Effect of cytofectins on the immune response of murine macrophages to mammalian DNA

Extracellular vesicles and lupus nephritis - New insights into pathophysiology and clinical implications

Lupus nephritis (LN) is a major cause for overall morbidity and mortality in patients with systemic lupus erythematosus (SLE), while its pathogenic mechanisms are still not well understood. Extracellular vesicles (EVs) are membrane vesicles that are released from almost all cell types. EVs can be subdivided into exosomes, microvesicles, and apoptotic bodies. Latest studies have shown that EVs can be released during several cellular events, including cell activation, autophagy, and several types of programed cell death, i.e. apoptosis, necroptosis, pyroptosis, and NETosis. Emerging evidence demonstrates that EVs harbor different bioactive molecules, including nucleic acids, proteins, lipids, cytokines, immune complexes (ICs), complements, and other molecules, some of which may contribute to pathogenesis of autoimmune diseases. EVs can serve as novel information shuttle to mediate local autocrine or paracrine signals to nearby cells, and distant endocrine signals to cells located far away. In LN, EVs may have pathogenic effects by transportation of autoantigens or complements, promotion of IC deposition or complement activation, and stimulation of inflammatory responses, renal tissue injury, or microthrombus formation. Additionally, EVs released from kidney cells may serve as specific biomarkers for diagnosis or monitoring of disease activity and therapeutic efficacy. In this review, we will summarize the latest progress about EV generation from basic research, their potential pathologic effects on LN, and their clinical implications. The cutting-edge knowledge about EV research provides insights into novel therapeutic strategy, new tools for diagnosis or prognosis, and evaluation approaches for treatment effectiveness in LN.

Toll-like receptor 2 promiscuity is responsible for the immunostimulatory activity of nucleic acid nanocarriers

Lipopolyamines (LPAs) are cationic lipids; they interact spontaneously with nucleic acids to form lipoplexes used for gene delivery. The main hurdle to using lipoplexes in gene therapy lies in their immunostimulatory properties, so far attributed to the nucleic acid cargo, while cationic lipids were considered as inert to the immune system. Here we demonstrate for the first time that di-C18 LPAs trigger pro-inflammatory responses through Toll-like receptor 2 (TLR2) activation, and this whether they are bound to nucleic acids or not. Molecular docking experiments suggest potential TLR2 binding modes reminiscent of bacterial lipopeptide sensing. The di-C18 LPAs share the ability of burying their lipid chains in the hydrophobic cavity of TLR2 and, in some cases, TLR1, at the vicinity of the dimerization interface; the cationic headgroups form multiple hydrogen bonds, thus crosslinking TLRs into functional complexes. Unravelling the molecular basis of TLR1 and TLR6-driven heterodimerization upon LPA binding underlines the highly collaborative and promiscuous ligand binding mechanism. The prevalence of non-specific main chain-mediated interactions demonstrates that potentially any saturated LPA currently used or proposed as transfection agent is likely to activate TLR2 during transfection. Hence our study emphasizes the urgent need to test the inflammatory properties of transfection agents and proposes the use of docking analysis as a preliminary screening tool for the synthesis of new non-immunostimulatory nanocarriers.

Microparticles as a source of extracellular DNA

Microparticles are small membrane-bound vesicles that display pro-inflammatory and pro-thrombotic activities important in the pathogenesis of a wide variety of diseases. These particles are released from activated and dying cells and incorporate nuclear and cytoplasmic molecules for extracellular export. Of these molecules, DNA is a central autoantigen in systemic lupus erythematosus (SLE). As studies in our laboratory show, DNA occurs prominently in microparticles, translocating into these structures during apoptotic cell death. This DNA is antigenically active and can bind to lupus anti-DNA autoantibodies. These findings suggest that microparticles are an important source of extracellular DNA to serve as an autoantigen and autoadjuvant in SLE.

Cytosolic DNA-activated human dendritic cells are potent activators of the adaptive immune response

Recent studies in cell lines and genetically engineered mice have demonstrated that cytosolic dsDNA could activate dendritic cells (DCs) to become effector APCs. Recognition of DNA might be a major factor in antimicrobial immune responses against cytosolic pathogens and also in human autoimmune diseases such as systemic lupus erythematosus. However, the role of cytosolic dsDNA in human DC activation and its effects on effector T and B cells are still elusive. In this study, we demonstrate that intracellular dsDNA is a potent activator of human monocyte-derived DCs as well as primary DCs. Activation by dsDNA depends on NF-κB activation, partially on the adaptor molecule IFN-promoter stimulator-1 and the novel cytosolic dsDNA receptor IFI16, but not on the previously recognized dsDNA sentinels absent in melanoma 2, DNA-dependent activator of IFN regulatory factor 3, RNA polymerase III, or high-mobility group boxes. More importantly, we report for the first time, to our knowledge, that human dsDNA-activated DCs, rather than LPS- or inflammatory cytokine mixture-activated DCs, represent the most potent inducers of naive CD4(+) T cells to promote Th1-type cytokine production and generate CD4(+) and CD8(+) cytotoxic T cells. dsDNA-DCs, but not LPS- or mixture-activated DCs, induce B cells to produce complement-fixing IgG1 and IgG3 Abs. We propose that cytosolic dsDNA represents a novel, more effective approach to generate DCs to enhance vaccine effectiveness in reprogramming the adaptive immune system to eradicate infectious agents, autoimmunity, allergy, and cancer.

Effect of double-stranded DNA on maturation of dendritic cells in vitro

A preparation of human genomic fragmented double-stranded DNA (dsDNA) was used as maturation stimulus in cultures of human dendritic cells (DCs) generated in compliance with the interferon protocol. Culturing of the DCs in medium with 5μg/ml of the DNA preparation was associated with a decrease in the relative proportion of CD14 + cells and an increase in that of CD83 + cells. These changes are markers of DC maturation. The efficiency with which the DNA preparation was able to elicit DC maturation was commensurate with that of lypopolysaccharide from bacterial cell, the standard inducer of DC maturation. Generated ex vivo, matured in the presence of the human DNA preparation, pulsed with tumor antigens mouse DCs were used as a vaccine in biological tests for its antitumor activity. The experimental results demonstrate that reinfusion of mature pulsed with tumor antigens DCs cause a statistically significant suppression of tumor graft growth.

Antibodies to DNA: infection or genetics?

Antibodies to DNA (anti-DNA) are the serological hallmark of systemic lupus erythematosus (SLE) and unique markers of the immunological disturbances critical to disease pathogenesis. In the form of immune complexes, anti-DNA autoantibodies can deposit in the tissue to incite inflammation and damage; in addition, these complexes can induce cytokine production, most prominently, type 1 interferon. Studies in both patients and animal models have implicated genetic as well as environmental factors in the aetiology of the anti-DNA response. Because bacterial DNA is a potent stimulant of innate immunity by both toll-like receptor (TLR) and non-TLR signalling pathways, foreign DNA introduced during the course of bacterial or viral infection could have a dual role in antibody induction. This DNA could serve as an adjuvant to activate innate immunity as well as an immunogen to drive an antigen-specific antibody response. In this scenario, the generation of cross-reactive autoantibodies, in contrast to highly specific antibodies to bacterial DNA, most likely depends on genetically determined abnormalities in the B-cell repertoire in patients with SLE. Given the universal expression of DNA, this model suggests that many different kinds of infections could trigger pathogenic autoantibody responses in SLE, as well as induce flare.

Immunotherapeutic applications of CpG oligodeoxynucleotide TLR9 agonists

Toll-like receptor 9 (TLR9) agonists have demonstrated substantial potential as vaccine adjuvants, and as mono- or combination therapies for the treatment of cancer and infectious and allergic diseases. Commonly referred to as CpG oligodeoxynucleotides (ODN), TLR9 agonists directly induce the activation and maturation of plasmacytoid dendritic cells and enhance differentiation of B cells into antibody-secreting plasma cells. Preclinical and early clinical data support the use of TLR9 agonists as vaccine adjuvants, where they can enhance both the humoral and cellular responses to diverse antigens. In mouse tumor models TLR9 agonists have shown activity not only as monotherapy, but also in combination with multiple other therapies including vaccines, antibodies, cellular therapies, other immunotherapies, antiangiogenic agents, radiotherapy, cryotherapy, and some chemotherapies. Phase I and II clinical trials have indicated that these agents have antitumor activity as single agents and enhance the development of antitumor T-cell responses when used as therapeutic vaccine adjuvants. CpG ODN have shown benefit in multiple rodent and primate models of asthma and other allergic diseases, with encouraging results in some early human clinical trials. Although their potential clinical contributions are enormous, the safety and efficacy of these TLR9 agonists in humans remain to be determined.

The induction of HMGB1 release from RAW 264.7 cells by transfected DNA

High mobility group protein 1 (HMGB1) is a non-histone nuclear protein that can activate innate immunity when in an extracellular location. As shown in in vitro studies, while polyinosinic-polycytidylic acid [poly (I:C)] and LPS, TLR3 and TLR4 ligands, respectively, can induce HMGB1 release from macrophages, CpG DNA, a TLR 9 ligand, does not. Since DNA displays distinct immunostimulatory activity when transfected into cells, we investigated whether transfected DNA can induce HMGB1 release from macrophages. In these experiments, using RAW 264.7 cells as model, we show that DNA, either natural DNA or synthetic oligonucleotides, can induce HMGB1 release when used to stimulate cells with the transfection reagent Lipofectamine 2000; release occurred irrespective of the intrinsic activity of the DNA. The induction of HMGB1 release by transfected DNA was dependent on IFN-beta as shown by the inhibitory effects of an antibody. In addition, JNK activation mediated HMGB1 release induced by a transfected phosphorothioate oligonucleotide but not by transfected natural DNA. Together, these findings indicate that transfected DNA can stimulate macrophages to release HMGB1 under conditions in which free DNA is inactive and suggest a role of DNA in inducing inflammation when bound to molecules that influence its entry into cells.

Apoptotic hepatocyte DNA inhibits hepatic stellate cell chemotaxis via toll-like receptor 9

Apoptosis of hepatocytes results in the development of liver fibrosis, but the molecular signals mediating this are poorly understood. Degradation and modification of nuclear DNA is a central feature of apoptosis, and DNA from apoptotic mammalian cells is known to activate immune cells via Toll-like receptor 9 (TLR9). We tested if DNA from apoptotic hepatocytes can induce hepatic stellate cell (HSC) differentiation. Our data show that apoptotic hepatocyte DNA and cytidine-phosphate-guanosine oligonucleotides induced up-regulation of transforming growth factor beta1 and collagen 1 messenger RNA both in the human HSC line LX-2 and in primary mouse HSCs. These effects were opposed by TLR9 antagonists. We have recently shown that adenosine inhibits HSC chemotaxis, and we now show that apoptotic hepatocyte DNA also inhibits platelet-derived growth factor (PDGF)-mediated HSC chemotaxis. Inhibition of HSC chemotaxis by PDGF was blocked by TLR9 antagonists, and was absent in primary HSCs from mice deficient in TLR9 or the TLR adaptor molecule MyD88. Stimulation of TLR9 on HSCs blocked signaling by the PDGF signaling molecule inositol 1,4,5-triphosphate and reduced PDGF-mediated increase in cytosolic Ca(2+).

CONCLUSION

DNA from apoptotic hepatocytes acts as an important mediator of HSC differentiation by (1) providing a stop signal to mobile HSCs when they have reached an area of apoptosing hepatocytes and (2) inducing a stationary phenotype-associated up-regulation of collagen production.

Elevated levels of endogenous apoptotic DNA and IFN-alpha in complement C4-deficient mice: implications for induction of systemic lupus erythematosus

Systemic lupus erythematosus (SLE), an autoimmune disease characterized by chronic nephritis, arthritis and dermatitis, and the presence of antinuclear autoantibodies, is associated with complement factor deficiencies in the classical activation pathway. In addition, IFN-alpha seems to be a key cytokine in SLE as an activated IFN-alpha system is regularly observed in patients with SLE. Here, we demonstrate that in lupus-susceptible, complement C4-deficient mice the lack of complement results in elevated intravascular levels of apoptotic DNA. The apoptotic DNA is targeted to the splenic marginal zone where it accumulates and induces IFN-alpha. As such, we present here a unifying hypothesis for the induction of SLE that incorporates the role of complement deficiency and elevated levels of IFN-alpha.

DNA and its cationic lipid complexes induce CpG motif-dependent activation of murine dendritic cells

Unmethylated CpG motifs in bacterial DNA, but not in vertebrate DNA, are known to trigger an inflammatory response of antigen-presenting cells (APC). In this study, we investigated the cytokine release from murine dendritic cells (DC) by the addition of various types of DNA in the free or complexed form with cationic lipids. Naked plasmid DNA and Escherichia coli DNA with immunostimulatory unmethylated CpG motifs induced pro-inflammatory cytokine secretion from granulocyte-macrophage colony-stimulating factor (GM-CSF)-cultured bone marrow-derived DC and the DC cell-line, DC2.4 cells, though vertebrate calf thymus DNA (CT DNA) with less CpG motifs did not. These characteristics differed from mouse peritoneal resident macrophages that do not respond to any naked DNA. The amount of cytokines released from the DC was significantly increased by complex formation with cationic lipids when CpG-motif positive DNAs were used. Unlike murine macrophages or Flt-3 L cultured DC, GM-CSF DC did not release inflammatory cytokines in response to the addition of CT DNA/cationic lipid complex, suggesting that the activation is completely dependent on CpG motifs. Taken together, the results of the present study demonstrate that murine DC produce pro-inflammatory cytokines upon stimulation with CpG-containing DNAs and the responses are enhanced by cationic lipids. These results also suggest that DC are the major cells that respond to naked CpG DNA in vivo, although both DC and macrophages will release inflammatory cytokines after the administration of a DNA/cationic lipid complex.

TLR9 in health and disease

Toll-like receptor 9 (TLR9) is specialized for the recognition of pathogenic nucleic acids. TLR9 is expressed in intracellular compartments where it responds specifically to pathogen DNA. Several factors contribute to the ability of TLR9 to discriminate between self and foreign DNA. Regulatory mechanisms of the innate and adaptive immune system exist that balance the immune responses mediated by TLR9. Short synthetic CpG oligodeoxynucleotides are used to induce controlled and directed TLR9-dependent stimulation and are effective immune modulators in preclinical and clinical studies. This review will summarize the interplay between TLR9-dependent opposing stimulatory and regulatory effects in innate and adaptive immunity.

Contribution of interferon-beta to the immune activation induced by double-stranded DNA

Introducing double-stranded DNA (dsDNA) into the cytoplasm of macrophages and dendritic cells triggers the activation of these professional antigen-presenting cells (APCs). This process is characterized by the up-regulation of costimulatory molecules and the production of various cytokines, chemokines, and antibacterial/viral factors. Current findings indicate that interferon-beta (IFN-beta) plays a key role in the stimulatory cascade triggered by dsDNA. Both immune and non-immune cells respond to intracytoplasmic dsDNA by up-regulating IFN-beta) expression, a process that reduces host susceptibility to infection. The immune activation induced by dsDNA is independent of MyD88, TRIF and DNA-PKcs, indicating that a Toll-like receptor-independent mechanism underlies the cellular activation mediated by intracytoplasmic dsDNA.

Progress in drug development of immunostimulatory CpG oligodeoxynucleotide ligands for TLR9

Oligodeoxynucleotides (ODNs) with unmethylated deoxycytosine-deoxyguanosine (CpG) motifs are recognised by Toll-like receptor (TLR)9 expressed in specialised cell subsets of the human immune system, B cells and plasmacytoid dendritic cells. TLR9-mediated stimulation of the immune system leads to a plethora of directed effects linking innate to adaptive immune responses. This allows the use of TLR9 agonists as highly effective targeted immunomodulatory drugs with broad potential applications as vaccine adjuvants, stand-alone therapy or in combination with other therapies in cancer, infectious diseases or asthma and allergy. TLR9 agonists represent a new class of small (8-30 bases long), easily synthesised, non-antisense ODN pharmaceuticals.

Lupus Nephritis

Lupus nephritis is one of the more serious manifestations of the systemic autoimmune disease, systemic lupus erythematosus, and is associated with considerable morbidity and even mortality. Treatment remains problematic, particularly in terms of controlling the underlying disease process while at the same time preventing unacceptable side effects of therapy. In recent years, clinical trials have started to define optimum regimens of the immunosuppressive agents presently in use. The etiology and pathogenesis of systemic lupus erythematosus and lupus nephritis still are understood incompletely. Nevertheless, insights gained from basic science research in both animals and human beings now are being translated into newer therapies that have the potential to be safer and more specific than those currently available.

Intracellular mammalian DNA stimulates myeloid dendritic cells to produce type I interferons predominantly through a toll-like receptor 9–independent pathway

OBJECTIVE

Exogenous nucleic acids, including bacterial unmethylated DNA and viral single-stranded RNA, are potent activators of innate immunity through interaction with the Toll-like receptors (TLRs). In contrast, mammalian DNA has been generally thought to have a limited activation effect, or even a suppressive effect, on innate immunity. Since DNA is a major component of dying cells and recent studies indicate that mammalian nucleic acids may be stimulatory under certain conditions, we undertook this study to examine the effect of intracellular mammalian DNA on myeloid dendritic cell (DC) activation.

METHODS

Mammalian DNA was introduced into murine bone marrow-derived DCs (BMDCs) by transfection. BMDC activation was determined by flow cytometry (CD40, CD86). Production of tumor necrosis factor alpha and interleukin-6 was measured by enzyme-linked immunosorbent assay, and production of type I interferons (IFNs) by bioassay. Parallel studies were conducted using BMDCs from mice deficient in myeloid differentiation 88 (MyD88), TLR-9, and IFNalpha/beta receptor.

RESULTS

Intracellular mammalian DNA activated immature BMDCs, as determined by the up-regulation of CD40 and CD86 as well as by the production of significant quantities of type I IFN. The interferogenic response was shown to be relatively independent of TLR-9, and the TLR adaptor MyD88. The IFN response to intracellular DNA was reduced in BMDCs lacking IFNalpha/beta receptor but was intact in embryonic fibroblasts lacking protein kinase R.

CONCLUSION

These results indicate that intracellular DNA stimulates BMDC maturation and IFN production predominantly through a TLR-independent pathway, and support a model whereby inefficient clearance and/or degradation of endogenous DNA may stimulate innate immune responses similar to the TLR-independent response to exogenous (i.e., viral) double-stranded RNA.

The binding of sera of patients with SLE to bacterial and mammalian DNA

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by antibodies to DNA (anti-DNA). Although these antibodies have features of antigen drive, the source of this DNA is not defined. To assess the potential role of foreign and self-DNA as driving antigens, the specificity of SLE sera for bacterial and mammalian DNA was evaluated. Micrococcus lysodeikticus (MC) and calf thymus (CT) DNA were tested as antigens, with absorption on CT DNA columns used to identify antibodies to antigenic sites on the two DNA. Among 9 sets of longitudinal sera tested, all showed binding to both DNA, and none showed exclusive or predominant binding to CT DNA. With absorbed sera, antibodies could be distinguished in terms of cross-reactive or selective binding to the DNAs. These findings suggest that anti-DNA antibodies vary in specificity and are consistent with a role of both foreign and self-DNA in anti-DNA induction.

Synthetic, self-assembly ABCD nanoparticles; a structural paradigm for viable synthetic non-viral vectors

Gene therapy research is still in trouble owing to a paucity of acceptable vector systems to deliver nucleic acids to patients for therapy. Viral vectors are efficient but may be too dangerous. Synthetic non-viral vectors are inherently safer but are currently not efficient enough to be clinically viable. The solution for gene therapy lies with improved synthetic non-viral vectors systems. This review is focused on synthetic cationic liposome/micelle-based non-viral vector systems and is a critical review written to illustrate the increasing importance of chemistry in gene therapy research. This review should be of primary interest to synthetic chemists and biomedical researchers keen to appreciate emerging technologies, but also to biological scientists who remain to be convinced about the relevance of chemistry to biology.

Toll-like receptors, endogenous ligands, and systemic autoimmune disease

The critical role of Toll-like receptors (TLRs) as mediators of pathogen recognition by the innate immune system is now firmly established. Such recognition results in the initiation of an inflammatory immune response and subsequent instruction of the adaptive immune system, both of which are designed to rid the host of the invading pathogen. More controversial is the potential role of TLRs in the recognition of endogenous ligands and what effect this might have on the consequent development of autoimmune or other chronic sterile inflammatory disorders. An increasing number of studies implicate TLRs as being involved in the immune response to self-molecules that have in some way been altered from their native state or accumulate in non-physiologic sites or amounts, although questions have been raised about possible contaminants in certain of these studies. In this review, we discuss the evidence for endogenous ligand-TLR interactions with particular emphasis on mammalian chromatin, systemic lupus erythematosus, and atherosclerosis. Overall, the data support the general concept of a role for TLRs in the recognition of endogenous ligands. However, the precise details of the interactions and the extent to which they may contribute to the pathogenesis of human disease remain to be clarified.

The stimulation of Toll-like receptors by nuclear antigens: a link between apoptosis and autoimmunity

As immunologists have long understood, effective responses to foreign antigens require adjuvants. It is now apparent that the initiation of autoimmune disease is comparably facilitated by adjuvant activity. In the case of antinuclear antibodies, it seems that DNA itself can serve as an endogenous adjuvant. Similar to many of the microbial adjuvants, mammalian DNA mediates its effect through a Toll-like receptor--in this case, TLR9.

Toll-like receptor 9-dependent and -independent dendritic cell activation by chromatin-immunoglobulin G complexes

Dendritic cell (DC) activation by nucleic acid-containing immunoglobulin (Ig)G complexes has been implicated in systemic lupus erythematosus (SLE) pathogenesis. However, the mechanisms responsible for activation and subsequent disease induction are not completely understood. Here we show that murine DCs are much more effectively activated by immune complexes that contain IgG bound to chromatin than by immune complexes that contain foreign protein. Activation by these chromatin immune complexes occurs by two distinct pathways. One pathway involves dual engagement of the Fc receptor FcgammaRIII and Toll-like receptor (TLR)9, whereas the other is TLR9 independent. Furthermore, there is a characteristic cytokine profile elicited by the chromatin immune complexes that distinguishes this response from that of conventional TLR ligands, notably the induction of BAFF and the lack of induction of interleukin 12. The data establish a critical role for self-antigen in DC activation and explain how the innate immune system might drive the adaptive immune response in SLE.

Mechanisms of activation of the RAW264.7 macrophage cell line by transfected mammalian DNA

Bacterial DNA can stimulate the production of cytokines and nitric oxide (NO), while mammalian DNA can block these responses. If mammalian DNA is transfected into macrophages, however, it can stimulate NO production, without inducing IL-12. To define further this activity, signaling pathways induced by transfected calf thymus (CT) DNA were studied. Using RAW264.7 cells as a model, CT DNA in the transfection agent FuGENE 6 activated cells through the NF-kappaB and MAPKs pathways, similar to bacterial DNA and LPS. The role of these pathways was further investigated using specific inhibitors, with studies indicating that NO production is blocked by inhibitors of NF-kappaB and p38 but not other MAPKs. These data indicate that the immune activity of DNA is influenced by context or intracellular location and that, when transfected into cells, mammalian DNA can activate cells through signaling pathways similar to those of bacterial DNA.

Activation of Autoreactive B Cells by CpG dsDNA

The proliferative response of autoreactive rheumatoid factor (RF) B cells to mammalian chromatin-containing immune complexes (ICs) results from the sequential engagement of the B cell receptor (BCR) and Toll-like receptor 9 (TLR9). We have used ICs constructed from anti-hapten antibodies and defined haptenated dsDNA fragments to determine the form of mammalian DNA that mediates this process. Despite their relatively low abundance in mammalian DNA, we found that inclusion of hypomethylated CpG motifs in these ICs was necessary for effective activation. In the absence of antibody, the same fragments could efficiently stimulate low-affinity hapten-specific and DNA-reactive 3H9 B cells, but not RF B cells. These results extend the BCR/TLR9 coengagement paradigm to a second major class of autoreactive B cells, further confirm the critical role of the BCR in chromatin ligand delivery to TLR9, and implicate hypomethylated CpG motifs as ligand elements necessary for the initiation of systemic autoimmune disease.