Apoptosis and immune responses to self

Circular and Circulating DNA in Inflammatory Bowel Disease: From Pathogenesis to Potential Molecular Therapies

Inflammatory bowel diseases (IBD), including Crohn's Disease (CD) and Ulcerative Colitis (UC) are chronic multifactorial disorders which affect the gastrointestinal tract with variable extent. Despite extensive research, their etiology and exact pathogenesis are still unknown. Cell-free DNAs (cfDNAs) are defined as any DNA fragments which are free from the origin cell and able to circulate into the bloodstream with or without microvescicles. CfDNAs are now being increasingly studied in different human diseases, like cancer or inflammatory diseases. However, to date it is unclear how IBD etiology is linked to cfDNAs in plasma. Extrachromosomal circular DNA (eccDNA) are non-plasmidic, nuclear, circular and closed DNA molecules found in all eukaryotes tested. CfDNAs appear to play an important role in autoimmune diseases, inflammatory processes, and cancer; recently, interest has also grown in IBD, and their role in the pathogenesis of IBD has been suggested. We now suggest that eccDNAs also play a role in IBD. In this review, we have comprehensively collected available knowledge in literature regarding cfDNA, eccDNA, and structures involving them such as neutrophil extracellular traps and exosomes, and their role in IBD. Finally, we focused on old and novel potential molecular therapies and drug delivery systems, such as nanoparticles, for IBD treatment.

B-1 cells in immunotoxicology: Mechanisms underlying their response to chemicals and particles

Since their discovery nearly 40 years ago, B-1 cells have continued to challenge the boundaries between innate and adaptive immunity, as well as myeloid and lymphoid functions. This B-cell subset ensures early immunity in neonates before the development of conventional B (B-2) cells and respond to immune injuries throughout life. B-1 cells are multifaceted and serve as natural- and induced-antibody-producing cells, phagocytic cells, antigen-presenting cells, and anti-/pro-inflammatory cytokine-releasing cells. This review retraces the origin of B-1 cells and their different roles in homeostatic and infectious conditions before focusing on pollutants comprising contact-sensitivity-inducing chemicals, endocrine disruptors, aryl hydrocarbon receptor (AHR) ligands, and reactive particles.

Immunogenic cell death and its therapeutic or prognostic potential in high-grade glioma

Immunogenic cell death (ICD) has emerged as a key component of therapy-induced anti-tumor immunity. Over the past few years, ICD was found to play a pivotal role in a wide variety of novel and existing treatment modalities. The clinical application of these techniques in cancer treatment is still in its infancy. Glioblastoma (GBM) is the most lethal primary brain tumor with a dismal prognosis despite maximal therapy. The development of new therapies in this aggressive type of tumors remains highly challenging partially due to the cold tumor immune environment. GBM could therefore benefit from ICD-based therapies stimulating the anti-tumor immune response. In what follows, we will describe the mechanisms behind ICD and the ICD-based (pre)clinical advances in anticancer therapies focusing on GBM.

Microvesicles in Autoimmune Diseases

During apoptosis or activation, cells can release a subcellular structure, called a membrane microvesicle (also known as microparticle) into the extracellular environment. Microvesicles bud-off as a portion of cell membrane with its associated proteins and lipids surrounding a cytosolic core that contains intracellular proteins, lipids, and nucleic acids (DNA, RNA, siRNA, microRNA, lncRNA). Biologically active molecules on the microvesicle surface and encapsulated within can act on recipient cells as a novel mode of intercellular communication. Apoptosis has long been known to be involved in the development of diseases of autoimmunity. Abnormally persistent microvesicles, particularly apoptotic microvesicles, can accelerate autoimmune responses locally in specific organs and tissues as well as systemically. In this review, we focus on studies implicating microvesicles in the pathogenesis of autoimmune diseases and their complications.

Divergent members of a single autoreactive B cell clone retain specificity for apoptotic blebs

Specificity for double-stranded DNA can arise due to somatic mutations within one of the branches of an autoreactive B cell clone. However, it is not known whether a different autospecificity predates anti-dsDNA and whether separate offshoots of an expanding B cell clone retain or evolve alternative specificities. We compared 3H9, an anti-dsDNA IgG, to 4H8 and 1A11, antibodies produced by hybridomas representing an alternative branch of the 3H9 B cell clone. All three IgG bound chromatin in ELISA and apoptotic cells in confocal microscopy, yet only 3H9 bound dsDNA, as measured by plasmon resonance. Moreover, we demonstrate that despite the unique specificity of 3H9 for dsDNA, all three clone members exhibited indistinguishable binding to chromatin. The binding to chromatin and apoptotic cells was unaffected by N-linked glycosylation in L chain CDR1, a modification that results from a replacement of serine 26 with asparagine in 4H8 and 1A11. These data provide the first evidence that specificity for nucleosome epitopes on apoptotic cells provides the initial positive stimulus for somatic variants that comprise a B cell clone, including those that subsequently acquire specificity for dsDNA. Conversely, selection of autoreactive B cells for binding to apoptotic cells leads to clonal expansion, antibody diversification, and the development of linked sets of anti-nuclear autoantibodies.

Heterogeneous nuclear ribonucleoprotein P2 is an autoantibody target in mice deficient for Mer, Axl, and Tyro3 receptor tyrosine kinases

Deficiencies in clearance of apoptotic cells predispose to the development of autoimmune disease. This is evident in mice lacking the receptor tyrosine kinases Tyro3, Axl, and Mer. Deficient mice exhibit an increased abundance of apoptotic cells in tissues and manifest diverse autoimmune conditions. To test these mice for the presence of autoantibodies to apoptotic cells, we generated spontaneous splenic B cell hybridomas and used a novel microscopy screen to detect Ab binding to apoptotic Jurkat cells. From hybridomas secreting IgG Abs reactive with apoptotic cells, we selected one that recreated the major serum specificity for apoptotic cells. The Ab LHC7.15 bound to an Ag that is differentially distributed between the nucleus and the cytoplasm in live and apoptotic cells. In late apoptotic cells, the Ag coalesces into aggregates that bleb from the cell surface. Immunopurification of the Ag, followed by mass spectrometry, identified a protein of 69 kDa whose partial sequence matched heterogeneous nuclear ribonucleoprotein P2. This multifunctional protein binds DNA, RNA, and several known ribonucleoprotein autoantigens. Our observations indicate that a ribonucleoprotein complex, formed and translocated to the cell surface in apoptosis, represents a potent stimulus for breaking tolerance and inducing systemic autoimmunity in mice with defective clearance of cell remnants.

Chaperone proteins and brain tumors: potential targets and possible therapeutics

Chaperone proteins are most notable for the proteo- and cyotoprotective capacities they afford during cellular stress. Under conditions of cellular normalcy, chaperones still play integral roles in the folding of nascent polypeptides into functional entities, in assisting in intracellular/intraorganellar transport, in assembly and maintenance of multi-subunit protein complexes, and in aiding and abetting the degradation of senescent proteins. Tumors frequently have relatively enhanced needs for chaperone number and activity because of the stresses of rapid proliferation, increased metabolism, and overall genetic instability. Thus, it may be possible to take advantage of this reliance that tumor cells have on chaperones by pharmacologic and biologic means. Certain chaperones are abundant in the brain, which implies important roles for them. While it is presumed that the requirements of brain tumors for chaperone proteins are similar to those of any other cell type, tumor or otherwise, very little inquiry has been directed at the possibility of using chaperone proteins as therapeutic targets or even as therapeutic agents against central nervous system malignancies. This review highlights some of the research on the functions of chaperone proteins, on what can be done to modify those functions, and on the physiological responses that tumors and organisms can have to chaperone-targeted or chaperone-based therapies. In particular, this review will also underscore areas of research where brain tumors have been part of the field, although in general those instances are few and far between. This relative dearth of research devoted to chaperone protein targets and therapeutics in brain tumors reveals much untrodden turf to explore for potential treatments of these dreadfully refractive diseases.

Transglutaminase type II is a key element in the regulation of the anti-inflammatory response elicited by apoptotic cell engulfment

A key feature of the macrophage-dependent clearance of apoptotic cells is the down-regulation of proinflammatory cytokines. Deficiency in the phagocytosis of apoptotic cells is often associated with the development of inflammatory reactions, resulting in chronic inflammatory and autoimmune diseases. The molecular mechanisms that regulate the engulfment process and particularly the immunomodulatory factors involved are still largely unknown in mammals. We have previously reported that the ablation of transglutaminase type II (TG2) in mice results in the defective clearance of apoptotic cells associated with the development of splenomegaly, autoantibodies, and glomerulonephritis. In this study we have investigated the mechanisms at the basis of the development of inflammation/autoimmunity associated with the defective clearance of apoptotic cells characterizing TG2 knockout mice. To this aim we compared the macrophage response to apoptotic cell exposure in wild-type vs TG2-null mice. We demonstrated that the lack of TG2 results in an impaired capacity of macrophages to engulf, but not to bind, apoptotic cells, which is paralleled by an abnormal inflammatory response both in vivo and in vitro. We have identified a differential response in the release of several cytokines in TG2(-/-) vs wild-type mice. Particularly relevant is the finding that both TGF-beta and IL-12 regulations were significantly altered in the absence of TG2. These results help explain the autoimmune phenotype developed by these mice and suggest that TG2 is a key regulatory element of the anti-inflammatory features of apoptosis.

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.

Defective ATM-p53-mediated apoptotic pathway in multiple sclerosis

Defective elimination of autoreactive cells is thought to play a role in the development of autoimmune diseases including multiple sclerosis (MS). We examined the activation of the ATM-CHK2-p53 pathway in MS patients after subjecting their peripheral blood mononuclear cells to gamma-irradiation. We found that peripheral blood mononuclear cells from a subset of MS patients show resistance to cell death induced by irradiation. This defect is due to impaired constitutive expression and activation of ATM (ataxia telangiectasia mutated), resulting in impaired stabilization of p53. We predict that these fundamental defects likely alter the regulation of the immune population of cells in MS and may contribute to the development or progression of the disease.

Involvement of lysosomal cathepsins in the cleavage of DNA topoisomerase I during necrotic cell death

OBJECTIVE

Autoantibodies to DNA topoisomerase I (topo I) are associated with diffuse systemic sclerosis (SSc), appear to be antigen driven, and may be triggered by cryptic epitopes exposed during in vivo topo I fragmentation. These autoantibodies recognize topo I and fragments of this autoantigen generated during apoptosis and necrosis. We undertook this study to determine whether lysosomal cathepsins are involved in topo I fragmentation during necrosis.

METHODS

Topo I cleavage during necrosis was assessed by immunoblotting of lysates from L929 fibroblasts exposed to tumor necrosis factor alpha (TNFalpha) and the broad caspase inhibitor Z-VAD-FMK, and by immunoblotting of lysates from endothelial cells treated with HgCl2. Purified topo I and L929 nuclei were incubated with cathepsins B, D, G, H, and L, and topo I cleavage was detected by immunoblotting. The intracellular localization of cathepsin L activity and topo I in necrotic cells was examined using fluorescence microscopy.

RESULTS

Treatment of L929 cells with TNFalpha and Z-VAD-FMK induced caspase-independent cell death with necrotic morphology. This cell death involved topo I cleavage into fragments of approximately 70 kd and 45 kd. This cleavage profile was reproduced in vitro by cathepsins L and H and was inhibited by the cathepsin L inhibitor Z-FY-CHO. During necrosis, cathepsin L activity diffused from lysosomes into the cytoplasm and nucleus, whereas topo I partially relocalized to the cytoplasm. Z-FY-CHO delayed necrosis and partially blocked topo I cleavage. The topo I cleavage fragments were also detected in necrotic endothelial cells and recognized by SSc sera containing anti-topo I antibodies.

CONCLUSION

These results implicate cathepsins, particularly cathepsin L, in the cleavage of topo I during necrosis. This cleavage may generate potentially immunogenic fragments that could trigger anti-topo I immune responses in SSc.