Integrin αvβ3 Limits Cytokine Production by Plasmacytoid Dendritic Cells and Restricts TLR-Driven Autoimmunity

Plasmacytoid dendritic cells (pDCs) are strongly implicated as a major source of IFN-I in systemic lupus erythematosus (SLE), triggered through TLR-mediated recognition of nucleic acids released from dying cells. However, relatively little is known about how TLR signaling and IFN-I production are regulated in pDCs. In this article, we describe a role for integrin αvβ3 in regulating TLR responses and IFN-I production by pDCs in mouse models. We show that αv and β3-knockout pDCs produce more IFN-I and inflammatory cytokines than controls when stimulated through TLR7 and TLR9 in vitro and in vivo. Increased cytokine production was associated with delayed acidification of endosomes containing TLR ligands, reduced LC3 conjugation, and increased TLR signaling. This dysregulated TLR signaling results in activation of B cells and promotes germinal center (GC) B cell and plasma cell expansion. Furthermore, in a mouse model of TLR7-driven lupus-like disease, deletion of αvβ3 from pDCs causes accelerated autoantibody production and pathology. We therefore identify a pDC-intrinsic role for αvβ3 in regulating TLR signaling and preventing activation of autoreactive B cells. Because αvβ3 serves as a receptor for apoptotic cells and cell debris, we hypothesize that this regulatory mechanism provides important contextual cues to pDCs and functions to limit responses to self-derived nucleic acids.

Lupus IgA1 autoantibodies synergize with IgG to enhance pDC responses to RNA-containing immune complexes

Autoantibodies to nuclear antigens are hallmarks of the autoimmune disease systemic lupus erythematosus (SLE) where they contribute to pathogenesis. However, there remains a gap in our knowledge regarding how different isotypes of autoantibodies contribute to disease, including the production of the critical type I interferon (IFN) cytokines by plasmacytoid dendritic cells (pDCs) in response to immune complexes (ICs). We focused on IgA, which is the second most prevalent isotype in serum, and along with IgG is deposited in glomeruli in lupus nephritis. Here, we show that individuals with SLE have IgA autoantibodies against most nuclear antigens, correlating with IgG against the same antigen. We investigated whether IgA autoantibodies against a major SLE autoantigen, Smith ribonucleoproteins (Sm/RNPs), play a role in IC activation of pDCs. We found that pDCs express the IgA-specific Fc receptor, FcαR, and there was a striking ability of IgA1 autoantibodies to synergize with IgG in RNA-containing ICs to generate robust pDC IFNα responses. pDC responses to these ICs required both FcαR and FcγRIIa, showing a potent synergy between these Fc receptors. Sm/RNP IC binding to and internalization by pDCs were greater when ICs contained both IgA1 and IgG. pDCs from individuals with SLE had higher binding of IgA1-containing ICs and higher expression of FcαR than pDCs from healthy control individuals. Whereas pDC FcαR expression correlated with blood ISG signature in SLE, TLR7 agonists, but not IFNα, upregulated pDC FcαR expression in vitro. Together, we show a new mechanism by which IgA1 autoantibodies contribute to SLE pathogenesis.

Rhinovirus infection of the airway epithelium enhances mast cell immune responses via epithelial-derived interferons

BACKGROUND

Mast cells (MCs) within the airway epithelium in asthma are closely related to airway dysfunction, but cross talk between airway epithelial cells (AECs) and MCs in asthma remains incompletely understood. Human rhinovirus (RV) infections are key triggers for asthma progression, and AECs from individuals with asthma may have dysregulated antiviral responses.

OBJECTIVE

We utilized primary AECs in an ex vivo coculture model system to examine cross talk between AECs and MCs after epithelial rhinovirus infection.

METHODS

Primary AECs were obtained from 11 children with asthma and 10 healthy children, differentiated at air-liquid interface, and cultured in the presence of laboratory of allergic diseases 2 (LAD2) MCs. AECs were infected with rhinovirus serogroup A 16 (RV16) for 48 hours. RNA isolated from both AECs and MCs underwent RNA sequencing. Direct effects of epithelial-derived interferons on LAD2 MCs were examined by real-time quantitative PCR.

RESULTS

MCs increased expression of proinflammatory and antiviral genes in AECs. AECs demonstrated a robust antiviral response after RV16 infection that resulted in significant changes in MC gene expression, including upregulation of genes involved in antiviral responses, leukocyte activation, and type 2 inflammation. Subsequent ex vivo modeling demonstrated that IFN-β induces MC type 2 gene expression. The effects of AEC donor phenotype were small relative to the effects of viral infection and the presence of MCs.

CONCLUSIONS

There is significant cross talk between AECs and MCs, which are present in the epithelium in asthma. Epithelial-derived interferons not only play a role in viral suppression but also further alter MC immune responses including specific type 2 genes.

Mediators of monocyte chemotaxis and matrix remodeling are associated with the development of fibrosis in patients with COVID-19

Acute respiratory distress syndrome (ARDS) has a fibroproliferative phase that may be followed by pulmonary fibrosis. This has been described in patients with COVID-19 pneumonia, but the underlying mechanisms have not been completely defined. We hypothesized that protein mediators of tissue remodeling and monocyte chemotaxis are elevated in the plasma and endotracheal aspirates of critically ill patients with COVID-19 who subsequently develop radiographic fibrosis. We enrolled COVID-19 patients admitted to the ICU who had hypoxemic respiratory failure, were hospitalized and alive for at least 10 days, and had chest imaging done during hospitalization ( n = 119). Plasma was collected within 24h of ICU admission and at 7d. In mechanically ventilated patients, endotracheal aspirates (ETA) were collected at 24h and 48-96h. Protein concentrations were measured by immunoassay. We tested for associations between protein concentrations and radiographic evidence of fibrosis using logistic regression adjusting for age, sex, and APACHE score. We identified 39 patients (33%) with features of fibrosis. Within 24h of ICU admission, plasma proteins related to tissue remodeling (MMP-9, Amphiregulin) and monocyte chemotaxis (CCL-2/MCP-1, CCL-13/MCP-4) were associated with the subsequent development of fibrosis whereas markers of inflammation (IL-6, TNF-α) were not. After 1 week, plasma MMP-9 increased in patients without fibrosis. In ETAs, only CCL-2/MCP-1 was associated with fibrosis at the later timepoint. This cohort study identifies proteins of tissue remodeling and monocyte recruitment that may identify early fibrotic remodeling following COVID-19. Measuring changes in these proteins over time may allow for early detection of fibrosis in patients with COVID-19.

Immune Mechanisms in Inflammatory Anemia

Maintaining the correct number of healthy red blood cells (RBCs) is critical for proper oxygenation of tissues throughout the body. Therefore, RBC homeostasis is a tightly controlled balance between RBC production and RBC clearance, through the processes of erythropoiesis and macrophage hemophagocytosis, respectively. However, during the inflammation associated with infectious, autoimmune, or inflammatory diseases this homeostatic process is often dysregulated, leading to acute or chronic anemia. In each disease setting, multiple mechanisms typically contribute to the development of inflammatory anemia, impinging on both sides of the RBC production and RBC clearance equation. These mechanisms include both direct and indirect effects of inflammatory cytokines and innate sensing. Here, we focus on common innate immune mechanisms that contribute to inflammatory anemias using examples from several diseases, including hemophagocytic lymphohistiocytosis/macrophage activation syndrome, severe malarial anemia during Plasmodium infection, and systemic lupus erythematosus, among others. Expected final online publication date for the Annual Review of Immunology, Volume 41 is April 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Evaluating the Immunopathogenesis of Diabetes After Acute Pancreatitis in the Diabetes RElated to Acute Pancreatitis and Its Mechanisms Study: From the Type 1 Diabetes in Acute Pancreatitis Consortium

ABSTRACT

The association between acute pancreatitis (AP) and diabetes mellitus (DM) has long been established, with the initial descriptions of AP patients presenting with DM after a bout of AP published in the 1940s and 50s. However, the potential mechanisms involved, particularly those components related to the immune system, have not been well defined. The Diabetes RElated to Acute pancreatitis and its Mechanisms (DREAM) study is a multicenter clinical study designed to understand the frequency and phenotype of DM developing after AP. This article describes one objective of the DREAM study: to determine the immunologic mechanisms of DM after AP, including the contribution of β-cell autoimmunity. This component of the study will assess the presence of islet autoimmunity, as well as the magnitude and kinetics of the innate and adaptive immune response at enrollment and during longitudinal follow-up after 1 or more episodes of AP. Finally, DREAM will evaluate the relationship between immune features, DM development, and pancreatitis etiology and severity.

IgA and FcαR are critical components of plasmacytoid DC response to autoantibody-containing immune complexes in SLE

Systemic lupus erythematosus (SLE) is a heterogenous autoimmune disease characterized by the presence of circulating autoreactive anti-nuclear antibodies (ANAs). ANAs form immune complexes (ICs) upon binding to nuclear antigens and these ANA-ICs promote a feedforward loop by enhancing immune responses in cells that recognize ANA-ICs via Fc receptors (FcRs). Plasmacytoid dendritic cells (pDCs) are FcR expressing cells that promote SLE pathology. ANA-ICs deliver nucleic acids to endosomal TLR7 and TLR9. These TLRs induce type I IFN secretion and immune activation upon nucleic acid recognition. Type I IFNs, including IFNα, and interferon stimulated genes are correlated with SLE disease activity. IgG isotype facilitated ANA-IC internalization via FcgRIIA has been the most studied antibody-mediated route for IFNα production in pDCs. The importance of IgA isotype ANAs remains under investigated despite these autoantibodies being present in half of SLE patients. We show here the novel result that human pDCs express the IgA-specific FcαR (CD89) by flow cytometry, that pDCs from SLE patients express increased FcαR compared to pDCs from healthy controls, and that increased pDC FcαR expression in SLE correlates with hallmark IFNα gene set. Additionally, we found that IgA autoantibodies were a critical component of pDC ANA-IC activation when these ANA-IC were generated with serum from IgA autoantibody positive SLE donors complexed with smRNP nuclear antigen. Therefore, our study shows that IgA-containing immune complexes and FαRI are important contributors to pDC type I IFN production in SLE.

Supported by LRA Lupus Mechanisms and Targets Award (JAH), R21AI154841 (JAH), and ITHS TL1 training grant (HRW).

Signals governing monocyte differentiation during inflammation

Monocytes are innate immune cells that develop in the bone marrow and are continually released into circulation, where they are poised to enter tissues in response to homeostatic or inflammatory cues. Monocytes are highly plastic cells that can differentiate in tissues into a variety of monocyte-derived cells to replace resident tissue macrophages, promote inflammatory responses, or resolution of inflammation. As such, monocytes can support tissue homeostasis as well as productive and pathogenic immune responses. Recent work shows previously unappreciated heterogeneity in monocyte development and differentiation in the steady state and during infectious, autoimmune, and inflammatory diseases. Monocyte-derived cells can differentiate via signals from cytokines, pattern recognition receptors or other factors, which can influence development in the bone marrow or in tissues. An improved understanding of these monocyte-derived cells and the signals that drive their differentiation in distinct inflammatory settings could allow for targeting these pathways in pathological inflammation.

Functional SARS-CoV-2-specific immune memory persists after mild COVID-19

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus is causing a global pandemic, and cases continue to rise. Most infected individuals experience mildly symptomatic coronavirus disease 2019 (COVID-19), but it is unknown whether this can induce persistent immune memory that could contribute to immunity. We performed a longitudinal assessment of individuals recovered from mild COVID-19 to determine whether they develop and sustain multifaceted SARS-CoV-2-specific immunological memory. Recovered individuals developed SARS-CoV-2-specific immunoglobulin (IgG) antibodies, neutralizing plasma, and memory B and memory T cells that persisted for at least 3 months. Our data further reveal that SARS-CoV-2-specific IgG memory B cells increased over time. Additionally, SARS-CoV-2-specific memory lymphocytes exhibited characteristics associated with potent antiviral function: memory T cells secreted cytokines and expanded upon antigen re-encounter, whereas memory B cells expressed receptors capable of neutralizing virus when expressed as monoclonal antibodies. Therefore, mild COVID-19 elicits memory lymphocytes that persist and display functional hallmarks of antiviral immunity.

The COVID-19 immune landscape is dynamically and reversibly correlated with disease severity

BACKGROUNDDespite a rapidly growing body of literature on coronavirus disease 2019 (COVID-19), our understanding of the immune correlates of disease severity, course, and outcome remains poor.METHODSUsing mass cytometry, we assessed the immune landscape in longitudinal whole-blood specimens from 59 patients presenting with acute COVID-19 and classified based on maximal disease severity. Hospitalized patients negative for SARS-CoV-2 were used as controls.RESULTSWe found that the immune landscape in COVID-19 formed 3 dominant clusters, which correlated with disease severity. Longitudinal analysis identified a pattern of productive innate and adaptive immune responses in individuals who had a moderate disease course, whereas those with severe disease had features suggestive of a protracted and dysregulated immune response. Further, we identified coordinate immune alterations accompanying clinical improvement and decline that were also seen in patients who received IL-6 pathway blockade.CONCLUSIONThe hospitalized COVID-19 negative cohort allowed us to identify immune alterations that were shared between severe COVID-19 and other critically ill patients. Collectively, our findings indicate that selection of immune interventions should be based in part on disease presentation and early disease trajectory due to the profound differences in the immune response in those with mild to moderate disease and those with the most severe disease.FUNDINGBenaroya Family Foundation, the Leonard and Norma Klorfine Foundation, Glenn and Mary Lynn Mounger, and the National Institutes of Health.

Functional SARS-CoV-2-Specific Immune Memory Persists after Mild COVID-19

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus is causing a global pandemic, and cases continue to rise. Most infected individuals experience mildly symptomatic coronavirus disease 2019 (COVID-19), but it is unknown whether this can induce persistent immune memory that could contribute to immunity. We performed a longitudinal assessment of individuals recovered from mild COVID-19 to determine whether they develop and sustain multifaceted SARS-CoV-2-specific immunological memory. Recovered individuals developed SARS-CoV-2-specific immunoglobulin (IgG) antibodies, neutralizing plasma, and memory B and memory T cells that persisted for at least 3 months. Our data further reveal that SARS-CoV-2-specific IgG memory B cells increased over time. Additionally, SARS-CoV-2-specific memory lymphocytes exhibited characteristics associated with potent antiviral function: memory T cells secreted cytokines and expanded upon antigen re-encounter, whereas memory B cells expressed receptors capable of neutralizing virus when expressed as monoclonal antibodies. Therefore, mild COVID-19 elicits memory lymphocytes that persist and display functional hallmarks of antiviral immunity.

B Cell αv Integrins Regulate TLR-Driven Autoimmunity

Systemic lupus erythematosus (SLE) is defined by loss of B cell tolerance, resulting in production of autoantibodies against nucleic acids and other cellular Ags. Aberrant activation of TLRs by self-derived RNA and DNA is strongly associated with SLE in patients and in mouse models, but the mechanism by which TLR signaling to self-ligands is regulated remains poorly understood. In this study, we show that αv integrin plays a critical role in regulating B cell TLR signaling to self-antigens in mice. We show that deletion of αv from B cells accelerates autoantibody production and autoimmune kidney disease in the Tlr7.1 transgenic mouse model of SLE. Increased autoimmunity was associated with specific expansion of transitional B cells, extrafollicular IgG2c-producing plasma cells, and activation of CD4 and CD8 T cells. Our data show that αv-mediated regulation of TLR signaling in B cells is critical for preventing autoimmunity and indicate that loss of αv promotes escape from tolerance. Thus, we identify a new regulatory pathway in autoimmunity and elucidate upstream signals that adjust B cell activation to prevent development of autoimmunity in a mouse model.

Functional SARS-CoV-2-specific immune memory persists after mild COVID-19

The recently emerged SARS-CoV-2 virus is currently causing a global pandemic and cases continue to rise. The majority of infected individuals experience mildly symptomatic coronavirus disease 2019 (COVID-19), but it is unknown whether this can induce persistent immune memory that might contribute to herd immunity. Thus, we performed a longitudinal assessment of individuals recovered from mildly symptomatic COVID-19 to determine if they develop and sustain immunological memory against the virus. We found that recovered individuals developed SARS-CoV-2-specific IgG antibody and neutralizing plasma, as well as virus-specific memory B and T cells that not only persisted, but in some cases increased numerically over three months following symptom onset. Furthermore, the SARS-CoV-2-specific memory lymphocytes exhibited characteristics associated with potent antiviral immunity: memory T cells secreted IFN-γ and expanded upon antigen re-encounter, while memory B cells expressed receptors capable of neutralizing virus when expressed as antibodies. These findings demonstrate that mild COVID-19 elicits memory lymphocytes that persist and display functional hallmarks associated with antiviral protective immunity.

Flightless-1 promotes lung CD103+ cDC phagocytosis and migration

Dendritic cells are specialized antigen-presenting cells integral for bridging the innate and adaptive immune responses. Critical to dendritic cell function is the need for a dynamic actin cytoskeleton. Flightless-1 is an actin capping protein linked to processes vital for dendritic cell immune functions including cell extension formation, phagocytosis, cell migration, and cell adhesion. Consistent with an important role in actin dynamics, whole body Flightless-1 knockouts are embryonic lethal. To enable further study of Flightless-1 in the immune response, we made mice with dendritic cell Flightless-1 deficiency using the CD11c-CRE driver. Homeostatic cDC1 and cDC2 populations in the spleen and lymph nodes were unchanged in DC-Flightless-1 knockouts relative to control animals. However, DC-Flightless-1 ablation led to a developmental disadvantage when in competition with WT DCs in mixed bone marrow chimeras. Upon LPS challenge in the airways, the Flightless-deficient cDC1 population showed reduced phagocytosis and migration to the lung draining lymph nodes. The DC migratory defect in the absence of Flightless-1 was supported by decreased CCR7 expression in both cDC1 and cDC2 populations. We hypothesize that the observed defects in phagocytosis and migration in Flightless-1-deficient dendritic cells are due to an altered actin cytoskeleton, which may also affect other actin-based immune structures. Current experiments are testing this hypothesis, and investigating the ability of Flightless-deficient DC to prime T cell responses.

Chronic TLR7 and TLR9 signaling drives anemia via differentiation of specialized hemophagocytes

Cytopenias are an important clinical problem associated with inflammatory disease and infection. We show that specialized phagocytes that internalize red blood cells develop in Toll-like receptor 7 (TLR7)-driven inflammation. TLR7 signaling caused the development of inflammatory hemophagocytes (iHPCs), which resemble splenic red pulp macrophages but are a distinct population derived from Ly6C^hi monocytes. iHPCs were responsible for anemia and thrombocytopenia in TLR7-overexpressing mice, which have a macrophage activation syndrome (MAS)-like disease. Interferon regulatory factor 5 (IRF5), associated with MAS, participated in TLR7-driven iHPC differentiation. We also found iHPCs during experimental malarial anemia, in which they required endosomal TLR and MyD88 signaling for differentiation. Our findings uncover a mechanism by which TLR7 and TLR9 specify monocyte fate and identify a specialized population of phagocytes responsible for anemia and thrombocytopenia associated with inflammation and infection.

The signaling adaptor BCAP inhibits NLRP3 and NLRC4 inflammasome activation in macrophages through interactions with Flightless-1

B cell adaptor for phosphoinositide 3-kinase (PI3K) (BCAP) is a signaling adaptor that activates the PI3K pathway downstream of B cell receptor signaling in B cells and Toll-like receptor (TLR) signaling in macrophages. BCAP binds to the regulatory p85 subunit of class I PI3K and is a large, multidomain protein. We used proteomic analysis to identify other BCAP-interacting proteins in macrophages and found that BCAP specifically associated with the caspase-1 pseudosubstrate inhibitor Flightless-1 and its binding partner leucine-rich repeat flightless-interacting protein 2. Because these proteins inhibit the NLRP3 inflammasome, we investigated the role of BCAP in inflammasome function. Independent of its effects on TLR priming, BCAP inhibited NLRP3- and NLRC4-induced caspase-1 activation, cell death, and IL-1β release from macrophages. Accordingly, caspase-1-dependent clearance of a Yersinia pseudotuberculosis mutant was enhanced in BCAP-deficient mice. Mechanistically, BCAP delayed the recruitment and activation of pro-caspase-1 within the NLRP3/ASC preinflammasome through its association with Flightless-1. Thus, BCAP is a multifunctional signaling adaptor that inhibits key pathogen-sensing pathways in macrophages.

Cutting Edge: BCAP Promotes Lupus-like Disease and TLR-Mediated Type I IFN Induction in Plasmacytoid Dendritic Cells

Systemic lupus erythematosus severity correlates with elevated serum levels of type I IFNs, cytokines produced in large quantities by plasmacytoid dendritic cells (pDC) in response to engagement of TLR7 and TLR9 with endocytosed nucleic acids. B cell adaptor for PI3K (BCAP) promoted many aspects of TLR7-driven lupus-like disease, including Isg15 and Ifit1 expression in blood and an immature pDC phenotype associated with higher IFN production. BCAP^-/- mice produced significantly less serum IFN-α than wild-type mice after injection of TLR9 agonist, and BCAP promoted TLR7 and TLR9-induced IFN-α production specifically in pDC. TLR-induced IFN-α production in pDC requires DOCK2-mediated activation of Rac1 leading to activation of IKKα, a mechanism we show was dependent on BCAP. BCAP^-/- pDC had decreased actin polymerization and Rac1 activation and reduced IKKα phosphorylation upon TLR9 stimulation. We show a novel role for BCAP in promoting TLR-induced IFN-α production in pDC and in systemic lupus erythematosus pathogenesis.

B cell adaptor for PI3-kinase (BCAP) modulates CD8+ effector and memory T cell differentiation

Singh et al. show that expression of B cell adaptor for PI3-kinase (BCAP) is induced upon T cell activation and that this helps control effector and memory CD8⁺ T cell differentiation.

CD8⁺ T cells respond to signals via the T cell receptor (TCR), costimulatory molecules, and immunoregulatory cytokines by developing into diverse populations of effector and memory cells. The relative strength of phosphoinositide 3-kinase (PI3K) signaling early in the T cell response can dramatically influence downstream effector and memory T cell differentiation. We show that initial PI3K signaling during T cell activation results in up-regulation of the signaling scaffold B cell adaptor for PI3K (BCAP), which further potentiates PI3K signaling and promotes the accumulation of CD8⁺ T cells with a terminally differentiated effector phenotype. Accordingly, BCAP-deficient CD8⁺ T cells have attenuated clonal expansion and altered effector and memory T cell development following infection with Listeria monocytogenes. Thus, induction of BCAP serves as a positive feedback circuit to enhance PI3K signaling in activated CD8⁺ T cells, thereby acting as a molecular checkpoint regulating effector and memory T cell development.

BCAP promotes Lupus-like disease and regulates IFNα production in pDC

Systemic lupus erythematosus (SLE) severity is correlated with elevated serum type I interferons (IFN), mainly IFNα, which is produced in large amounts by pDC in response to nucleic acid sensing by TLR7 and TLR9. TLR7 and TLR9-induced IRF7 translocation to the nucleus and subsequent IFNα production by pDC is dependent on phosphatidylinositol-3 kinase (PI3K), but how PI3K regulates this process remains undefined. We showed that B cell adaptor for PI3K (BCAP) links TLRs to PI3K activation in macrophages, thus we asked if BCAP plays a role in pDC IFNα production and SLE pathogenesis. We show BCAP promoted many aspects of TLR7-driven lupus-like disease including interferon-stimulated gene expression in blood. BCAP promoted TLR7 and TLR9-induced IFNα production in pDC, and BCAP−/− mice produced significantly less serum IFNα after injection of TLR9 agonist than WT mice, consistent with a pDC IFNα defect. TLR-induced IFNα production in pDC involves dual signaling pathways that run in parallel and converge upon the phosphorylation and activation of IKKα. There is a TLR-independent pathway initiated by nucleic acid recognition at the plasma membrane that involves Dock2-mediated activation of Rac1, required for phosphorylation of IKKα. The TLR-dependent pathway occurs upon nucleic acid recognition by endosomal TLR7 or 9 and leads to MyD88 activation. BCAP regulated IFNα production independently of the TLR-MyD88 pathway and both BCAP and PI3K were required for CpG DNA-induced early actin remodeling, a readout of Rac1 activation, and IKKα phosphorylation in pDC. Our data suggest BCAP and PI3K specifically regulate the TLR-parallel pathway in pDC. Overall, we show a novel role for BCAP in regulating IFNα production in pDC and SLE pathogenesis.

A Novel Strategy to Prevent Advanced Atherosclerosis and Lower Blood Glucose in a Mouse Model of Metabolic Syndrome

Cardiovascular disease caused by atherosclerosis is the leading cause of mortality associated with type 2 diabetes and metabolic syndrome. Insulin therapy is often needed to improve glycemic control, but it does not clearly prevent atherosclerosis. Upon binding to the insulin receptor (IR), insulin activates distinct arms of downstream signaling. The IR-Akt arm is associated with blood glucose lowering and beneficial effects, whereas the IR-Erk arm might exert less desirable effects. We investigated whether selective activation of the IR-Akt arm, leaving the IR-Erk arm largely inactive, would result in protection from atherosclerosis in a mouse model of metabolic syndrome. The insulin mimetic peptide S597 lowered blood glucose and activated Akt in insulin target tissues, mimicking insulin's effects, but only weakly activated Erk and even prevented insulin-induced Erk activation. Strikingly, S597 retarded atherosclerotic lesion progression through a process associated with protection from leukocytosis, thereby reducing lesional accumulation of inflammatory Ly6C^hi monocytes. S597-mediated protection from leukocytosis was accompanied by reduced numbers of the earliest bone marrow hematopoietic stem cells and reduced IR-Erk activity in hematopoietic stem cells. This study provides a conceptually novel treatment strategy for advanced atherosclerosis associated with metabolic syndrome and type 2 diabetes.

cGAS-mediated control of blood-stage malaria promotes Plasmodium-specific germinal center responses

Sensing of pathogens by host pattern recognition receptors is essential for activating the immune response during infection. We used a nonlethal murine model of malaria (Plasmodium yoelii 17XNL) to assess the contribution of the pattern recognition receptor cyclic GMP-AMP synthase (cGAS) to the development of humoral immunity. Despite previous reports suggesting a critical, intrinsic role for cGAS in early B cell responses, cGAS-deficient (cGAS-/-) mice had no defect in the early expansion or differentiation of Plasmodium-specific B cells. As the infection proceeded, however, cGAS-/- mice exhibited higher parasite burdens and aberrant germinal center and memory B cell formation when compared with littermate controls. Antimalarial drugs were used to further demonstrate that the disrupted humoral response was not B cell intrinsic but instead was a secondary effect of a loss of parasite control. These findings therefore demonstrate that cGAS-mediated innate-sensing contributes to parasite control but is not intrinsically required for the development of humoral immunity. Our findings highlight the need to consider the indirect effects of pathogen burden in investigations examining how the innate immune system affects the adaptive immune response.

Negative regulation of TLR signaling in myeloid cells--implications for autoimmune diseases

Toll-like receptors (TLR) are transmembrane pattern recognition receptors that recognize microbial ligands and signal for production of inflammatory cytokines and type I interferon in macrophages and dendritic cells (DC). Whereas TLR-induced inflammatory mediators are required for pathogen clearance, many are toxic to the host and can cause pathological inflammation when over-produced. This is demonstrated by the role of TLR-induced cytokines in autoimmune diseases, such as rheumatoid arthritis, inflammatory bowel disease, and systemic lupus erythematosus. Because of the potent effects of TLR-induced cytokines, we have diverse mechanisms to dampen TLR signaling. Here, we highlight three pathways that participate in inhibition of TLR responses in macrophages and DC, and their implications in autoimmunity; A20, encoded by the TNFAIP3 gene, Lyp encoded by the PTPN22 gene, and the BCAP/PI3K pathway. We present new findings that Lyp promotes TLR responses in primary human monocytes and that the autoimmunity risk Lyp620W variant is more effective at promoting TLR-induced interleukin-6 than the non-risk Lyp620R protein. This suggests that Lyp serves to downregulate a TLR inhibitory pathway in monocytes, and we propose that Lyp inhibits the TREM2/DAP12 inhibitory pathway. Overall, these pathways demonstrate distinct mechanisms of negative regulation of TLR responses, and all impact autoimmune disease pathogenesis and treatment.

Cutting Edge: Direct Sensing of TLR7 Ligands and Type I IFN by the Common Myeloid Progenitor Promotes mTOR/PI3K-Dependent Emergency Myelopoiesis

During infection, recognition of pathogens and inflammatory cytokines skews hematopoiesis toward myeloid development, although the precise mechanisms responsible for this are unclear. In this study, we show that accelerated myeloid differentiation, known as emergency myelopoiesis, involves recognition of pathogen-associated molecular patterns by the common myeloid progenitor (CMP) and is dependent on type I IFN for monocyte/macrophage differentiation. Direct sensing of TLR agonists by CMP induced rapid proliferation and induction of myeloid-differentiation genes. Lack of type I IFN signaling in CMP abrogated macrophage differentiation in response to TLR stimuli, whereas exogenous type I IFN amplified this process. Mechanistically, TLR7 induced PI3K/mammalian target of rapamycin signaling in CMP, which was enhanced by type I IFN, and this pathway was essential for emergency myelopoiesis. This work identifies a novel mechanism by which TLR and type I IFN synergize to promote monocyte/macrophage development from hematopoietic progenitors, a process critical in triggering rapid immune responses during infection.

The adaptor molecule BCAP is essential for the formation of CD8+ T cell memory

Generation of effector and memory CD8+ T cells is pivotal for the clearance of intracellular infections and forms the basis for vaccination strategies against a wide range of pathogens and anti-cancer immunotherapy. We have generated novel data which shows that the signaling scaffold protein B cell adaptor for PI3-kinase (BCAP) is upregulated in activated CD8+ T cells, links TCR/CD3 engagement to the PI3K signaling cascade and is essential for CD8+ T cell memory formation. To analyze the cell intrinsic properties of BCAP in CD8+ T cells, we used OT-1+ mice which have a high affinity CD8+ T Cell Receptor (TCR) for the ova peptide. We transferred 5,000 WT OT-1+CD45.1+cells and 5,000 BCAPKO OT-1+CD45.2+ cells into CD45.1+CD45.2+ recipients, then infected with Listeria Monocytogenes expressing the ova peptide. We found that BCAP is essential for the proper clonal expansion of CD8+ T cells and BCAP deficiency in CD8+ T cells resulted in a loss of KLRG1+CD127− Terminal Effector Cells (TECs) in favor of KLRG1− CD127+ Memory Precursor Cells (MPCs) in vivo. Strikingly, we observed an enhanced CD8+ T cell contraction phenotype in BCAP deficient CD8+ T cells which led to a significant loss of antigen-specific memory CD8+ T cells. BCAPKO CD8+ T cells were undetectable in 50% of all mice analyzed at day 100 post infection. Therefore BCAP is critical for the formation of CD8+ T cell memory. We found the greatest loss of cells was confined to the recently described KLRG1+CD27loCXCR3loEomeslo effector memory cells, which have potent cytolytic function and anti-cancer capabilities. Finally we found that the ability of BCAPKO memory CD8+ T cells to respond to secondary infection was dysregulated. To conclude, BCAP is essential for the formation of CD8+ T cell memory.

BCAP inhibits myeloid cell development from hematopoietic progenitors

B cell adaptor for PI3-kinase (BCAP) is a signaling adaptor expressed in hematopoietic cells including macrophages, monocytes, and neutrophils. Here we asked if BCAP plays a role in development of these myeloid cells. We found that BCAP was expressed in bone marrow (BM) hematopoietic progenitors, including LSK (Lin−Sca1+cKit+), CMP (Common Myeloid Progenitor) and GMP (Granulocyte/Macrophage Progenitor) cells, suggesting that BCAP may impact myelopoiesis. BCAP−/− mice had more BM monocytes than WT mice, and in mixed chimeras generated with a 1:1 ratio of WT and BCAP−/− BM, monocytes and neutrophils in the BM, blood and spleen exhibited skewing towards BCAP−/− origin, showing a competitive advantage for BCAP−/− myeloid cells. Thus we hypothesized that BCAP inhibits myeloid development. Consistent with this hypothesis, BCAP-deficient BM LSK, CMP and GMP cells out-competed WT progenitors in mixed chimeras. In an in vitro myeloid colony-forming-unit assay, sorted BCAP−/− progenitors produced more myeloid cells than WT progenitors, supporting a cell-intrinsic role of BCAP in inhibiting myeloid differentiation. Furthermore, BCAP−/− progenitors were more mature than WT progenitors, indicating that BCAP−/− progenitors display accelerated myeloid development. During cyclophosphamide-induced myeloablation or specific monocyte depletion, BCAP−/− mice replenish circulating monocytes earlier than WT mice. Lastly, BCAP−/− progenitors showed increased expression of the myeloid-differentiating transcription factors IRF8 and C/EBPα compared to WT progenitors. Together, these data identify BCAP as an inhibitor of myeloid development from BM hematopoietic progenitors in the steady state and during demand conditions.

TLR7 signaling drives development of a unique population of hemophagocytic macrophages associated with anemia

Toll-like receptor (TLR) 7 activity is associated with myeloid expansion in the autoimmune disease Systemic Lupus Erythematosus (SLE), yet how this contributes to severe anemia, often seen in SLE complication, is unclear. Transcriptional and functional assays indicate that TLR7 signaling in the common myeloid progenitor (CMP) preferentially skews lineage choice towards a hemophagocytic macrophage. Transgenic mice that overexpress Tlr7 and develop SLE-like disease exhibited increased hemophagocytic macrophages and progressively developed anemia. Interestingly, in these mice, we observed not only conventional hemophagocytic macrophage populations known as red pulp macrophages (RPM), but also an inflammation-induced hemophagocytic macrophage population (HPM). Unlike RPM that are primarily yolk sac-derived and require the transcription factor Spi-C for their development, we have found that HPM are monocyte-derived and do not require Spi-C. Additionally, TLR7 drives an expansion of hemophagocytic CD11bhi bone marrow macrophages (BMM) that are not found in the absence of TLR7 signaling. Although it has been shown that pathological conditions including excess heme overload can drive monocytes to differentiate to hemophagocytic macrophages, this is the first account of inflammation driving this process. These findings uncover a novel mechanism by which TLR7 signaling participates in hemophagocytic macrophage differentiation from monocytes and may account for one of the mechanisms driving anemia during chronic inflammation and chronic autoimmune diseases such as SLE.

Hematopoietic and nonhematopoietic cells promote Type I interferon- and TLR7-dependent monocytosis during low-dose LCMV infection

Release of inflammatory monocytes from the bone marrow (BM) into the blood is an important physiological response to infection, but the mechanisms regulating this phenomenon during viral infection are not completely defined. Here, we show that low-dose infection with lymphocytic choriomeningitis virus (LCMV) caused rapid, transient inflammatory monocytosis that required type I interferon (IFN) and Toll-like receptor (TLR) 7 signaling. Type I IFN and TLR7 signals were critical for induction of IFN-stimulated gene expression and CCR2 ligand upregulation in the BM microenvironment in response to LCMV infection. Experiments utilizing BM chimeric mice demonstrated that type I IFN and TLR7 signaling on either hematopoietic or nonhematopoietic cells was sufficient to initiate monocytosis in response to LCMV infection. BM plasmacytoid dendritic cells (pDCs) generated type I IFN directly ex vivo, suggesting that pDCs are a hematopoietic contributor of type I IFN in the BM early during LCMV infection. Overall, we describe novel roles for type I IFN and TLR7 signaling in nonhematopoietic cells and BM pDCs in directing IFN-stimulated gene and CCR2 ligand expression in the BM to initiate an increase in blood inflammatory monocytes during viral infection.

The Tec Kinase-Regulated Phosphoproteome Reveals a Mechanism for the Regulation of Inhibitory Signals in Murine Macrophages

Previous work has shown conflicting roles for Tec family kinases in regulation of TLR-dependent signaling in myeloid cells. In the present study, we performed a detailed investigation of the role of the Tec kinases Btk and Tec kinases in regulating TLR signaling in several types of primary murine macrophages. We demonstrate that primary resident peritoneal macrophages deficient for Btk and Tec secrete less proinflammatory cytokines in response to TLR stimulation than do wild-type cells. In contrast, we found that bone marrow-derived and thioglycollate-elicited peritoneal macrophages deficient for Btk and Tec secrete more proinflammatory cytokines than do wild-type cells. We then compared the phosphoproteome regulated by Tec kinases and LPS in primary peritoneal and bone marrow-derived macrophages. From this analysis we determined that Tec kinases regulate different signaling programs in these cell types. In additional studies using bone marrow-derived macrophages, we found that Tec and Btk promote phosphorylation events necessary for immunoreceptor-mediated inhibition of TLR signaling. Taken together, our results are consistent with a model where Tec kinases (Btk, Tec, Bmx) are required for TLR-dependent signaling in many types of myeloid cells. However, our data also support a cell type-specific TLR inhibitory role for Btk and Tec that is mediated by immunoreceptor activation and signaling via PI3K.

Overexpression of TLR7 promotes cell-intrinsic expansion and autoantibody production by transitional T1 B cells

Transgenic expression of TLR7 results in the expansion and hyperactivation of T1 B cells in response to endogenous RNA complexes, leading to increased autoantibody production.

Toll-like receptor (TLR), a ligand for single-stranded RNA, has been implicated in the development of pathogenic anti-RNA autoantibodies both in systemic lupus erythematous (SLE) patients and in murine models of lupus. It is still unclear, however, where and how TLR7-mediated interactions affect the development of autoreactive B cells. We found that overexpression of TLR7 in transgenic mice (TLR7.1Tg) leads to marked alterations of transitional (T1) B cells, associated with their expansion and proliferation within the splenic red pulp (RP). This phenotype was intrinsic to the T1 subset of B cells and occurred independently of type 1 IFN signals. Overexpression of RNase in TLR7.1Tg mice significantly limited the expansion and proliferation of T1 cells, indicating that endogenous RNA complexes are driving their activation. TLR7.1Tg T1 cells were hyper-responsive to anti-IgM and TLR7 ligand stimulation in vitro and produced high concentrations of class-switched IgG2b and IgG2c, including anti-RNA antibodies. Our results demonstrate that initial TLR7 stimulation of B cells occurs at the T1 stage of differentiation in the splenic RP and suggest that dysregulation of TLR7 expression in T1 cells can result in production of autoantibodies.

β(2) integrins inhibit TLR responses by regulating NF-κB pathway and p38 MAPK activation

Outside-in signals from β(2) integrins require immunoreceptor tyrosine-based activation motif adapters in myeloid cells that are known to dampen TLR responses. However, the relationship between β(2) integrins and TLR regulation is unclear. Here we show that deficiency in β(2) integrins (Itgb2(-/-) ) causes hyperresponsiveness to TLR stimulation, demonstrating that β(2) integrins inhibit signals downstream of TLR ligation. Itgb2(-/-) macrophages and dendritic cells produced more IL-12 and IL-6 than WT cells when stimulated with TLR agonists and Itgb2(-/-) mice produced more inflammatory cytokines than WT mice when injected with LPS. TLR hypersensitivity was not the result of insufficient ABIN-3, A20, Hes-1, or IRAK-M expression, nor to changes in IL-10 production or sensitivity, though Itgb2(-/-) macrophages had reduced p38 MAPK phosphorylation after LPS treatment. Furthermore, a Cbl-b-MyD88 regulatory axis is not required for TLR inhibition in macrophages. Instead, Itgb2(-/-) macrophages presented with enhanced IκBα degradation, leading to changes in NF-κB recruitment to target promoters and elevated cytokine, chemokine, and anti-apoptotic gene transcription. Thus, β(2) integrins limit TLR signaling by inhibiting NF-κB pathway activation and promoting p38 MAPK activation, thereby fine-tuning TLR-induced inflammatory responses.

Increased ribonuclease expression reduces inflammation and prolongs survival in TLR7 transgenic mice

TLR7 activation is implicated in the pathogenesis of systemic lupus erythematosus. Mice that overexpress TLR7 develop a lupus-like disease with autoantibodies and glomerulonephritis and early death. To determine whether degradation of the TLR7 ligand RNA would alter the course of disease, we created RNase A transgenic (Tg) mice. We then crossed the RNase Tg to TLR7 Tg mice to create TLR7 × RNase double Tg (DTg) mice. DTg mice had a significantly increased survival associated with reduced activation of T and B lymphocytes and reduced kidney deposition of IgG and C3. We observed massive hepatic inflammation and cell death in TLR7 Tg mice. In contrast, hepatic inflammation and necrosis were strikingly reduced in DTg mice. These findings indicate that high concentrations of serum RNase protect against immune activation and inflammation associated with TLR7 stimulation and that RNase may be a useful therapeutic strategy in the prevention or treatment of inflammation in systemic lupus erythematosus and, possibly, liver diseases.

Cutting edge: Type I IFN drives emergency myelopoiesis and peripheral myeloid expansion during chronic TLR7 signaling

Mice overexpressing TLR7 (TLR7.1 mice) are a model of systemic lupus erythematosus pathogenesis and exhibit peripheral myeloid expansion. We show that TLR7.1 mice have a dramatic expansion of splenic cells that derive from granulocyte/macrophage progenitors (GMP) compared with wild-type mice. In the bone marrow, TLR7.1 mice exhibited hallmarks of emergency myelopoiesis and contained a discrete population of Sca-1(+) GMP, termed emergency GMP, which are more proliferative and superior myeloid precursors than classical Sca-1(-) GMP. The emergency myelopoiesis and peripheral myeloid expansion in TLR7.1 mice was dependent on type I IFN signaling. TLR7 agonist administration to nontransgenic mice also drove type I IFN-dependent emergency myelopoiesis. TLR7.1 plasmacytoid dendritic cells were cell-intrinsically activated by TLR7 overexpression and constitutively produced type I IFN mRNA. This study shows that type I IFN can act upon myeloid progenitors to promote the development of emergency GMP, which leads to an expansion of their progeny in the periphery.

TREM-2, triggering receptor expressed on myeloid cell-2, negatively regulates TLR responses in dendritic cells

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.

S100A9 differentially modifies phenotypic states of neutrophils, macrophages, and dendritic cells: implications for atherosclerosis and adipose tissue inflammation

BACKGROUND

S100A9 is constitutively expressed in neutrophils, dendritic cells, and monocytes; is associated with acute and chronic inflammatory conditions; and is implicated in obesity and cardiovascular disease in humans. Most of the constitutively secreted S100A9 is derived from myeloid cells. A recent report demonstrated that mice deficient in S100A9 exhibit reduced atherosclerosis compared with controls and suggested that this effect was due in large part to loss of S100A9 in bone marrow-derived cells.

METHODS AND RESULTS

To directly investigate the role of bone marrow-derived S100A9 in atherosclerosis and insulin resistance in mice, low-density lipoprotein receptor-deficient, S100A9-deficient bone marrow chimeras were generated. Neither atherosclerosis nor insulin resistance was reduced in S100A9-deficient chimeras fed a diet rich in fat and carbohydrates. To investigate the reason for this lack of effect, myeloid cells were isolated from the peritoneal cavity or bone marrow. S100A9-deficient neutrophils exhibited a reduced secretion of cytokines in response to toll-like receptor-4 stimulation. In striking contrast, S100A9-deficient dendritic cells showed an exacerbated release of cytokines after toll-like receptor stimulation. Macrophages rapidly lost S100A9 expression during maturation; hence, S100A9 deficiency did not affect the inflammatory status of macrophages.

CONCLUSIONS

S100A9 differentially modifies phenotypic states of neutrophils, macrophages, and dendritic cells. The effect of S100A9 deficiency on atherosclerosis and other inflammatory diseases is therefore predicted to depend on the relative contribution of these cell types at different stages of disease progression. Furthermore, S100A9 expression in nonmyeloid cells is likely to contribute to atherosclerosis.

Short-term IL-1beta blockade reduces monocyte CD11b integrin expression in an IL-8 dependent fashion in patients with type 1 diabetes

OBJECTIVE

Interleukin 1-beta (IL-1beta) is a major inflammatory cytokine. Blockade of the IL-1beta pathway is therapeutically efficacious in type 2 diabetes, but the mechanistic effects on the immune system are incompletely understood.

RESEARCH DESIGN

We administered an IL-1 receptor antagonist, anakinra, to 7 type 1 diabetes patients in order to investigate the immunologic and metabolic effects of this drug. Mechanistic assays were performed before and after drug administration.

RESULTS

A novel signature was observed, with reduced serum interleukin 8 (IL-8) levels and reduced CD11b integrin expression on monocytes associated with increased CXCR1 expression.

CONCLUSIONS

This set of linked phenotypes suggests that blockade of the IL-1beta pathway results in the reduced ability of mononuclear cells to traffic to sites of inflammation. Mechanistic studies from large scale trials using IL-1 blockade in type 1 diabetes should focus on changes in monocyte trafficking and the IL-8 pathway.

DAP12 is required for macrophage recruitment to the lung in response to cigarette smoke and chemotaxis toward CCL2

DAP12 is an adapter protein that associates with several receptors in macrophages. Little is known about the biological role of DAP12 in alveolar macrophages. In genome-wide profiling, we previously found that two DAP12-associated receptors, myeloid DAP12-associated lectin-1 and triggering receptor expressed on myeloid cells 2 (TREM2), were highly induced in alveolar macrophages from habitual smokers. Here, we found that transcript levels for these receptors in alveolar macrophages increased with packs per day of cigarettes smoked and expression of TREM2 protein was increased in lung macrophages of former smokers with emphysema compared with that in controls. In vitro, cigarette smoke directly induced expression of myeloid DAP12-associated lectin-1 and TREM2 and activation of DAP12 signaling in mouse macrophages. To determine whether DAP12 plays a role in cigarette smoke-induced pulmonary inflammation, we exposed wild-type and DAP12-deficient mice to chronic cigarette smoke and found significant reduction in recruitment of alveolar macrophages in DAP12-deficient mice. Because cigarette smoking induces the macrophage chemoattractant CCL2, we tested the chemotactic ability of DAP12-deficient macrophages and found abrogation of chemotaxis toward CCL2 in vitro. Airway administration of CCL2 also resulted in a significant reduction of macrophage recruitment to the lungs of DAP12-deficient mice compared with that in controls. DAP12 was also required for normal macrophage migration in a "scratch" assay. Reconstitution studies revealed that phosphorylation of the DAP12 ITAM was required for normal migration in vitro and association with TREM2 was sufficient for normal migration. These findings indicate that DAP12, possibly through association with TREM2, contributes to alveolar macrophage chemotaxis and recruitment to the lung and may mediate macrophage accumulation in lung diseases such as emphysema.

The expanding roles of ITAM adapters FcRgamma and DAP12 in myeloid cells

The adapter proteins DAP12 and FcRgamma associate with a wide spectrum of receptors in a variety of innate immune cells to mediate intracellular signaling pathways when their cognate receptor is engaged. These adapter proteins are coupled to their receptors through charged residues within the transmembrane regions of the adapter and receptor. DAP12 and FcRgamma contain specific protein domains (referred to as immunoreceptor tyrosine-based activation motifs) that serve as the substrates and docking sites for kinases, allowing amplification of intracellular signaling reactions. Recent research has broadened the repertoire of receptors that utilize these adapters for signaling to include not only novel immunoglobulin superfamily members but also cytokine receptors, integrins, and other adhesion molecules. There is abundant evidence that these multifunctional signaling adapters also mediate inhibitory activity, downmodulating signaling from Toll-like receptors and other heterologous receptors. In this review, we discuss the newly described receptors that utilize DAP12 and/or FcRgamma adapters to modulate innate immune responses.

TREM-2 (triggering receptor expressed on myeloid cells 2) is a phagocytic receptor for bacteria

Phagocytosis, which is essential for the immune response to pathogens, is initiated by specific interactions between pathogens and cell surface receptors expressed by phagocytes. This study identifies triggering receptor expressed on myeloid cells 2 (TREM-2) and its signaling counterpart DAP12 as a molecular complex that promotes phagocytosis of bacteria. Expression of TREM-2–DAP12 enables nonphagocytic Chinese hamster ovary cells to internalize bacteria. This function depends on actin cytoskeleton dynamics and the activity of the small guanosine triphosphatases Rac and Cdc42. Internalization also requires src kinase activity and tyrosine phosphorylation. In bone marrow–derived macrophages, phagocytosis is decreased in the absence of DAP12 and can be restored by expression of TREM-2–DAP12. Depletion of TREM-2 inhibits both binding and uptake of bacteria. Finally, TREM-2–dependent phagocytosis is impaired in Syk-deficient macrophages. This study highlights a novel role for TREM-2–DAP12 in the immune response to bacterial pathogens.

Interleukin-15/interleukin-15R alpha complexes promote destruction of established tumors by reviving tumor-resident CD8+ T cells

Tumors often escape immune-mediated destruction by suppressing lymphocyte infiltration or effector function. New approaches are needed that overcome this suppression and thereby augment the tumoricidal capacity of tumor-reactive lymphocytes. The cytokine interleukin-15 (IL-15) promotes proliferation and effector capacity of CD8(+) T cells, natural killer (NK) cells, and NKT cells; however, it has a short half-life and high doses are needed to achieve functional responses in vivo. The biological activity of IL-15 can be dramatically increased by complexing this cytokine to its soluble receptor, IL-15R alpha. Here, we report that in vivo delivery of IL-15/IL-15R alpha complexes triggers rapid and significant regression of established solid tumors in two murine models. Despite a marked expansion of IL-2/IL-15R beta(+) cells in lymphoid organs and peripheral blood following treatment with IL-15/IL-15R alpha complexes, the destruction of solid tumors was orchestrated by tumor-resident rather than newly infiltrating CD8(+) T cells. Our data provide novel insights into the use of IL-15/IL-15R alpha complexes to relieve tumor-resident T cells from functional suppression by the tumor microenvironment and have significant implications for cancer immunotherapy and treatment of chronic infections.

Increased TLR responses in dendritic cells lacking the ITAM-containing adapters DAP12 and FcRgamma

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.

The activating immunoreceptor NKG2D and its ligands are involved in allograft transplant rejection

Although the linkage between innate and adaptive immunity in transplantation has been recognized, the mechanisms underlying this cooperation remain to be fully elucidated. In this study, we show that early "danger" signals associated with transplantation lead to rapid up-regulation of NKG2D ligands. A second wave of NKG2D ligand up-regulation is mediated by the adaptive immune response to allografts. Treatment with an Ab to NKG2D was highly effective in preventing CD28-independent rejection of cardiac allografts. Notably, NKG2D blockade did not deplete CD8(+) T cells or NK1.1(+) cells nor affect their migration to the allografts. These results establish a functional role of NKG2D and its ligands in the rejection of solid organ transplants.

Inhibition of Toll-like receptor and Fc receptor responses in macrophages by TREM-2 and DAP12 (44.7)

DAP12 is a transmembrane signaling adapter that contains an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic domain and is expressed in myeloid cells and natural killer cells. DAP12 associates with receptors that do not have intrinsic signaling ability and therefore use DAP12 to propagate signals. We examined the inflammatory response in macrophages lacking DAP12. Surprisingly, DAP12-deficient macrophages produced increased amounts of inflammatory cytokines in response to Toll-like receptor and Fc receptor stimulation, demonstrating that DAP12 inhibits inflammatory responses in wild-type macrophages. This inhibition was dependent upon the DAP12 ITAM. In vivo, DAP12-deficient mice were more susceptible to LPS/D-galactosamine-induced shock and more resistant to Listeria monocytogenes infection. TREM-2, a DAP12-associated Ig superfamily receptor, was responsible for the DAP12-mediated inhibition of TLR and FcR responses in macrophages. Additionally, a fusion protein containing the extracellular domain of TREM-2 bound to macrophages. These results suggest that the interaction of TREM-2 and its ligand on macrophages results in an inhibitory signal that can reduce the macrophage inflammatory response.

Cutting edge: inhibition of TLR and FcR responses in macrophages by triggering receptor expressed on myeloid cells (TREM)-2 and DAP12

DAP12 is an ITAM-containing adapter that associates with receptors in myeloid and NK cells. DAP12-associated receptors can give activation signals leading to cytokine production; however, in some situations, DAP12 inhibits cytokine production stimulated through TLRs and FcRs. Here we show that Triggering Receptor Expressed on Myeloid cells (TREM)-2 is responsible for the DAP12-mediated inhibition in mouse macrophages. A chimeric receptor composed of the extracellular domain of TREM-2 and the cytoplasmic domain of DAP12 inhibited the TLR- and FcR-induced TNF production of DAP12-deficient macrophages, whereas a TREM-1 chimera did not. In wild-type macrophages, TREM-2 knockdown increased TLR-induced TNF production. A TREM-2 Fc fusion protein bound to macrophages, indicating that macrophages express a TREM-2 ligand. Thus, the interaction of TREM-2 and its ligand results in an inhibitory signal that can reduce the inflammatory response.

Inhibition of Immune Responses by ITAM-Bearing Receptors

Cells of the immune system possess many multisubunit receptors that are composed of a ligand-binding subunit associated with distinct signaling adaptors containing one or more immunoreceptor tyrosine-based activation motifs (ITAMs). These receptors include the T cell receptor, the B cell receptor, and many Fc receptors, as well as families of activating receptors on myeloid and natural killer cells. Receptors that associate with ITAM-containing adaptors classically have been viewed as transducing activating signals involving phosphorylation of the tyrosines within the ITAM and recruitment of Syk family tyrosine kinases. Receptors associated with ITAM-containing adaptors in myeloid cells have also been implicated in inhibition of cellular activation. Here, we discuss these new negative roles for signaling by receptors that associate with ITAM-bearing adaptors in myeloid and other cell types within the immune system.

Enhanced Toll-like receptor responses in the absence of signaling adaptor DAP12

DAP12 is a signaling adaptor containing an immunoreceptor tyrosine-based activation motif (ITAM) that pairs with receptors on myeloid cells and natural killer cells. We examine here the responses of mice lacking DAP12 to stimulation through Toll-like receptors (TLRs). Unexpectedly, DAP12-deficient macrophages produced higher concentrations of inflammatory cytokines in response to a variety of pathogenic stimuli. Additionally, macrophages deficient in spleen tyrosine kinase (Syk), which signals downstream of DAP12, showed a phenotype identical to that of DAP12-deficient macrophages. DAP12-deficient mice were more susceptible to endotoxic shock and had enhanced resistance to infection by the intracellular bacterium Listeria monocytogenes. These data suggest that one or more DAP12-pairing receptors negatively regulate signaling through TLRs.