Apoptotic cell death and lupus

Experimental Analysis of Apoptotic Thymocyte Engulfment by Macrophages

Cell apoptosis is a natural process and plays a critical role in embryonic development, homeostatic regulation, immune tolerance induction, and resolution of inflammation. Accumulation of apoptotic debris in the body may trigger chronic inflammatory responses that lead to systemic autoimmune diseases over time. Impaired apoptotic cell clearance has been implicated in a variety of autoimmune diseases. Apoptotic clearance is a complex process rarely detected under physiological conditions. It involves abundant surface receptors and signaling molecules. Studying the process of apoptotic cell clearance provides insightful molecular mechanisms and subsequent biological responses, which may lead to the development of new therapeutics. Here, we describe protocols for the induction of apoptotic thymocytes, the preparation of peritoneal macrophages, and the analysis of apoptotic cell clearance by flow cytometry and microscopy. All cells will undergo apoptosis at a certain stage, and many residential and circulating cells can uptake apoptotic debris. Therefore, the protocol described here can be used in many applications to characterize apoptotic cell binding and ingestion by many other cell types.

Early endosome autoantigen 1 regulates IL-1β release upon caspase-1 activation independently of gasdermin D membrane permeabilization

Unconventional protein secretion represents an important process of the inflammatory response. The release of the pro-inflammatory cytokine interleukin (IL)-1β which burst during pyroptosis as a consequence of gasdermin D plasma membrane pore formation, can also occur through other unconventional secretion pathways dependent on caspase-1 activation. However, how caspase-1 mediates cytokine release independently of gasdermin D remains poorly understood. Here we show that following caspase-1 activation by different inflammasomes, caspase-1 cleaves early endosome autoantigen 1 (EEA1) protein at Asp^127/132. Caspase-1 activation also results in the release of the endosomal EEA1 protein in a gasdermin D-independent manner. EEA1 knock-down results in adecreased release of caspase-1 and IL-1β, but the pyroptotic release of other inflammasome components and lactate dehydrogenase was not affected. This study shows how caspase-1 control the release of EEA1 and IL-1β in a pyroptotic-independent manner.

Retracted: Long noncoding RNA THRIL contributes in lipopolysaccharide-induced HK-2 cells injury by sponging miR-34a

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease with unknown etiology. Nowadays, several long noncoding RNAs (lncRNAs) have been reported as molecular alterations involved in SLE. This study aimed to reveal the function of TNF-related and HNRNPL-related immunoregulatory lncRNA (THRIL) in SLE. Human epithelial HK-2 cells were exposed to lipopolysaccharide (LPS) to mimic an in vitro SLE model. Then, the functions of THRIL, miR-34a, and monocyte chemoattractant protein-1 (MCP-1), as well as their correlations were detected. LncRNA THRIL was highly expressed in the LPS-stimulated cells, and THRIL overexpression aggravated LPS-induced cell damage as cell viability was decreased, and apoptosis and the release of proinflammatory cytokines were increased. THRIL worked as a sponge of microRNA-34a (miR-34a) and it could directly target MCP-1. Furthermore, MCP-1-activated JNK and Wnt/β-catenin signaling pathways. In conclusion, this study suggested that lncRNA THRIL might be a key regulator participating in LPS-induced injury in HK-2 cells. THRIL overexpression aggravated LPS-induced injury possibly via sponging miR-34a, and thus preventing MCP-1 from degradation by miR-34a. The THRIL/miR-34a/MCP-1 axis might play critical roles in SLE.

Toxicological Implications and Inflammatory Response in Human Lymphocytes Challenged with Oxytetracycline

Antibiotics are widely used in zoo technical and veterinary practices as feed supplementation to ensure wellness of farmed animals and livestock. Several evidences have been suggesting both the toxic role for tetracyclines, particularly for oxytetracycline (OTC). This potential toxicity appears of great relevance for human nutrition and for domestic animals. This study aimed to extend the evaluation of such toxicity. The biologic impact of the drug was assessed by evaluating the proinflammatory effect of OTC and their bone residues on cytokine secretion by in vitro human peripheral blood lymphocytes. Our results showed that both OTC and OTC‐bone residues significantly induced the T lymphocyte and non‐T cell secretion of interferon (IFN)‐γ, as cytokine involved in inflammatory responses in humans as well as in animals. These results may suggest a possible implication for new potential human and animal health risks depending on the entry of tetracyclines in the food‐processing chain.

Increased serum IL-10 in lupus patients promotes apoptosis of T cell subsets via the caspase 8 pathway initiated by Fas signaling

We sought to investigate the expression of Fas and FasL on T cell surface and caspase 8 involvement in T cell apoptosis promoted by serum IL-10 in systemic lupus erythematosus (SLE) patients. Cells and sera were obtained from 35 SLE patients. Apoptosis of T cells in patients with SLE was increased and associated with the SLE disease activity index (SLEDAI). Elevated expression of Fas and FasL on T cell surface contributed to increased apoptosis of T cells. Increased IL-10 in the sera of SLE patients was capable of inducing Fas and FasL expression on CD4⁺T cell surface, promoting apoptosis of this cell subset. Decreased IL-10 serum levels and low expression of Fas were found in 5 patients of the first follow-up group after 2-month treatment. In another group with one-year treatment, the SLEDAI declined to inactive scores. Serum IL-10 was decreased significantly, and expression of Fas and FasL on T cells was also reduced. Declined apoptosis was predominant only in CD4⁺T cell subset. When sera with high level of IL-10 were used to culture PBMCs from healthy controls, activated caspase 8 was elevated in CD3⁺T, CD4⁺T and CD8⁺T cells. The study showed that serum IL-10 induced apoptosis of T cell subsets via the caspase 8 pathway initiated by Fas signaling. Increased apoptosis of T cells contributes to autoantigen burden, which is pathogenic in the development of SLE.

Induction of PP2A Bβ, a regulator of IL-2 deprivation-induced T-cell apoptosis, is deficient in systemic lupus erythematosus

The activity and substrate specificity of the ubiquitously expressed phosphatase PP2A is determined by the type of regulatory (B) subunit that couples to the catalytic/scaffold core of the enzyme. We determined that the Bβ subunit (PPP2R2B) is expressed in resting T cells, its transcription is down-regulated during T-cell activation, and up-regulated in conditions of low IL-2. Specifically, high levels of PP2A Bβ were produced during IL-2 deprivation-induced apoptosis, whereas Fas ligation had no effect. Forced expression of the Bβ subunit in primary human T cells was sufficient to induce apoptosis, whereas silencing using siRNA protected activated T cells from IL-2 withdrawal-induced cell death. Because T-cell apoptosis is known to be altered in T cells from patients with systemic lupus erythematosus, we analyzed the regulation of PP2A Bβ in this autoimmune disease. We found that levels of PP2A Bβ did not increase upon IL-2 deprivation in 50% of the patients. Remarkably, this defect was accompanied by resistance to apoptosis. Importantly, kinetics of cell death were normal in cells of patients that up-regulated PP2A Bβ in a normal manner. We have identified a unique role for the phosphatase PP2A, particularly the holoenzyme formed by PP2A Bβ. Bβ appears to trigger apoptosis of T cells in the absence of IL-2 and probably contributes to the termination of a no-longer-needed immune response. We propose that defective production of PP2A Bβ upon IL-2 deprivation results in apoptosis resistance and longer survival of autoreactive T cells, in a subset of SLE patients.

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.

Disrupted Mer receptor tyrosine kinase expression leads to enhanced MZ B-cell responses

Control of lymphocyte homeostasis is essential to ensure efficient immune responses and to prevent autoimmunity. Splenic marginal zone B cells are important producers of autoantibodies, and are subject to stringent tolerance mechanisms to prevent autoimmunity. In this paper, we explore the role of the Mer tyrosine kinase (Mertk) in regulating autoreactive B cells. This receptor tyrosine kinase serves to bind apoptotic cells, to mediate their phagocytosis, and to regulate subsequent cytokine production. Mice lacking Mertk suffer from impaired apoptotic cell clearance and develop a lupus-like autoimmune syndrome. Here we show that such Mertk-KO mice have expanded numbers of splenic marginal zone B cells. Mertk-KO mice bearing a DNA-specific immunoglobulin heavy-chain transgene (3H9) produced anti-DNA antibodies that appeared to be secreted largely by marginal zone B cells. Finally, Mertk-KO mice developed greater antibody responses after NP-Ficoll immunization than their B6 counterparts. Taken together, our data show that Mertk has a major effect on the development of the marginal zone B-cell compartment. Mertk is also important in establishing DNA-specific B-cell tolerance in 3H9 anti-DNA transgenic mice.

Proteomic analysis for the anti-apoptotic effects of cystamine on apoptosis-prone macrophage

Increased macrophage vulnerability is associated with progression of systemic lupus erythematosus. Our previous studies have shown that cystamine, an inhibitor of transglutaminase 2 (TG2), alleviated the apoptosis of hepatocyte and brain cell in lupus-prone mice NZB/W-F1. In present study, we further investigated the effects of cystamine on apoptosis-prone macrophages (APMs) in the lupus mice. Using two-dimensional gel electrophoresis (2-DE) analysis, we found that cystamine induced a differential protein expression pattern of APM as comparing to the PBS control. The protein spots presenting differential level between cystamine and PBS treatment were then identified by peptide-mass fingerprinting (PMF). After bioinformatic analysis, these identified proteins were found involved in mitochondrial apoptotic pathway, oxidative stress, and mitogen-activated protein (MAP) kinase-mediated pathway. Further investigation revealed that cystamine significantly decreased the levels of apoptotic Bax and Apaf-1 and the activity of caspase-3, and increased the levels of anti-apoptotic Bcl-2 in APM. We also found that these apoptotic mediators were up-regulated in a correlation with the progression of lupus severity in NZB/W-F1, which were little affected in BALB/c mice. We also found that the reduced serum glutathione was restored by cystamine in NZB/W-F1. Interestingly, the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in APM and the phagocytic ability was diminished in presence of cystamine. In conclusion, our findings indicate that cystamine significantly inhibited mitochondrial pathway, induced antioxidant proteins, and diminished phosphorylation of extracellular ERK1/2, which may alleviate the apoptosis and the phagocytic ability of APM.

The Bcl-2 family antagonist ABT-737 significantly inhibits multiple animal models of autoimmunity

The Bcl-2 family of proteins plays a critical role in controlling immune responses by regulating the expansion and contraction of activated lymphocyte clones by apoptosis. ABT-737, which was originally developed for oncology, is a potent inhibitor of Bcl-2, Bcl-x(L), and Bcl-w protein function. There is evidence that Bcl-2-associated dysregulation of lymphocyte apoptosis may contribute to the pathogenesis of autoimmunity and lead to the development of autoimmune diseases. In this study, we report that ABT-737 treatment resulted in potent inhibition of lymphocyte proliferation as measured by in vitro mitogenic or ex vivo Ag-specific stimulation. More importantly, ABT-737 significantly reduced disease severity in tissue-specific and systemic animal models of autoimmunity. Bcl-2 family antagonism by ABT-737 was efficacious in treating animal models of arthritis and lupus. Our results suggest that treatment with a Bcl-2 family antagonist represents a novel and potentially attractive therapeutic approach for the clinical treatment of autoimmunity.

Upregulated BclG(L) expression enhances apoptosis of peripheral blood CD4+ T lymphocytes in patients with systemic lupus erythematosus

Increased lymphocyte apoptosis has been suggested to contribute to the development of systemic lupus erythematosus (SLE), but the critical factors involved in the apoptotic pathways are still unknown. By long serial analysis of gene expression (LongSAGE) profiles and microarray analyses, a novel apoptosis-related gene BclG(L) expression was found significantly increased in peripheral blood CD4+ T cells of SLE patients, which was correlated with the enhanced CD4+ T cells apoptosis, anti-nuclear antibody (ANA) titer and proteinuria. In vitro, BclG(L) expression could be specially upregulated by SLE serum stimulation and positively correlated with induced CD4+ T cell apoptosis. Enforcing BclG(L) overexpression by lentivirus could directly enhance CD4+ T cell apoptosis, but these apoptosis-inducing effects could be partially inhibited by knockdown of BclG(L) expression. Collectively, these results indicate that increased BclG(L) expression may contribute to the aberrant CD4+ T cell apoptosis which causes an inappropriate immune response and impaired homeostasis in SLE.

The genetics of SLE: an update in the light of genome-wide association studies

Understanding the pathogenesis of SLE remains a considerable challenge. Multiple abnormalities of both the innate and adaptive immune system have been described and, furthermore, immunological dysfunction precedes clinical presentation by many years. There is a strong genetic basis to SLE, which means that genetic studies can play a key role in furthering our understanding of this disease. Since susceptibility variants are present from birth and are unaffected by the course of the disease, or by its treatment, genetic analysis is, perhaps uniquely, capable of identifying fundamental, causative, disease mechanisms. Over the last 12 months, there has been a staggering increase in our understanding of SLE genetics. We have seen the identification of new and important SLE susceptibility genes through candidate gene studies, and we have seen the publication of two whole-genome association analyses. The 'hypothesis free' whole-genome studies have provided additional evidence in support of a number of existing susceptibility genes and have identified novel gene candidates. In this article, we review the current SLE genetics literature in the light of these recent advances and we discuss our current understanding of the functional role of the key susceptibility genes. By considering how these genes fall into clusters with shared function we can begin to understand how dysregulation at a number of key immunological steps may predispose to the development of SLE.

Sequential shrinkage and swelling underlie P2X7-stimulated lymphocyte phosphatidylserine exposure and death

Patterns of change in cell volume and plasma membrane phospholipid distribution during cell death are regarded as diagnostic means of distinguishing apoptosis from necrosis, the former being associated with cell shrinkage and early phosphatidylserine (PS) exposure, whereas necrosis is associated with cell swelling and consequent lysis. We demonstrate that cell volume regulation during lymphocyte death stimulated via the purinergic receptor P2X7 is distinct from both. Within seconds of stimulation, murine lymphocytes undergo rapid shrinkage concomitant with, but also required for, PS exposure. However, within 2 min shrinkage is reversed and swelling ensues ending in cell rupture. P2X7-induced shrinkage and PS translocation depend upon K+ efflux via KCa3.1, but use a pathway of Cl- efflux distinct from that previously implicated in apoptosis. Thus, P2X7 stimulation activates a novel pathway of cell death that does not conform to those conventionally associated with apoptosis and necrosis. The mixed apoptotic/necrotic phenotype of P2X7-stimulated cells is consistent with a potential role for this death pathway in lupus disease.

Heterozygous mutations in TREX1 cause familial chilblain lupus and dominant Aicardi-Goutieres syndrome

TREX1 constitutes the major 3'-->5' DNA exonuclease activity measured in mammalian cells. Recently, biallelic mutations in TREX1 have been shown to cause Aicardi-Goutieres syndrome at the AGS1 locus. Interestingly, Aicardi-Goutieres syndrome shows overlap with systemic lupus erythematosus at both clinical and pathological levels. Here, we report a heterozygous TREX1 mutation causing familial chilblain lupus. Additionally, we describe a de novo heterozygous mutation, affecting a critical catalytic residue in TREX1, that results in typical Aicardi-Goutieres syndrome.