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Terminal differentiation of B cells into antibody-secreting cells is the foundation of humoral immune response. B-1 cells, which are different from B-2 cells, preferentially differentiate into plasma cells. CMTM7 is a MARVEL-domain-containing membrane protein predominantly expressed in B cells that plays an important role in B-1a cell development. The present study assessed CMTM7 function in response to antigen stimulation. Following immunization with T cell-dependent and T cell-independent antigens, Cmtm7-deficient mice exhibited decreased IgM but normal IgG responses in vivo. In vitro stimulation with LPSs induced Cmtm7 -/- B-1 cell activation, whereas proliferation was marginally reduced. Notably, Cmtm7 deficiency markedly suppressed plasma cell differentiation in response to TLR agonists, accompanied by a decrease in IgM and IL-10 production. At the molecular level, loss of Cmtm7 repressed the downregulation of Pax5 and the upregulation of Xbp1, Irf4, and Prdm1. Furthermore, p38 phosphorylation was inhibited in Cmtm7 -/- B-1 cells. Experiments using a p38 inhibitor revealed that p38 activation was essential for the terminal differentiation of B-1 cells, suggesting that Cmtm7 contributes to B-1 cell differentiation by maintaining p38 activation. Overall, the data reveal the crucial functions of CMTM7 in TLR-induced terminal differentiation and p38 activation in B-1 cells., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Natural killer T (NKT) cells recognize glycolipids presented on CD1d. They share features of adaptive T lymphocytes and innate NK cells, and mediate immunoregulatory functions via rapid production of cytokines. Invariant (iNKT) and diverse (dNKT) NKT cell subsets are defined by their TCR. The immunological role of dNKT cells, that do not express the invariant TCRα-chain used by iNKT cells, is less well explored than that of iNKT cells. Here, we investigated signals driving Toll-like receptor (TLR) ligand activation of TCR-transgenic murine dNKT cells. IFN-γ production by dNKT cells required dendritic cells (DC), cell-to-cell contact and presence of TLR ligands. TLR-stimulated DC activated dNKT cells to secrete IFN-γ in a CD1d-, CD80/86- and type I IFN-independent manner. In contrast, a requirement for IL-12p40, and a TLR ligand-selective dependence on IL-18 or IL-15 was observed. TLR ligand/DC stimulation provoked early secretion of pro-inflammatory cytokines by both CD62L + and CD62L - dNKT cells. However, proliferation was limited. In contrast, TCR/co-receptor-mediated activation resulted in proliferation and delayed production of a broader cytokine spectrum preferentially in CD62L - dNKT cells. Thus, innate (TLR ligand/DC) and adaptive (TCR/co-receptor) stimulation of dNKT cells resulted in distinct cellular responses that may contribute differently to the formation of immune memory., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Increasing evidence suggests a role of CD8 T cells in autoimmune demyelinating CNS disease, which, however, is still controversially discussed. Mice, which express ovalbumin (OVA) as cytosolic self-antigen in oligodendrocytes (ODC-OVA mice), respond to CNS infection induced by OVA-expressing attenuated Listeria with CD8 T cell-mediated inflammatory demyelination. This model is suitable to decipher the contribution of CD8 T cells and the pathogen in autoimmune CNS disease. Here, we show that both antigen and pathogen are required in the CNS for disease induction, though not in a physically linked fashion. Intracerebral challenge with combined toll like receptor (TLR) TLR2 and TLR9 as well as TLR7 and TLR9 agonists substituted for the bacterial stimulus, but not with individual TLR agonists (TLR2, TLR3,TLR5,TLR7, TLR9). Furthermore, MyD88 inactivation rendered ODC-OVA mice resistant to disease induction. Collectively, CD8 T cell-mediated destruction of oligodendrocytes is activated if (i) an antigen shared with an infectious agent is provided in the CNS microenvironment and (ii) innate immune signals inform the CNS microenvironment that pathogen removal warrants an immune attack by CD8 T cells, even at the expense of locally restricted demyelination., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Systemic lupus erythematosus is a debilitating autoimmune disease in which autoantibodies and autoreactive T cells destroy kidneys and other organs. Disease is clinically and genetically heterogeneous, suggesting that underlying mechanisms vary between patients. We previously used an autoantibody transgenic mouse reporter system to examine the effect of different autoimmune backgrounds on B-cell tolerance, failure of which is a fundamental defect in lupus. We identified a defect consistent with reversible anergy induced by endotoxin stimulation of B cells from Ig transgenic New Zealand Black (NZB) mice. Herein we report that the tolerance defect is revealed by TLR7 and TLR9 as well as TLR4 ligands, with additive effect, and is partially reversed by Mek inhibition. Gene expression analysis reveals significant differences in transcription of multiple TLR pathway genes and ptpn22 in stimulated NZB compared to B6 B cells. Additionally, the defect is detected in Ig transgenic NZB F1 hybrid strains (NZBxNZW)F1 and (B6xNZB)F1. These results implicate an inherited defect wherein NZB anergic B cells maintain coordinated TLR/BCR signaling that permits autoantibody production. Agents targeting these pathways may have therapeutic benefit in the subset of lupus patients that manifest similar defects in B-cell regulation., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Toll-like receptors (TLRs) orchestrate immune responses to a wide variety of danger- and pathogen-associated molecular patterns. Compared to the central nervous system (CNS), expression profile and function of TLRs in the human peripheral nervous system (PNS) are ill-defined. We analyzed TLR expression of satellite glial cells (SGCs) and microglia, glial cells predominantly involved in local immune responses in ganglia of the human PNS and normal-appearing white matter (NAWM) of the CNS, respectively. Ex vivo flow cytometry analysis of cell suspensions obtained from human cadaveric trigeminal ganglia (TG) and NAWM showed that both SGCs and microglia expressed TLR1-5, TLR7, and TLR9, although expression levels varied between these cell types. Immunohistochemistry confirmed expression of TLR1-TLR4 and TLR9 by SGCs in situ. Stimulation of TG- and NAWM-derived cell suspensions with ligands of TLR1-TLR6, but not TLR7 and TLR9, induced interleukin 6 (IL-6) secretion. We identified CD45 LOW CD14 POS SGCs and microglia, but not CD45 HIGH leukocytes and CD45 NEG cells as the main source of IL-6 and TNF-α upon stimulation with TLR3 and TLR5 ligands. In conclusion, human TG-resident SGCs express a broad panel of functional TLRs, suggesting their role in initiating and orchestrating inflammation to pathogens in human sensory ganglia., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Leukocyte recruitment is an important process in combating pathogens. The largest class of circulating leukocytes are neutrophils, which rapidly invade inflamed tissue, followed by inflammatory Ly6C + monocytes. Ly6C low monocytes patrol the endothelial wall routinely in the steady state. We recently reported early luminal recruitment of Ly6C low monocytes, which preceded and orchestrated neutrophil arrival and extravasation in response to TLR7/8-mediated vascular inflammation. Here we dissected the kinetics of recruitment of monocytes and neutrophils and examined the dynamics of Ly6C low monocytes in response to several other Toll-like receptor (TLR) agonists, using intravital confocal microscopy. We observed two types of kinetics in mesenteric veins. TLR2, TLR5 and TLR9 agonists caused early monocyte and neutrophil influx whereas TLR3 and TLR4 agonists rapidly recruited neutrophils and caused Ly6C low monocytes to arrive at low levels later on. All TLR agonists, except TLR9, led Ly6C low monocytes to meticulously patrol the vascular wall. Finally, these monocytes released pro-inflammatory cytokines and chemokines implicated in neutrophil recruitment in response to TLR2, TLR4, and TLR9 stimulation but not to TLR3 and TLR5 agonists. These results refine our understanding of the early events in the leukocyte recruitment cascade, including the patrolling behavior of Ly6C low monocytes, in TLR-mediated acute vascular inflammation., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Interleukin-1 receptor-associated kinase (IRAK) 4 mediates host defense against infections. As an active kinase, IRAK4 elicits full spectra of myeloid differentiation primary response protein (MyD) 88-dependent responses, while kinase-inactive IRAK4 induces a subset of cytokines and negative regulators whose expression is not regulated by mRNA stability. IRAK4 kinase activity is critical for resistance against Streptococcus pneumoniae, but its involvement in autoimmunity is incompletely understood. In this study, we determined the role of IRAK4 kinase activity in murine lupus. Lupus development in BXSB mice expressing the Y chromosome autoimmunity accelerator (Yaa) increased basal and Toll-like receptor (TLR) 4/7-induced phosphorylation of mitogen-activated protein kinases, p65 nuclear factor-κB (NF-κB), enhanced tumor necrosis factor (TNF)-α and C-C motif chemokine ligand (CCL) 5 gene expression in splenic macrophages, but decreased levels of Toll-interacting protein and IRAK-M, without affecting IRAK4 or IRAK1 expression. Mice harboring kinase-inactive IRAK4 on the lupus-prone Yaa background manifested blunted TLR signaling in macrophages and reduced glomerulonephritis, splenomegaly, serum anti-nuclear antibodies, numbers of splenic macrophages, total and TNF-α + dendritic cells, activated T- and B-lymphocytes, and lower TNF-α expression in macrophages compared with lupus-prone mice with functional IRAK4. Thus, IRAK4 kinase activity contributes to murine lupus and could represent a new therapeutic target., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

IL-37 is unique in the IL-1 family in that unlike other members of the family, IL-37 broadly suppresses innate immunity. IL-37 can be elevated in humans with inflammatory and autoimmune diseases where it likely functions to limit inflammation. Transgenic mice expressing human IL-37 (IL37-tg) exhibit less severe inflammation in models of endotoxin shock, colitis, myocardial infarction, lung, and spinal cord injury. IL37-tg mice have reduced antigen-specific responses and dendritic cells (DCs) from these mice exhibit characteristics of tolerogenic DCs. Compared to aging wild-type (WT) mice, aging IL37-tg mice are protected against B-cell leukemogenesis and heart failure. Treatment of WT mice with recombinant human IL-37 has been shown to be protective in several models of inflammation and injury. IL-37 binds to the IL-18 receptor but then recruits the orphan IL-1R8 (formerly TIR8 or SIGIRR) in order to function as an inhibitor. Here, we review the discovery of IL-37, its production, release, and mechanisms by which IL-37 reduces inflammation and suppresses immune responses. The data reviewed here suggest a therapeutic potential for IL-37., Competing Interests: Conflict of interest. The authors declare no financial or commercial conflict of interest., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Pentraxin-3 (PTX3), an acute-phase protein released during inflammation, aids phagocytic clearance of pathogens and apoptotic cells, and plays diverse immunoregulatory roles in tissue injury. In neuroinflammatory diseases, like MS, resident microglia could become activated by endogenous agonists for Toll like receptors (TLRs). Previously we showed a strong TLR2-mediated induction of PTX3 in cultured human microglia and macrophages by HspB5, which accumulates in glia during MS. Given the anti-inflammatory effects of HspB5, we examined the contribution of PTX3 to these effects in MS and its animal model EAE. Our data indicate that TLR engagement effectively induces PTX3 expression in human microglia, and that such expression is readily detectable in MS lesions. Enhanced PTX3 expression is prominently expressed in microglia in preactive MS lesions, and in microglia/macrophages engaged in myelin phagocytosis in actively demyelinating lesions. Yet, we did not detect PTX3 in cerebrospinal fluid of MS patients. PTX3 expression is also elevated in spinal cords during chronic relapsing EAE in Biozzi ABH mice, but the EAE severity and time course in PTX3-deficient mice did not differ from WT mice. Moreover, systemic PTX3 administration did not alter the disease onset or severity. Our findings reveal local functions of PTX3 during neuroinflammation in facilitating myelin phagocytosis, but do not point to a role for PTX3 in controlling the development of autoimmune neuroinflammation., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Intramacrophage protozoan parasite Leishmania donovani, causative agent of visceral leishmaniasis, escapes Toll-like receptor (TLR) dependent early host immune response by inducing the deubiquitinating enzyme A20, which is sustained up to 6 h postinfection only. Therefore, Leishmania must apply other means to deactivate late host responses. Here, we elucidated the role of IL-1 receptor-associated kinase M (IRAK-M), a negative regulator of TLR signaling, in downregulating macrophage proinflammatory response during late hours of in vitro infection. Our data reveal a sharp decline in IRAK1 and IRAK4 phosphorylation at 24 h postinfection along with markedly reduced association of IRAK1-TNF receptor associated factor 6, which is mandatory for TLR activation. In contrast, IRAK-M was induced after A20 levels decreased and reached a maximum at 24 h postinfection. IRAK-M induction coincided with increased stimulation of TGF-β, a hallmark cytokine of visceral infection. TGF-β-dependent signaling-mediated induction of SMAD family of proteins, 2, 3, and 4 plays important roles in transcriptional upregulation of IRAK-M. In infected macrophages, siRNA-mediated silencing of IRAK-M displayed enhanced IRAK1 and IRAK4 phosphorylation with a concomitant increase in downstream NF-κB activity and reduced parasite survival. Taken together, the results suggest that IRAK-M may be targeted by L. donovani to inhibit TLR-mediated proinflammatory response late during in vitro infection., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Toll-like receptors (TLRs) play a major part in providing innate immunity against pathogenic microorganisms. Recent studies show that these receptors are also expressed on T cells, which are the sentinels of adaptive immunity. Here, we have investigated the regulatory role of the T-cell receptor in the functioning of these innate receptors in T cells. We show that freshly isolated human CD4(+) T cells readily secrete the neutrophil chemoattractant CXCL8 upon activation with the TLR ligands Pam3CSK and flagellin. In contrast, TCR-activated cells secrete considerably less CXCL8 but start producing IFN-γ upon stimulation with TLR agonists in the absence of concomitant TCR engagement. These T cells show increased activation of p38 and JNK MAP-kinases in response to TLR stimulation, and inhibition of p38 abrogates TLR-induced IFN-γ secretion. The shifting of the T-cell innate immune response from CXCL8(hi) IFN-γ(null) in freshly isolated to CXCL8(lo) IFN-γ(hi) in activated T cells is also observed in response to endogenous innate stimulus, IL-1. These results suggest that the innate immune response of human CD4(+) T cells switches from a proinflammatory to an effector type following activation of these cells through the antigen receptor., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Dendritic cells (DCs) are key regulators of both innate and adaptive immunity. During infection, DCs recognise pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors (PRRs) including the Toll-like receptor (TLR) family. TLRs mainly signal via the adaptor protein MyD88. This signalling pathway is required for immune protection during many infections, which are lethal in the absence of MyD88. However, the cell type specific importance of this pathway during both innate and adaptive immune responses against pathogens in vivo remains ill-defined. We discuss recent findings from conditional KO or gain-of-function mouse models targeting TLR/MyD88 signalling pathways in DCs and other myeloid cells during infection. While the general assumption that MyD88-dependent recognition by DCs is essential for inducing protective immunity holds true in some instances, the results surprisingly indicate a much more complex context-dependent requirement for this pathway in DCs and other myeloid or lymphoid cell-types in vivo. Furthermore, we highlight the advantages of Cre-mediated DC targeting approaches and their possible limitations. We also present future perspectives on the development of new genetic mouse models to target distinct DC subsets in vivo. Such models will serve to understand the functional heterogeneity of DCs in vivo., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Interleukin-1β (IL-1β) production is impaired in cord blood monocytes. However, the mechanism underlying this developmental attenuation remains unclear. Here, we analyzed the extent of variability within the Toll-like receptor (TLR)/NLRP3 inflammasome pathways in human neonates. We show that immature low CD14 expressing/CD16(pos) monocytes predominate before 33 weeks of gestation, and that these cells lack production of the pro-IL-1β precursor protein upon LPS stimulation. In contrast, high levels of pro-IL-1β are produced within high CD14 expressing monocytes, although these cells are unable to secrete mature IL-1β. The lack of secreted IL-1β in these monocytes parallels a reduction of NLRP3 induction following TLR stimulation resulting in a lack of caspase-1 activity before 29 weeks of gestation, whereas expression of the apoptosis-associated speck-like protein containing a CARD and function of the P2×7 receptor are preserved. Our analyses also reveal a strong inhibitory effect of placental infection on LPS/ATP-induced caspase-1 activity in cord blood monocytes. Lastly, secretion of IL-1β in preterm neonates is restored to adult levels during the neonatal period, indicating rapid maturation of these responses after birth. Collectively, our data highlight important developmental mechanisms regulating IL-1β responses early in gestation, in part due to a downregulation of TLR-mediated NLRP3 expression. Such mechanisms may serve to limit potentially damaging inflammatory responses in a developing fetus., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The innate immune system detects infection and tissue injury through different families of pattern-recognition receptors (PRRs), such as Toll-like receptors. Most PRR-mediated responses initiate elaborate processes of signaling, transcription, translation, and secretion of effector mediators, which together require time to achieve. Therefore, PRR-mediated processes are not active in the early phases of infection. These considerations raise the question of how the host limits microbial replication and invasion during this critical period. Here, we examine the crucial defense mechanisms, such as antimicrobial peptides or extracellular traps, typically activated within minutes of the initial infection phase, which we term the "immediate protective response". Deficiencies in different components of the immediate protective response are often associated with severe and recurrent infectious diseases in humans, highlighting their physiologic importance., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We analyzed the functional outcome of the interaction between tumor-associated macrophages (TAMs) and natural killer (NK) cells. TAMs from ascites of ovarian cancer patients displayed an alternatively activated functional phenotype (M2) characterized by a remarkably high frequency and surface density of membrane-bound IL-18. Upon TLR engagement, TAMs acquired a classically activated functional phenotype (M1), released immunostimulatory cytokines (IL-12, soluble IL-18), and efficiently triggered the cytolytic activity of NK cells. TAMs also induced the release of IFN-γ from NK cells, which however was significantly lower compared with that induced by in vitro-polarized M2 cells. Most tumor-associated NK cells displayed a CD56(bright) , CD16(neg) or CD56(bright) , CD16(dim) phenotype, and very poor cytolytic activities, despite an increased expression of the activation marker CD69. They also showed downregulation of DNAM-1, 2B4, and NTB-A activating receptors, and an altered chemokine receptor repertoire. Importantly however, when appropriately stimulated, NK cells from the patients, including those cells isolated from ascites, efficiently killed autologous TAMs that expressed low, "nonprotective" levels of HLA class I molecules. Overall, our data show the existence of a complex tumor microenvironment in which poorly cytolytic/immature NK cells deal with immunosuppressive tumor-educated macrophages., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Basophils are mostly known for their involvement in allergic reactions. Recent studies in mice indicate a role for basophils in the induction of adaptive immunity, especially T helper 2 (Th2) responses. Therefore, it would be highly important to understand how basophils respond to pathogen-associated molecules, such as ligands for toll-like receptors (TLRs), and if the basophils could promote Th2 responses via these stimuli. To this end, the activation of basophils via TLRs in combination with activation via IgE was studied, as well as its effect on T helper cell skewing. Using quantitative PCR, we demonstrated the presence of mRNA for TLRs 1-8 in human basophils. Basophils responded to TLR triggering with differential cytokine production, but not with degranulation. Simultaneous triggering of TLRs and IgE led to synergy in production of IL-4, IL-8, IL-13, and RANTES. Furthermore, the synergistic effects on basophils mediated by IgE and TLR-4 triggering allowed robust Th2 skewing upon activation of naïve human CD4⁺ T cells. Our data show that human basophils respond to TLR ligands in synergy with IgE-mediated activation and that the cytokines produced can promote Th2 differentiation. These results indicate a role for basophils in the regulation of T-cell responses in humans., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Recent research has shown that (i) Toll-like receptor (TLR) agonists drive hematopoietic stem and progenitor cells (HSPCs) to proliferate and differentiate along the myeloid lineage in vitro, and (ii) direct TLR-mediated stimulation of HSPCs also promotes macrophage differentiation in vivo following infection. These new insights demonstrate that TLR signaling in HSPCs, in addition to other TLR-dependent mechanisms, can contribute to HSPC expansion and myeloid differentiation after infection. Evidence is, therefore, mounting that direct TLR-induced programming of hematopoiesis plays a key role in host defense by rapidly replenishing the innate immune system with the cells needed to deal with pathogens., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Human multipotent mesenchymal stromal cells (MSCs) are clinically applied to treat autoimmune diseases and graft-versus-host disease due to their immunomodulatory properties. Several molecules have been identified to mediate these effects, including constitutively expressed galectin-1. However, there are indications in the literature that MSCs exert enhanced immunosuppressive functions after interaction with an inflammatory environment. Therefore, we analyzed how inflammatory stimuli influence the expression of the galectin network in MSCs and functionally tested the relevance for the immunomodulatory effects of MSCs. We found that galectin-9 was strongly induced in MSCs upon interaction with activated PBMCs. Proinflammatory cytokines, such as interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α), and also ligands of the Toll-like receptors (TLRs) TLR2, TLR3, and TLR4 elicited similar induction of galectin-9 in activated PBMCs. Galectin-9 was not only upregulated intracellularly, but also released by MSCs in significant amounts into the supernatant after exposure to proinflammatory stimuli. In proliferation assays, MSCs with a galectin-9 knockdown lost a significant portion of their antiproliferative effects on T cells. In conclusion, we found that unlike constitutively expressed galectin-1, galectin-9 is induced by several proinflammatory stimuli and released by MSCs. Thus, galectin-9 contributes to the inducible immunomodulatory functions of MSCs., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Most Toll-like-receptors (TLRs) and interleukin-1 receptors (IL-1Rs) signal via myeloid differentiation primary response 88 (MyD88) and interleukin-1 receptor-associated kinase 4 (IRAK-4). The combined roles of these two receptor families in the course of experimental infections have been assessed in MyD88- and IRAK-4-deficient mice for almost fifteen years. These animals have been shown to be susceptible to 46 pathogens: 27 bacteria, eight viruses, seven parasites, and four fungi. Humans with inborn MyD88 or IRAK-4 deficiency were first identified in 2003. They suffer from naturally occurring life-threatening infections caused by a small number of bacterial species, although the incidence and severity of these infections decrease with age. Mouse TLR- and IL-1R-dependent immunity mediated by MyD88 and IRAK-4 seems to be vital to combat a wide array of experimentally administered pathogens at most ages. By contrast, human TLR- and IL-1R-dependent immunity mediated by MyD88 and IRAK-4 seems to be effective in the natural setting against only a few bacteria and is most important in infancy and early childhood. The roles of TLRs and IL-1Rs in protective immunity deduced from studies in mutant mice subjected to experimental infections should therefore be reconsidered in the light of findings for natural infections in humans carrying mutations as discussed in this review., (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Induction of the glucocorticoid-induced leucine zipper (GILZ) by glucocorticoids plays a role in their antiinflammatory action, whereas GILZ expression is reduced under inflammatory conditions. The mechanisms regulating GILZ expression during inflammation, however, have not yet been characterized. Here, we investigated GILZ expression in human alveolar macrophages (AMs) following Toll-like receptor (TLR) activation. Macrophages were shown to predominantly express GILZ transcript variant 2. Lipopolysaccharide-treated AMs, THP-1 cells, and lungs of lipopolysaccharide-exposed mice displayed decreased GILZ protein and mRNA levels. The effect was strictly dependent on the adapter molecule MyD88, as shown by using specific ligands or a knockdown strategy. Investigations on the functional significance of GILZ downregulation performed by GILZ knockdown revealed a proinflammatory response, as indicated by increased cytokine expression and NF-κB activity. We found that TLR activation reduced GILZ mRNA stability, which was mediated via the GILZ 3'-untranslated region. Finally, involvement of the mRNA-binding protein tristetraprolin (TTP) is suggested, since TTP overexpression or knockdown modulated GILZ expression and TTP was induced in a MyD88-dependent fashion. Taken together, our data show a MyD88- and TTP-dependent GILZ downreg-ulation in human macrophages upon TLR activation. Suppression of GILZ is mediated by mRNA destabilization, which might represent a regulatory mechanism in macrophage activation., (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The 2011 Nobel Prize in Physiology/Medicine to Ralph Steinmann, Jules Hoffmann, and Bruce Beutler recognized a paradigm shift in our understanding of innate immunity, and its impact on adaptive immunity. The Prize highlighted the initial discoveries of Toll's role in immunity in flies, Toll-like receptors in mammals, and the establishment of dendritic cells as the initiators of adaptive immunity. This historical Commentary focuses on the developments in our understanding of innate immunity., (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

DCs play a key role in defense against infections and also in preventing inflammatory and autoimmune diseases. The response of DCs to pathogens is tightly regulated by many mechanisms to allow for appropriate, but not pathogenic, responses. We previously showed that DCs with deficiencies for two ITAM-bearing signaling adapters, DAP12 and FcRγ, produce more inflammatory cytokines upon treatment with Toll-like receptor (TLR) agonists than WT DCs. Here, we investigated whether the TREM-2 receptor pairs with DAP12 to inhibit TLR responses in DCs. TREM-2-deficient BMDCs showed increased inflammatory cytokine and type I IFN production in response to TLR ligation. Additionally, TREM-2-deficient BMDCs had increased TLR-induced maturation and were more efficient at inducing antigen-specific T-cell proliferation upon CpG DNA stimulation compared with WT BMDCs. Finally, we showed that a TREM-2 ligand is expressed on the surface of BMDCs, suggesting that the TREM-2 receptor transduces inhibitory signals due to recognition of an endogenous ligand., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The generation of plasma cells (PCs) is key for proper humoral immune responses. The transcription factors IRF-4 and BLIMP-1 (B-lymphocyte induce maturation protein-1) control PC commitment, but the underlying regulatory mechanisms are incompletely understood. Here we have identified SWAP-70 as being critically involved in Toll-like receptor (TLR)-triggered PC differentiation. Upon activation through various TLRs, Swap-70(-/-) B cells were activated and proliferated normally. However, expression of BLIMP-1 was markedly reduced and PC differentiation was impaired. Four hours of LPS stimulation were sufficient to drive PC differentiation, and SWAP-70 was required during this initial period. Swap-70(-/-) B cells pre-activated in vitro failed to efficiently differentiate into PCs upon adoptive transfer into recipient mice. Re-introduction of SWAP-70 into Swap-70(-/-) B cells rescued their development into PCs, and SWAP-70 over-expression in wild-type (WT) B cells increased PC generation. In the absence of SWAP-70, IRF-4 protein levels were reduced and the IRF-4(high) B220(+) CD138(-) compartment, including PC precursors, was strongly diminished. Ectopic expression of SWAP-70 increases IRF-4 protein levels and PC differentiation in WT and Swap-70(-/-) B cells, and IRF-4 over-expression in Swap-70(-/-) B cells elevates PC differentiation to WT levels. Thus, in a dose-dependent manner, SWAP-70 controls IRF-4 protein expression and thereby regulates the initiation of PC differentiation., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Toll-like receptors (TLRs) signal the presence of pathogens and tissue injury, triggering the inflammatory process in macrophages. The goal of inflammation is to resolve the injury and return the body to homeostasis. MicroRNAs are an important group of regulators of TLR signaling and several are induced by TLRs in macrophages. These TLR-induced microRNAs target signaling components in the TLR pathway, thereby producing a negative feedback loop, and they are therefore prime candidates for the initiation of repair. Importantly, their dysregualtion may be important for chronic inflammation, which in turn can lead to autoimmunity and cancer, as discussed in this Viewpoint., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

TLR agonists have been suggested to directly impact Tregs, thereby enhancing or reversing their suppressive function. Here, in order to select TLR agonists leading to potent effector T-cell responses, while minimizing Treg inhibitory function, we used a model antigen, covalently linked to an inert delivery system, combined with a large panel of TLR agonists, for the immunization of mice with an attenuated/depleted or intact Treg subset. We observed that the negative modulation of effector CD4(+) T cells exerted by Tregs cannot be circumvented, whatever the TLR agonist used as adjuvant. To better understand the impact of TLR agonists on Tregs, we investigated (i) the TLR expression profile of highly purified CD4(+) Foxp3(+) Tregs, at steady state or subsequent to in vivo activation by TLR agonists and (ii) the Treg phenotype after in vivo and in vitro activation by TLR agonists. Our results demonstrate that TLR agonists, as single signal inducers, are not able to directly activate Tregs. The phenotypic Treg activation observed in vivo, following TLR administration, does not result from cross-talk with conventional T cells but is rather a consequence of the interaction with other immune cell type(s)., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

DEC-205 is a type I C-type lectin receptor (CLR) that is expressed on various APC subsets and has been suggested to bind necrotic and apoptotic cells. Here we study DEC-205 characteristics in plasmacytoid DCs (pDCs) obtained from healthy individuals and assess its ability to mediate antigen presentation by isolating sufficient numbers of pDCs from apheresis material obtained from stage III/IV melanoma patients. The results demonstrate that DEC-205 is expressed on human pDCs. Internalization of DEC-205 after antibody ligation is clathrin- and dynamin-dependent as it is blocked by hypertonic shock or by inhibition of dynamin activity. Antibody targeting to DEC-205 does not affect TLR-induced expression levels of co-stimulatory and MHC molecules, but clearly impairs TLR-induced IFN-α secretion by 40%. We observed that TLR-mediated signaling increases DEC-205 expression levels without affecting receptor internalization. Moreover, human pDCs retained the capacity to present antigens via DEC-205 following TLR activation., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Multiple physiological and pathological conditions interfere with the function of the endoplasmic reticulum (ER). However, much remains unknown regarding the impact of ER stress on inflammatory responses in dendritic cells (DCs) upon the recognition of pathogen molecules. We show that ER stress greatly potentiates the expression of inflammatory cytokines and IFN-β in murine DCs stimulated by polyIC, a synthetic mimic of virus dsRNA. Both toll-like receptor 3 and melanoma differentiation-associated gene-5 are involved in the enhanced IFN-β production, which is associated with increased activation of NF-κB and IRF3 signaling as well as the splicing of X-box-binding protein-1 (XBP-1), an important regulator involved in ER stress response. Surprisingly, silencing of XBP-1 reduces polyIC-stimulated IFN-β expression in the presence or absence of ER stress, indicating that XBP-1 may be essential for polyIC signaling and ER stress-amplified IFN-β production. Overexpression of a spliced form of XBP-1 (XBP-1s) synergistically augments polyIC-induced inflammatory response. For the first time, we show that XBP-1s overexpression-enhanced IFN-β production in DCs markedly suppresses vesicular stomatitis virus infection, revealing a previously unrecognized role for XBP-1 in an antiviral response. Our findings suggest that evolutionarily conserved ER stress response and XBP-1 may function collaboratively with innate immunity to maintain cellular homeostasis., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Cross-presentation is an important mechanism by which DCs present exogenous antigens on MHC-I molecules, and activate CD8(+) T cells, cells that are crucial for the elimination of tumors. We investigated the feasibility of exploiting the capacity of the mannose receptor (MR) to improve both cross-presentation of tumor antigens and Th polarization, processes that are pivotal for the anti-tumor potency of cytotoxic T cells. To this end, we selected two glycan ligands of the MR, 3-sulfo-Lewis(A) and tri-GlcNAc (N-acetylglucosamine), to conjugate to the model antigen OVA and assessed in vitro the effect on antigen presentation and Th differentiation. Our results demonstrate that conjugation of either 3-sulfo-Lewis(A) or tri-GlcNAc specifically directs antigen to the MR. Both neo-glycoconjugates showed, even at low doses, improved uptake as compared with native OVA, resulting in enhanced cross-presentation. Using MR(-/-) and MyD88-TRIFF(-/-) bone marrow-derived DCs (BMDCs), we show that the cross-presentation of the neo-glycoconjugates is dependent on MR and independent of TLR-mediated signaling. Whereas proliferation of antigen-specific CD4(+) T cells was unchanged, stimulation with neo-glycoconjugate-loaded DCs enhanced the generation of IFN-γ-producing T cells. We conclude that modification of antigen with either 3-sulfo-Lewis(A) or tri-GlcNAc enhances cross-presentation and permits Th1 skewing, through specific targeting of the MR, which may be beneficial for DC-based vaccination strategies to treat cancer., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A balance of inhibitory and activating signals determines the function of dendritic cells (DCs) in the immune response, which may be regulatory or stimulatory. Defects of inhibitory receptor FcγRIIb are involved in the pathogenesis of autoimmune diseases such as systemic lupus erythematosus (SLE), in which high levels of circulating immune complexes (IC) exist. Our previous study showed that IC/Ig can suppress TLR4-triggered inflammatory responses in macrophages via FcγRIIb. This led us to question whether IC/Ig can polarize FcγRIIb-overexpressing DCs (DC-FcγRIIb) to be tolerogenic, thus attenuating lupus progression once infused in vivo. First, we found that IC/Ig markedly inhibited LPS- or CpG-induced DC maturation, enhanced tolerogenicity of DCs via FcγRIIb, and induced massive prostaglandin E2 (PGE2) secretion from DCs, both contributing to T-cell hyporesponsiveness. Endogenous Ig and lupus-derived IC also exhibited the same effect. DC-FcγRIIb, transfected with recombinant adenovirus encoding FcγRIIb, displayed enhanced tolerogenic function and produced more PGE2 in the presence of IC, thus further inhibiting T-cell responses. Importantly, in vivo infusion with DC-FcγRIIb significantly reduced kidney damage and prolonged the survival of lupus-prone MRL/lpr mice either before or after the onset of clinic lupus. Therefore, administration of DC-FcγRIIb may be a new approach to attenuate lupus progression., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Cystatin could completely cure experimental visceral leishmaniasis by switching the differentiation of Th2 cells to Th1 type, as well as upregulating NO, and activation of NF-κB played a major role in these processes. Analysis of upstream signaling events revealed that TLR 2/4-mediated MyD88-dependent participation of IL-1R-activated kinase (IRAK)1, TNF receptor-associated factor (TRAF)6 and TGFβ-activated kinase (TAK)1 is essential to induce cystatin-mediated IκB kinase (IKK)/NF-κB activation in macrophages. Cystatin plus IFN-γ activated the IKK complex to induce phosphorylation-mediated degradation of p105, the physiological partner and inhibitor of the MEK kinase, tumor progression locus 2 (Tpl-2). Consequently, Tpl-2 was liberated from p105, thereby stimulating activation of the MEK/ERK MAPK cascade. Cystatin plus IFN-γ-induced IKK-β post-transcriptionally modified p65/RelA subunit of NF-κB by dual phosphorylation in infected phagocytic cells. IKK induced the phosphorylation of p65 directly on Ser-536 residue whereas phosphorylation on Ser 276 residue was by sequential activation of Tpl-2/MEK/ERK/MSK1. Collectively, the present study indicates that cystatin plus IFN-γ-induced MyD88 signaling may bifurcate at the level of IKK, leading to a divergent pathway regulating NF-κB activation by IκBα phosphorylation and by p65 transactivation through Tpl-2/MEK/ERK/MSK1., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A healthy intestinal tract is characterized by controlled homeostasis due to the balanced interaction between commensal bacteria and the host mucosal immune system. Human and animal model studies have supported the hypothesis that breakdown of this homeostasis may underlie the pathogenesis of inflammatory bowel diseases. However, it is not well understood how intestinal microflora stimulate the intestinal mucosal immune system and how such activation is regulated. Using a spontaneous, commensal bacteria-dependent colitis model in IL-10-deficient mice, we investigated the role of TLR and their negative regulation in intestinal homeostasis. In addition to IL-10(-/-) MyD88(-/-) mice, IL-10(-/-) TLR4(-/-) mice exhibited reduced colitis compared to IL-10(-/-) mice, indicating that TLR4 signaling plays an important role in inducing colitis. Interestingly, the expression of IRAK-M, a negative regulator of TLR signaling, is dependent on intestinal commensal flora, as IRAK-M expression was reduced in mice re-derived into a germ-free environment, and introduction of commensal bacteria into germ-free mice induced IRAK-M expression. IL-10(-/-) IRAK-M(-/-) mice exhibited exacerbated colitis with increased inflammatory cytokine gene expression. Therefore, this study indicates that intestinal microflora stimulate the colitogenic immune system through TLR and negative regulation of TLR signaling is essential in maintaining intestinal homeostasis., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Chitin is a highly abundant glycopolymer, which serves as structural component in fungi, arthropods and crustaceans but is not synthesized by vertebrates. However, vertebrates express chitinases and chitinase-like proteins, some of which are induced by infection with helminths suggesting that chitinous structures may be targets of the immune system. The chitin-induced modulations of the innate and adaptive immune responses are not well understood. Here, we demonstrate that intranasal administration of OVA and chitin resulted in diminished T-cell expansion and Th2 polarization as compared with OVA administration alone. Chitin did not promote nor attenuate Th2 polarization in vitro. Chitin-exposed macrophages inhibited proliferation of CD4(+) T cells in a cell-cell contact-dependent manner. Chitin induced upregulation of the inhibitory ligand B7-H1 (PD-L1) on macrophages independently of MyD88, TRIF, TLR2, TLR3, TLR4 and Stat6. Inhibition of T-cell proliferation was largely dependent on B7-H1, as the effect was not observed in cocultures with cells from B7-H1-deficient mice.

B cells are now acknowledged to play multiple roles in the immune response, in addition to making antibodies. Their role in regulating T-cell responses during inflammation has come into focus recently. Thus, IL-10 production by B cells has been shown to be important in limiting auto-reactive and pathogen-driven immune pathology; however, the exact identity of the regulatory B cells remains elusive; do they belong to a committed subset or can all B cells regulate given the appropriate inducing stimuli? A study in this issue of the European Journal of Immunology provides insight into the IL-10-producing B cells in humans, suggesting that many B cells have the capacity to make IL-10 when optimally stimulated via the BCR and TLR9. Despite producing significant amounts of inflammatory cytokines as well, these B cells suppress T-cell proliferation. This Commentary places this study in the context of what we think we know about regulatory B cells and more importantly highlights the questions we still need to answer.

Plasmacytoid DC (pDC) are type-I IFN-producing cells known for their capacity to promote anti-viral innate and adaptive immune responses. Despite their potent anti-viral function, when compared with conventional DC, pDC exhibit poor immunostimulatory ability and their interaction with T cells often favors the generation of Treg. pDC are activated primarily in response to ssRNA and ssDNA through TLR7 and TLR9, respectively, but also through TLR-independent mechanisms. Non-lymphoid tissue pDC, such as those residing in the airways, gut, and liver, play a significant role in regulating mucosal immunity and are critical for the development of tolerance to inhaled or ingested antigens. Herein we discuss properties that define tolerogenic pDC and how their unique characteristics translate into an ability to regulate immunity and promote the development of tolerance. We cover the importance of pDC during intrathymic Treg development and the maintenance of peripheral tolerance, as well as their regulatory role in transplantation, autoimmunity, and cancer. We highlight recent findings regarding danger-associated molecular pattern and PAMP signaling in the regulation of pDC function, and how the ability of pDC to promote tolerance translates into the potential clinical applications of these cells as therapeutic targets to regulate immune reactivity.

NK cells are cytotoxic cells of the innate immune system. They have been found to be critical in the defense against infections and also against some tumors. Recent studies have shown that NK cells require signals from accessory cells to induce their recruitment and activation at the site of infection or tumor growth. In this study, we examined whether plasmacytoid DC (pDC) could recruit and activate NK cells in vivo. When CpG-stimulated pDC were injected i.p. to C57BL/6 mice, they efficiently recruited NK cells, a process that was dependent on NK cell CXCR3 and CD62L and in part on CCR5. NK cells isolated from the peritoneum of mice inoculated with TLR7/8 or TLR9-stimulated pDC exhibited greater cytotoxicity against YAC-1 tumor cells than NK cells recovered from mice inoculated with control pDC. The present results are discussed in relation to pDC-induced NK cell migration and activation in vivo.

The Drosophila Toll-signaling pathway controls the systemic antifungal host response. Gram-negative binding protein 3 (GNBP3), a member of the beta-glucan recognition protein family senses fungal infections and activates this pathway. A second detection system perceives the activity of proteolytic fungal virulence factors and redundantly activates Toll. GNBP3(hades) mutant flies succumb more rapidly to Candida albicans and to entomopathogenic fungal infections than WT flies, despite normal triggering of the Toll pathway via the virulence detection system. These observations suggest that GNBP3 triggers antifungal defenses that are not dependent on activation of the Toll pathway. Here, we show that GNBP3 agglutinates fungal cells. Furthermore, it can activate melanization in a Toll-independent manner. Melanization is likely to be an essential defense against some fungal infections given that the entomopathogenic fungus Beauveria bassiana inhibits the activity of the main melanization enzymes, the phenol oxidases. Finally, we show that GNBP3 assembles "attack complexes", which comprise phenoloxidase and the necrotic serpin. We propose that Drosophila GNBP3 targets fungi immediately at the inception of the infection by bringing effector molecules in direct contact with the invading microorganisms.

Innate stimuli, such as TLR ligands, are known to greatly facilitate cross-priming. Currently it is unclear whether innate stimuli enhance cross-priming at the level of cross-presentation or at the level of T-cell priming. In this study, we addressed this question by measuring cross-presentation as well as cross-priming by virus-like particles (VLP) displaying peptide p33 derived of lymphocytic choriomeningitis virus. Innate stimuli were varied by either packaging different TLR ligands into virus-like particles or using mice deficient in two key molecules of TLR-signaling, namely the adaptor molecule MyD88 as well as IFN-alpha/beta receptor. While efficient cross-presentation occurred despite strongly reduced activation of DC in the absence of TLR ligand-mediated signals, T-cell priming was abolished. Thus, innate stimuli regulate cross-priming at the level of DC licensing for T-cell activation and not antigen presentation.

Oral Salmonella infection recruits phagocytes to Peyer's patches (PP) and MLN. The chemokines induced in infected PP and MLN, the cellular sources during infection and the TLR signaling pathways involved in vivo are not known. Here, we show that CCL2, CXCL9 and CXCL2 mRNA are up-regulated in PP and MLN coincident with the first arrival of monocytes and neutrophils. Laser capture microdissection microscopy revealed that chemokine mRNA up-regulation was differently distributed in PP. Despite this, recruited monocytes and neutrophils formed inflammatory cell clusters throughout PP. Monocytes and neutrophils purified from infected mice preferentially produced CXCL2 and small amounts of CCL2, and neutrophils from infected mice migrated towards CXCL2 and CCL3. Furthermore, phagocyte recruitment to PP and MLN was intact in mice lacking TLR4 alone and when signaling through TLR4 and TLR5 was simultaneously absent; however, recruitment was compromised in MyD88(-/-) and more so in MyD88(-/-)TLR4(-/-) double knockout mice. Phagocyte release into the blood, however, was only marginally reduced in MyD88(-/-)TLR4(-/-) mice. Defective phagocyte recruitment to PP and MLN of MyD88(-/-)TLR4(-/-) mice was paralleled by low chemokine induction. These data provide insight into the chemokines and TLR signaling pathways that orchestrate the early phagocyte response to oral Salmonella infection.

The analysis of the human memory B-cell repertoire is of both fundamental and practical significance. We developed a simple method for the selective activation of memory B cells in total fresh or frozen PBMC using a combination of R848 and IL-2. In these conditions, 30-40% of memory B cells generated clones producing on average 200 ng IgG in 10 days. This method was used to measure the frequency of antigen-specific memory B cells as well as the fine specificity, cross-reactivity and neutralizing activity of the secreted antibodies. Following influenza vaccination, specific B cells expanded dramatically, reaching up to 50% of total clonable memory B cells on day 14. Specific B-cell expansions were detected also in individuals that did not show a significant serological response. Dynamic changes and persistence of B cells specific for a variety of pathogens were documented in serial PBMC samples collected over almost two decades. These results reveal novel aspects of memory B-cell kinetics and provide a powerful tool to monitor immune responses following infection and vaccination.

Chemokines mediate the inflammatory response by attracting various leukocyte types. MCP-2/CC chemokine ligand 8 (CCL8) was induced at only suboptimal levels in fibroblasts and endothelial cells by IL-1beta or IFN-gamma, unless these cytokines were combined. IFN-gamma also synergized with the TLR ligands peptidoglycan (TLR2), dsRNA (TLR3) or LPS (TLR4). Under these conditions, intact MCP-2/CCL8(1-76) produced by fibroblasts was found to be processed into MCP-2/CCL8(6-75), which lacked chemotactic activity for monocytic cells. Furthermore, the capacity of MCP-2/CCL8(6-75) to increase intracellular calcium levels through CCR1, CCR2, CCR3 and CCR5 was severely reduced. However, the truncated isoform still blocked these receptors for other ligands. MCP-2/CCL8(6-75) induced internalization of CCR2, inhibited MCP-1/CCL2 and MCP-2/CCL8 ERK signaling and antagonized the chemotactic activity of several CCR2 ligands (MCP-1/CCL2, MCP-2/CCL8, MCP-3/CCL7). In contrast to MCP-3/CCL7, parvoviral delivery of MCP-2/CCL8 into B78/H1 melanoma failed to inhibit tumor growth, partially due to proteolytic cleavage into inactive MCP-2/CCL8 missing five NH(2)-terminal residues. However, in an alternative tumor model, using HeLa cells, MCP-2/CCL8 retarded tumor development. These data indicate that optimal induction and delivery of MCP-2/CCL8 is counteracted by converting this chemokine into a receptor antagonist, thereby losing its anti-tumoral potential.

The ability of TLR agonists to promote adaptive immune responses is attributed to their ability to robustly activate innate immunity. However, it has been observed that, for adjuvants in actual use in research and vaccination, TLR signaling is dispensable for generating humoral immunity. Here, we examined the role of TLR5 and MyD88 in promoting innate and humoral immunity to flagellin using a prime/boost immunization regimen. We observed that eliminating TLR5 greatly reduced flagellin-induced cytokine production, except for IL-18, and ablated DC maturation but did not significantly impact flagellin's ability to promote humoral immunity. Elimination of MyD88, which will ablate signaling through TLR and IL-1beta/IL-18 generated by Nod-like receptors, reduced, but did not eliminate flagellin's promotion of humoral immunity. In contrast, loss of the innate immune receptor for profilin-like protein (PLP), TLR11, greatly reduced the ability of PLP to elicit humoral immunity. Together, these results indicate that, firstly, the degree of innate immune activation induced by TLR agonists may be in great excess of that needed to promote humoral immunity and, secondly, there is considerable redundancy in mechanisms that promote the humoral immune response upon innate immune recognition of flagellin. Thus, it should be possible to design innate immune activators that are highly effective vaccine adjuvants yet avoid the adverse events associated with systemic TLR activation.

Newborns face complex physical and immunological changes before and after birth. Although the uterus is a sterile environment for the fetus, it also contains non-self material from the mother. Birth involves the transition from the sterile intra-uterine environment to an environment rich in microbes and requires rapid induction of appropriate responses to control these microbes. In this review we focus on the similarities and differences of human and murine neonatal DC and their reaction to various stimuli. A better understanding of the newborn immune system--in particular, the DC-T-cell interaction--will be beneficial for the development of improved strategies to prevent or treat infections in this vulnerable population and prepare the immune system to cope with allergens and tumors later in life.

Certain viral nucleic acids aggravate autoimmunity through nucleic acid-specific TLR. Viral 5'-triphosphate RNA (3P-RNA) and double-stranded non-CpG DNA induce antiviral immunity via TLR-independent pathways but their role in autoimmunity is unknown. Transient exposure of 16-wk-old MRL(lpr/lpr) mice to 3P-RNA aggravated lupus nephritis by increasing IFN signaling and decreasing CD4(+)CD25(+) T cells. By contrast, transient exposure to non-CpG DNA exacerbate lupus nephritis in association with splenomegaly, lymphoproliferation, hypergammaglobulinaemia and increased B220(+)CD138(+) plasma cells. Both, 3P-RNA and non-CpG DNA increased glomerular complement factor C3c deposits but both nucleic acid formats were less potent in aggravating renal pathology as compared with CpG DNA. 3P-RNA and non-CpG DNA also localized to the glomerular mesangial cells and activated cultured mesangial cells to produce IL-6. We conclude, 3P-RNA or non-CpG DNA both trigger autoimmune disease in MRL(lpr/lpr) mice by specifically activating adaptive immunity but similarly enhance inflammation on the tissue level.

Since enteric microbial composition is a distinctive and stable individual trait, microbial heterogeneity may confer lifelong, non-genetic differences between individuals. Here we report that C57BL/6 mice bearing restricted flora microbiota, a distinct but diverse resident enteric microbial community, are numerically and functionally deficient in marginal zone (MZ) B cells. Surprisingly, MZ B-cell levels are minimally affected by germ-free conditions or null mutations of various TLR signaling molecules. In contrast, MZ B-cell depletion is exquisitely dependent on cytolytic CD8(+) T cells, and includes targeting of a cross-reactive microbial/endogenous MHC class 1B antigen. Thus, members of certain enteric microbial communities link with CD8(+) T cells as a previously unappreciated mechanism that shapes innate immunity dependent on innate-like B cells.

Recent reports have provided convincing evidence that IL-17-producing T cells play a key role in the pathogenesis of organ-specific autoimmune diseases, a function previously attributed exclusively to IFN-gamma-secreting Th1 cells. Furthermore, it appears that IL-17-producing T cells can also function with Th1 cells to mediate protective immunity to pathogens. Although much of the focus has been on IL-17-secreting CD4+ T cells, termed Th17 cells, CD8+ T cells, gammadelta T cells and NKT cells are also capable of secreting IL-17. The differentiation of Th17 cells from naïve T cells appears to involve signals from TGF-beta, IL-6, IL-21, IL-1beta and IL-23. Furthermore, IL-1alpha or IL-1beta in synergy with IL-23 can promote IL-17 secretion from memory T cells. The induction or function of Th17 cells is regulated by cytokines secreted by the other major subtypes of T cells, including IFN-gamma, IL-4, IL-10 and at high concentrations, TGF-beta. The main function of IL-17-secreting T cells is to mediate inflammation, by stimulating production of inflammatory cytokines, such as TNF-alpha, IL-1beta and IL-6, and inflammatory chemokines that promote the recruitment of neutrophils and macrophages.

Cancer immunotherapy aims at inducing immune responses against tumour-associated antigens that mediate the eradication of tumour cells. For successful vaccination against antigens expressed by the tumour, the immune system has to be provided with sufficient amounts of these antigens in connection with strong immunostimulatory signals such as toll-like receptor (TLR) ligands. Tumour cells represent a convenient source of relevant tumour-associated antigens but can have suppressive properties. In this study, we explored how different forms of tumour cell material influence the activation of dendritic cells (DC), which play a crucial role in the induction of anti-tumour immune responses. We show that freeze-and-thaw-disrupted tumour cells inhibit DC activation in response to TLR stimulation, a phenomenon that is only partially seen with non-disrupted control cells. This suppression of DC stimulation is independent of tumour cell- and species-specific factors. We tested the hypothesis that phosphatidylserine on cells with disrupted membrane integrity mediates inhibition of TLR-induced DC activation. Our experimental evidence indicates that phosphatidylserine is not involved in the inhibition of TLR-mediated DC activation by freeze-and-thaw-disrupted cells. The inhibitory activity associated with disrupted tumour cells could explain why such preparations are less effective tumour vaccines than apoptotic tumour cells.

Notch signaling is a well-conserved pathway involved in cell fate decisions, proliferation and apoptosis. We report on the involvement of Notch signaling in regulating gene expression in activated macrophages. Toll-like receptors (TLR) agonists such as bacterial lipopeptide, polyI:C, lipopolysaccharide and unmethylated CpG DNA all induced up-regulation of Notch1 in primary and macrophage-like cell lines. Notch1 up-regulation was dependent on the MyD88 pathway when stimulated through TLR2, but not TLR4. Activated Notch1 and expression of the Notch target genes, Hes1 and Deltex, were detected in activated macrophages, suggesting that Notch signaling was activated upon stimulation. Inhibiting processing of Notch receptor by gamma-secretase using a gamma-secretase inhibitor (GSI), the expression of Notch1 was down-regulated to basal levels. This treatment significantly modulated expression of TNF-alpha, IL-6, and IL-10. In addition, the amount of nitric oxide produced was significantly lower and the expression of MHC class II was up-regulated in GSI-treated cells. Treatment with GSI or silencing Notch1 resulted in decreased translocation of NF-kappaBp50 into nucleus upon stimulation. Taken together, stimulation of macrophages through the TLR signaling cascade triggered activation of Notch signaling, which in turn regulated gene expression patterns involved in pro-inflammatory responses, through activation of NF-kappaB.

The inhibitory effect of DAP12 on macrophages has been revealed by examining myeloid cells from DAP12-deficient mice. In this report, we demonstrate that both DAP12 and the FcepsilonRIgamma-chain (FcRgamma) are required for negative regulation of TLR responses in bone marrow-derived dendritic cells (DC). Loss of both DAP12 and FcRgamma enhanced the pro-inflammatory cytokine production and maturation of DC after TLR stimulation, resulting in a greater percentage of DC that produced IL-12 p40, TNF, and IL-6, and expressed high levels of MHC class II, CD80, and CD86. Whereas DC lacking only DAP12 showed some increased TLR responses, those lacking only FcRgamma had a greater enhancement of maturation and cytokine production, though to a lesser extent than DC lacking both DAP12 and FcRgamma. Additionally, antigen-specific T cell proliferation was enhanced by DAP12(-/-)FcRgamma(-/-) DC relative to wild-type DC after maturation. Similar to DAP12(-/-)FcRgamma(-/-) DC, Syk-deficient DC also had increased inflammatory cytokine production, maturation, and antigen presentation. These results confirm the inhibitory effect of immunoreceptor tyrosine-based activation motif (ITAM) signaling in myeloid cells and show that DC and macrophages differ in their dependence on the ITAM-containing adapters DAP12 and FcRgamma for negative regulation of TLR signaling.