Generation of novel covalent RNA-protein complexes in cells by ultraviolet B irradiation: implications for autoimmunity

Systemic Lupus Erythematosus Risk: The Role of Environmental Factors

Systemic lupus erythematosus (SLE) is a complex, chronic autoimmune disease. The etiology of SLE is multifactorial and includes potential environmental triggers, which may occur sequentially (the "multi-hit" hypothesis). This review focuses on SLE risk potentially associated with environmental factors including infections, the microbiome, diet, respirable exposures (eg, crystalline silica, smoking, air pollution), organic pollutants, heavy metals, and ultraviolet radiation.

Benzophenone-3 and antinuclear antibodies in U.S. adolescents and adults ages 12-39 years

Background

Between 1988 and 2012, prevalence of antinuclear antibodies (ANA) increased in the U.S., especially in adolescents and non-Hispanic Whites. Female predominance of ANA suggests a role for hormonal factors, including xenobiotic exposures that may disrupt endocrine signaling. Benzophenone-3 (BP-3) is one such chemical with increasing exposure through sunscreen use. We investigated whether urinary BP-3 levels were related to ANA in adolescents and young adults.

Methods

In a sample of 1,785 individuals ages 12-39 years in the National Health and Nutrition Examination Survey (NHANES; 2003-4, 2011-12), we examined cross-sectional associations of ANA (N=192; 3+ or 4+ at the 1:80 dilution, measured by HEp-2 immunofluorescence) with urinary BP-3, and other phenols bisphenol-A, triclosan, and parabens. Adjusted prevalence odds ratios (POR) were calculated in season-stratified models [winter (November-April) and summer (May-October)], given differences in sunscreen use and BP-3 concentrations.

Results

BP-3 concentrations (detected in >98.5% of individuals) did not differ by ANA positivity in the summer (geometric mean, GM 30.6 ng/ml ANA-positive vs. 35.3 ANA-negative; GM ratio 1.15), but in winter were higher among ANA-positives (50.2 vs. 20.1 ANA-negative; GM ratio 2.50). ANA was associated with log₁₀BP-3 in winter (POR 1.57; 95%CI 1.07-2.30 per unit increase) but not summer (0.94; 0.61, 1.44; interaction p=0.09). Triclosan, parabens, and bisphenol-A levels were unrelated to ANA overall or by season (ORs 0.64 to 1.33).

Conclusions

The association of urinary BP-3 with ANA in the winter may reflect different exposure patterns or unmeasured confounders. Findings warrant replication in prospective studies and including past and year-round exposures.

The role of environmental exposures and gene-environment interactions in the etiology of systemic lupus erythematous

Systemic lupus erythematosus (SLE) is a complex, chronic autoimmune disease, whose etiology includes both genetic and environmental factors. Individual genetic risk factors likely only account for about one-third of observed heritability among individuals with a family history of SLE. A large portion of the remaining risk may be attributable to environmental exposures and gene-environment interactions. This review focuses on SLE risk associated with environmental factors, ranging from chemical and physical environmental exposures to lifestyle behaviors, with the weight of evidence supporting positive associations between SLE and occupational exposure to crystalline silica, current smoking, and exogenous estrogens (e.g., oral contraceptives and postmenopausal hormones). Other risk factors may include lifestyle behaviors (e.g., dietary intake and sleep) and other exposures (e.g., ultraviolet [UV] radiation, air pollution, solvents, pesticides, vaccines and medications, and infections). Alcohol use may be associated with decreased SLE risk. We also describe the more limited body of knowledge on gene-environment interactions and SLE risk, including IL-10, ESR1, IL-33, ITGAM, and NAT2 and observed interactions with smoking, UV exposure, and alcohol. Understanding genetic and environmental risk factors for SLE, and how they may interact, can help to elucidate SLE pathogenesis and its clinical heterogeneity. Ultimately, this knowledge may facilitate the development of preventive interventions that address modifiable risk factors in susceptible individuals and vulnerable populations.

Environmental exposures and the development of systemic lupus erythematosus

PURPOSE OF REVIEW

This review examines evidence relating environmental factors to the development of systemic lupus erythematosus (SLE).

RECENT FINDINGS

The strongest epidemiologic evidence exists for the associations of silica, cigarette smoking, oral contraceptives, postmenopausal hormone therapy and endometriosis, with SLE incidence. Recent studies have also provided robust evidence of the association between alcohol consumption and decreased SLE risk. There are preliminary, conflicting or unsubstantiated data that other factors, including air pollution, ultraviolet light, infections, vaccinations, solvents, pesticides and heavy metals such as mercury, are related to SLE risk. Biologic mechanisms linking environmental exposures and SLE risk include increased oxidative stress, systemic inflammation and inflammatory cytokine upregulation, and hormonal triggers, as well as epigenetic modifications resulting from exposure that could lead to SLE.

SUMMARY

Identifying the environmental risk factors related to risk of SLE is essential as it will lead to increased understanding of pathogenesis of this complex disease and will also make risk factor modification possible for those at increased risk.

Understanding the role of environmental factors in the development of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a multisystem disease with a complex etiology. Its risk is higher among women, racial and ethnic minorities, and individuals with a family history of SLE or related autoimmune diseases. It is believed that genetic factors interact with environmental exposures throughout the lifespan to influence susceptibility to developing SLE. The strongest epidemiologic evidence exists for increased risk of SLE associated with exposure to crystalline silica, current cigarette smoking, use of oral contraceptives, and postmenopausal hormone replacement therapy, while there is an inverse association with alcohol use. Emerging research results suggest possible associations of SLE risk with exposure to solvents, residential and agricultural pesticides, heavy metals, and air pollution. Ultraviolet light, certain infections, and vaccinations have also been hypothesized to be related to SLE risk. Mechanisms linking environmental exposures and SLE include epigenetic modifications resulting from exposures, increased oxidative stress, systemic inflammation and inflammatory cytokine upregulation, and hormonal effects. Research needs to include new studies of environmental risk factors for SLE in general, with a focus on lifetime exposure assessment. In addition, studies in susceptible subgroups, such as family members, studies based on genetic risk profiles, and studies in individuals with evidence of pre-clinical autoimmunity based on the detection of specific auto-antibodies are also required. Understanding the role of environmental exposures in the development of SLE may help identify modifiable risk factors and potential etiological mechanisms.

Human autoimmune diseases: a comprehensive update

There have been significant advances in our understanding of human autoimmunity that have led to improvements in classification and diagnosis and, most importantly, research advances in new therapies. The importance of autoimmunity and the mechanisms that lead to clinical disease were first recognized about 50 years ago following the pioneering studies of Macfarlane Burnett and his Nobel Prize-winning hypothesis of the 'forbidden clone'. Such pioneering efforts led to a better understanding not only of autoimmunity, but also of lymphoid cell development, thymic education, apoptosis and deletion of autoreactive cells. Contemporary theories suggest that the development of an autoimmune disease requires a genetic predisposition and environmental factors that trigger the immune pathways that lead, ultimately, to tissue destruction. Despite extensive research, there are no genetic tools that can be used clinically to predict the risk of autoimmune disease. Indeed, the concordance of autoimmune disease in identical twins is 12-67%, highlighting not only a role for environmental factors, but also the potential importance of stochastic or epigenetic phenomena. On the other hand, the identification of cytokines and chemokines, and their cognate receptors, has led to novel therapies that block pathological inflammatory responses within the target organ and have greatly improved the therapeutic effect in patients with autoimmune disease, particularly rheumatoid arthritis. Further advances involving the use of multiplex platforms for diagnosis and identification of new therapeutic agents should lead to major breakthroughs within the next decade.

Ultraviolet radiation and systemic lupus erythematosus

Exposure to ultraviolet (UV) radiation is among the environmental factors that have been proposed and studied in association with systemic lupus erythematosus (SLE). While it is known that UV radiation exposure may exacerbate pre-existing lupus, it remains unclear whether UV exposure is a risk factor for the development of SLE. Experimental studies show a significant immunomodulatory role for UV radiation, but strong epidemiologic data regarding its role in triggering SLE onset are lacking. Further studies are needed to assess the role of UV radiation in relation to development of incident SLE, yet they are challenging to design due to difficulties in accurate exposure assessment, the heterogeneous nature of SLE, and the challenge of assessing photosensitivity, a feature of SLE, which often precedes its diagnosis.

Immune-mediated pore-forming pathways induce cellular hypercitrullination and generate citrullinated autoantigens in rheumatoid arthritis

Autoantibodies to citrullinated protein antigens are specific markers of rheumatoid arthritis (RA). Although protein citrullination can be activated by numerous stimuli in cells, it remains unclear which of these produce the prominent citrullinated autoantigens targeted in RA. In these studies, we show that RA synovial fluid cells have an unusual pattern of citrullination with marked citrullination of proteins across the broad range of molecular weights, which we term cellular hypercitrullination. Although histone citrullination is a common event during neutrophil activation and death induced by different pathways including apoptosis, NETosis, and necroptosis/autophagy, hypercitrullination is not induced by these stimuli. However, marked hypercitrullination is induced by two immune-mediated membranolytic pathways, mediated by perforin and the membrane attack complex (MAC), which are active in the RA joint and of importance in RA pathogenesis. We further demonstrate that perforin and MAC activity on neutrophils generate the profile of citrullinated autoantigens characteristic of RA. These data suggest that activation of peptidylarginine deiminases during complement and perforin activity may be at the core of citrullinated autoantigen production in RA. These pathways may be amenable to monitoring and therapeutic modulation.

NETs: the missing link between cell death and systemic autoimmune diseases?

For almost 20 years, apoptosis and secondary necrosis have been considered the major source of autoantigens and endogenous adjuvants in the pathogenic model of systemic autoimmune diseases. This focus is justified in part because initial evidence in systemic lupus erythematosus (SLE) guided investigators toward the study of apoptosis, but also because other forms of cell death were unknown. To date, it is known that many other forms of cell death occur, and that they vary in their capacity to stimulate as well as inhibit the immune system. Among these, NETosis (an antimicrobial form of death in neutrophils in which nuclear material is extruded from the cell forming extracellular traps), is gaining major interest as a process that may trigger some of the immune features found in SLE, granulomatosis with polyangiitis (formerly Wegener’s granulomatosis) and Felty’s syndrome. Although there have been volumes of very compelling studies published on the role of cell death in autoimmunity, no unifying theory has been adopted nor have any successful therapeutics been developed based on this important pathway. The recent inclusion of NETosis into the pathogenic model of autoimmune diseases certainly adds novel insights into this paradigm, but also reveals a previously unappreciated level of complexity and raises many new questions. This review discusses the role of cell death in systemic autoimmune diseases with a focus on apoptosis and NETosis, highlights the current short comings in our understanding of the vast complexity of cell death, and considers the potential shift in the cell death paradigm in autoimmunity. Understanding this complexity is critical in order to develop tools to clearly define the death pathways that are active in systemic autoimmune diseases, identify drivers of disease propagation, and develop novel therapeutics.

Abnormal production of pro- and anti-inflammatory cytokines by lupus monocytes in response to apoptotic cells

Monocytes are a key component of the innate immune system involved in the regulation of the adaptive immune response. Previous studies have focused on apoptotic cell clearance abnormalities in systemic lupus erythematosus (SLE) monocytes. However, whether SLE monocytes might express unique patterns of cytokine secretion in response to apoptotic cells is still unknown. Here, we used monocytes from healthy controls and SLE patients to evaluate the production of TNF-α and TGF-β in response to apoptotic cells. Upon recognition of apoptotic material, monocytes from healthy controls showed prominent TGF-β secretion (mean ± SD: 824.6±144.3 pg/ml) and minimal TNF-α production (mean ± SD: 32.6±2.1 pg/ml). In contrast, monocytes from SLE patients had prominent TNF-α production (mean ± SD: 302.2±337.5 pg/ml) and diminished TGF-β secretion (mean ± SD: 685.9±615.9 pg/ml), a difference that was statistically significant compared to normal monocytes (p≤10(-6) for TNF-α secretion, and p = 0.0031 for TGF-β, respectively). Interestingly, the unique cytokine response by SLE monocytes was independent of their phagocytic clearance efficiency, opsonizing autoantibodies and disease activity. We further showed that nucleic acids from apoptotic cells play important role in the induction of TNF-α by lupus monocytes. Together, these observations suggest that, in addition to potential clearance defects, monocytes from SLE patients have an abnormal balance in the secretion of anti- and pro-inflammatory cytokines in response to apoptotic cells. Since the abnormal cytokine response to apoptotic material in SLE is not related to disease activity and opsonizing autoantibodies, it is possible that this response might be an intrinsic property of lupus monocytes. The studies focus attention on toll-like receptors (TLRs) and their downstream pathways as mediators of this response.

Monocyte activation by apoptotic cells removal in systemic lupus erythematosus patients

Decreased apoptotic cells (ACs) removal has been described as relevant in systemic lupus erythematosus (SLE) pathogenesis. Binding/phagocytosis of ACs was decreased in SLE patients. Blocking experiments suggested a role for CD36 in ACs clearance in healthy controls, not observed in SLE patients. Binding/phagocytosis of ACs induced the production of IL-6, CXCL8 and CCL22 in patients and controls and IL-1β, TNF-α and CCL3 only in healthy controls. ACs clearance induced an increase in CD80 and a decrease in CD86 expression in healthy controls and atherosclerotic patients. However, SLE patients did not up-regulate CD80 expression. The number and expression of CD36 and CD163 in monocytes was not different between the groups. ACs removal induced a down-regulation of CD36 expression in adherent HLA-DR(+) cells in SLE patients but not healthy controls. The decreased binding/phagocytosis of ACs observed in SLE patients, induces a distinct immune response compared with healthy controls.

Occupational and environmental exposures and risk of systemic lupus erythematosus: silica, sunlight, solvents

OBJECTIVES

We examined occupational and non-occupational exposures in relation to risk of SLE in a case-control study conducted through the Canadian Network for Improved Outcomes in SLE (CaNIOS).

METHODS

SLE cases (n = 258) were recruited from 11 rheumatology centres across Canada. Controls (without SLE, n = 263) were randomly selected from phone number listings and matched to cases by age, sex and area of residence. Data were collected using a structured telephone interview.

RESULTS

An association was seen with outdoor work in the 12 months preceding diagnosis [odds ratio (OR) 2.0; 95% CI 1.1, 3.8]; effect modification by sun reaction was suggested, with the strongest effect among people who reported reacting to midday sun with a blistering sunburn or a rash (OR 7.9; 95% CI 0.97, 64.7). Relatively strong but imprecise associations were seen with work as an artist working with paints, dyes or developing film (OR 3.9; 95% CI 1.3, 12.3) and work that included applying nail polish or nail applications (OR 10.2; 95% CI 1.3, 81.5). Patients were more likely than controls to report participation in pottery or ceramics work as a leisure activity, with an increased risk among individuals with a total frequency of at least 26 days (OR 2.1; 95% CI 1.1, 3.9). Analyses of potential respirable silica exposures suggested an exposure-response gradient (OR 1.0, 1.4. and 2.1 for zero, one and two or more sources of exposure, respectively; trend test P < 0.01).

CONCLUSIONS

This study supports the role of specific occupational and non-occupational exposures in the development of SLE.

Recent advances and opportunities in research on lupus: environmental influences and mechanisms of disease

We summarize research on mechanisms through which environmental agents may affect the pathogenesis of lupus, discuss three exposures that have been the focus of research in this area, and propose recommendations for new research initiatives. We examined studies pertaining to key mechanistic events and specific exposures. Apoptosis leading to increased production or decreased clearance of immunogenic intracellular self-antigens and defective apoptosis of autoreactive immune cells both have been implicated in the loss of self-tolerance. The adjuvant or bystander effect is also needed to produce a sustained autoimmune response. Activation of toll-like receptors is one mechanism through which these effects may occur. Abnormal DNA methylation may contribute to the pathogenesis of lupus. Each of the specific exposures has been shown, in humans or in mice, to act upon one or more of these pathogenic steps. Specific recommendations for the continued advancement of our understanding of environmental influences on lupus and other autoimmune diseases include the development and use of mouse models with varying degrees of penetrance and manifestations of disease, identification of molecular or physiologic targets of specific exposures, development and use of improved exposure assessment methodologies, and multisite collaborations designed to examine understudied environmental exposures in humans.

The U1-snRNP complex: structural properties relating to autoimmune pathogenesis in rheumatic diseases

The U1 small nuclear ribonucleoprotein particle (snRNP) is a target of autoreactive B cells and T cells in several rheumatic diseases including systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD). We propose that inherent structural properties of this autoantigen complex, including common RNA-binding motifs, B and T-cell epitopes, and a unique stimulatory RNA molecule, underlie its susceptibility as a target of the autoimmune response. Immune mechanisms that may contribute to overall U1-snRNP immunogenicity include epitope spreading through B and T-cell interactions, apoptosis-induced modifications, and toll-like receptor (TLR) activation through stimulation by U1-snRNA. We conclude that understanding the interactions between U1-snRNP and the immune system will provide insights into why certain patients develop anti-U1-snRNP autoimmunity, and more importantly how to effectively target therapies against this autoimmune response.

Inflammatory clearance of apoptotic cells after UVB challenge

In the human body, every day billions of apoptotic cells are produced. Removal of these cells is necessary, to prevent the release of intracellular toxic constituents, and occurs very effectively via phagocytosis by (semi)-professional phagocytes. This elimination process occurs rapidly and without inflammation. In systemic lupus erythematosus (SLE) a disturbed elimination of apoptotic cells has been implicated in the induction and reactivation of the disease. Accumulation of apoptotic cells may result in autoantibody formation. A delayed, pro-inflammatory clearance is also thought to play a crucial role in the development of inflammatory lesions once the disease has manifested. One of the hallmarks of patients with SLE is the development of cutaneous lesions upon exposure to sunlight. In this review, we will focus on apoptotic cells, their elimination, and the consequences of a disturbed elimination of apoptotic cells on the development of UVB induced inflammatory skin lesions.

Cleavage of La protein by granzyme H induces cytoplasmic translocation and interferes with La-mediated HCV-IRES translational activity

Granzymes are key components of the cytotoxic arm of the immune response, which play critical roles in eliminating host cells infected by intracellular pathogens and transformed cells. Although the induction of cell death is likely a central process underlying the function of these enzymes, little is known about whether granzymes use additional mechanisms to exert their antipathogen activity. This study identifies La, a phosphoprotein involved in multiple roles in cellular and viral RNA metabolism, as the first nonapoptotic substrate of granzyme H (gzmH), a cytotoxic granule protease that is constitutively expressed by NK cells. Cleavage of La by gzmH occurs at Phe-364 (P(1) site) and generates a COOH-terminal truncated form of La that loses nuclear localization and decreases HCV (hepatitis C virus)-internal ribosome entry site (IRES)-mediated translational activity. The ability of gzmH to cleave host proteins involved in essential viral functions provides a novel mechanism by which granzymes can mediate direct antiviral activities.

Pathogenesis of cutaneous lupus erythematosus

Although for decades sunlight was suspected to be involved in the development of cutaneous lupus erythematosus (CLE), only in recent years research on the effects of ultraviolet irradiation on the skin of patients with CLE has resulted in a more comprehensive model for the pathogenesis of the disease. In this model, exposure to UV light induces apoptosis of keratinocytes and the release of pro-inflammatory cytokines. In susceptible patients, the presence or even accumulation of apoptotic cells results in the induction of characteristic inflammatory skin lesions, which might be due to a delayed and pro-inflammatory clearance of these apoptotic cells. Many other factors, in part genetically determined, are involved in CLE resulting in a very heterogeneous clinical manifestation. Among these factors, presence of autoantibodies, a decreased number of regulatory T cells at the site of inflammation and increased expression of pro-inflammatory cytokines like TNFalpha and IFN-inducible protein myxovirus protein A have been shown to play a role in the pathogenesis of CLE.

Recent advances and opportunities in research on lupus: environmental influences and mechanisms of disease

OBJECTIVES

In this review we summarize research on mechanisms through which environmental agents may affect the pathogenesis of lupus, discuss three exposures that have been the focus of research in this area, and propose recommendations for new research initiatives.

DATA SOURCES AND SYNTHESIS

We examined studies pertaining to key mechanistic events and specific exposures. Apoptosis leading to increased production or decreased clearance of immunogenic intracellular self-antigens and defective apoptosis of autoreactive immune cells both have been implicated in the loss of self-tolerance. The adjuvant or bystander effect is also needed to produce a sustained autoimmune response. Activation of toll-like receptors is one mechanism through which these effects may occur. Abnormal DNA methylation may also contribute to the pathogenesis of lupus. Each of the specific exposures we examined--Epstein-Barr virus, silica, and trichloroethylene--has been shown, in humans or in mice, to act upon one or more of these pathogenic steps. Specific recommendations for the continued advancement of our understanding of environmental influences on lupus and other autoimmune diseases include the development and use of mouse models with varying degrees of penetrance and manifestations of disease, identification of molecular or physiologic targets of specific exposures, development and use of improved exposure assessment methodologies, and multisite collaborations designed to examine understudied environmental exposures in humans.

CONCLUSIONS

The advances made in the past decade concerning our understanding of mechanisms involved in the development of lupus and the influence of environmental agents on this process provide a strong foundation for further developments in this field.

Ultraviolet light and cutaneous lupus

Exposure to ultraviolet (UV) light is one of the major factors known to trigger cutaneous disease activity in (systemic) lupus erythematosus patients. UV light, UVB in particular, is a potent inducer of apoptosis. Currently, disturbed clearance of apoptotic cells is one of the concepts explaining the development of inflammation in lupus patients. We review the role of apoptotic cells and autoantibodies in the pathogenesis of UVB induced skin lesions.

Toll-like receptors in systemic autoimmune disease

Toll-like receptors (TLRs) have a crucial role in the early detection of pathogen-associated molecular patterns and the subsequent activation of the adaptive immune response. Whether TLRs also have an important role in the recognition of endogenous ligands has been more controversial. Numerous in vitro studies have documented activation of both autoreactive B cells and plasmacytoid dendritic cells by mammalian TLR ligands. The issue of whether these in vitro observations translate to an in vivo role for TLRs in either the initiation or the progression of systemic autoimmune disease is a subject of intense research; data are beginning to emerge showing that this is the case.

Apoptotic cell death and lupus

Programmed cell death and the disposal of cell corpses by phagocytic cells are highly regulated ongoing processes essential for the survival and well-being of higher organisms. Abnormalities in the susceptibility of certain cells to receptor-induced death are known to lead to certain human diseases (e.g., autoimmune lymphoproliferative syndrome) and may contribute to the pathogenesis of systemic lupus erythematosus. Impaired clearance of apoptotic cells is also likely to be an important factor in lupus pathogenesis, though the biological basis of such a defect remains elusive. Finally, the process of apoptosis has been shown to contribute to lupus disease effector mechanisms. A better understanding of the role of apoptosis in lupus very likely will lead to improved diagnosis and therapy.