Plasmacytoid dendritic cells (pDCs) have long been implicated in the pathogenesis of lupus. However, this conclusion has been largely based on a correlative link between the copious production of IFN-?/? by pDCs and the IFN-?/? "signature" often seen in human lupus patients. The specific contribution of pDCs to disease in vivo has not been investigated in detail. For this reason, we generated a strain of BXSB lupus-prone mice in which pDCs can be selectively depleted in vivo. Early, transient ablation of pDCs before disease initiation resulted in reduced splenomegaly and lymphadenopathy, impaired expansion and activation of T and B cells, reduced antibodies against nuclear autoantigens and improved kidney pathology. Amelioration of pathology coincided with decreased transcription of IFN-?/?-induced genes in tissues. PDC depletion had an immediate impact on the activation of immune cells, and importantly, the beneficial effects on pathology were sustained even though pDCs later recovered, indicating an early pDC contribution to disease. Together, our findings demonstrate a critical function for pDCs during the IFN-?/?-dependent initiation of autoimmune lupus and point to pDCs as an attractive therapeutic target for the treatment of SLE.
Phagocytosis and inflammation within the lungs is crucial for host defense during bacterial pneumonia. Triggering receptor expressed on myeloid cells (TREM)-2 was proposed to negatively regulate TLR-mediated responses and enhance phagocytosis by macrophages, but the role of TREM-2 in respiratory tract infections is unknown. Here, we established the presence of TREM-2 on alveolar macrophages (AM) and explored the function of TREM-2 in the innate immune response to pneumococcal infection in vivo. Unexpectedly, we found Trem-2-/- AM to display augmented bacterial phagocytosis in vitro and in vivo compared to WT AM. Mechanistically, we detected that in the absence of TREM-2, pulmonary macrophages selectively produced elevated complement component 1q (C1q) levels. We found that these increased C1q levels depended on peroxisome proliferator-activated receptor-? (PPAR-?) activity and were responsible for the enhanced phagocytosis of bacteria. Upon infection with S. pneumoniae, Trem-2-/- mice exhibited an augmented bacterial clearance from lungs, decreased bacteremia and improved survival compared to their WT counterparts. This work is the first to disclose a role for TREM-2 in clinically relevant respiratory tract infections and demonstrates a previously unknown link between TREM-2 and opsonin production within the lungs.
Nfil3 is viewed as an obligate transcription factor for NK cell development. However, mouse CMV (MCMV) infection recently was shown to bypass the requirement for Nfil3 by inducing the appearance of NK cells that express the MCMV-specific receptor Ly49H. Thus, signals transmitted by Ly49H and proinflammatory cytokines are sufficient to promote NK cell differentiation in the absence of Nfil3. In this study, we report that salivary gland (SG) NK cells develop in an Nfil3-independent fashion in the steady-state in the absence of MCMV or any infection. Moreover, we show that SG NK cells have an integrin profile reminiscent of tissue-resident lymphocytes and express TRAIL for killing target cells. These results demonstrate that SG NK cells, although related to conventional NK cells, are a distinct subset of innate lymphoid cells that deviates from the conventional developmental pathway, perhaps under the influence of tissue-specific factors.
Retention of lymphocytes in the intestinal mucosa requires specialized chemokine receptors and adhesion molecules. We find that both CD4(+)CD8(+) and CD4(+) T cells in the intestinal epithelium, as well as CD8(+) T cells in the intestinal mucosa and mesenteric lymph nodes, express the cell adhesion molecule class I-restricted T cell-associated molecule (Crtam) upon activation, whereas the ligand of Crtam, cell adhesion molecule 1 (Cadm1), is expressed on gut CD103(+)DCs. Lack of Crtam-Cadm1 interactions in Crtam(-/-) and Cadm1(-/-) mice results in loss of CD4(+)CD8(+) T cells, which arise from mucosal CD4(+) T cells that acquire a CD8 lineage expression profile. After acute oral infection with Toxoplasma gondii, both WT and Crtam(-/-) mice mounted a robust TH1 response, but markedly fewer TH17 cells were present in the intestinal mucosa of Crtam(-/-) mice. The almost exclusive TH1 response in Crtam(-/-) mice resulted in more efficient control of intestinal T. gondii infection. Thus, Crtam-Cadm1 interactions have a major impact on the residency and maintenance of CD4(+)CD8(+) T cells in the gut mucosa in the steady state. During pathogenic infection, Crtam-Cadm1 interactions regulate the dynamic equilibrium between newly formed CD4(+) T cells and their retention in the gut, thereby shaping representation of disparate CD4(+) T cell subsets and the overall quality of the CD4(+) T cell response.
Plasmacytoid dendritic cells (pDC) produce IFN-I in response to viruses and are routinely identified in mice by SiglecH expression. SiglecH is a sialic acid-binding Ig-like lectin that has an immunomodulatory role during viral infections. In this study, we evaluated the impact of SiglecH deficiency on cytokine responses in the presence and absence of pDC. We found that lack of SiglecH enhanced IFN-I responses to viral infection, regardless of whether pDC were depleted. We also examined the expression pattern of SiglecH and observed that it was expressed by specialized macrophages and progenitors of classical dendritic cells and pDC. Accordingly, marginal zone macrophages and pDC precursors were eliminated in newly generated SiglecH-diphtheria toxin receptor (DTR)-transgenic (Tg) mice but not in CLEC4C-DTR-Tg mice after diphtheria toxin (DT) treatment. Using two bacterial models, we found that SiglecH-DTR-Tg mice injected with DT had altered bacterial uptake and were more susceptible to lethal Listeria monocytogenes infection than were DT-treated CLEC4C-DTR-Tg mice. Taken together, our findings suggest that lack of SiglecH may affect cytokine responses by cell types other than pDC during viral infections, perhaps by altering viral distribution or burden, and that cell depletion in SiglecH-DTR-Tg mice encompasses more than pDC.
CD96, CD226 (DNAM-1) and TIGIT belong to an emerging family of receptors that interact with nectin and nectin-like proteins. CD226 activates natural killer (NK) cell-mediated cytotoxicity, whereas TIGIT reportedly counterbalances CD226. In contrast, the role of CD96, which shares the ligand CD155 with CD226 and TIGIT, has remained unclear. In this study we found that CD96 competed with CD226 for CD155 binding and limited NK cell function by direct inhibition. As a result, Cd96(-/-) mice displayed hyperinflammatory responses to the bacterial product lipopolysaccharide (LPS) and resistance to carcinogenesis and experimental lung metastases. Our data provide the first description, to our knowledge, of the ability of CD96 to negatively control cytokine responses by NK cells. Blocking CD96 may have applications in pathologies in which NK cells are important.
Plasmacytoid dendritic cells (pDC) produce type I interferons (IFN-I) and proinflammatory cytokines in response to viruses; however, their contribution to antiviral immunity in vivo is unclear. In this study, we investigated the impact of pDC depletion on local and systemic antiviral responses to herpes simplex virus (HSV) infections using CLEC4C-DTR transgenic mice. We found that pDC do not appear to influence viral burden or survival after vaginal HSV-2 infection, nor do they seem to contribute to virus-specific CD8 T cell responses following subcutaneous HSV-1 infection. In contrast, pDC were important for early IFN-I production, proinflammatory cytokine production, NK cell activation and CD8 T cell responses during systemic HSV-2 and HSV-1 infections. Our data also indicate that unlike pDC, TLR3-expressing cells are important for promoting antiviral responses to HSV-1 regardless of the route of virus administration.
Mucosal innate lymphoid cell (ILC) subsets promote immune responses to pathogens by producing distinct signature cytokines in response to changes in the cytokine microenvironment. We previously identified human ILC3 distinguished by interleukin-22 (IL-22) secretion. Here we characterized a human ILC1 subset that produced interferon-? (IFN-?) in response to IL-12 and IL-15 and had a unique integrin profile, intraepithelial location, hallmarks of TGF-? imprinting, and a memory-activated phenotype. Because tissue-resident memory CD8(+) T cells share this profile, intraepithelial ILC1 may be their innate counterparts. In mice, intraepithelial ILC1 were distinguished by CD160 expression and required Nfil3- and Tbx21-encoded transcription factors for development, but not IL-15 receptor-?, indicating that intraepithelial ILC1 are distinct from conventional NK cells. Intraepithelial ILC1 were amplified in Crohns disease patients and contributed to pathology in the anti-CD40-induced colitis model in mice. Thus, intraepithelial ILC1 may initiate IFN-? responses against pathogens but contribute to pathology when dysregulated.
Plasmacytoid dendritic cells (pDCs) specialize in the secretion of type I interferons (IFN-I) and thus are considered critical mediators of antiviral responses. We recently reported that pDCs have a very early but limited and transient capacity to curtail viral infections. Additionally, pDC numbers are not sustained in human infections caused by Hepatitis B or C viruses (HBV and HCV) and HIV. Thus, the numbers and/or function of pDCs appear to be regulated during the course of viral infection. In this study, we show that splenic pDCs are reduced in vivo during several systemic viral infections and after administration of synthetic toll-like receptor ligands. We demonstrate that IFN-I, regardless of the source, contributes to this decline and mediates pDC death via the intrinsic apoptosis pathway. These findings demonstrate a feedback control mechanism by which IFN-I modulates pDC numbers, thus fine-tuning systemic IFN-I response to viruses. IFN-I-mediated control of pDCs may explain the loss of pDCs during human infections caused by HBV, HCV, or HIV and has important therapeutic implications for settings in which IFN-I is used to treat infections and autoimmune diseases.
Viral infections have been linked to the onset of type I diabetes (T1D), with viruses postulated to induce disease directly by causing ? cell injury and subsequent release of autoantigens and indirectly via the host type I interferon (IFN-I) response triggered by the virus. Consistent with this, resistance to T1D is associated with polymorphisms that impair the function of melanoma differentiation associated gene-5 (MDA5), a sensor of viral RNA that elicits IFN-I responses. In animal models, triggering of another viral sensor, TLR3, has been implicated in diabetes. Here, we found that MDA5 and TLR3 are both required to prevent diabetes in mice infected with encephalomyocarditis virus strain D (EMCV-D), which has tropism for the insulin-producing ? cells of the pancreas. Infection of Tlr3-/- mice caused diabetes due to impaired IFN-I responses and virus-induced ? cell damage rather than T cell-mediated autoimmunity. Mice lacking just 1 copy of Mda5 developed transient hyperglycemia when infected with EMCV-D, whereas homozygous Mda5-/- mice developed severe cardiac pathology. TLR3 and MDA5 controlled EMCV-D infection and diabetes by acting in hematopoietic and stromal cells, respectively, inducing IFN-I responses at kinetically distinct time points. We therefore conclude that optimal functioning of viral sensors and prompt IFN-I responses are required to prevent diabetes when caused by a virus that infects and damages the ? cells of the pancreas.
The activating C-type lectin-like receptor NKG2D, which is expressed by mouse NK cells and activated CD8 T cells, was previously demonstrated to be involved in tumor rejection and as a defense mechanism against viral and bacterial infections. Because CD8 T cells are important for protective immune responses during chronic Mycobacterium tuberculosis (Mtb) infection and represent a promising target for new vaccine strategies to prevent human pulmonary tuberculosis (TB), we studied the immune response in mice deficient for the NKG2D adapter molecule DAP10 during experimental TB. After aerosol infection, DAP10-defcient mice displayed an unimpaired recruitment, activation and development of antigen-specific CD8 T cells. Whereas the frequency of interferon-gamma-producing CD8 T cells from Mtb-infected DAP10-defcient mice was not affected, CD8 T cell-mediated cytotoxicity was significantly reduced in the absence of DAP10. The loss of cytotoxic activity in DAP10-deficient CD8 T cells was associated with an impaired release of cytotoxic granules. Together, our results suggest that during Mtb infection DAP10 is required for maximal cytolytic activity of CD8 T cells.
FcR-like (FcRL) proteins comprise a family of lymphocyte receptors with homology to FcgammaRI. Among these receptors, FcRLA is uniquely interesting due to its intracellular localization, unusual structural features, and high expression within human germinal center and marginal zone B cells. Our analysis of human cell lines has confirmed that this receptor is not secreted but is maintained as an intracellular protein in B cells where it interacts with Igs, consistent with a possible role in Ab assembly. By generating FcRLA-specific antisera as well as knockout mice, we were able to unequivocally demonstrate that FcRLA protein is expressed exclusively in all mouse B cells. We also found that FcRLA is not required for the generation of Ag-specific humoral immune responses to T-dependent or T-independent Ags. However, given its highly conserved structure and universal expression within B cells, it is probable that FcRLA functions similarly in humans and mice. Cumulatively, our data suggest that FcRLA plays a role in Ig assembly that can be compensated for by other proteins.
Immature dendritic cells (DCs) specialize in antigen capture and maintain a highly dynamic pool of intracellular major histocompatibility complex class II (MHCII) that continuously recycles from peptide loading compartments to the plasma membrane and back again. This process facilitates sampling of environmental antigens for presentation to T helper cells. Here, we show that a signaling pathway mediated by the DC immunoreceptor tyrosine-based activation motif (ITAM)-containing adaptors (DAP12 and FcR?) and Vav family guanine nucleotide exchange factors controls the half-life of surface peptide-MHCII (pMHCII) complexes and is critical for CD4 T-cell triggering in vitro. Strikingly, mice with disrupted DC ITAMs show defective T helper cell priming in vivo and are protected from experimental autoimmune encephalitis. Mechanistically, we show that deficiency in ITAM signaling results in increased pMHCII internalization, impaired recycling, and an accumulation of ubiquitinated MHCII species that are prematurely degraded in lysosomes. We propose a novel mechanism for control of T helper cell priming.
Plasmacytoid dendritic cells (pDCs) mediate type I interferon (IFN-I) responses to viruses that are recognized through the Toll-like receptor 7 (TLR7) or TLR9 signaling pathway. However, it is unclear how pDCs regulate the antiviral responses via innate and adaptive immune cells. We generated diphtheria toxin receptor transgenic mice to selectively deplete pDCs by administration of diphtheria toxin. pDC-depleted mice were challenged with viruses known to activate pDCs. In murine cytomegalovirus (MCMV) infection, pDC depletion reduced early IFN-I production and augmented viral burden facilitating the expansion of natural killer (NK) cells expressing the MCMV-specific receptor Ly49H. During vesicular stomatitis virus (VSV) infection, pDC depletion enhanced early viral replication and impaired the survival and accumulation of virus-specific cytotoxic T lymphocytes. We conclude that pDCs mediate early antiviral IFN-I responses and influence the accrual of virus-specific NK or CD8(+) T cells in a virus-dependent manner.
Polyinosinic:polycytidylic acid (poly I:C), a synthetic analog of double-stranded viral RNA, serves as a potent adjuvant for vaccination against soluble proteins, pathogens, and tumors. Poly I:C is sensed by both TLR3 in the endosomes and melanoma differentiation-associated protein 5 (MDA5) in the cytoplasm. Although it is known that TLR3 is required for cross-priming of CD8 T cells specific for viral Ags, the role of MDA5 in inducing CD8 T cell responses is still unclear. In this study, we demonstrate that in mice lacking MDA5, the majority of CD8 T cells do not survive after primary immunization with poly I:C and Ag, impairing memory response to subsequent Ag challenge. Furthermore, bone marrow chimera experiments revealed that MDA5 expression in radiation-resistant stromal cells, but not in radiation-sensitive hematopoietic cells, is essential for establishing CD8 T cell memory. We conclude that MDA5 and TLR3 mediate substantially distinct yet complementary functions during poly I:C-mediated activation of Ag-specific CD8 T cell responses.
The early host response to pathogens is mediated by several distinct pattern recognition receptors. Cytoplasmic RNA helicases including RIG-I and MDA5 have been shown to respond to viral RNA by inducing interferon (IFN) production. Previous in vitro studies have demonstrated a direct role for MDA5 in the response to members of the Picornaviridae, Flaviviridae and Caliciviridae virus families ((+) ssRNA viruses) but not to Paramyxoviridae or Orthomyxoviridae ((-) ssRNA viruses). Contrary to these findings, we now show that MDA5 responds critically to infections caused by Paramyxoviridae in vivo. Using an established model of natural Sendai virus (SeV) infection, we demonstrate that MDA5(-/-) mice exhibit increased morbidity and mortality as well as severe histopathological changes in the lower airways in response to SeV. Moreover, analysis of viral propagation in the lungs of MDA5(-/-) mice reveals enhanced replication and a distinct distribution involving the interstitium. Though the levels of antiviral cytokines were comparable early during SeV infection, type I, II, and III IFN mRNA expression profiles were significantly decreased in MDA5(-/-) mice by day 5 post infection. Taken together, these findings indicate that MDA5 is indispensable for sustained expression of IFN in response to paramyxovirus infection and provide the first evidence of MDA5-dependent containment of in vivo infections caused by (-) sense RNA viruses.
A role for NK cells in therapeutic intervention for hematologic malignancies, such as acute myeloid leukemia and multiple myeloma, and nonhematologic malignancies, such as melanoma, is becoming more apparent. DNAM-1 is an NK cell receptor whose importance in facilitating activation signals received by NK cells in natural and cytokine-driven responses to tumor metastases in vivo is poorly explored. In this study, we used matched tumor lines expressing a variety of relevant ligands, neutralizing monoclonal Abs, and DNAM-1 gene-targeted mice to determine the relative importance of DNAM-1-ligand interactions in controlling tumor metastases. Our results demonstrate that NK cells require DNAM-1 for natural or cytokine (IL-2, IL-12, or IL-21) suppression of tumor metastases or their variants expressing CD70 or CD80. In contrast, DNAM-1 was dispensable when tumor cells were targets of Ab-dependent cellular cytotoxicity or presented ligands for NKG2D. CD155 appeared to be a key ligand recognized by DNAM-1 in NK cell-mediated suppression of metastases, and DNAM-1-mediated suppression coincided with perforin activity. Overall, these data implied a general role for DNAM-1-CD155 interactions in NK cell-mediated killing of tumors, even in the presence of tumor CD70 or CD80 expression, and further defined the optimal efficacy requirements of cytokines that directly activate NK cells.
The double-stranded RNA (dsRNA) analogue poly(I:C) is a promising adjuvant for cancer vaccines because it activates both dendritic cells (DCs) and natural killer (NK) cells, concurrently promoting adaptive and innate anticancer responses. Poly(I:C) acts through two dsRNA sensors, Toll-like receptor 3 (TLR3) and melanoma differentiation-associated protein-5 (MDA5). Here, we investigated the relative contributions of MDA5 and TLR3 to poly(I:C)-mediated NK cell activation using MDA5(-/-), TLR3(-/-), and MDA5(-/-)TLR3(-/-) mice. MDA5 was crucial for NK cell activation, whereas TLR3 had a minor impact most evident in the absence of MDA5. MDA5 and TLR3 activated NK cells indirectly through accessory cells and induced the distinct stimulatory cytokines interferon-alpha and interleukin-12, respectively. To identify the relevant accessory cells in vivo, we generated bone marrow chimeras between either wild-type (WT) and MDA5(-/-) or WT and TLR3(-/-) mice. Interestingly, multiple accessory cells were implicated, with MDA5 acting primarily in stromal cells and TLR3 predominantly in hematopoietic cells. Furthermore, poly(I:C)-mediated NK cell activation was not notably impaired in mice lacking CD8alpha DCs, providing further evidence that poly(I:C) acts through diverse accessory cells rather than solely through DCs. These results demonstrate distinct yet complementary roles for MDA5 and TLR3 in poly(I:C)-mediated NK cell activation.
Sterile alpha and HEAT/Armadillo motif (SARM) is a highly conserved Toll/interleukin-1 receptor (TIR)-containing adaptor protein that is believed to negatively regulate signaling of the pathogen recognition receptors Toll-like receptor 3 (TLR3) and TLR4. To test its physiological function in the context of a microbial infection, we generated SARM(-/-) mice and evaluated the impact of this deficiency on the pathogenesis of West Nile virus (WNV), a neurotropic flavivirus that requires TLR signaling to restrict infection. Although SARM was preferentially expressed in cells of the central nervous system (CNS), studies with primary macrophages, neurons, or astrocytes showed no difference in viral growth kinetics. In contrast, viral replication was increased specifically in the brainstem of SARM(-/-) mice, and this was associated with enhanced mortality after inoculation with a virulent WNV strain. A deficiency of SARM was also linked to reduced levels of tumor necrosis factor alpha (TNF-alpha), decreased microglia activation, and increased neuronal death in the brainstem after WNV infection. Thus, SARM appears to be unique among the TIR adaptor molecules, since it functions to restrict viral infection and neuronal injury in a brain region-specific manner, possibly by modulating the activation of resident CNS inflammatory cells.
DC recognize microbial components through an array of receptors known as PRR. PRR initiate intracellular signals, which engender DC with the capacity to stimulate T-cell responses. Dectin-1 is a PRR that recognizes beta-glucan, a major constituent of many fungis outer cell wall. Here we show that Dectin-1 activates DC through phospholipase (PLC)gamma2 signaling. PLCgamma2-deficient DC were unable to expand antigen-specific T cells and induce T(H)1 and T(H)17 differentiation in response to beta-glucan. Mechanistically, PLCgamma2-deficiency impaired the capacity of DC to secrete polarizing cytokines following exposure to beta-glucan. Dectin-1 required PLCgamma2 to activate MAPK, AP-1 and NF-kappaB, which induce cytokine gene expression. Moreover, PLCgamma2 controlled Dectin-1-mediated NFAT activation and induction of NFAT-dependent genes such as IL-2, cyclooxigenase-2 and Egr transcription factors. We conclude that PLCgamma2 is a crucial signaling mediator that modifies DC gene expression program to activate DC responses to beta-glucan-containing pathogens.
NK cells recognize target cells through activating receptors, many of which rely on the transmembrane adaptors DAP10, DAP12 and FcR-gamma to deliver intracellular signals. Because these adaptors initiate distinct signaling pathways, they dictate the type of response mediated by receptor engagement. DAP10, for example, primarily triggers cytotoxicity, whereas DAP12 induces both cytotoxicity and IFN-gamma secretion. In mice, NKG2D signals through both DAP10 and DAP12, which broadens and modulates the type of response engendered by encounter with ligand. Although initial studies indicated that Ly49H and Ly49D recruit only DAP12, a recent report suggested that they also associate with DAP10. We asked whether this association occurs and is functionally significant under physiologic conditions. Our data demonstrate that DAP10 does associate with Ly49H and Ly49D in primary NK cells. While this association contributes slightly to cell surface expression of both receptors, it has no significant impact on Ly49H-mediated control of murine cytomegalovirus infection. Thus, while many activating NK-cell receptors are promiscuous in terms of adaptor association, our data indicate that the functional consequences of such promiscuity may vary widely and may not be evident in all cases.
Acute respiratory infections are responsible for more than 4 million deaths each year. Neutrophils play an essential role in the innate immune response to lung infection. These cells have an armamentarium of pattern recognition molecules and antimicrobial agents that identify and eliminate pathogens. In the setting of infection, neutrophil triggering receptor expressed on myeloid cells 1 (TREM-1) amplifies inflammatory signaling. Here we demonstrate for the first time that TREM-1 also plays an important role in transepithelial migration of neutrophils into the airspace. We developed a TREM-1/3-deficient mouse model of pneumonia and found that absence of TREM-1/3 markedly increased mortality following Pseudomonas aeruginosa challenge. Unexpectedly, TREM-1/3 deficiency resulted in increased local and systemic cytokine production. TREM-1/3-deficient neutrophils demonstrated intact bacterial killing, phagocytosis, and chemotaxis; however, histologic examination of TREM-1/3-deficient lungs revealed decreased neutrophil infiltration of the airways. TREM-1/3-deficient neutrophils effectively migrated across primary endothelial cell monolayers but failed to migrate across primary airway epithelia grown at the air-liquid interface. These data define a new function for TREM-1 in neutrophil migration across airway epithelial cells and suggest that it amplifies inflammation through targeted neutrophil migration into the lung.
The differentiation of bone marrow-derived progenitor cells into monocytes, tissue macrophages and some dendritic cell (DC) subtypes requires the growth factor CSF1 and its receptor, CSF1R. Langerhans cells (LCs) and microglia develop from embryonic myeloid precursor cells that populate the epidermis and central nervous system (CNS) before birth. Notably, LCs and microglia are present in CSF1-deficient mice but absent from CSF1R-deficient mice. Here we investigated whether an alternative CSF1R ligand, interleukin 34 (IL-34), is responsible for this discrepancy. Through the use of IL-34-deficient (Il34(LacZ/LacZ)) reporter mice, we found that keratinocytes and neurons were the main sources of IL-34. Il34(LacZ/LacZ) mice selectively lacked LCs and microglia and responded poorly to skin antigens and viral infection of the CNS. Thus, IL-34 specifically directs the differentiation of myeloid cells in the skin epidermis and CNS.
Type I interferon (IFN-I) promotes antiviral CD8(+)T cell responses, but the contribution of different IFN-I sources and signaling pathways are ill defined. While plasmacytoid dendritic cells (pDCs) produce IFN-I upon TLR stimulation, IFN-I is induced in most cells by helicases like MDA5. Using acute and chronic lymphocytic choriomeningitis virus (LCMV) infection models, we determined that pDCs transiently produce IFN-I that minimally impacts CD8(+)T cell responses and viral persistence. Rather, MDA5 is the key sensor that induces IFN-I required for CD8(+)T cell responses. In the absence of MDA5, CD8(+)T cell responses to acute infection rely on CD4(+)T cell help, and loss of both CD4(+)T cells and MDA5 results in CD8(+)T cell exhaustion and persistent infection. Chronic LCMV infection rapidly attenuates IFN-I responses, but early administration of exogenous IFN-I rescues CD8(+)T cells, promoting viral clearance. Thus, effective antiviral CD8(+)T cell responses depend on the timing and magnitude of IFN-I production.
Bone marrow stromal Ag 2 (BST2) is a transmembrane protein that prevents virus release from infected cells. It was also reported that BST2 inhibits type I IFN production by plasmacytoid dendritic cells. To determine BST2 impact on antiviral responses in vivo, we generated BST2(-/-) mice. Following infection with a murine retrovirus, BST2(-/-) mice had slightly elevated viral loads; however, infection with other enveloped viruses revealed unexpected roles of BST2. BST2(-/-) mice showed reduced type I IFN production by plasmacytoid dendritic cells. Moreover, BST2(-/-) mice had lower viral titers in lungs following intranasal infection with vesicular stomatitis virus expressing OVA and influenza B and increased numbers of virus-specific CD8 T cells in the lungs, suggesting that BST2 may facilitate entry and/or replication of enveloped viruses and modulate priming of CD8 T cells. These findings suggest complex roles of BST2 beyond retroviral control in vivo, possibly reflecting the involvement of BST2 in endocytosis and intracellular trafficking of viruses, viral nucleic acids, and Ags.
TREM2 is an immunoreceptor expressed on osteoclasts (OC) and microglia that transmits intracellular signals through the adaptor DAP12. Individuals with genetic mutations inactivating TREM2 or DAP12 develop the Nasu-Hakola disease (NHD) with cystic-like lesions of the bone and brain demyelination that lead to fractures and presenile dementia. The mechanisms of this disease are poorly understood. In this study, we report that TREM2-deficient mice have an osteopenic phenotype reminiscent of NHD. In vitro, lack of TREM2 impairs proliferation and ?-catenin activation in osteoclast precursors (OcP) in response to M-CSF. This defect results in accelerated differentiation of OcP into mature OC. Corroborating the importance of a balanced proliferation and differentiation of OcP for bone homeostasis, we show that conditional deletion of ?-catenin in OcP also results in reduced OcP proliferation and accelerated osteoclastogenesis in vitro as well as osteopenia in vivo. These results reveal that TREM2 regulates the rate of osteoclastogenesis and provide a mechanism for the bone pathology in NHD.
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