cells. The results of the in situ study were confirmed by RT‐PCR; LTB was significantly more abundant in reactive lymph nodes than in follicular lymphoma, with the lowest values detected in predominantly diffuse follicular lymphoma. In neoplastic follicles, low production of LTB by tumour B cells was associated with weaker expression of CD21+/CD23+ by FDCs. Our findings detail for the first time the distribution of LTA‐, LTB‐, and TNFA‐producing cells in human reactive germinal centres and in follicular lymphoma. They suggest the possibility that impaired tumour‐cell LTB production may represent a determinant of FDC phenotype loss and for defective follicular organization in follicular lymphoma.

Lymphotoxin in physiology of lymphoid tissues - Implication for antiviral defense

Lymphotoxin (LT) is a member of the tumor necrosis factor (TNF) superfamily of cytokines which serves multiple functions, including the control of lymphoid organ development and maintenance, as well as regulation of inflammation and autoimmunity. Although the role of LT in organogenesis and maintenance of lymphoid organs is well established, the contribution of LT pathway to homeostasis of lymphoid organs during the immune response to pathogens is less understood. In this review, we highlight recent advances on the role of LT pathway in antiviral immune responses. We discuss the role of LT signaling in lymphoid organ integrity, type I IFN production and regulation of protection and immunopathology during viral infections. We further discuss the potential of therapeutic targeting LT pathway for controlling immunopathology and antiviral protection.

A dendritic-cell-stromal axis maintains immune responses in lymph nodes

Within secondary lymphoid tissues, stromal reticular cells support lymphocyte function, and targeting reticular cells is a potential strategy for controlling pathogenic lymphocytes in disease. However, the mechanisms that regulate reticular cell function are not well understood. Here we found that during an immune response in lymph nodes, dendritic cells (DCs) maintain reticular cell survival in multiple compartments. DC-derived lymphotoxin beta receptor (LTβR) ligands were critical mediators, and LTβR signaling on reticular cells mediated cell survival by modulating podoplanin (PDPN). PDPN modulated integrin-mediated cell adhesion, which maintained cell survival. This DC-stromal axis maintained lymphocyte survival and the ongoing immune response. Our findings provide insight into the functions of DCs, LTβR, and PDPN and delineate a DC-stromal axis that can potentially be targeted in autoimmune or lymphoproliferative diseases.

Deficiency of the B cell-activating factor receptor results in limited CD169+ macrophage function during viral infection

The B cell-activating factor (BAFF) is critical for B cell development and humoral immunity in mice and humans. While the role of BAFF in B cells has been widely described, its role in innate immunity remains unknown. Using BAFF receptor (BAFFR)-deficient mice, we characterized BAFFR-related innate and adaptive immune functions following infection with vesicular stomatitis virus (VSV) and lymphocytic choriomeningitis virus (LCMV). We identified a critical role for BAFFR signaling in the generation and maintenance of the CD169(+) macrophage compartment. Consequently, Baffr(-) (/) (-) mice exhibited limited induction of innate type I interferon production after viral infection. Lack of BAFFR signaling reduced virus amplification and presentation following viral infection, resulting in highly reduced antiviral adaptive immune responses. As a consequence, BAFFR-deficient mice showed exacerbated and fatal disease after viral infection. Mechanistically, transient lack of B cells in Baffr(-) (/) (-) animals resulted in limited lymphotoxin expression, which is critical for maintenance of CD169(+) cells. In conclusion, BAFFR signaling affects both innate and adaptive immune activation during viral infections.

IMPORTANCE

Viruses cause acute and chronic infections in humans resulting in millions of deaths every year. Innate immunity is critical for the outcome of a viral infection. Innate type I interferon production can limit viral replication, while adaptive immune priming by innate immune cells induces pathogen-specific immunity with long-term protection. Here, we show that BAFFR deficiency not only perturbed B cells, but also resulted in limited CD169(+) macrophages. These macrophages are critical in amplifying viral particles to trigger type I interferon production and initiate adaptive immune priming. Consequently, BAFFR deficiency resulted in reduced enforced viral replication, limited type I interferon production, and reduced adaptive immunity compared to BAFFR-competent controls. As a result, BAFFR-deficient mice were predisposed to fatal viral infections. Thus, BAFFR expression is critical for innate immune activation and antiviral immunity.

T lymphocytes maintain structure and function of fibroblastic reticular cells via lymphotoxin (LT)-B

Background

Although a lot is known about how Fibroblastic Reticular Cells (FRCs) can regulate T lymphocytes (T cells), little is understood about whether or how T cells can regulate FRCs.

Results

This study shows that the absence of T cells inhibited the secretion of ER-TR7 by splenic FRCs, induced the structural disorder of FRCs, down-regulated the expression of the chemokine ligands CCL21 and CCL19, and weakened the homing ability of T cells to the spleen of nude mice. Transfusion of T cells from BALB/c mice restored the structure and functions of FRCs and recovered them. The expression of lymphotoxin (LT)-B was significantly downregulated in the absence of T cells from nude mice and was recovered after the transfusion of T cells. After the occlusion of the LT-B receptor, the FRCs’ structure and functions were not restored by transfusion of T cells.

Conclusions

These data reveal that the absence of T cells will subject spleen FRCs to structural and functional abnormality, and weaken the homing ability of T cells to the spleen. These changes are attributed to the T-cell- derived LT-B.

Tolerogenic properties of lymphatic endothelial cells are controlled by the lymph node microenvironment

Peripheral self-tolerance eliminates lymphocytes specific for tissue-specific antigens not encountered in the thymus. Recently, we demonstrated that lymphatic endothelial cells in mice directly express peripheral tissue antigens, including tyrosinase, and induce deletion of specific CD8 T cells via Programmed Death Ligand-1 (PD-L1). Here, we demonstrate that high-level expression of peripheral tissue antigens and PD-L1 is confined to lymphatic endothelial cells in lymph nodes, as opposed to tissue (diaphragm and colon) lymphatics. Lymphatic endothelial cells in the lymph node medullary sinus express the highest levels of peripheral tissue antigens and PD-L1, and are the only subpopulation that expresses tyrosinase epitope. The representation of lymphatic endothelial cells in the medullary sinus expressing high-level PD-L1, which is necessary for normal CD8 T cell deletion kinetics, is controlled by lymphotoxin-β receptor signaling and B cells. Lymphatic endothelial cells from neonatal mice do not express high-level PD-L1 or present tyrosinase epitope. This work uncovers a critical role for the lymph node microenvironment in endowing lymphatic endothelial cells with potent tolerogenic properties.

Sialoadhesin - a macrophage-restricted marker of immunoregulation and inflammation

Sialoadhesin (Sn, also known as Siglec-1 and CD169) is a macrophage-restricted cell surface receptor that is conserved across mammals. Sn is a member of the sialic acid-binding IgG-like lectin (Siglec) family of proteins characterized by affinity to specifically sialylated ligands, and under normal conditions is expressed on subsets of macrophages in secondary lymphoid tissues, such as lymph node and spleen. However, Sn-positive macrophages can also be found in a variety of pathological conditions, including (autoimmune) inflammatory infiltrates and tumours. Sn has been shown to contribute to sialylated pathogen uptake, antigen presentation and lymphocyte proliferation, and to influence both immunity and tolerance. This review presents Sn as a macrophage-specific marker of inflammation and immunoregulation with the potential to becoming an important biomarker for immunologically active macrophages and a target for therapy.

Lymphotoxin-beta receptor expression and its related signaling pathways govern dendritic cell homeostasis and function

Dendritic cells (DCs) play a fundamental function, either positive or detrimental, in regulating immune responses. Numerous specialized DC subsets exist in different organs. However, the trophic factors regulating their origin, location, homeostasis and functions remains to be fully understood. Recent evidence indicates that signaling via the lymphotoxin β receptor (LTβR) can function as a trophic signaling system for specific DCs. LTβR is part of a complex signaling network that provides both positive and inhibitory signals to DC subsets. In this review, we focus on the role of LTβR expressed in DC subsets and its associated signaling pathways that regulate DC homeostasis and function. Therapeutically targeting the LTβR signaling pathway could support the development of a beneficial immune response for the host.

B cell maintenance of subcapsular sinus macrophages protects against a fatal viral infection independent of adaptive immunity

Neutralizing antibodies have been thought to be required for protection against acutely cytopathic viruses, such as the neurotropic vesicular stomatitis virus (VSV). Utilizing mice that possess B cells but lack antibodies, we show here that survival upon subcutaneous (s.c.) VSV challenge was independent of neutralizing antibody production or cell-mediated adaptive immunity. However, B cells were absolutely required to provide lymphotoxin (LT) α1β2, which maintained a protective subcapsular sinus (SCS) macrophage phenotype within virus draining lymph nodes (LNs). Macrophages within the SCS of B cell-deficient LNs, or of mice that lack LTα1β2 selectively in B cells, displayed an aberrant phenotype, failed to replicate VSV, and therefore did not produce type I interferons, which were required to prevent fatal VSV invasion of intranodal nerves. Thus, although B cells are essential for survival during VSV infection, their contribution involves the provision of innate differentiation and maintenance signals to macrophages, rather than adaptive immune mechanisms.

B cells regulate antibody responses through the medullary remodeling of inflamed lymph nodes

Lymph node (LN) structure is remodeled during immune responses, a process which is considered to play an important role in the regulation of immune function. To date, little attention has been paid to the remodeling of the medullary region, despite its proposed role as a niche for antibody-producing plasma cells. Here, we show that B cells mediate medullary remodeling of antigen-draining LNs during inflammation. This process occurs with kinetics similar to changes in plasma cell number and is accompanied by stromal renetworking which manifests as the segregation of B cells and plasma cells. Medullary remodeling depends on signaling via the lymphotoxin-β receptor and the presence of B cells but occurs independently of T-dependent humoral responses or other immune cell subsets including T cells, monocytes and neutrophils. Moreover, reconstitution of non-cognate polyclonal B cells in B cell-deficient mice restores not only the medullary remodeling but also the antibody response by separately transferred cognate B cells, suggesting that non-cognate B cells contribute to antibody responses through medullary remodeling. We propose that non-cognate B cells mediate the expansion of the plasma cell niche in LN through medullary remodeling, thereby regulating the size of the LN plasma cell pool.

Enforced viral replication activates adaptive immunity and is essential for the control of a cytopathic virus

The innate immune system limits viral replication via type I interferon and also induces the presentation of viral antigens to cells of the adaptive immune response. Using infection of mice with vesicular stomatitis virus, we analyzed how the innate immune system inhibits viral propagation but still allows the presentation of antigen to cells of the adaptive immune response. We found that expression of the gene encoding the inhibitory protein Usp18 in metallophilic macrophages led to lower type I interferon responsiveness, thereby allowing locally restricted replication of virus. This was essential for the induction of adaptive antiviral immune responses and, therefore, for preventing the fatal outcome of infection. In conclusion, we found that enforced viral replication in marginal zone macrophages was an immunological mechanism that ensured the production of sufficient antigen for effective activation of the adaptive immune response.

Growth-factor receptor-bound protein-2 (Grb2) signaling in B cells controls lymphoid follicle organization and germinal center reaction

Grb2 (growth-factor receptor-bound protein-2) is a signaling adaptor that interacts with numerous receptors and intracellular signaling molecules. However, its role in B-cell development and function remains unknown. Here we show that ablation of Grb2 in B cells results in enhanced B-cell receptor signaling; however, mutant B cells do not form germinal centers in the spleen after antigen stimulation. Furthermore, mutant mice exhibit defects in splenic architecture resembling that observed in B-cell-specific lymphotoxin-β-deficient mice, including disruption of marginal zone and follicular dendritic cell networks. We find that grb2(-/-) B cells are defective in lymphotoxin-β expression. Although lymphotoxin can be up-regulated by chemokine CXCL13 and CD40 ligand stimulation in wild-type B cells, elevation of lymphotoxin expression in grb2(-/-) B cells is only induced by anti-CD40 but not by CXCL13. Our results thus define Grb2 as a nonredundant regulator that controls lymphoid follicle organization and germinal center reaction. Loss of Grb2 has no effect on B-cell chemotaxis to CXCL13, indicating that Grb2 executes this function by connecting the CXCR5 signaling pathway to lymphotoxin expression but not to chemotaxis.

Long-term immunologically competent human peripheral lymphoid tissue cultures in a 3D bioreactor

Peripheral lymphoid organs (PLOs), the primary sites of development of adaptive immune responses, display a complex structural organization reflecting separation of cellular subsets (e.g., T and B lymphocytes) and functional compartments which is critical for immune function. The generation of in vitro culture systems capable of recapitulating salient features of PLOs for experimental, biotechnological, and clinical applications would be highly desirable, but has been hampered so far by the complexity of these systems. We have previously developed a three-dimensional bioreactor system for long-term, functional culture of human bone marrow cells on macroporous microspheres in a packed-bed bioreactor with frequent medium change. Here we adapt the same system for culture of human primary cells from PLOs (tonsil) in the absence of specific exogenous growth factors or activators. Cells in this system displayed higher viability over several weeks, and maintain population diversity and cell surface markers largely comparable to primary cells. Light microscopy showed cells organizing in large diverse clusters within the scaffold pores and presence of B cell-enriched areas. Strikingly, these cultures generated a significant number of antibody-producing B cells when challenged with a panel of diverse antigens, as expected from a lymphoid tissue. Thus the three-dimensional tonsil bioreactor culture system may serve as a useful model of PLOs by recapitulating their structural organization and function ex vivo.

Transcriptional repression and DNA looping associated with a novel regulatory element in the final exon of the lymphotoxin-β gene

Transcriptional regulation has a critical role in the coordinate and context-specific expression of a cluster of genes encoding members of the tumour necrosis factor (TNF) superfamily found at chromosome 6p21, comprising TNF, LTA (encoding lymphotoxin-α) and LTB (encoding lymphotoxin-β). This is important, as dysregulated expression of these genes is implicated in susceptibility to many autoimmune, inflammatory and infectious diseases. We describe here a novel regulatory element in the fourth exon of LTB, which is highly conserved, localises to the only CpG island in the locus, and is associated with a DNase I hypersensitive site and specific histone modifications. We find evidence of binding by Yin Yang 1 (YY1), cyclic AMP response element (CRE)-binding protein (CREB) and CCCTC-binding factor (CTCF) to this region in Jurkat T cells, which is associated with transcriptional repression on reporter gene analysis. Chromatin conformation capture experiments show evidence of DNA looping, involving interaction of this element with the LTB promoter, LTA promoter and TNF 3′ untranslated region (UTR). Small interfering RNA (siRNA) experiments demonstrate a functional role for YY1 and CREB in LTB expression. Our findings provide evidence of additional complexity in the transcriptional regulation of LTB with implications for coordinate expression of genes in this important genomic locus.

Cytokine/stromal cell networks and lymphoid tissue environments

Initiation of an effective adaptive immune response against a foreign pathogen requires orchestrated encounters between lymphocytes and antigen-presenting cells. The tissues of the lymphoid system provide the ideal environment for increasing the efficiency of these encounters. Within the spleen, the mucosal-associated lymphoid tissues, and the lymph nodes, an intricate network of stromal cells, collagen fibers, and extracellular matrix exists that effectively compartmentalizes immune cells as they transit through these tissues. The stromal cells within lymphoid tissues are by no means homogenous, and it is now clear that these cells are not merely sessile bystanders during immune responses. Indeed, stromal cells within lymphoid tissues are the source of important cytokines and chemokines that guide and polarize immune cells. Here, we review the cytokines that maintain the integrity of this important stromal scaffold system within the lymphoid tissue, paying particular attention to the Lymphotoxin pathway, which is an important player in stromal cell biology. How cytokines maintain the organization of lymphoid tissues during development, in the adult animal, during inflammation and during disease will be discussed in sequence, and the clinical implications of targeting cytokines that regulate lymphoid tissue stroma will be considered.

Tumor necrosis factor, lymphotoxin and cancer

Initially TNF has been discovered as an anti-tumor factor, but it is now considered as one of the universal effectors of innate signaling implicating its key role in host defense and inflammation. Other physiological functions of TNF are primarily linked to organization of lymphoid tissues. TNF can exert deleterious effects on the organism when its local or systemic concentrations exceed certain levels. This is the main reason for the failure of TNF therapy in oncology. Moreover, in certain experimental models TNF to TNFRp55 signaling axis was found to play a pro-tumorigenic role. On the other hand, anti-TNF therapy proved to be beneficial in rheumatic and other autoimmune diseases. Taking into consideration the pivotal function of TNF in the immune system, it is obvious that such therapy cannot be entirely free of adverse effects including suppression of host defense and, possibly, predisposition to lymphomas. Lymphotoxins alpha and beta are the two related cytokines that exist in distinct trimeric forms which can signal through TNFR I and TNFR II, as well LTbetaR receptors, depending on the composition of the trimer. These signals have important functions in the development and homeostasis of the immune system. Importantly, there is a recently uncovered link between the LTalpha/LTbeta to LTbetaR signaling axis and cancer. Here we review the current status of the field with the focus on one particular issue: are TNF and lymphotoxins intrinsically anti-cancer or pro-tumorigenic.

Lymphotoxin beta receptor signaling in intestinal epithelial cells orchestrates innate immune responses against mucosal bacterial infection

Epithelial cells provide the first line of defense against mucosal pathogens; however, their coordination with innate and adaptive immune cells is not well understood. Using mice with conditional gene deficiencies, we found that lymphotoxin (LT) from innate cells expressing transcription factor RORgammat, but not from adaptive T and B cells, was essential for the control of mucosal C. rodentium infection. We demonstrate that the LTbetaR signaling was required for the regulation of the early innate response against infection. Furthermore, we have revealed that LTbetaR signals in gut epithelial cells and hematopoietic-derived cells coordinate to protect the host from infection. We further determined that LTbetaR signaling in intestinal epithelial cells was required for recruitment of neutrophils to the infection site early during infection via production of CXCL1 and CXCL2 chemokines. These results support a model wherein LT from RORgammat(+) cells orchestrates the innate immune response against mucosal microbial infection.

Accelerated thymic atrophy as a result of elevated homeostatic expression of the genes encoded by the TNF/lymphotoxin cytokine locus

TNF, lymphotoxin (LT)-alpha, LT-beta and LIGHT are members of a larger superfamily of TNF-related cytokines that can cross-utilize several receptors. Although LIGHT has been implicated in thymic development and function, the role of TNF and LT remains incompletely defined. To address this, we created a model of modest homeostatic overexpression of TNF/LT cytokines using the genomic human TNF/LT locus as a low copy number Tg. Strikingly, expression of Tg TNF/LT gene products led to profound early thymic atrophy characterized by decreased numbers of thymocytes and cortical thymic epithelial cells, partial block of thymocyte proliferation at double negative (DN) 1 stage, increased apoptosis of DN2 thymocytes and severe decline of T-cell numbers in the periphery. Results of backcrossing to TNFR1-, LTbetaR- or TNF/LT-deficient backgrounds and of reciprocal bone marrow transfers implicated both LT-alpha/LT-beta to LTbetaR and TNF/LT-alpha to TNFR1 signaling in accelerated thymus degeneration. We hypothesize that chronic infections can promote thymic atrophy by upregulating LT and TNF production.

Form follows function: lymphoid tissue microarchitecture in antimicrobial immune defence

Secondary lymphoid organs (SLOs) are tissues that facilitate the induction of adaptive immune responses. These organs capture pathogens to limit their spread throughout the body, bring antigen-presenting cells into productive contact with their cognate lymphocytes and provide niches for the differentiation of immune effector cells. Therefore, the microanatomy of SLOs defines the ability of an organism to respond to pathogens. SLO microarchitecture is, at the same time, extremely adaptable to environmental changes. In this Review, we discuss recent insights into the function and plasticity of the SLO microenvironment with regards to antimicrobial immune defence.

The TNF receptor and Ig superfamily members form an integrated signaling circuit controlling dendritic cell homeostasis

Dendritic cells (DC) constitute the most potent antigen presenting cells of the immune system, playing a key role bridging innate and adaptive immune responses. Specialized DC subsets differ depending on their origin, tissue location and the influence of trophic factors, the latter remain to be fully understood. Myeloid-associated lymphotoxin-beta receptor (LTbetaR) signaling is required for the local proliferation of lymphoid tissue DC. This review focuses on the LTbetaR signaling cascade as a crucial positive trophic signal in the homeostasis of DC subsets. The noncanonical coreceptor pathway comprised of the immunoglobulin (Ig) superfamily member, B and T lymphocyte attenuator (BTLA) and TNFR superfamily member, herpesvirus entry mediator (HVEM) counter regulates the trophic signaling by LTbetaR. Together both pathways form an integrated signaling circuit achieving homeostasis of DC subsets.

Restoration of lymphoid organ integrity through the interaction of lymphoid tissue-inducer cells with stroma of the T cell zone

The generation of lymphoid microenvironments in early life depends on the interaction of lymphoid tissue-inducer cells with stromal lymphoid tissue-organizer cells. Whether this cellular interface stays operational in adult secondary lymphoid organs has remained elusive. We show here that during acute infection with lymphocytic choriomeningitis virus, antiviral cytotoxic T cells destroyed infected T cell zone stromal cells, which led to profound disruption of secondary lymphoid organ integrity. Furthermore, the ability of the host to respond to secondary antigens was lost. Restoration of the lymphoid microanatomy was dependent on the proliferative accumulation of lymphoid tissue-inducer cells in secondary lymphoid organs during the acute phase of infection and lymphotoxin alpha(1)beta(2) signaling. Thus, crosstalk between lymphoid tissue-inducer cells and stromal cells is reactivated in adults to maintain secondary lymphoid organ integrity and thereby contributes to the preservation of immunocompetence.

Lymphotoxin-mediated crosstalk between B cells and splenic stroma promotes the initial type I interferon response to cytomegalovirus

Toll-like receptor (TLR)-dependent pathways control the production of IFNalphabeta, a key cytokine in innate immune control of viruses including mouse cytomegalovirus (MCMV). The lymphotoxin (LT) alphabeta-LTbeta receptor signaling pathway is also critical for defense against MCMV and thought to aid in the IFNbeta response. We find that upon MCMV infection, mice deficient for lymphotoxin (LT)alphabeta signaling cannot mount the initial part of a biphasic IFNalphabeta response, but show normal levels of IFNalphabeta during the sustained phase of infection. Significantly, the LTalphabeta-dependent, IFNalphabeta response is independent of TLR signaling. B, but not T, cells expressing LTbeta are essential for promoting the initial IFNalphabeta response. LTbetaR expression is required strictly in splenic stromal cells for initial IFNalphabeta production to MCMV and is dependent upon the NF-kappaB-inducing kinase (NIK). These results reveal a TLR-independent innate host defense strategy directed by B cells in communication with stromal cells via the LTalphabeta cytokine system.

An Alternative Pathway of NF-κB Activation Results in Maturation and T Cell Priming Activity of Dendritic Cells Overexpressing a Mutated IκBα

Maturation of dendritic cells (DC) is a critical step in the induction of T cell responses and depends on the activation of NF-kappaB transcription factors. Therefore, inhibition of NF-kappaB activation has been proposed as a strategy to maintain DC in an immature stage and to promote immune tolerance. Herein, we generated murine myeloid DC expressing a mutated IkappaBalpha acting as a superrepressor of the classical NF-kappaB pathway (s-rIkappaB DC) to investigate the consequences of NF-kappaB inhibition on the ability of DC to prime T cell responses. Upon in vitro LPS activation, maturation of s-rIkappaB DC was profoundly impaired as indicated by defective up-regulation of MHC class II and costimulatory molecules and reduced secretion of IL-12 p70 and TNF-alpha. In contrast, after injection, s-rIkappaB DC had the same capacity as control DC to migrate to draining lymph node and to induce Th1- and Th2-type cytokine production in a MHC class II-incompatible host mice. Likewise, s-rIkappaB DC pulsed with OVA were as efficient as control DC to induce Ag-specific T cell responses in vivo. Indeed, further in vitro experiments established that s-rIkappaB DC undergo efficient maturation upon prolonged contact with activated T cells via the alternative pathway of NF-kappaB activation triggered at least partly by lymphotoxin beta receptor ligation and involving processing of p100/RelB complexes.