Distinct contributions of TNF and LT cytokines to the development of dendritic cells in vitro and their recruitment in vivo

Superior low-immunogenicity of tilapia type I collagen based on unique secondary structure with single calcium binding motif over terrestrial mammals by inhibiting activation of DC intracellular Ca2+-mediated STIM1-Orai1/NF-кB pathway

The reason for low- or non-immunogenicity of fish collagens is still in doubt, which, to some extent, bottlenecks their production and clinical application as biomaterials. Employing bovine or porcine type I collagens (BCI or PCI) as controls in this paper, we intensively investigate the influence of tilapia type I collagens (TCI) on the function of dendritic cells (DCs) and T cells. From bio-informatic analyses, as well as data obtained in vitro and in vivo, we find the variations in amino acid sequences lead to only one calcium binding motif in the secondary structure of TCI, compared with three in BCI or PCI. So when TCI (together with the minor amount of Ca²⁺ they take) are uptaken, intracellular [Ca²⁺] remains stable and DCs maintain immature. On the contrary, those that have uptaken PCI or BCI experience not only increased [Ca²⁺] in the plasma but also phosphorylation of p65, resulting in activation of STIM1-Orai1/NF-кB signaling pathway and DC maturation. We fully prove our results on mice models, with no obvious cellular and humoral immune reactions. Our study primarily reveal the underlying mechanisms why TCI, different from BCI or PCI, show almost non-immunogenicity. Our findings are of great importance for the promotion and wide application of TCI in biomedicine.

Immune control of Mycobacterium tuberculosis is dependent on both soluble TNFRp55 and soluble TNFRp75

Tuberculosis presents a global health challenge, and tumour necrosis factor (TNF) signalling is required for host immunity against Mycobacterium tuberculosis (Mtb). TNF receptor shedding, however, compromises effective immunity by reducing bioactive TNF through the formation of inactive complexes. In this study, we first compared the effect of total soluble TNF receptors using a transgenic p55^ΔNS /p75^-/- murine strain on host protection during a low-dose aerosol Mtb H37Rv challenge. We report that the presence of membrane-bound TNFRp55 alone in the absence of TNFRp75 results in superior control of a primary Mtb infection where p55^ΔNS /p75^-/- hyperactive dendritic cells displayed an increased capacity to induce a hyperactive Mtb-specific CD4⁺ T-cell response. p55^ΔNS /p75^-/- dendritic cells expressed a higher frequency of MHCII and increased MFIs for both CD86 and MHCII, while CD4⁺ T cells had higher expression of CD44 and IFN-γ. Next, the relative contributions of soluble TNFRp55 and soluble TNFRp75 to host protection against either primary Mtb infection or during reactivation of latent tuberculosis were delineated by comparing the experimental outcomes of control C57BL/6 mice to transgenic p55^ΔNS /p75^-/- , p55^ΔNS and p75^-/- mouse strains. We found that soluble TNFRp55 is redundant for immune regulation during the chronic stages of a primary Mtb infection. However, TNFRp55 together with soluble TNFRp75 has a crucial role in immune regulation of reactivation of latent tuberculosis.

ILC3s control splenic cDC homeostasis via lymphotoxin signaling

The spleen contains a myriad of conventional dendritic cell (cDC) subsets that protect against systemic pathogen dissemination by bridging antigen detection to the induction of adaptive immunity. How cDC subsets differentiate in the splenic environment is poorly understood. Here, we report that LTα1β2-expressing Rorgt+ ILC3s, together with B cells, control the splenic cDC niche size and the terminal differentiation of Sirpα+CD4+Esam+ cDC2s, independently of the microbiota and of bone marrow pre-cDC output. Whereas the size of the splenic cDC niche depended on lymphotoxin signaling only during a restricted time frame, the homeostasis of Sirpα+CD4+Esam+ cDC2s required continuous lymphotoxin input. This latter property made Sirpα+CD4+Esam+ cDC2s uniquely susceptible to pharmacological interventions with LTβR agonists and antagonists and to ILC reconstitution strategies. Together, our findings demonstrate that LTα1β2-expressing Rorgt+ ILC3s drive splenic cDC differentiation and highlight the critical role of ILC3s as perpetual regulators of lymphoid tissue homeostasis.

Deficiency of the Intramembrane Protease SPPL2a Alters Antimycobacterial Cytokine Responses of Dendritic Cells

Signal peptide peptidase-like 2a (SPPL2a) is an aspartyl intramembrane protease essential for degradation of the invariant chain CD74. In humans, absence of SPPL2a leads to Mendelian susceptibility to mycobacterial disease, which is attributed to a loss of the dendritic cell (DC) subset conventional DC2. In this study, we confirm depletion of conventional DC2 in lymphatic tissues of SPPL2a^-/- mice and demonstrate dependence on CD74 using SPPL2a^-/- CD74^-/- mice. Upon contact with mycobacteria, SPPL2a^-/- bone marrow-derived DCs show enhanced secretion of IL-1β, whereas production of IL-10 and IFN-β is reduced. These effects correlated with modulated responses upon selective stimulation of the pattern recognition receptors TLR4 and Dectin-1. In SPPL2a^-/- bone marrow-derived DCs, Dectin-1 is redistributed to endosomal compartments. Thus, SPPL2a deficiency alters pattern recognition receptor pathways in a CD74-dependent way, shifting the balance from anti- to proinflammatory cytokines in antimycobacterial responses. We propose that in addition to the DC reduction, this altered DC functionality contributes to Mendelian susceptibility to mycobacterial disease upon SPPL2a deficiency.

Molecular Programming of Perivascular Stem Cell Precursors

Pericytes have been shown to act as precursors of resident adult stem cells in stromal tissues in vivo. When expanded in vitro these cells are capable of giving rise to multiple mesenchymal cell types, irrespective of their tissue of origin. This phenomenon of multi-lineage differentiation is only observed in culture, whereas in vivo, stromal stem cell differentiation is restricted to tissue-specific cell types. An important unanswered question is how a single, widely distributed cell type (a pericyte) gives rise to stem cells with tissue-specific functions and attributes. Using a combination of transcriptomics and epigenomics we have compared the molecular status of two populations of stromal stem cell precursors. Using a LacZ transgene insertion that is expressed in pericytes but not in stem cells, we were able to compare pericyte populations from two different tissues, mouse incisors and bone marrow. Pericytes, freshly isolated from mouse incisors and bone marrow, exhibited transcriptomes and epigenetic landscapes that were extensively different, reflecting their tissue of origin and future in vivo differentiation potential. Dspp, an odontoblast differentiation gene, as well as additional odontogenic genes, are shown to be expressed in dental pulp-derived pericytes. These genetic loci are also decorated with histone modifications indicative of a transcriptionally active chromatin state. In bone marrow pericytes, a major osteogenic differentiation gene, Runx2, is not expressed but is marked by both active and repressive histones and therefore primed to be expressed. Polycomb repressor complex 1 analysis showed that key genes involved in the induction of adipogenesis, chondrogenesis, and myogenesis are targeted by Ring1b and therefore stably repressed. This indicates that pericyte populations are molecularly obstructed from differentiating down certain lineages in vivo. Stem Cells 2018;36:1890-15.

The Lymphotoxin β Receptor Is Essential for Upregulation of IFN-Induced Guanylate-Binding Proteins and Survival after Toxoplasma gondii Infection

Lymphotoxin β receptor (LTβR) signaling plays an important role in efficient initiation of host responses to a variety of pathogens, encompassing viruses, bacteria, and protozoans via induction of the type I interferon response. The present study reveals that after Toxoplasma gondii infection, LTβR^−/− mice show a substantially reduced survival rate when compared to wild-type mice. LTβR^−/− mice exhibit an increased parasite load and a more pronounced organ pathology. Also, a delayed increase of serum IL-12p40 and a failure of the protective IFNγ response in LTβR^−/− mice were observed. Serum NO levels in LTβR^−/− animals rose later and were markedly decreased compared to wild-type animals. At the transcriptional level, LTβR^−/− animals exhibited a deregulated expression profile of several cytokines known to play a role in activation of innate immunity in T. gondii infection. Importantly, expression of the IFNγ-regulated murine guanylate-binding protein (mGBP) genes was virtually absent in the lungs of LTβR^−/− mice. This demonstrates clearly that the LTβR is essential for the induction of a type II IFN-mediated immune response against T. gondii. The pronounced inability to effectively upregulate host defense effector molecules such as GBPs explains the high mortality rates of LTβR^−/− animals after T. gondii infection.

Suppression of Canine Dendritic Cell Activation/Maturation and Inflammatory Cytokine Release by Mesenchymal Stem Cells Occurs Through Multiple Distinct Biochemical Pathways

Mesenchymal stem cells (MSC) represent a readily accessible source of cells with potent immune modulatory activity. MSC can suppress ongoing inflammatory responses by suppressing T cell function, while fewer studies have examined the impact of MSC on dendritic cell (DC) function. The dog spontaneous disease model represents an important animal model with which to evaluate the safety and effectiveness of cellular therapy with MSC. This study evaluated the effects of canine MSC on the activation and maturation of canine monocyte-derived DC, as well as mechanisms underlying these effects. Adipose-derived canine MSC were cocultured with canine DC, and the MSC effects on DC maturation and activation were assessed by flow cytometry, cytokine ELISA, and confocal microscopy. We found that canine MSC significantly suppressed lipopolysaccharide (LPS)-stimulated upregulation of DC activation markers such as major histocompatibility class II (MHCII), CD86, and CD40. Furthermore, pretreatment of MSC with interferon gamma (IFNγ) augmented this suppressive activity. IFNγ-activated MSC also significantly reduced LPS-elicited DC secretion of tumor necrosis factor alpha without reducing secretion of interleukin-10. The suppressive effect of IFNγ-treated MSC on LPS-induced DC activation was mediated by soluble factors secreted by both MSC and DC. Pathways of DC functional suppression included programmed death ligand-1 expression and secretion of nitrous oxide, prostaglandin E2, and adenosine by activated MSC. Coculture of DC with IFNγ-treated MSC maintained DC in an immature state and prolonged DC antigen uptake during LPS maturation stimulus. Taken together, canine MSC are capable of potently suppressing DC function in a potentially inflammatory microenvironment through several separate immunological pathways and confirm the potential for immune therapy with MSC in canine immune-mediated disease models.

Common Genetic Polymorphisms within NFκB-Related Genes and the Risk of Developing Invasive Aspergillosis

Invasive Aspergillosis (IA) is an opportunistic infection caused by Aspergillus, a ubiquitously present airborne pathogenic mold. A growing number of studies suggest a major host genetic component in disease susceptibility. Here, we evaluated whether 14 single-nucleotide polymorphisms within NFκB1, NFκB2, RelA, RelB, Rel, and IRF4 genes influence the risk of IA in a population of 834 high-risk patients (157 IA and 677 non-IA) recruited through a collaborative effort involving the aspBIOmics consortium and four European clinical institutions. No significant overall associations between selected SNPs and the risk of IA were found in this large cohort. Although a hematopoietic stem cell transplantation (HSCT)-stratified analysis revealed that carriers of the IRF4_{rs12203592T/T} genotype had a six-fold increased risk of developing the infection when compared with those carrying the C allele (OR_REC = 6.24, 95%CI 1.25–31.2, P = 0.026), the association of this variant with IA risk did not reach significance at experiment-wide significant threshold. In addition, we found an association of the IRF4_AATC and IRF4_GGTC haplotypes (not including the IRF4_rs12203592T risk allele) with a decreased risk of IA but the magnitude of the association was similar to the one observed in the single-SNP analysis, which indicated that the haplotypic effect on IA risk was likely due to the IRF4_rs12203592 SNP. Finally, no evidence of significant interactions among the genetic markers tested and the risk of IA was found. These results suggest that the SNPs on the studied genes do not have a clinically relevant impact on the risk of developing IA.

Dendritic cell maturation and survival are differentially regulated by TNFR1 and TNFR2

The capacity of dendritic cells (DC) to regulate adaptive immunity is controlled by their maturation state and lifespan. Although TNF is a well-known maturation and survival factor for DC, the role of the two TNFR, TNFR1 and TNFR2, in mediating these effects is poorly understood. By using unique TNF variants that selectively signal through TNFR1 and/or TNFR2, we demonstrate differential functions of TNFR in human monocyte-derived and blood CD1c(+) DC. Activation of TNFR1, but not TNFR2, efficiently induced DC maturation, as defined by enhanced expression of cell surface maturation markers (CD83, CD86, and HLA-DR) as well as enhanced T cell stimulatory capacity. In contrast, both TNFR1 and TNFR2 significantly protected DC against cell death, indicating that innate signals can promote DC survival in the absence of DC maturation. We further show differential activation of NF-κB signaling pathways by the TNFR: TNFR1 activated both the p65 and p52 pathways, whereas TNFR2 triggered p52, but not p65, activation. Accordingly, the p65 NF-κB pathway only played a role in the prosurvival effect of TNFR1. However, cell death protection through both TNFR was mediated through the Bcl-2/Bcl-xL pathway. Taken together, our data show that TNFR1, but not TNFR2, signaling induces DC maturation, whereas DC survival can be mediated independently through both TNFR. These data indicate differential but partly overlapping responses through TNFR1 and TNFR2 in both inflammatory and conventional DC, and they demonstrate that DC maturation and DC survival can be regulated through independent signaling pathways.

LTβR expression on hematopoietic cells regulates acute inflammation and influences maturation of myeloid subpopulations

Lymphotoxin beta-receptor (LTβR) is involved in the formation and maintenance of secondary lymphoid structures, as well as in the regulation of inflammatory responses. Because LTβR lymphoid structure formation continues to develop in infants, we compared two different chimera models: one using adult mice and the other using a transplantation model of neonatal mice. To elucidate the function of LTβR on lymphoid and non-lymphoid cells, we generated bone marrow chimeras on the wild type C57Bl/6 and the LTβR-deficient (LTβR(-/-)) background, and reconstituted the mice with bone marrow cells reciprocally. These chimeric mice were analyzed in the experimental model of acute dextran sulfate sodium-induced colitis. Interestingly, both models revealed not only equal reconstitution levels but also similar immunological responses: LTβR expression on stromal cells is essential for lymph node formation, whereas LTBR on hematopoietic cells is crucial for a decrease in inflammation. In addition, mice lacking LTβR on hematopoietic cells revealed (a) an increase of immature granulocytic cells in the spleen and (b) a reduced proportion of myeloid cells in peripheral blood and spleen expressing CD11b(+)Ly6C(+)Ly6G(-) (myeloid-derived suppressor cells expression profile). In conclusion, LTβR expression on hematopoietic cells seems to be involved in the down-regulation of acute inflammatory reactions paralleled by the appearance of immature myeloid cells.

Functional study of immature dendritic cells co-transfected with IL-10 and TGF-beta 1 genes in vitro

Dendritic cells (DC) have important functions in T cell immunity and T cell tolerance. Previous studies suggest that immature dendritic cells (imDCs) might be involved in the induction of peripheral T cell tolerance. While interleukin-10 (IL-10) functions at different levels of the immune response, transforming growth factor-beta 1 (TGF-beta 1) is considered to be a key factor in immune tolerance. In this study, we investigated the effects of immature DC (imDC) co-transfected with IL-10 and TGF-beta 1 genes (IL-10-TGF-beta 1-imDC) on inducing immune tolerance. Moreover, we compared the effects of IL-10-TGF-beta 1-imDC with IL-10 transfected imDC (IL-10-imDC) and TGF-beta 1-transfected imDC (TGF-beta 1-imDC), respectively. IL-10-TGF-beta 1-imDC resulted in the down-regulation of MHC class II, CD80 and CD86. IL-10-TGF-beta 1-imDC could induce T cell hyporesponsiveness, and was reluctant to proliferate. IL-10-TGF-beta 1-imDC was more effective than IL-10-imDC and TGF-beta 1-imDC, respectively. In summary, co-expression of IL-10 and TGF-beta 1 affected the immunity of imDCs and enhanced their tolerogenicity. It might be a promising therapy for donor-specific tolerance after organ transplantation.

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.

Higher-order chromatin regulation and differential gene expression in the human tumor necrosis factor/lymphotoxin locus in hepatocellular carcinoma cells

The three-dimensional context of endogenous chromosomal regions may contribute to the regulation of gene clusters by influencing interactions between transcriptional regulatory elements. In this study, we investigated the effects of tumor necrosis factor (TNF) signaling on spatiotemporal enhancer-promoter interactions in the human tumor necrosis factor (TNF)/lymphotoxin (LT) gene locus, mediated by CCCTC-binding factor (CTCF)-dependent chromatin insulators. The cytokine genes LTα, TNF, and LTβ are differentially regulated by NF-κB signaling in inflammatory and oncogenic responses. We identified at least four CTCF-enriched sites with enhancer-blocking activities and a TNF-responsive TE2 enhancer in the TNF/LT locus. One of the CTCF-enriched sites is located between the early-inducible LTα/TNF promoters and the late-inducible LTβ promoter. Depletion of CTCF reduced TNF expression and accelerated LTβ induction. After TNF stimulation, via intrachromosomal dynamics, these insulators mediated interactions between the enhancer and the LTα/TNF promoters, followed by interaction with the LTβ promoter. These results suggest that insulators mediate the spatiotemporal control of enhancer-promoter associations in the TNF/LT gene cluster.

TNF receptor 1 mediates dendritic cell maturation and CD8 T cell response through two distinct mechanisms

TNF-α and its two receptors (TNFR1 and 2) are known to stimulate dendritic cell (DC) maturation and T cell response. However, the specific receptor and mechanisms involved in vivo are still controversial. In this study, we show that in response to an attenuated mouse hepatitis virus infection, DCs fail to mobilize and up-regulate CD40, CD80, CD86, and MHC class I in TNFR1(-/-) mice as compared with the wild-type and TNFR2(-/-) mice. Correspondingly, virus-specific CD8 T cell response was dramatically diminished in TNFR1(-/-) mice. Adoptive transfer of TNFR1-expressing DCs into TNFR1(-/-) mice rescues CD8 T cell response. Interestingly, adoptive transfer of TNFR1-expressing naive T cells also restores DC mobilization and maturation and endogenous CD8 T cell response. These results show that TNFR1, not TNFR2, mediates TNF-α stimulation of DC maturation and T cell response to mouse hepatitis virus in vivo. They also suggest two mechanisms by which TNFR1 mediates TNF-α-driven DC maturation, as follows: a direct effect through TNFR1 expressed on immature DCs and an indirect effect through TNFR1 expressed on naive T cells.

Control of TNF-induced dendritic cell maturation by hybrid-type N-glycans

The activity of α-1,2-mannosidase I is required for the conversion of high-mannose to hybrid-type (ConA reactive) and complex-type N-glycans (Phaseolus vulgaris-leukoagglutinin [PHA-L] reactive) during posttranslational protein N-glycosylation. We recently demonstrated that α-1,2-mannosidase I mRNA decreases in graft-infiltrating CD11c(+) dendritic cells (DCs) prior to allograft rejection. Although highly expressed in immature DCs, little is known about its role in DC functions. In this study, analysis of surface complex-type N-glycan expression by lectin staining revealed the existence of PHA-L(low) and PHA-L(high) subpopulations in murine splenic conventional DCs, as well as in bone marrow-derived DC (BMDCs), whereas plasmacytoid DCs are nearly exclusively PHA-L(high). Interestingly, all PHA-L(high) DCs displayed a strongly reduced responsiveness to TNF-α-induced p38-MAPK activation compared with PHA-L(low) DCs, indicating differences in PHA-L-binding capacities between DCs with different inflammatory properties. However, p38 phosphorylation levels were increased in BMDCs overexpressing α-1,2-mannosidase I mRNA. Moreover, hybrid-type, but not complex-type, N-glycans are required for TNF-α-induced p38-MAPK activation and subsequent phenotypic maturation of BMDCs (MHC-II, CD86, CCR7 upregulation). α-1,2-mannosidase I inhibitor-treated DCs displayed diminished transendothelial migration in response to CCL19, homing to regional lymph nodes, and priming of IFN-γ-producing T cells in vivo. In contrast, the activity of α-1,2-mannosidase I is dispensable for LPS-induced signaling, as well as the DCs' general capability for phenotypic and functional maturation. Systemic application of an α-1,2-mannosidase I inhibitor was able to significantly prolong allograft survival in a murine high-responder corneal transplantation model, further highlighting the importance of N-glycan processing by α-1,2-mannosidase I for alloantigen presentation and T cell priming.

NF-κB in immunobiology

NF-κB was first discovered and characterized 25 years ago as a key regulator of inducible gene expression in the immune system. Thus, it is not surprising that the clearest biological role of NF-κB is in the development and function of the immune system. Both innate and adaptive immune responses as well as the development and maintenance of the cells and tissues that comprise the immune system are, at multiple steps, under the control of the NF-κB family of transcription factors. Although this is a well-studied area of NF-κB research, new and significant findings continue to accumulate. This review will focus on these areas of recent progress while also providing a broad overview of the roles of NF-κB in mammalian immunobiology.

Hypothermia-induced neurite outgrowth is mediated by tumor necrosis factor-alpha

Systemic or brain-selective hypothermia is a well-established method for neuroprotection after brain trauma. There is increasing evidence that hypothermia exerts beneficial effects on the brain and may also support regenerative responses after brain damage. Here, we have investigated whether hypothermia influences neurite outgrowth in vitro via modulation of the post-injury cytokine milieu. Organotypic brain slices were incubated: deep hypothermia (2 h at 17 degrees C), rewarming (2 h up to 37 degrees C), normothermia (20 h at 37 degrees C). Neurite density and cytokine release (IL 1beta, IL-6, IL-10, and TNF-alpha) were investigated after 24 h. For functional analysis mice deficient in NT-3/NT-4 and TNF-alpha as well as the TNF-alpha inhibitor etanercept were used. Hypothermia led to a significant increase of neurite outgrowth, which was independent of neurotrophin signaling. In contrast to other cytokines investigated, TNF-alpha secretion by organotypic brain slices was significantly increased after deep hypothermia. Moreover, hypothermia-induced neurite extension was abolished after administration of the TNF-alpha inhibitor and in TNF-alpha knockout mice. We demonstrate that TNF-alpha is responsible for inducing neurite outgrowth in the context of deep hypothermia and rewarming. These data suggest that hypothermia not only exerts protective effects in the CNS but may also support neurite outgrowth as a potential mechanism of regeneration.

Targeting TNF for Treatment of Cancer and Autoimmunity

Tumor necrosis factor-alpha (TNF-alpha) was first isolated two decades ago as a macrophageproduced protein that can effectively kill tumor cells. TNF-alpha is also an essential component of the immune system and is required for hematopoiesis, for protection from bacterial infection and for immune cell-mediated cytotoxicity. Extensive research, however, has revealed that TNF-alpha is one of the major players in tumor initiation, proliferation, invasion, angiogenesis and metastasis. The proinflammatory activities link TNF-alpha with a wide variety of autoimmune diseases, including psoriasis, inflammatory bowel disease, rheumatoid arthritis, systemic sclerosis, systemic lupus erythematosus, multiple sclerosis, diabetes and ankylosing spondylitis. Systemic inhibitors of TNF such as etanercept (Enbrel) (a soluble TNF receptor) and infliximab (Remicade) and adalimumab (Humira) (anti-TNF antibodies) have been approved for the treatment inflammatory bowel disease, psoriasis and rheumatoid arthritis. These drugs, however, exhibit severe side effects and are expensive. Hence orally active blockers of TNF-alpha that are safe, efficacious and inexpensive are urgently needed. Numerous products from fruits, vegetable and traditional medicinal plants have been described which can suppress TNF expression and TNF signaling but their clinical potential is yet uncertain.

Tumor necrosis factor negative bone marrow-derived dendritic cells exhibit deficient IL-10 expression

The effective maturation of dendritic cells (DC) is complex and highly regulated and requires the presence of a variety of signals. Tumor necrosis factor (TNF) and its receptors or innate pattern recognition receptors such as the toll-like receptors have been shown to contribute to this process. DC derived from bone marrow cells in the presence of granulocyte/macrophage colony-stimulating factor can be used as a model to ascertain the contribution of different signals to DC maturation. Analysis of DC activated by addition of the mycobacterial vaccine strain Bacillus Calmette-Guérin showed that of the effector molecules studied only interleukin-10 expression was significantly reduced in TNF-negative (B6.TNF(-/-)) DC. Another effector molecule produced by DC, inducible nitric oxide synthase, was largely unchanged.

IRF5 is required for late-phase TNF secretion by human dendritic cells

Spatially and temporally controlled expression of inflammatory mediators is critical for an appropriate immune response. In this study, we define the role for interferon regulatory factor 5 (IRF5) in secretion of tumor necrosis factor (TNF) by human dendritic cells (DCs). We demonstrate that DCs but not macrophages have high levels of IRF5 protein, and that IRF5 is responsible for the late-phase expression of TNF, which is absent in macrophages. Sustained TNF secretion is essential for robust T-cell activation by DCs. Systematic bioinformatic and biochemical analyses of the TNF gene locus map 2 sites of IRF5 recruitment: 5' upstream and 3' downstream of the TNF gene. Remarkably, while IRF5 can directly bind to DNA in the upstream region, its recruitment to the downstream region depends on the protein-protein interactions with NF-kappaB RelA. This study provides new insights into diverse molecular mechanisms employed by IRF5 to regulate gene expression and implicates RelA-IRF5 interactions as a putative target for cell-specific modulation of TNF expression.

Lack of TNFalpha expression protects anaplastic lymphoma kinase-positive T-cell lymphoma (ALK+ TCL) cells from apoptosis

Here we report that T-cell lymphomas characterized by the expression of anaplastic lymphoma kinase (ALK+ TCL) fail to express the TNFalpha and frequently display DNA methylation of the TNFalpha gene promoter. While only a subset of the ALK+ TCL-derived cell lines showed a high degree of the promoter methylation, all 6 showed low to nondetectable expression of the TNFalpha mRNA, and none expressed the TNFalpha protein. All 14 ALK+ TCL tissue samples examined displayed some degree of the TNFalpha promoter methylation, which was the most prominent in the distal portion of the the promoter. Treatment with a DNA methyltransferase inhibitor, 5'-aza-2'-deoxy-cytidine (5-ADC), reversed the promoter methylation and led to the expression of TNFalpha mRNA and protein. Furthermore, in vitro DNA methylation of the promoter impaired its transcriptional activity in the luciferase reporter assay. This impairment was seen even if only either distal or proximal portion were methylated, with methylation of the former exerting a more profound inhibitory effect. Notably, the ALK+ TCL cell lines uniformly expressed the type 1 TNFalpha receptor (TNF-R1) protein known to transduce the TNFalpha-induced pro-apoptotic signals. Moreover, exogenous TNFalpha inhibited growth of the ALK+ TCL cell lines in a dose-dependent manner and induced activation of the members of the cell apoptotic pathway: Caspase 8 and caspase 3. These findings provide additional rationale for the therapeutic inhibition of DNA methyltransferases in ALK+ TCL. They also suggest that treatment with TNFalpha may be highly effective in this type of lymphoma.

Mesenchymal stem cells can affect solid organ allograft survival

It has recently been recognized that mesenchymal stem cells (MSCs) isolated from adult bone marrow are able to modify the alloimmune response in vitro and in vivo. MSCs can be expanded into large quantities in culture, thereby facilitating potential future applications in solid organ transplantation. To develop novel MSC-based antirejection treatments, the mechanism behind the immunomodulatory ability of MSCs has to be elucidated further. At present, a variety of possible in vitro effects of MSCs on immune system effector cells have been reported, but little is known about their in vivo properties. Here, we discuss recent findings regarding the influence of MSCs on different effector cell populations in vitro and summarize the available data describing their in vivo properties.

Targeting lymphocyte activation through the lymphotoxin and LIGHT pathways

SUMMARY

Cytokines mediate key communication pathways essential for regulation of immune responses. Full activation of antigen-responding lymphocytes requires cooperating signals from the tumor necrosis factor (TNF)-related cytokines and their specific receptors. LIGHT, a lymphotoxin-beta (LTbeta)-related TNF family member, modulates T-cell activation through two receptors, the herpesvirus entry mediator (HVEM) and indirectly through the LT-beta receptor. An unexpected finding revealed a non-canonical binding site on HVEM for the immunoglobulin superfamily member, B and T lymphocyte attenuator (BTLA), and an inhibitory signaling protein suppressing T-cell activation. Thus, HVEM can act as a molecular switch between proinflammatory and inhibitory signaling. The non-canonical HVEM-BTLA pathway also acts to counter LTbetaR signaling that promotes the proliferation of antigen-presenting dendritic cells (DCs) within lymphoid tissue microenvironments. These results indicate LTbeta receptor and HVEM-BTLA pathways form an integrated signaling circuit. Targeting these cytokine pathways with specific antagonists (antibody or decoy receptor) can alter lymphocyte differentiation and activation. Alternately, agonists directed at their cell surface receptors can restore homeostasis and potentially reset immune and inflammatory processes, which may be useful in treating autoimmune and infectious diseases and cancer.

Effects of chronic ethanol feeding on murine dendritic cell numbers, turnover rate, and dendropoiesis

BACKGROUND

Chronic alcoholics have increased susceptibility to and severity of infection, which are likely to be a result of impaired immune defense mechanisms. The contribution of dendritic cells (DC) to these immune defense changes is not well understood. Alterations in DC numbers, dendropoiesis, and lifespan have not been specifically studied in vivo in chronic ethanol (EtOH) exposure models. As DC play an essential role in initiating immune responses, alterations in these DC characteristics would help explain changes observed in adaptive immune responses.

METHODS

Mice received 20% EtOH (w/v) in the drinking water for up to 28 weeks, with mouse chow ad libitum. In EtOH-fed and water control mice, DC were enumerated by flow cytometry. The effect of EtOH on DC precursor numbers was determined by differentiation in vitro in the presence of granulocyte-macrophage colony-stimulating factor and interleukin-4, and the effect of an EtOH environment on untreated DC differentiation was measured following bone marrow transfer to irradiated hosts. DC turnover rate was also examined by bromodeoxyuridine incorporation and loss.

RESULTS

The percentage and absolute numbers of DC were decreased in spleen and increased in thymus beginning as early as 4 weeks of EtOH feeding. In addition, the overall cellularity of spleen and thymus were altered by this regimen. However, chronic EtOH consumption did not adversely affect DC precursor numbers, differentiation abilities, or turnover rates.

CONCLUSIONS

Decreased splenic DC numbers observed following chronic murine EtOH consumption are not because of altered DC precursor numbers or differentiation, nor increased DC turnover rate. Similarly, increased thymic DC numbers are not the result of alterations in DC precursor differentiation or turnover rate. Compartment size plays a role in determining splenic and thymic DC numbers following chronic EtOH feeding. EtOH-induced alterations in total DC numbers provide several mechanisms to partially explain why chronic alcoholics have increased susceptibility to infections.

The absence of cutaneous lymph nodes results in a Th2 response and increased susceptibility to Leishmania major infection in mice

Lymph nodes (LNs) are important sentinel organs where antigen-presenting cells interact with T cells to induce adaptive immune responses. In cutaneous infection of mice with Leishmania major, resistance depends on the induction of a T-helper-cell-1 (Th1)-mediated cellular immune response in draining, peripheral LNs. We investigated whether draining, peripheral LNs are absolutely required for resistance against L. major infection. We investigated the course of experimental leishmaniasis in wild-type (wt) mice lacking peripheral LNs (pLNs), which we generated by in utero blockade of membrane-bound lymphotoxin, and in mice lacking pLNs or all LNs due to genetic deletion of lymphotoxin ligands or receptors. wt mice of the resistant C57BL/6 strain without local skin-draining LNs were still able to generate specific T-cell responses, but this yielded Th2 cells. This switch to a Th2 response resulted in severe systemic infection. We also confirmed these results with mice lacking pLNs due to genetic depletion of lymphotoxin-beta. The complete absence of LNs due to a genetic depletion of the lymphotoxin-beta receptor also resulted in a marked deterioration of disease and a Th2 response. Thus, in the absence of pLNs, an L. major-specific Th2 response is induced in the remaining secondary lymphoid organs, such as the spleen and non-skin-draining LNs. This indicates a critical requirement for pLNs to induce protective Th1 immunity and suggests that whether Th1 or Th2 priming to the same antigen occurs depends on the site of the primary antigen recognition.

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.

The inhibitory HVEM-BTLA pathway counter regulates lymphotoxin receptor signaling to achieve homeostasis of dendritic cells

Proliferation of dendritic cells (DC) in the spleen is regulated by positive growth signals through the lymphotoxin (LT)-beta receptor; however, the countering inhibitory signals that achieve homeostatic control are unresolved. Mice deficient in LTalpha, LTbeta, LTbetaR, and the NFkappaB inducing kinase show a specific loss of CD8- DC subsets. In contrast, the CD8alpha- DC subsets were overpopulated in mice deficient in the herpesvirus entry mediator (HVEM) or B and T lymphocyte attenuator (BTLA). HVEM- and BTLA-deficient DC subsets displayed a specific growth advantage in repopulating the spleen in competitive replacement bone marrow chimeric mice. Expression of HVEM and BTLA were required in DC and in the surrounding microenvironment, although DC expression of LTbetaR was necessary to maintain homeostasis. Moreover, enforced activation of the LTbetaR with an agonist Ab drove expansion of CD8alpha- DC subsets, overriding regulation by the HVEM-BTLA pathway. These results indicate the HVEM-BTLA pathway provides an inhibitory checkpoint for DC homeostasis in lymphoid tissue. Together, the LTbetaR and HVEM-BTLA pathways form an integrated signaling network regulating DC homeostasis.

Lymphotoxin αβ2 (Membrane Lymphotoxin) Is Critically Important for Resistance toLeishmania majorInfection in Mice

Although the essential role of TNF-alpha in the control of intracellular pathogens including Leishmania major is well established, it is uncertain whether the related cytokine lymphotoxin alphabeta2 (LTalpha1beta2, membrane lymphotoxin) plays any role in this process. In this study, we investigated the contribution of membrane lymphotoxin in host response to L. major infection by using LTbeta-deficient (LTbeta(-/-)) mice on the resistant C57BL/6 background. Despite mounting early immune responses comparable to those of wild-type (WT) mice, LTbeta(-/-) mice developed chronic nonhealing cutaneous lesions due to progressive and unresolving inflammation that is accompanied by uncontrolled parasite proliferation. This chronic disease was associated with striking reduction in IL-12 and Ag-specific IFN-gamma production by splenocytes from infected mice. Consistent with defective cellular immune response, infected LTbeta(-/-) mice had significantly low Ag-specific serum IgG1 and IgG2a levels compared with WT mice. Although administration of rIL-12 to L. major-infected LTbeta(-/-) mice caused complete resolution of chronic lesions, it only partially (but significantly) reduced parasite proliferation. In contrast, blockade of LIGHT signaling in infected LTbeta(-/-) mice resulted in acute and progressive lesion development, massive parasite proliferation, and dissemination to the visceral organs. Although infected LTbeta(-/-) WT bone marrow chimeric mice were more resistant than LTbeta(-/-) mice, they still had reduced ability to control parasites and showed defective IL-12 and IFN-gamma production compared with infected WT mice. These results suggest that membrane lymphotoxin plays critical role in resistance to L. major by promoting effective T cell-mediated anti-Leishmania immunity.

Antitumour necrosis factor-? chimeric antibody (infliximab) inhibits activation of skin-homing CD4+ and CD8+ T lymphocytes and impairs dendritic cell function

BACKGROUND

Psoriasis is a chronic inflammatory skin disease characterized by hyperproliferation and altered differentiation of keratinocytes in reply to cytokines such as interferon (IFN)-gamma and tumour necrosis factor (TNF)-alpha, provided by infiltrating CD4+ and CD8+ T cells and natural killer cells. Infliximab is a chimeric monoclonal antibody that neutralizes both soluble and membrane-bound TNF-alpha, and that may give a long-term disease remission.

OBJECTIVES

To determine the in vitro effects of infliximab on CD4+ and CD8+ T cells derived from lesional skin, and on dendritic cells (DCs).

METHODS

Psoriatic T-cell lines were isolated from lesional skin of four patients with psoriasis and assayed for their proliferation, cytokine release and susceptibility to apoptotic stimuli in the presence of graded (1-100 microg mL(-1)) concentrations of infliximab. DCs were differentiated in the presence of infliximab from peripheral blood monocytes. Phenotype was assessed by fluorescence-activated cell sorting and antigen-presenting capacity in functional assays.

RESULTS

In vitro activation of psoriatic as well as antigen (nickel)-specific skin-homing T cells was strongly and dose-dependently impaired by infliximab, in terms both of proliferation and of IFN-gamma release. Despite the significant reduction of IFN-gamma secretion, infliximab only marginally affected the release of interleukin (IL)-10 by skin T cells, thus determining a reduction of the IFN-gamma/IL-10 ratio at the site of inflammation. The effects were maximal when T-cell activation occurred in the absence of costimulation, or when T cells were activated by immature compared with mature DCs. In addition, skin-homing CD8+ T cells were more prominently affected by infliximab compared with CD4+ T lymphocytes, both in terms of inhibition of activation and in their susceptibility to apoptosis. Finally, infliximab directly affected the differentiation of monocyte-derived DCs, by inhibiting the expression of CD1a and CD86, and strongly impaired the antigen-presenting capacity of immature and, to a lesser extent, mature DCs.

CONCLUSIONS

Infliximab directly affects psoriatic T cells and impairs the antigen-presenting capacity of DCs. These effects may help to explain the long-term disease remission obtained with the drug.

Blocking lymphotoxin beta receptor signalling exacerbates acute DSS-induced intestinal inflammation--opposite functions for surface lymphotoxin expressed by T and B lymphocytes

The lymphotoxin beta receptor (LTbetaR) signalling pathway is involved in the development of secondary lymphoid organs and the maintenance of organized lymphoid tissues. Additionally, previous studies clearly demonstrated the involvement of the LTbetaR interaction with its ligands in promoting intestinal inflammation. In order to dissect the role of LTbetaR activation in the mouse model of acute DSS-induced colitis we treated mice with a functional inhibitor of LTbetaR activation (LTbetaR:Ig) and compared it to disease in LTbetaR-deficient and LTalphabeta-deficient mice. All these modes of LTbetaR signalling ablation resulted in significant aggravation of the disease and in release of inflammatory cytokines such as TNF, IL-6, and IFNgamma. Finally, using mice with conditionally ablated expression of membrane bound LTbeta on T or B cells, respectively, distinct and opposite contributions of surface LTbeta expressed on T or B cells was found. Thus, activation of LTbetaR by LTalphabeta mainly expressed on T lymphocytes is crucial for the down regulation of the inflammatory response in this experimental model.

Expression of lymphotoxin-alphabeta on antigen-specific T cells is required for DC function

During an immune response, activated antigen (Ag)-specific T cells condition dendritic cells (DCs) to enhance DC function and survival within the inflamed draining lymph node (LN). It has been difficult to ascertain the role of the tumor necrosis factor (TNF) superfamily member lymphotoxin-αβ (LTαβ) in this process because signaling through the LTβ-receptor (LTβR) controls multiple aspects of lymphoid tissue organization. To resolve this, we have used an in vivo system where the expression of TNF family ligands is manipulated only on the Ag-specific T cells that interact with and condition Ag-bearing DCs. We report that LTαβ is a critical participant required for optimal DC function, independent of its described role in maintaining lymphoid tissue organization. In the absence of LTαβ or CD40L on Ag-specific T cells, DC dysfunction could be rescued in vivo via CD40 or LTβR stimulation, respectively, suggesting that these two pathways cooperate for optimal DC conditioning.

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.

LIGHT-related molecular network in the regulation of innate and adaptive immunity

The LIGHT-related molecular network is composed of at least seven interacting receptors and ligands. Recent studies reveal that this network has profound immune regulatory functions for both innate and adaptive immunity. Experimental data support the concept that this network may also play roles in the pathogenesis of human diseases including cancer, infection, transplantation tolerance, and autoimmune diseases. In this review, we attempt to dissect each molecular interaction in detail and assemble them in the context of their roles in the pathogenesis and possible therapeutic potential in human diseases.

NF-kappaB and the immune response

One of the primary physiological roles of nuclear factor-kappa B (NF-kappaB) is in the immune system. In particular, NF-kappaB family members control the transcription of cytokines and antimicrobial effectors as well as genes that regulate cellular differentiation, survival and proliferation, thereby regulating various aspects of innate and adaptive immune responses. In addition, NF-kappaB also contributes to the development and survival of the cells and tissues that carry out immune responses in mammals. This review, therefore, describes the role of the NF-kappaB pathway in the development and functioning of the immune system.

Restoring immune defenses via lymphotoxin signaling: lessons from cytomegalovirus

Although primary infection with human cytomegalovirus (HCMV), a beta-herpesvirus, is widespread and acquired early in life, it rarely causes disease in immune-competent individuals. However, in immune-compromised patients HCMV infection or reactivation invariably leads to serious disease, the effective treatment of which remains a difficult clinical problem. Current antiviral therapy is limited not only by toxicity but also by the continual emergence of drugresistant viruses. The limitations of these current therapeutics provides a strong impetus to develop novel approaches that will enhance the host's immune responsiveness while at the same time effectively controlling virus replication. Type I interferon (IFN) plays a critical role in initiating innate antiviral defenses and promoting adaptive responses and lymphotoxin (LT)-alphabeta has recently been identified as an essential effector cytokine regulating the induction of type I IFN during CMV infection. In particular, CMV infection of immune-compromised mice has revealed the immunotherapeutic potential of the lymphotoxin-beta receptor (LTbetaR) signaling pathway to restore immune function and provide protection from CMV mortality. In this review, we discuss the potential benefits and risks associated with LTbetaR-directed immunotherapy for CMV disease and other persistent viral infections.

Expression of lymphotoxin beta governs immunity at two distinct levels

Interaction of lymphotoxin alpha(1)beta(2) (LTalpha(1)beta(2)) with its receptor is key for the generation and maintenance of secondary lymphoid organ microstructure. We used mice conditionally deficient for LTbeta on different lymphocyte subsets to determine how the LTbeta-dependent lymphoid structure influences immune reactivity. All conditionally LTbeta-deficient mice mounted normal immune responses against vesicular stomatitis virus (VSV), and were protected against lymphocytic choriomeningitis virus (LCMV). In contrast, they exhibited reduced immune responses against non-replicating antigens. Completely LTbeta-deficient mice failed to retain VSV in the marginal zone and died from VSV infections, and they became virus carriers following infection with the non-cytopathic LCMV, which was correlated with defective virus replication in dendritic cells. It was ruled out that LTbeta expression on lymphocytes influenced their activation, homing capacity, or maturation. We therefore conclude that LTbeta expression influences immune reactivity at two distinct levels: (i) Expression of LTbeta on lymphocytes enhances the induction of immune responses against limiting amounts of antigen. (ii) Expression of LTbeta on non-lymphocytes governs antiviral immunity by enhancing antigen presentation on antigen-presenting cells. This prevents cytotoxic T lymphocytes exhaustion or death of the host by uncontrolled virus spread.

Therapeutic efficacy of tumor-targeted IL2 in LTalpha(-/-) mice depends on conditioned T cells

An effective immunological eradication of tumors by the adaptive immune system depends on T cell priming, expansion of specific T cells and their effector function. It has been shown that either step may be impaired in the tumor-bearing host, and several strategies have been used to improve antitumor immune responses. In this regard, tumor-targeted IL2 therapy leads to the destruction of established melanoma metastases in fully immune competent mice as previously demonstrated. This effect has been attributed, but never directly confirmed, to the boost of antigen-experienced T cells. To this end, we demonstrate the absence of any antitumor effect of targeted IL2 in mice characterized by an impaired priming of T cell responses. Notably, in these animals tumor-targeted IL2 therapy induced tumor regression only after adoptive transfer of tumor-conditioned splenocytes. A detailed analysis revealed that T cells present within the transferred splenocytes were actively participating in the immune response as these were clonally expanded after targeted IL2 therapy. In summary, we demonstrate here that in LTalpha(-/-) mice lacking sufficient numbers of tumor-specific T cells only the passive transfer of such cells prior to therapy restores the efficacy of tumor-targeted IL2 therapy. Thus, the antitumor effect of tumor-targeted IL2 is indeed based on the boost of pre-existing T cell responses.

Novel lymphotoxin alpha (LTalpha) knockout mice with unperturbed tumor necrosis factor expression: reassessing LTalpha biological functions

Lymphotoxin alpha (LTalpha) can exist in soluble form and exert tumor necrosis factor (TNF)-like activity through TNF receptors. Based on the phenotypes of knockout (KO) mice, the physiological functions of LTalpha and TNF are considered partly redundant, in particular, in supporting the microarchitecture of the spleen and in host defense. We exploited Cre-LoxP technology to generate a novel neomycin resistance gene (neo) cassette-free LTalpha-deficient mouse strain (neo-free LTalpha KO [LTalphaDelta/Delta]). Unlike the "conventional" LTalpha-/- mice, new LTalphaDelta/Delta animals were capable of producing normal levels of systemic TNF upon lipopolysaccharide (LPS) challenge and were susceptible to LPS/D-galactosamine (D-GalN) toxicity. Activated neutrophils, monocytes, and macrophages from LTalphaDelta/Delta mice expressed TNF normally at both the mRNA and protein levels as opposed to conventional LTalpha KO mice, which showed substantial decreases in TNF. Additionally, the spleens of the neo-free LTalpha KO mice displayed several features resembling those of LTbeta KO mice rather than conventional LTalpha KO animals. The phenotype of the new LTalphaDelta/Delta mice indicates that LTalpha plays a smaller role in lymphoid organ maintenance than previously thought and has no direct role in the regulation of TNF expression.

Progenitor cells in liver regeneration: molecular responses controlling their activation and expansion

Although normally quiescent, the adult mammalian liver possesses a great capacity to regenerate after different types of injuries in order to restore the lost liver mass and ensure maintenance of the multiple liver functions. Major players in the regeneration process are mature residual cells, including hepatocytes, cholangiocytes and stromal cells. However, if the regenerative capacity of mature cells is impaired by liver-damaging agents, hepatic progenitor cells are activated and expand into the liver parenchyma. Upon transit amplification, the progenitor cells may generate new hepatocytes and biliary cells to restore liver homeostasis. In recent years, hepatic progenitor cells have been the subject of increasing interest due to their therapeutic potential in numerous liver diseases as alternative or supportive/complementary tools to liver transplantation. While the first investigations on hepatic progenitor cells have focused on their origin and phenotypic characterization, recent attention has focused on the influence of the hepatic microenvironment on their activation and proliferation. This microenvironment comprises the extracellular matrix, epithelial and non-epithelial resident liver cells, and recruited inflammatory cells as well as the variety of growth-modulating molecules produced and/or harboured by these elements. The cellular and molecular responses to different regenerative stimuli seem to depend on the injury inflicted and consequently on the molecular microenvironment created in the liver by a certain insult. This review will focus on molecular responses controlling activation and expansion of the hepatic progenitor cell niche, emphasizing similarities and differences in the microenvironments orchestrating regeneration by recruitment of progenitor cell populations or by replication of mature cells.

IL-3-mediated TNF production is necessary for mast cell development

Mouse mast cell development and survival are largely controlled by the cytokines IL-3 and stem cell factor (SCF). We have found that IL-3 stimulation of bone marrow cells induces the production of TNF via a PI3K- and MAPK kinase/ERK-dependent pathway. Specifically, Mac-1-positive cells were responsible for TNF production, which peaked on days 7-10 of culture and decreased rapidly thereafter. The importance of IL-3-induced TNF secretion was demonstrated by the failure of TNF-deficient bone marrow cells to survive for >3 wk when cultured in IL-3 and SCF, a defect that was reversed by the addition of soluble TNF. The development of human mast cells from bone marrow progenitors was similarly hampered by the addition of TNF-blocking Abs. Cell death was due to apoptosis, which occurred with changes in mitochondrial membrane potential and caspase activation. Apoptosis appeared to be due to loss of IL-3 signaling, because TNF-deficient cells were less responsive than their wild-type counterparts to IL-3-mediated survival. In vitro cultured mast cells from TNF-deficient mice also demonstrated reduced expression of the high affinity IgE receptor, which was restored to normal levels by the addition of soluble TNF. Finally, TNF-deficient mice demonstrated a 50% reduction in peritoneal mast cell numbers, indicating that TNF is an important mast cell survival factor both in vitro and in vivo.

Stimulating lymphotoxin beta receptor on the dendritic cells is critical for their homeostasis and expansion

The increased number of dendritic cells (DCs) inside lymphoid tissue may contribute to the enhanced priming of lymphocytes. The homeostasis of splenic DCs has mostly been attributed to their migration to the spleen via the chemokine microenvironment induced by lymphotoxin beta receptor (LTbetaR) signaling on splenic stromal cells. In this study we show that the lack of direct LTbetaR signaling on DCs is associated with the reduction of the number of DCs in the spleen independently of chemokine gradients. LTbetaR-/- mice have reduced DCs and reduced BrdU incorporation on DCs, and fewer DCs from LTbetaR-/- mice are detected in the spleen. Furthermore, increased expression of LIGHT (homologous to lymphotoxin, exhibits inducible expression, competes with herpesvirus glycoprotein D for herpes virus entry mediator on T cells) on T cells, a member of the TNF family (TNFSF14) and a ligand for LTbetaR, could dramatically increase the number of T cells and DCs, which leads to severe autoimmune diseases in a LTbetaR-dependent fashion. In vitro, LIGHT could directly promote accumulation of bone marrow-derived DCs. Furthermore, intratumor expression of LIGHT can dramatically expand DCs in situ, and inoculation of DCs into tumor tissues enhanced tumor immunity. Therefore, LTbetaR signaling on DCs is required for their homeostasis during physiology and pathological conditions, and increased LIGHT-LTbetaR interaction could stimulate DC expansion for T cell-mediated immunity.

Cytokine and Anti-Cytokine Therapies for Psoriasis and Atopic Dermatitis

Since the discovery of cytokines as key mediators in inflammation, targeting the cytokine network has represented a promising therapeutic approach. Psoriasis and atopic dermatitis, as T cell-mediated diseases with a strong cytokine component and a high unmet medical need, have moved into the focus of experimental therapies. Whereas pro-inflammatory cytokines such as tumor necrosis factor (TNF)-alpha are overexpressed in both diseases, a type 1 cytokine pattern predominates in psoriasis and a type 2 cytokine pattern is of pathophysiological importance at least in the initial stages of atopic dermatitis. Strategies for intervention into the cytokine network have included antagonism of pro-inflammatory cytokines (e.g. TNFalpha, interleukin [IL]-1, IL-8, IL-12, IL-18, IL-23) with neutralizing antibodies and soluble receptors, application of recombinant cytokines (e.g. IL-4, IL-10, IL-11, interferon [IFN]-gamma) to shift the cytokine balance, and administration of small molecules to modulate cytokine expression or signaling. Results from the clinic have led to novel therapeutic options as well as a better understanding of the pathophysiology of inflammatory skin diseases. This review highlights the various therapeutic strategies, results from the clinic (that are in some cases preliminary), and insights that can be drawn from the more advanced clinical studies and the use of approved cytokine-directed therapies.

Inhibitory effects of tumor necrosis factor on hematopoiesis seen in vitro are translated to increased numbers of both committed and multipotent progenitors in TNF-deficient mice

OBJECTIVES

The effects of TNF deficiency on myelopoiesis were evaluated in long-term (LTBMC) and short-term bone marrow cultures (STBMC) and compared to hematopoietic activity in vivo in TNF-deficient mice.

METHODS

LTBMC and STBMC were established from bone marrow of TNF-deficient mice in the presence or absence of soluble TNF. Total cell production was measured over time, as well as the number of colony-forming units in culture (CFU-C). Morphology of nonadherent (NA) cells in LTBMC was assessed after 10 weeks. Bone marrow cells (BMC) and peripheral blood (PB) cells were used to determine lineage distribution within the hematopoietic system. BMC were sorted to obtain Lin(-)c-kit(+)Sca-1- and Lin(-)c-kit(+)Sca-1+ cells, which were plated in semisolid media to determine CFU-C numbers or injected into irradiated recipients to determine colony formation in the spleen (CFU-S).

RESULTS

TNF-deficient LTBMC and STBMC show increased proliferative capacity, which can be inhibited by exogenous TNF to wild-type levels. Morphological analysis of NA cells from TNF-deficient LTBMC revealed increased numbers of cells at early stages of granulocytic differentiation (myeloblasts/promyelocytes) paralleled by a sharp decrease in the number of terminally differentiated polymorphonuclear neutrophils. Slightly elevated numbers of leukocytes, mainly neutrophils, were detected in PB of TNF-deficient mice. In bone marrow of TNF-deficient mice a significant increase in the number of both CFU-GM within Lin(-)c-kit(+)Sca-1- population and CFU-S within Lin(-)c-kit(+)Sca-1+ population was observed.

CONCLUSIONS

TNF has inhibitory effects on granulocyte-macrophage progenitors in vitro and on committed and primitive hematopoietic progenitors in vivo. However, in adult organism TNF deficiency is mostly compensated and controlled.

Independent protective effects for tumor necrosis factor and lymphotoxin alpha in the host response to Listeria monocytogenes infection

Although the essential role of tumor necrosis factor (TNF) in resistance to Listeria monocytogenes infection is well established, the roles of the related cytokines lymphotoxin alpha (LTalpha) and lymphotoxin beta (LTbeta) are unknown. Using C57BL/6 mice in which the genes for these cytokines were disrupted, we examined the contributions of TNF, LTalpha, and LTbeta in the host response to Listeria. To overcome the lack of peripheral lymph nodes in LTalpha(-/-) and LTbeta(-/-) mice, bone marrow chimeras were constructed. TNF(-/-) and LTalpha(-/-) chimeras that lacked both secreted LTalpha(3) and membrane-bound LTalpha(1)beta(2) and LTalpha(2)beta(1) were highly susceptible and succumbed 4.5 and 6 days, respectively, after a low-dose infection (200 CFU). LTbeta(-/-) chimeras, which lacked only membrane-bound LT, controlled the infection in a manner comparable to wild-type (WT) chimeras. The Listeria-specific proliferative and gamma interferon T-cell responses were equivalent in all five groups of infected mice (LTalpha(-/-) and LTbeta(-/-) chimeras, WT chimeras, and TNF(-/-) and WT mice). TNF(-/-) mice and LTalpha(-/-) chimeras, however, failed to generate the discrete foci of lymphocytes and macrophages that are essential for bacterial elimination. Rather, aberrant necrotic lesions comprised predominantly of neutrophils with relatively few lymphocytes and macrophages were observed in the livers and spleens of TNF(-/-) and LTalpha(-/-) chimeras. Therefore, in addition to TNF, soluble LTalpha(3) plays a separate essential role in control of listerial infection through control of leukocyte accumulation and organization in infected organs.