Apoptotic cells induce Mer tyrosine kinase-dependent blockade of NF-kappaB activation in dendritic cells

Adoptive transfer of bone marrow dendritic cells failed to localize in the renal cortex and to improve renal injury in adriamycin nephropathy

BACKGROUND AND AIMS

Murine bone marrow (BM) dendritic cells (DCs) can be modulated to be tolerogenic by cytokines, such as interleukin (IL)-10 and transforming growth factor (TGF)-β, and may play a regulatory role and sustain immune hemostasis in cognate kidney disease. However, it is unknown whether BM-DCs can be used to protect against renal injury in murine Adriamycin nephropathy (AN).

METHODS

In this study, by adoptive in vivo transfer of BM-DCs, including immature DCs, mature DCs (lipopolysaccharide-stimulated DCs) and BM regulatory DCs (IL-10/TGF-β-modified DCs, DCregs), we addressed the potential benefits of BM-DCs in chronic kidney disease.

RESULTS

We found that after adoptive transfer of DCregs, renal injury, including glomerulosclerosis, interstitial fibrosis and tubular atrophy, was not changed compared to AN controls. Correspondingly, renal functions measured by serum creatinine, 12-hour urine protein and creatinine clearance were also not improved by transfusion with DCregs compared to AN controls.

CONCLUSION

This study showed that the adoptive transfer of BM-DCs was unable to improve renal injury in an AN model, and this failure related to their inability to access the kidney.

TAM receptors affect adult brain neurogenesis by negative regulation of microglial cell activation

TAM tyrosine kinases play multiple functional roles, including regulation of the target genes important in homeostatic regulation of cytokine receptors or TLR-mediated signal transduction pathways. In this study, we show that TAM receptors affect adult hippocampal neurogenesis and loss of TAM receptors impairs hippocampal neurogenesis, largely attributed to exaggerated inflammatory responses by microglia characterized by increased MAPK and NF-κB activation and elevated production of proinflammatory cytokines that are detrimental to neuron stem cell proliferation and neuronal differentiation. Injection of LPS causes even more severe inhibition of BrdU incorporation in the Tyro3(-/-)Axl(-/-)Mertk(-/-) triple-knockout (TKO) brains, consistent with the LPS-elicited enhanced expression of proinflammatory mediators, for example, IL-1β, IL-6, TNF-α, and inducible NO synthase, and this effect is antagonized by coinjection of the anti-inflammatory drug indomethacin in wild-type but not TKO brains. Conditioned medium from TKO microglia cultures inhibits neuron stem cell proliferation and neuronal differentiation. IL-6 knockout in Axl(-/-)Mertk(-/-) double-knockout mice overcomes the inflammatory inhibition of neurogenesis, suggesting that IL-6 is a major downstream neurotoxic mediator under homeostatic regulation by TAM receptors in microglia. Additionally, autonomous trophic function of the TAM receptors on the proliferating neuronal progenitors may also promote progenitor differentiation into immature neurons.

Dendritic cells generated with Flt3L and exposed to apoptotic cells lack induction of T cell anergy and Foxp3⁺ regulatory T cell conversion in vitro

Removal of apoptotic cells, which appear during the steady state, is a pre-requisite to prevent generation of secondary necrotic cells that may lead to autoimmunity. The recognition of apoptotic material by dendritic cells (DCs) has been proposed to convert them into tolerogenic DCs equipped with specialized tolerogenic mechanisms on T cells. However, comparative studies to demonstrate functional alterations of DCs upon exposure to apoptotic cells have not been performed so far. Here we show that immature murine bone marrow-derived DCs generated with GM-CSF (GM-DCs) or Flt3L (FL-DCs) interact with live or apoptotic syngeneic thymocytes. As expected, GM-DCs phagocytose apoptotic but not live cells, FL-DCs only show trogocytosis of membrane parts. Interaction with live or apoptotic thymocytes did not lead to DC maturation. Both GM-DCs and FL-DCs present OVA as protein, peptide and membrane-associated antigens. Interestingly, only GM-DCs were able to induce T cell anergy or convert naïve T cells into FoxP3⁺ regulatory T cells (Tregs) but FL-DCs did not show either of these effects. Unexpectedly, exposure of immature GM-DCs to live or apoptotic thymocytes did not improve DC functions in both types of in vitro T cell tolerance induction assays. Together, our data suggest that these tolerogenic in vitro measures of immature BM-DCs are not further enhanced by exposure to apoptotic cells and may depend on the generating cytokine.

Protein S: A multifunctional anticoagulant vitamin K-dependent protein at the crossroads of coagulation, inflammation, angiogenesis, and cancer

Since its discovery in 1970, protein S (PS) has emerged as a key vitamin K-dependent natural anticoagulant protein at the crossroads of multiple biological processes, including coagulation, apoptosis, atherosclerosis, angiogenesis/vasculogenesis, and cancer progression. Following the binding to a unique family of protein tyrosine kinase receptors referred to as Tyro-3, Axl and Mer (TAM) receptors, PS can lead to regulation of coagulation, phagocytosis of apoptotic cells, cell survival, activation of innate immunity, vessel integrity and angiogenesis, and local invasion and metastasis. Because of these dynamics and multiple functions of PS, which are largely lost following invalidation of the mouse PROS1 gene, this molecule is currently intensively studied in biomedical research. The purpose of this review is to provide a brief chronicle of the discovery and current understanding of the mechanisms of PS signaling, and how PS and their signaling partners regulate various cellular functions, with a particular focus on TAM receptors.

Loss of RORγt DNA binding activity inhibits IL-17 expression in HIV-1 infected Indian individuals

Abstract IL-17 producing CD4 T cells have recently been shown to play an important role in mucosal immunity in HIV infection. But its role in peripheral immunity and the molecular mechanism underlying its regulation during HIV-1 infection are ill defined. In this study, we report a significant negative correlation between IL-17 production in peripheral blood and HIV-1 plasma viral load (pVL). On further investigation, we observe a marked reduction in retinoid-related orphan nuclear receptor (RORγt; Th17 lineage specific transcription factor) binding at IL-17 promoter in HIV patients with high viremia (pVL>10,000 copies/mL) in contrast to relatively low viremic patients which indicate the magnitude of viral copy number on RORγt binding at IL-17 promoter. Additionally, our study highlights that FoxP3 influences IL-17 production by binding to and acting together with RORγt, consequently inhibiting RORγt binding to IL-17 promoter with growing viremia in HIV infection. Collectively, our data suggest that FoxP3 interacts with RORγt transcription factor in a viral load-dependent fashion and brings about negative impact on IL-17 production in HIV-1 infection.

TAM receptors in apoptotic cell clearance, autoimmunity, and cancer

Receptor tyrosine kinases, Tyro-3, Axl and Mer, collectively designated as TAM, are involved in the clearance of apoptotic cells. TAM ligands, Gas6 and Protein S, bind to the surfaces of apoptotic cells, and at the same time, interact directly with TAM expressed on phagocytes, impacting the engulfment and clearance of apoptotic cells and debris. The well-tuned and balanced actions of TAM may affect a variety of human pathologies including autoimmunity, retinal degeneration, and cancer. This article emphasizes some of the emerging findings and mechanistic insights into TAM functions that are clinically relevant and possibly therapeutically targeted.

Paradoxical role of the proto-oncogene Axl and Mer receptor tyrosine kinases in colon cancer

The receptor tyrosine kinases Axl and Mer, belonging to the Tyro3, Axl and Mer (TAM) receptor family, are expressed in a number of tumor cells and have well-characterized oncogenic roles. The therapeutic targeting of these kinases is considered an anticancer strategy, and various inhibitors are currently under development. At the same time, Axl and Mer are expressed in dendritic cells and macrophages and have an essential function in limiting inflammation. Inflammation is an enabling characteristic of multiple cancer hallmarks. These contrasting oncogenic and anti-inflammatory functions of Axl and Mer posit a potential paradox in terms of anticancer therapy. Here we demonstrate that azoxymethane (AOM) and dextran sulfate sodium (DSS)-induced inflammation-associated cancer is exacerbated in mice lacking Axl and Mer. Ablation of Axl and Mer signaling is associated with increased production of proinflammatory cytokines and failure to clear apoptotic neutrophils in the intestinal lamina propria, thereby favoring a tumor-promoting environment. Interestingly, loss of these genes in the hematopoietic compartment is not associated with increased colitis. Axl and Mer are expressed in radioresistant intestinal macrophages, and the loss of these genes is associated with an increased inflammatory signature in this compartment. Our results raise the possibility of potential adverse effects of systemic anticancer therapies with Axl and Mer inhibitors, and underscore the importance of understanding their tissue and cell type-specific functions in cancer.

Growth arrest specific gene 6 protein concentration in cerebrospinal fluid correlates with relapse severity in multiple sclerosis

Background. Growth arrest specific gene 6 (Gas6) protein enhances survival of oligodendrocytes and neurons, and it is involved in autoimmunity. Therefore, we aimed to verify whether cerebrospinal-fluid (CSF) Gas6 concentration may represent a biomarker of disease activity in multiple sclerosis. Methods. Sixty-five patients who underwent a spinal tap during relapse of relapsing/remitting multiple sclerosis (RR-MS)(McDonald-criteria) were studied. Forty patients affected by noninflammatory/nonautoimmune neurological diseases served as controls. Relapse was defined according to Schumacher criteria. Symptoms were grouped according to Kurtzke-Functional System (FS). Clinical characteristics of the relapse, duration, Expanded-Disability-Status Scale (EDSS) change, number of FS involved, completeness of recovery, age, steroid therapy, were categorised. Patients were followed at 6-month intervals to assess relapse rate and EDSS progression. Gas6 was measured (CSF, plasma) by in-house-enzyme-linked immunoassay (ELISA). Results. Higher CSF Gas6 concentrations were observed in relapses lasting ≤60 days (8.7 ± 3.9 ng/mL) versus >60 days (6.5 ± 2.6) or controls (6.5 ± 2.4; P = 0.05), with ≤2 FS involved (8.5 ± 3.8) versus >2 FS (5.6 ± 2.5) (P < 0.05) and EDSS change ≤2.5 points (8.8 ± 3.7) versus >2.5 (6.5 ± 3.5) (P = 0.04). Conversely, CSF Gas6 was not predictive of the completeness of recovery. Plasma and CSF concentrations were not related (R² = 0.0003), and neither were predictive of relapse rate or EDSS progression after first relapse. Conclusions. CSF concentration of Gas6 is inversely correlated with the severity of relapse in RR-MS patients but does not predict the subsequent course of the disease.

MERTK interactions with SH2-domain proteins in the retinal pigment epithelium

The receptor tyrosine kinase MERTK plays an essential role in the phagocytic uptake of shed photoreceptor membranes by the retinal pigment epithelium (RPE). A fundamental aspect of signal transduction by receptor tyrosine kinases involves autophosphorylation of tyrosine residues that recruit Src-homology 2 (SH2)-domain proteins to the receptor intracellular domain. The goal of the current study was to evaluate the interactions of human MERTK with SH2-domain proteins present in the RPE. The MERTK intracellular domain was expressed as a 6xHis-fusion protein (6xHis-rMERTK(571-999)), purified and phosphorylated. Ni(2+)-NTA pull downs were performed using 6xHis-rMERTK(571-999) in incubations with recombinant phosphotyrosine-recognition sequences expressed as GST-fusion proteins. In addition, pull downs of native SH2-domain proteins were performed using 6xHis-rMERTK(571-999) and protein homogenates from rat RPE/choroid. For both recombinant and native proteins, western analysis detected MERTK interactions with GRB2, PIK3R1 (P85α), VAV3, and SRC. Immunohistochemical analysis localized each protein to mouse RPE. In cultured RPE-J cells incubated with rod outer segments (OS), siRNA knockdown of Grb2 had no effect on OS binding, but significantly reduced OS uptake. Pik3r1 localized to early phagosomes along with Rab5 and Eea1. Phosphorylation and activation of Src was detected downstream of phagocytosis and Mertk activation. These findings suggest that MERTK signaling in the RPE involves a cohort of SH2-domain proteins with the potential to regulate both cytoskeletal rearrangement and membrane movement. Identification of the SH2-domain signaling partners of MERTK is an important step toward further defining the mechanism of RPE phagocytosis that is central to the function and survival of the retina.

O death where is thy sting? Immunologic tolerance to apoptotic self

In higher organisms, innate scavenging cells maintain physiologic homeostasis by removal of the billions of apoptotic cells generated on a daily basis. Apoptotic cell removal requires efficient recognition and uptake by professional and non-professional phagocytic cells, which are governed by an array of soluble and apoptotic cell-integral signals resulting in immunologically silent clearance. While apoptosis is associated with profound suppression of adaptive and innate inflammatory immunity, we have only begun to scratch the surface in understanding how immunologic tolerance to apoptotic self manifest at either the molecular or cellular level. In the last 10 years, data has emerged implicating professional phagocytes, most notably stromal macrophages and CD8α(+)CD103(+) dendritic cells, as critical in initiation of the regulatory cascade that will ultimately lead to long-term whole-animal immune tolerance. Importantly, recent work by our lab and others has shown that alterations in apoptotic cell perception by the innate immune system either by removal of critical phagocytic sentinels in secondary lymphoid organs or blockage of immunosuppressive pathways leads to pronounced inflammation with a breakdown of tolerance towards self. This challenges the paradigm that apoptotic cells are inherently immunosuppressive, suggesting that apoptotic cell tolerance is a "context-dependent" event.

Upregulation of Mer receptor tyrosine kinase signaling attenuated lipopolysaccharide-induced lung inflammation

Mer receptor tyrosine kinase (Mer) signaling plays a central role in the intrinsic inhibition of the inflammatory response to Toll-like receptor activation. Previously, we found that lung Mer protein expression decreased after lipopolysaccharide (LPS) treatment due to enhanced Mer cleavage. The purpose of the present study was to examine whether pharmacologically restored membrane-bound Mer expression upregulates the Mer signaling pathways and suppresses lung inflammatory responses. Pretreatment with the ADAM17 (a disintegrin and metalloproteinase-17) inhibitor TAPI-0 (tumor necrosis factor alpha protease inhibitor-0) reduced LPS-induced production of soluble Mer protein in bronchoalveolar lavage (BAL) fluid, restored membrane-bound Mer expression, and increased Mer activation in alveolar macrophages and lungs after LPS treatment. TAPI-0 also enhanced Mer downstream signaling, including phosphorylation of protein kinase b, focal adhesion kinase, and signal transducer and activator of transcription 1. As expected from enhanced Mer signaling, TAPI-0 also augmented suppressor of cytokine signaling-1 and -3 mRNA and protein levels and inhibited nuclear factor κB activation at 4 and 24 hours after LPS treatment. TAPI-0 suppressed LPS-induced inflammatory cell accumulation, total protein level elevation in BAL fluid, and production of inflammatory mediators, including tumor necrosis factor-α, interleukin-1β, and macrophage inflammatory protein-2. Additionally, the effects of TAPI-0 on the activation of Mer signaling and the production of inflammatory responses could be reversed by cotreatment with specific Mer-neutralizing antibody. Restored Mer protein expression by treatment with TAPI-0 efficiently prevents the inflammatory cascade during acute lung injury.

Molecular control of steady-state dendritic cell maturation and immune homeostasis

Dendritic cells (DCs) are specialized sentinels responsible for coordinating adaptive immunity. This function is dependent upon coupled sensitivity to environmental signs of inflammation and infection to cellular maturation-the programmed alteration of DC phenotype and function to enhance immune cell activation. Although DCs are thus well equipped to respond to pathogens, maturation triggers are not unique to infection. Given that immune cells are exquisitely sensitive to the biological functions of DCs, we now appreciate that multiple layers of suppression are required to restrict the environmental sensitivity, cellular maturation, and even life span of DCs to prevent aberrant immune activation during the steady state. At the same time, steady-state DCs are not quiescent but rather perform key functions that support homeostasis of numerous cell types. Here we review these functions and molecular mechanisms of suppression that control steady-state DC maturation. Corruption of these steady-state operatives has diverse immunological consequences and pinpoints DCs as potent drivers of autoimmune and inflammatory disease.

Murine myeloid dendritic cells that phagocytose apoptotic T cells inhibit the immune response via NO

The contraction phase of antigen-specific immune responses involves the apoptotic loss of numerous activated lymphocytes. While apoptotic cells are known to induce immune suppression, the mechanisms involved therein are still ambiguous. Some reports have speculated that macrophages can induce regulatory T cells (Tregs) after engulfing apoptotic cells. In this study, we showed that dendritic cells (DCs) that phagocytose apoptotic T cells acquire inhibitory function (named DCapos) toward CD4⁺ and CD8⁺ T cells. These inhibitory DCs could not induce the generation of Tregs, but they were found to directly inhibit mDCs that initiate CD4⁺ and CD8⁺ T cell proliferation both in vitro and in vivo. Soluble factors including NO play a role in the DCapos-induced suppression of CD4⁺ and CD8⁺ T cell proliferation. Further results showed that STAT3 phosphorylation and inducible nitric oxide synthase (iNOS) generation were enhanced when DCs were co-cultured with apoptotic cells. Both iNOS transcription and NO secretion were inhibited in the presence of the specific p-STAT3 inhibitor JSI-124. All the data indicated that apoptotic cells could turn DCs to inhibitory DCs, which might play important roles in the suppression of immune responses. STAT3 activation and the consequent release of NO are responsible for the inhibitory functions of DCapos.

Intrinsic unresponsiveness of Mertk-/- B cells to chronic graft-versus-host disease is associated with unmodulated CD1d expression

Activation and migration of marginal zone B (MZB) cells into follicular (FO) regions of the spleen has been proposed as one of the mechanisms that regulate the development of autoreactive B cells. The mer receptor tyrosine kinase (Mertk) mediates apoptotic cell clearance and regulates activation and cytokine secretion. In the well-studied class II chronic GVH model of bm12 cells into B6 hosts, we observed that Mertk deficient B6 mice did not generate autoantibodies in response to this allogeneic stimulus. We posited that Mertk is important in MHC-II-mediated B cell signaling. In the present study, we show that B cells from Mertk(-/-) mice but not WT B6 mice exhibited decreased calcium mobilization and tyrosine phosphorylation when stimulated by MHC-II cross-linking. The finding that Mertk was important for class II signaling in B cells was further supported by the preponderance of a-allotype autoantibodies in cGVH in RAG-KO mice reconstituted with a mixture of bone marrow from Mertk(-/-) mice (b-allotype) and C20 mice (a-allotype). MZB cells from Mertk(-/-) mice were unable to down regulate surface CD1d expression and subsequent inclusion in the MZ, associated with significantly lower germinal center responses compared to MZB cells from WT. Moreover, Mertk(-/-) mice treated with an anti-CD1d down regulating antibody responded significantly to bm12 cells, while no response was observed in Mertk(-/-) mice treated with control antibodies. Taken together, these findings extend the role of Mertk to include CD1d down regulation on MZB cells, a potential mechanism limiting B cell activation in cGVH.

The HIV-1 protease inhibitor nelfinavir activates PP2 and inhibits MAPK signaling in macrophages: a pathway to reduce inflammation

The HIV-1 PI NFV has off-target effects upon host enzymes, including inhibition of the 20S proteasome, resulting in activation of PP1. HIV-1-associated monocyte/macrophage activation, in part a result of systemically elevated levels of microbial products including LPS, is associated with risk of mortality, independent of viremia or CD4 T cell loss. This study tested the hypothesis that activation of protein phosphatases by NFV would reduce activation of monocytes/macrophages through dephosphorylation of signal transduction proteins. NFV uniquely blocked LPS-induced production by human monocyte-derived macrophages of the inflammatory cytokines TNF and IL-6, as well as sCD14. Although NFV failed to modulate NF-κB, NFV treatment reduced phosphorylation of AKT and MAPKs. Inhibition of PP2 with okadaic acid blocked the anti-inflammatory effect of NFV, whereas the PP1 inhibitor calyculin A failed to counter the anti-inflammatory effects of NFV. For in vivo studies, plasma sCD14 and LPS were monitored in a cohort of 31 pediatric HIV-1 patients for over 2 years of therapy. Therapy, including NFV, reduced sCD14 levels significantly compared with IDV or RTV, independent of ΔLPS levels, VL, CD4 T cell frequency, or age. The hypothesis was supported as NFV induced activation of PP2 in macrophages, resulting in disruption of inflammatory cell signaling pathways. In vivo evidence supports that NFV may offer beneficial effects independent of antiviral activity by reducing severity of chronic innate immune activation in HIV-1 infection.

The TAM-family receptor Mer mediates production of HGF through the RhoA-dependent pathway in response to apoptotic cells

The TAM receptor protein tyrosine kinases Tyro3, Axl, and Mer play important roles in macrophage function. We investigated the roles of the TAM receptors in mediating the induction of hepatocyte growth factor (HGF) during the interaction of macrophages with apoptotic cells. Mer-specific neutralizing antibody, small interfering RNA (siRNA), and a recombinant Mer protein (Mer/Fc) inhibited HGF mRNA and protein expression, as well as activation of RhoA, Akt, and specific mitogen-activated protein (MAP) kinases in response to apoptotic cells. Inhibition of Axl or Tyro3 with specific antibodies, siRNA, or Fc-fusion proteins did not prevent apoptotic cell-induced HGF mRNA and protein expression and did not inhibit activation of the postreceptor signaling molecules RhoA and certain MAP kinases, including extracellular signal-regulated protein kinase and c-Jun NH(2)-terminal kinase. However, Axl- and Tyro3-specific blockers did inhibit the activation of Akt and p38 MAP kinase in response to apoptotic cells. In addition, none of the TAM receptors mediated the effects of apoptotic cells on transforming growth factor-β or epidermal growth factor mRNA expression. However, they were involved in the induction of vascular endothelial growth factor mRNA expression. Our data provide evidence that when macrophages interact with apoptotic cells, only Mer of the TAM-family receptors is responsible for mediating transcriptional HGF production through a RhoA-dependent pathway.

Preventing cleavage of Mer promotes efferocytosis and suppresses acute lung injury in bleomycin treated mice

Mer receptor tyrosine kinase (Mer) regulates macrophage activation and promotes apoptotic cell clearance. Mer activation is regulated through proteolytic cleavage of the extracellular domain. To determine if membrane-bound Mer is cleaved during bleomycin-induced lung injury, and, if so, how preventing the cleavage of Mer enhances apoptotic cell uptake and down-regulates pulmonary immune responses. During bleomycin-induced acute lung injury in mice, membrane-bound Mer expression decreased, but production of soluble Mer and activity as well as expression of disintegrin and metalloproteinase 17 (ADAM17) were enhanced . Treatment with the ADAM inhibitor TAPI-0 restored Mer expression and diminished soluble Mer production. Furthermore, TAPI-0 increased Mer activation in alveolar macrophages and lung tissue resulting in enhanced apoptotic cell clearance in vivo and ex vivo by alveolar macrophages. Suppression of bleomycin-induced pro-inflammatory mediators, but enhancement of hepatocyte growth factor induction were seen after TAPI-0 treatment. Additional bleomycin-induced inflammatory responses reduced by TAPI-0 treatment included inflammatory cell recruitment into the lungs, levels of total protein and lactate dehydrogenase activity in bronchoalveolar lavage fluid, as well as caspase-3 and caspase-9 activity and alveolar epithelial cell apoptosis in lung tissue. Importantly, the effects of TAPI-0 on bleomycin-induced inflammation and apoptosis were reversed by coadministration of specific Mer-neutralizing antibodies. These findings suggest that restored membrane-bound Mer expression by TAPI-0 treatment may help resolve lung inflammation and apoptosis after bleomycin treatment.

Impaired clearance of apoptotic cells in germinal centers: implications for loss of B cell tolerance and induction of autoimmunity

Germinal centers (GCs) comprise lymphoid microenvironments where antigen-stimulated B cells undergo rapid proliferation and somatic hypermutation (SHM), resulting in the generation of B cells with high affinity for antigen. However, this process also generates B cell clones with low antigen affinity and with the potential for autoreactivity. It has been suggested that GC B cells with low antigen affinity and autoreactivity are eliminated via apoptosis and are rapidly cleared by tingible body macrophages (TBMφs). Inefficient clearance of apoptotic cells (ACs) results in autoimmunity that is thought to be mediated by various intracellular molecules possessing danger-associated molecular patterns (DAMPs), including nuclear self-Ags. DAMPs can be released from ACs undergoing "secondary necrosis" due to a disruption in AC clearance within GCs. This review discusses the role and mechanisms associated with impaired clearance of ACs in GCs in loss of B cell tolerance leading to autoantibody production and the development of autoimmunity.

Inhibiting Mer receptor tyrosine kinase suppresses STAT1, SOCS1/3, and NF-κB activation and enhances inflammatory responses in lipopolysaccharide-induced acute lung injury

Mer signaling participates in a novel inhibitory pathway in TLR activation. The purpose of the present study was to examine the role of Mer signaling in the down-regulation of TLR4 activation-driven immune responses in mice, i.t.-treated with LPS, using the specific Mer-blocking antibody. At 4 h and 24 h after LPS treatment, expression of Mer protein in alveolar macrophages and lung tissue decreased, sMer in BALF increased significantly, and Mer activation increased. Pretreatment with anti-Mer antibody did not influence the protein levels of Mer and sMer levels. Anti-Mer antibody significantly reduced LPS-induced Mer activation, phosphorylation of Akt and FAK, STAT1 activation, and expression of SOCS1 and -3. Anti-Mer antibody enhanced LPS-induced inflammatory responses, including activation of the NF-κB pathway; the production of TNF-α, IL-1β, and MIP-2 and MMP-9 activity; and accumulation of inflammatory cells and the total protein levels in BALF. These results indicate that Mer plays as an intrinsic feedback inhibitor of the TLR4- and inflammatory mediator-driven immune responses during acute lung injury.

PTEN negatively regulates engulfment of apoptotic cells by modulating activation of Rac GTPase

Efficient clearance of apoptotic cells by phagocytes (efferocytosis) is critical for normal tissue homeostasis and regulation of the immune system. Apoptotic cells are recognized by a vast repertoire of receptors on macrophage that lead to transient formation of phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P(3)] and subsequent cytoskeletal reorganization necessary for engulfment. Certain PI3K isoforms are required for engulfment of apoptotic cells, but relatively little is known about the role of lipid phosphatases in this process. In this study, we report that the activity of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a phosphatidylinositol 3-phosphatase, is elevated upon efferocytosis. Depletion of PTEN in macrophage results in elevated PtdIns(3,4,5)P(3) production and enhanced phagocytic ability both in vivo and in vitro, whereas overexpression of wild-type PTEN abrogates this process. Loss of PTEN in macrophage leads to activation of the pleckstrin homology domain-containing guanine-nucleotide exchange factor Vav1 and subsequent activation of Rac1 GTPase, resulting in increased amounts of F-actin upon engulfment of apoptotic cells. PTEN disruption also leads to increased production of anti-inflammatory cytokine IL-10 and decreased production of proinflammatory IL-6 and TNF-α upon engulfment of apoptotic cells. These data suggest that PTEN exerts control over efferocytosis potentially by regulating PtdIns(3,4,5)P(3) levels that modulate Rac GTPase and F-actin reorganization through Vav1 exchange factor and enhancing apoptotic cell-induced anti-inflammatory response.

Retinal self-antigen induces a predominantly Th1 effector response in Axl and Mertk double-knockout mice

The TAM family of receptors (Tyro3, Axl, and Mertk) plays an important role in the negative regulation of response of dendritic cells (DCs) and macrophages to pathogenic stimuli, and mice lacking this receptor family develop spontaneous lupus-like systemic autoimmunity against a variety of tissues, including retina. To study the molecular mechanism underlying the TAM regulation of APC functions and subsequent effects on the induction of an autoimmune response against the eye, we examined CD4 T cell differentiation following retinal self-antigen immunization. CD4 T cells prepared from naive or interphotoreceptor retinoid-binding protein (IRBP)1-20-immunized Axl and Mertk double-knockout (dko) mice reacted to activation using anti-CD3 and anti-CD28 Abs or to bolster by self-antigen in vitro with a predominantly Th1 effector response, as characterized by increased IFN-γ production and higher frequency of IFN-γ-positive CD4 T cells. The Th17 effector response to IRBP immunization was similar in dko mice to that in wild-type controls, as shown by ELISA measurement of IL-17A in the culture medium and flow cytometric analysis of IL-17A-secreting CD4 T cells. Interestingly, APCs or DCs isolated from IRBP-immunized dko mice exhibited a greater ability to drive the Th1 response. The production of two driving cytokines for Th1 differentiation, IL-12 and IL-18, was dramatically increased in dko DCs and macrophages, and LPS stimulation bolstered their production. The preferential development into the Th1 subset in dko mice suggests that the cytokine milieu produced by the mutant mice in vivo or by mutant APCs in vitro selectively creates a differentiation environment favoring the Th1 effector response.

How to improve the immunogenicity of chemotherapy and radiotherapy

Chemotherapy or radiotherapy could induce various tumor cell death modalities, releasing tumor-derived antigen as well as danger signals that could either be captured for triggering antitumor immune response or ignored. Exploring the interplay among therapeutic drugs, tumor cell death and the immune cells should improve diagnostic, prognostic, predictive, and therapeutic management of tumor. We summarized some of the cell death-derived danger signals and the mechanism for host to sense and response to cell death in the tumor microenvironment. Based on the recent clinical or experimental findings, several strategies have been suggested to improve the immunogenicity of cell death and augment antitumor immunity.

Polymorphisms in the receptor tyrosine kinase MERTK gene are associated with multiple sclerosis susceptibility

Multiple sclerosis (MS) is a debilitating, chronic demyelinating disease of the central nervous system affecting over 2 million people worldwide. The TAM family of receptor tyrosine kinases (TYRO3, AXL and MERTK) have been implicated as important players during demyelination in both animal models of MS and in the human disease. We therefore conducted an association study to identify single nucleotide polymorphisms (SNPs) within genes encoding the TAM receptors and their ligands associated with MS. Analysis of genotype data from a genome-wide association study which consisted of 1618 MS cases and 3413 healthy controls conducted by the Australia and New Zealand Multiple Sclerosis Genetics Consortium (ANZgene) revealed several SNPs within the MERTK gene (Chromosome 2q14.1, Accession Number NG_011607.1) that showed suggestive association with MS. We therefore interrogated 28 SNPs in MERTK in an independent replication cohort of 1140 MS cases and 1140 healthy controls. We found 12 SNPs that replicated, with 7 SNPs showing p-values of less than 10⁻⁵ when the discovery and replication cohorts were combined. All 12 replicated SNPs were in strong linkage disequilibrium with each other. In combination, these data suggest the MERTK gene is a novel risk gene for MS susceptibility.

Epithelial cell-directed efferocytosis in the post-partum mammary gland is necessary for tissue homeostasis and future lactation

Background

Mammary glands harbor a profound burden of apoptotic cells (ACs) during post-lactational involution, but little is known regarding mechanisms by which ACs are cleared from the mammary gland, or consequences if this process is interrupted. We investigated AC clearance, also termed efferocytosis, during post-lactational remodeling, using mice deficient for MerTK, Axl, and Tyro3, three related receptor tyrosine kinases (RTKs) regulating macrophage-mediated efferocytosis in monocytes. MerTK expression, apoptosis and the accumulation of apoptotic debris were examined in histological sections of MerTK-deficient, Axl/Tyro3-deficient, and wild-type mammary glands harvested at specific time points during lactation and synchronized involution. The ability of primary mammary epithelial cells (MECs) to engulf ACs was assessed in culture. Transplant of MerTK-deficient mammary epithelium into cleared WT mammary fat pads was used to assess the contribution of WT mammary macrophages to post-lactational efferocytosis.

Results

ACs induced MerTK expression in MECs, resulting in elevated MerTK levels at the earliest stages of involution. Loss of MerTK resulted in AC accumulation in post-lactational MerTK-deficient mammary glands, but not in Axl and Tyro3-deficient mammary glands. Increased vascularization, fibrosis, and epithelial hyperproliferation were observed in MerTK-deficient mammary glands through at least 60 days post-weaning, due to failed efferocytosis after lactation, but did not manifest in nulliparous mice. WT host-derived macrophages failed to rescue efferocytosis in transplanted MerTK-deficient mammary epithelium.

Conclusion

Efferocytosis by MECs through MerTK is crucial for mammary gland homeostasis and function during the post-lactational period. Efferocytosis by MECs thus limits pathologic consequences associated with the apoptotic load following lactation.

Apoptotic cell-based therapies against transplant rejection: role of recipient's dendritic cells

One of the ultimate goals in transplantation is to develop novel therapeutic methods for induction of donor-specific tolerance to reduce the side effects caused by the generalized immunosuppression associated to the currently used pharmacologic regimens. Interaction or phagocytosis of cells in early apoptosis exerts potent anti-inflammatory and immunosuppressive effects on antigen (Ag)-presenting cells (APC) like dendritic cells (DC) and macrophages. This observation led to the idea that apoptotic cell-based therapies could be employed to deliver donor-Ag in combination with regulatory signals to recipient’s APC as therapeutic approach to restrain the anti-donor response. This review describes the multiple mechanisms by which apoptotic cells down-modulate the immuno-stimulatory and pro-inflammatory functions of DC and macrophages, and the role of the interaction between apoptotic cells and APC in self-tolerance and in apoptotic cell-based therapies to prevent/treat allograft rejection and graft-versus-host disease in murine experimental systems and in humans. It also explores the role that in vivo-generated apoptotic cells could have in the beneficial effects of extracorporeal photopheresis, donor-specific transfusion, and tolerogenic DC-based therapies in transplantation.

Ectosomes as immunomodulators

Considerable progress has been made in recognizing microvesicles as important mediators of intercellular communication rather than irrelevant cell debris. Microvesicles released by budding directly from the cell membrane surface (i.e., ectocytosis) either spontaneously or in response to various stimuli are called shed vesicles or ectosomes. Ectosomes are rightside-out vesicles with cytosolic content, and they expose phosphatidylserine in the outer leaflet of their membrane. Depending on their cellular origin, ectosomes have been associated with a broad spectrum of biological activities. In the light of recent findings, we now know that ectosomes derived from polymorphonuclear leukocytes, erythrocytes, platelets, and tumor cells have profound effects on the innate immune system, as well as on the induction of the adaptive immunity, globally reprogramming cells such as macrophages or dendritic cells toward an immunosuppressive and possibly tolerogenic phenotype. Although the effects observed in the circulation are mainly procoagulant and pro-inflammatory, ectosomes might be anti-inflammatory/immunosuppressive in local inflammation.

Recent advances in antigen-loaded dendritic cell-based strategies for treatment of minimal residual disease in acute myeloid leukemia

Therapeutic vaccination with dendritic cells (DCs) is recognized as an important experimental therapy for the treatment of minimal residual disease in acute myeloid leukemia. Many sources of leukemia-associated antigens and different methods for antigen loading of DCs have been used in an attempt to optimize anti-tumor responses. For instance, monocyte-derived DCs have been loaded with apoptotic whole-cell suspensions, necrotic cell lysates, tumor-associated peptides, eluted peptides and cellular DNA or RNA. Furthermore, monocyte-derived DCs can be chemically or electrically fused with leukemic blasts, and DCs have been cultured out of leukemic blasts. However, it remains a challenge in cancer immunotherapy to identify which of these methods is the most optimal for antigen loading and activation of DCs. This review discusses recent advances in DC research and the application of this knowledge towards new strategies for antigen loading of DCs in the treatment of minimal residual disease in acute myeloid leukemia.

Rethinking the red wolf disease: does Protein S suppress systemic lupus erythematosus clinical activity?

In systemic lupus erythematosus, the forces responsible for disease initiation and self-perpetuation in these clinically heterogeneous populations remain poorly understood. Recent studies of the TAM (Tyro3, Axl and MerTK) family of receptor tyrosine kinases may lead to a better understanding of the fundamental control system responsible for the clearance of apoptotic cells and the regulation of inflammation. In a recent report, serum levels of the TAM ligand, Protein S, was found to correlate with certain disease manifestations and with C3 and C4 levels. Protein S levels could provide a quantitative clinical biomarker but it remains to be determined whether this factor directly affects disease activity.

TAM receptor signaling and autoimmune disease

The TAM receptor tyrosine kinases Tyro3, Axl, and Mer and their ligands Gas6 and Protein S are essential for the phagocytosis of apoptotic cells and membranes in the adult immune, nervous, and reproductive systems. Genetic studies indicate that this receptor-ligand system is central to apoptotic cell engulfment that is triggered by the 'eat-me' signal phosphatidylserine (PtdSer). At the same time, TAM signaling is normally activated by Toll-like receptor (TLR) and type I interferon signaling, as part of the innate inflammatory response in dendritic cells (DCs) and macrophages, where it inhibits this response. Deficiencies in TAM signaling result in human retinal dystrophies and may contribute to lupus and other human autoimmune diseases.

Ectosomes released by polymorphonuclear neutrophils induce a MerTK-dependent anti-inflammatory pathway in macrophages

At the earliest stage of activation, human polymorphonuclear neutrophils release vesicles derived directly from the cell surface. These vesicles, called ectosomes (PMN-Ect), expose phosphatidylserine in the outer membrane leaflet. They inhibit the inflammatory response of human monocyte-derived macrophages and dendritic cells to zymosan A (ZymA) and LPS and induce TGF-β1 release, suggesting a reprogramming toward a tolerogenic phenotype. The receptors and signaling pathways involved have not yet been defined. Here, we demonstrate that PMN-Ect interfered with ZymA activation of macrophages via inhibition of NFκB p65 phosphorylation and NFκB translocation. The MerTK (Mer receptor tyrosine kinase) and PI3K/Akt pathways played a key role in this immunomodulatory effect as shown using specific MerTK-blocking antibodies and PI3K inhibitors LY294002 and wortmannin. As a result, PMN-Ect reduced the transcription of many proinflammatory genes in ZymA-activated macrophages. In sum, PMN-Ect interacted with the macrophages by activation of the MerTK pathway responsible for down-modulation of the proinflammatory signals generated by ZymA.

Impaired apoptotic cell clearance in the germinal center by Mer-deficient tingible body macrophages leads to enhanced antibody-forming cell and germinal center responses

Germinal centers (GCs) are specialized microenvironments that generate high-affinity Ab-forming cells (AFCs) and memory B cells. Many B cells undergo apoptosis during B cell clonal selection in GCs. Although the factors that regulate the AFC and GC responses are not precisely understood, it is widely believed that dysregulated AFCs and GCs contribute to autoimmunity. The Mer receptor tyrosine kinase (Mer) facilitates macrophage clearance of apoptotic cells. The Tyro-3, Axl, and Mer receptors, including Mer, suppress TLRs and cytokine-mediated inflammatory responses. We report in this study that tingible body macrophages (TBMφs) in GCs express Mer. Compared to C57BL/6 (B6) controls, Mer-deficient (Mer(-/-)) mice had significantly higher AFC, GC, and Th1-skewed IgG2 Ab (especially IgG2c) responses against the T cell-dependent Ag (4-hydroxy-3-nitrophenyl) acetyl-chicken γ globulin. Mer(-/-) mice had a significantly higher percentage of GC B cells on days 9, 14, and 21 postimmunization compared with B6 controls. Significantly increased numbers of apoptotic cells accumulated in Mer(-/-) GCs than in B6 GCs, whereas the number of TBMφs remained similar in both strains. Our data are the first, to our knowledge, to demonstrate a critical role for Mer in GC apoptotic cell clearance by TBMφs and have interesting implications for Mer in the regulation of B cell tolerance operative in the AFC and GC pathways.

Dysregulation of thymic clonal deletion and the escape of autoreactive T cells

Events ongoing in the thymus are critical for deleting developing thymocytes specific for tissue antigens, and establishing self-tolerance within the T cell compartment. Aberrant thymic negative selection, however, is believed to generate a repertoire with increased self-reactivity, which in turn can contribute to the development of T cell-mediated autoimmunity. In this review, mechanisms that regulate the efficacy of negative selection and influence the deletion of autoreactive thymocytes will be discussed.

Dendritic cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) persists as a chronic inflammatory autoimmune disease and is characterized by the production of autoantibodies and immune complexes that affect multiple organs. The underlying mechanism that triggers and sustains disease are complex and involve certain susceptibility genes and environmental factors. There have been several immune mediators linked to SLE including cytokines and chemokines that have been reviewed elsewhere [ 1-3 ]. A number of articles have reviewed the role of B cells and T cells in SLE [ 4-10 ]. Here, we focus on the role of dendritic cells (DC) and innate immune factors that may regulate autoreactive B cells.

Regulation of phagocytosis by TAM receptors and their ligands

The TAM family of receptors is preferentially expressed by professional and non-professional phagocytes, including macrophages, dendritic cells and natural killer cells in the immune system, osteoclasts in bone, Sertoli cells in testis, and retinal pigmental epithelium cells in the retina. Mutations in the Mertk single gene or in different combinations of the double or triple gene mutations in the same cell cause complete or partial impairment in phagocytosis of their preys; and as a result, either the normal apoptotic cells cannot be efficiently removed or the tissue neighbor cells die by apoptosis. This scenario of TAM regulation represents a widely adapted model system used by phagocytes in all different tissues. The present review will summarize current known functional roles of TAM receptors and their ligands, Gas 6 and protein S, in the regulation of phagocytosis.

Leishmania donovani isolates with antimony-resistant but not -sensitive phenotype inhibit sodium antimony gluconate-induced dendritic cell activation

The inability of sodium antimony gluconate (SAG)-unresponsive kala-azar patients to clear Leishmania donovani (LD) infection despite SAG therapy is partly due to an ill-defined immune-dysfunction. Since dendritic cells (DCs) typically initiate anti-leishmanial immunity, a role for DCs in aberrant LD clearance was investigated. Accordingly, regulation of SAG-induced activation of murine DCs following infection with LD isolates exhibiting two distinct phenotypes such as antimony-resistant (Sb^RLD) and antimony-sensitive (Sb^SLD) was compared in vitro. Unlike Sb^SLD, infection of DCs with Sb^RLD induced more IL-10 production and inhibited SAG-induced secretion of proinflammatory cytokines, up-regulation of co-stimulatory molecules and leishmanicidal effects. Sb^RLD inhibited these effects of SAG by blocking activation of PI3K/AKT and NF-κB pathways. In contrast, Sb^SLD failed to block activation of SAG (20 µg/ml)-induced PI3K/AKT pathway; which continued to stimulate NF-κB signaling, induce leishmanicidal effects and promote DC activation. Notably, prolonged incubation of DCs with Sb^SLD also inhibited SAG (20 µg/ml)-induced activation of PI3K/AKT and NF-κB pathways and leishmanicidal effects, which was restored by increasing the dose of SAG to 40 µg/ml. In contrast, Sb^RLD inhibited these SAG-induced events regardless of duration of DC exposure to Sb^RLD or dose of SAG. Interestingly, the inhibitory effects of isogenic Sb^SLD expressing ATP-binding cassette (ABC) transporter MRPA on SAG-induced leishmanicidal effects mimicked that of Sb^RLD to some extent, although antimony resistance in clinical LD isolates is known to be multifactorial. Furthermore, NF-κB was found to transcriptionally regulate expression of murine γglutamylcysteine synthetase heavy-chain (mγGCS_hc) gene, presumably an important regulator of antimony resistance. Importantly, Sb^RLD but not Sb^SLD blocked SAG-induced mγGCS expression in DCs by preventing NF-κB binding to the mγGCS_hc promoter. Our findings demonstrate that Sb^RLD but not Sb^SLD prevents SAG-induced DC activation by suppressing a PI3K-dependent NF-κB pathway and provide the evidence for differential host-pathogen interaction mediated by Sb^RLD and Sb^SLD.

Kala-azar, a life-threatening parasitic disease caused by Leishmania donovani (LD), is widening its base in different parts of the world. Currently, there is no effective vaccine against kala-azar. The antimonial drugs like sodium antimony gluconate (SAG) have been the mainstay of therapy for this disease. Recently, due to the emergence of antimony-resistance in parasites, SAG often fails to cure kala-azar patients, which is compounding the disaster further. It is still unknown how infection with LD exhibiting antimony-resistant phenotype, in contrast to antimony-sensitive phenotype, is handled by the kala-azar patients upon SAG treatment. This demands an understanding of the nature of host immune responses against these two distinct categories of parasites. Accordingly, we compared the impact of infection with LD exhibiting antimony-resistant versus antimony-sensitive phenotype on dendritic cells (DCs). DCs upon activation/maturation initiate anti-leishmanial immunity. We showed that parasites with antimony-resistant but not antimony-sensitive phenotype prevented SAG-induced DC activation/maturation by blocking activation of NF-κB. The latter is a key signaling pathway regulating DC activation/maturation. Our studies for the first time provide both a cellular and molecular basis for differential response of host cells to parasite isolates with antimony-resistant and antimony-sensitive phenotype, which may influence the outcome of the disease.

Increased hematopoietic cells in the mertk-/- mouse peritoneal cavity: a result of augmented migration

The peritoneal cavity is recognized as an important site for autoreactive B cells prior to their transit to other immune tissues; however, little is known of the genes that may regulate this process. Mice lacking the receptor tyrosine kinase, Mertk, display a lupus-like autoimmune phenotype with splenomegaly and high autoantibodies titers. In this study, we investigate whether Mertk regulates the composition of peritoneal cells that favor an autoimmune phenotype. We found an increase in the number of macrophages, dendritic cells (DCs), plasmacytoid DCs, T cells, and B cells in the peritoneal cavity of mertk-/- mice when compared with wild-type mice. This disparity in cell numbers was not due to changes in cell proliferation or cell death. In adoptive transfer experiments, we showed an increase in migration of labeled donor cells into the mertk-/- peritoneal cavity. In addition, bone marrow chimeric mice showed hematopoietic-derived factors were also critical for T cell migration. Consistent with this migration and the increase in the number of cells, we identified elevated expression of CXCL9, its receptor CXCR3, and IL-7R on peritoneal cells from mertk-/- mice. To corroborate the migratory function of CXCR3 on cells, the depletion of CXCR3 donor cells significantly reduced the number of adoptively transferred cells that entered into the peritoneum of mertk-/- mice. This control of peritoneal cells numbers correlated with autoantibody production and was exclusively attributed to Mertk because mice lacking other family members, Axl or Tyro 3, did not display dysregulation in peritoneal cell numbers or the autoimmune phenotype.

Effect of Dachengqi decoction on NF-kappaB p65 expression in lung of rats with partial intestinal obstruction and the underlying mechanism

To investigate the effect of Dachengqi decoction on NF-kappaB p65 expression in lung of rats with partial intestinal obstruction and the underlying mechanism, 30 SD rats were randomly divided into three groups: sham-operation group, model group and Dachengqi decoction treatment group (Dachengqi group), with 10 animals in each group. The models were made by partially ligating their large intestines outside the body. The pathological changes were analyzed by HE staining. The expression of NF-kappaB p65 in rats lung were measured by using real-time polymerase chain reaction and immunohistochemistry respectively. Moreover, the expression of caveolin-1 in rats lung was also measured to. Increased edema, interstitial thickening, hemorrhage, and infiltration of inflammatory cells were found in the model group. In contrast, this change was significantly reduced in Dachengqi group as compared with model group. In addition, the up-regulated caveolin-1 and NF-kappaB p65 were also suppressed by Dachengqi decoction in lung of rats with partial intestinal obstruction. We are led to concluded that the caveolin-1-NF-kappaB pathway plays an important role in the development of lung injury of rats with partial intestinal obstruction and Dachengqi decoction could down-regulate the expression of caveolin-1 and NF-kappaB p65 in lung of rats with partial intestinal obstruction.

Extracorporeal photopheresis-induced immune tolerance: a focus on modulation of antigen-presenting cells and induction of regulatory T cells by apoptotic cells

PURPOSE OF REVIEW

This review is intended to introduce recent advances in the research surrounding extracorporeal photopheresis (ECP) with a focus on how apoptotic cells modulate antigen-presenting cells and induce regulatory T cells, given that ECP therapy induces apoptosis of leukocytes collected through leukapheresis.

RECENT FINDINGS

It has been suggested that ECP therapy, unlike other immunosuppressive regimens, does not cause global immunosuppression, but induces immune tolerance. Recent clinical and animal studies demonstrate that ECP therapy induces antigen-specific regulatory T cells, including CD4+CD25+FoxP3+ T cells and IL-10-producing Tr1 cells, that may arise secondarily to the induction of tolerogenic antigen-presenting cells (APCs) by infusion of apoptotic cells. It has also been suggested that ECP therapy may induce IL-10-producing regulatory B cells and regulatory CD8+ T cells. Finally, several recent studies, which examined the cellular elements involved in the uptake of apoptotic cells, demonstrated that apoptotic cells modulate APCs through binding to specific receptors, particularly TAM receptors that provide inhibitory signals that block APC activation.

SUMMARY

ECP therapy induces immune tolerance through modulation of antigen-presenting cells as well as induction of regulatory T cells. ECP therapy has great potential in the management of allogeneic transplantation and autoimmune diseases.

Suppression of FVIII inhibitor formation in hemophilic mice by delivery of transgene modified apoptotic fibroblasts

The development of inhibitory antibodies to factor VIII (FVIII) is currently the most significant complication of FVIII replacement therapy in the management of patients with severe hemophilia A. Immune tolerance protocols for the eradication of inhibitors require daily delivery of intravenous FVIII for at least 6 months and are unsuccessful in 20-40% of treated patients. We hypothesize that tolerance can be induced more efficiently and reliably by delivery of FVIII antigen within autologous apoptotic cells (ACs). In this study, we demonstrated suppression of the T cell and inhibitor responses to FVIII by infusion of FVIII expression vector modified apoptotic syngeneic fibroblasts in both naive and preimmunized hemophilia A mice. ACs without FVIII antigen exerted modest generalized immune suppression mediated by anti-inflammatory signals. However, FVIII expressing apoptotic syngeneic fibroblasts produced much stronger antigen-specific immune suppression. Mice treated with these fibroblasts generated CD4+ T cells that suppressed the immune response to FVIII after adoptive transfer into naive recipients and antigen-specific CD4+CD25+ regulatory T cells (Tregs) that inhibited the proliferation of FVIII responsive effector T cells in vitro. These preclinical results demonstrate the potential for using FVIII vector modified autologous ACs to treat high-titer inhibitors in patients with hemophilia A.

A novel role for c-Src and STAT3 in apoptotic cell-mediated MerTK-dependent immunoregulation of dendritic cells

Dendritic cells (DCs) play an instrumental role in regulating tolerance to self-antigens and preventing autoimmunity. One mechanism by which "tolerogenic" DCs are established is through the inhibitory effects of apoptotic cells (ACs). Immature DCs encountering ACs are resistant to stimuli that activate and mature DCs. We have shown that the Mer receptor tyrosine kinase (MerTK) plays a key role in transducing inhibitory signals upon binding of ACs, which in turn involve the phosphatidylinositol 3-kinase (PI3K) pathway. Nevertheless, the molecular basis for AC-induced inhibition of DCs is ill defined. In the current study, the proximal signaling events induced by MerTK after AC binding were studied. AC treatment of bone marrow-derived or splenic DCs established a complex consisting of MerTK, the nonreceptor tyrosine kinase c-Src, the transcription factor STAT3, and PI3K. In contrast, AC treatment of DCs lacking MerTK expression failed to increase c-Src and STAT3 activation. In addition, the inhibitory effects of ACs were blocked by treating DCs with pharmacologic inhibitors or siRNA specific for c-Src and STAT3. These findings demonstrate that AC-induced inhibition of DCs requires MerTK-dependent activation of c-Src and STAT3, and provide evidence for novel roles for c-Src and STAT3 in the immunoregulation of DCs.

Apoptotic cells promote their own clearance and immune tolerance through activation of the nuclear receptor LXR

Effective clearance of apoptotic cells by macrophages is essential for immune homeostasis. The transcriptional pathways that allow macrophages to sense and respond to apoptotic cells are poorly defined. We found that liver X receptor (LXR) signaling was important for both apoptotic cell clearance and the maintenance of immune tolerance. Apoptotic cell engulfment activated LXR and thereby induced the expression of Mer, a receptor tyrosine kinase critical for phagocytosis. LXR-deficient macrophages exhibited a selective defect in phagocytosis of apoptotic cells and an aberrant proinflammatory response to them. As a consequence of these defects, mice lacking LXRs manifested a breakdown in self-tolerance and developed autoantibodies and autoimmune glomerulonephritis. Treatment with an LXR agonist ameliorated disease progression in a mouse model of lupus-like autoimmunity. Thus, activation of LXR by apoptotic cells engages a virtuous cycle that promotes their own clearance and couples engulfment to the suppression of inflammatory pathways.

The liaison between apoptotic cells and macrophages--the end programs the beginning

The efficient execution of apoptotic cell death with the clearance of apoptotic debris by phagocytes is a key regulatory mechanism ensuring tissue homeostasis. Failure in this execution program contributes to the pathogenesis of many human diseases. In this review, we describe the current knowledge regarding the interaction of apoptotic cells with their professional 'captors', the macrophages, with special emphasis on the immunological outcome. Removal of apoptotic cells must be considered as a process that actively delivers signals to polarize macrophages, which are fundamental for the resolution of inflammation. However, the sculpting of macrophage responses by apoptotic cells can be misused under certain inflammatory disease conditions, including tumor development.

The receptor tyrosine kinase MerTK regulates dendritic cell production of BAFF

The MerTK receptor tyrosine kinase is an important negative regulator of dendritic cell function and is required to prevent B cell autoimmunity in vivo. It is not currently known however, if any causal relationship exists between these two aspects of MerTK function. We sought to determine if dendritic cells (DC) from mice lacking MerTK (mertk(- / - ) mice) have characteristics that may aid in the development of B cell autoimmunity. Specifically, we found that mertk(- / - ) mice contain an elevated number of splenic DC, and this population contains an elevated proportion of cells secreting the critical B cell pro-survival factor, B cell activating factor (BAFF). Elevated numbers of BAFF-secreting cells were also detected among mertk(- / - ) bone marrow-derived dendritic cell (BMDC) populations. This was observed in both resting BMDC, and BMDC stimulated with lipopolysaccharide (LPS) or treated with exogenous apoptotic cells. We also found that DC in general have a pro-survival effect on resting B cells in co-culture. However, despite containing more BAFF-secreting cells, mertk(- / - ) BMDC were not superior to C57BL/6 or baff-deficient BMDC at promoting B cell survival. Furthermore, using decoy receptors, we show that DC may promote B cell survival and autoimmunity through a BAFF-and a proliferation-inducing ligand-independent mechanism.

Regulation of dendritic cells and macrophages by an anti-apoptotic cell natural antibody that suppresses TLR responses and inhibits inflammatory arthritis

Although natural Abs (NAbs) are present from birth, little is known about what drives their selection and whether they have housekeeping functions. The prototypic T15-NAb, first identified because of its protective role in infection, is representative of a special type of NAb response that specifically recognizes and forms complexes with apoptotic cells and which promotes cell-corpse engulfment by phagocytes. We now show that this T15-NAb IgM-mediated clearance process is dependent on the recruitment of C1q and mannose-binding lectin, which have known immune modulatory activities that also provide "eat me" signals for enhancing phagocytosis. Further investigation revealed that the addition of T15-NAb significantly suppressed in vitro LPS-induced TNF-alpha and IL-6 secretion by the macrophage-like cell line, RAW264.7, as well as TLR3-, TLR4-, TLR7-, and TLR9-induced maturation and secretion of a range of proinflammatory cytokines and chemokines by bone marrow-derived conventional dendritic cells. Significantly, high doses of this B-1 cell produced NAb also suppressed in vivo TLR-induced proinflammatory responses. Although infusions of apoptotic cells also suppressed such in vivo inflammatory responses and this effect was associated with the induction of high levels of IgM antiapoptotic cell Abs, apoptotic cell treatment was not effective at suppressing such TLR responses in B cell-deficient mice. Moreover, infusions of T15-NAb also efficiently inhibited both collagen-induced arthritis and anti-collagen II Ab-mediated arthritis. These studies identify and characterize a previously unknown regulatory circuit by which a NAb product of innate-like B cells aids homeostasis by control of fundamental inflammatory pathways.