Monomethylarsonous acid (MMA+3) Inhibits IL-7 Signaling in Mouse Pre-B Cells

Advances in IL-7 Research on Tumour Therapy

Interleukin-7 (IL-7) is a versatile cytokine that plays a crucial role in regulating the immune system's homeostasis. It is involved in the development, proliferation, and differentiation of B and T cells, as well as being essential for the differentiation and survival of naïve T cells and the production and maintenance of memory T cells. Given its potent biological functions, IL-7 is considered to have the potential to be widely used in the field of anti-tumour immunotherapy. Notably, IL-7 can improve the tumour microenvironment by promoting the development of Th17 cells, which can in turn promote the recruitment of effector T cells and NK cells. In addition, IL-7 can also down-regulate the expression of tumour growth factor-β and inhibit immunosuppression to promote anti-tumour efficacy, suggesting potential clinical applications for anti-tumour immunotherapy. This review aims to discuss the origin of IL-7 and its receptor IL-7R, its anti-tumour mechanism, and the recent advances in the application of IL-7 in tumour therapy.

Arsenite Methyltransferase Is an Important Mediator of Hematotoxicity Induced by Arsenic in Drinking Water

Chronic arsenic exposures via the consumption of contaminated drinking water are clearly associated with many deleterious health outcomes, including anemia. Following exposure, trivalent inorganic arsenic (As^III) is methylated through a series of arsenic (+III oxidation state) methyltransferase (As3MT)-dependent reactions, resulting in the production of several intermediates with greater toxicity than the parent inorganic arsenicals. The extent to which inorganic vs. methylated arsenicals contribute to As^III-induced hematotoxicity remains unknown. In this study, the contribution of As3MT-dependent biotransformation to the development of anemia was evaluated in male As3mt-knockout (KO) and wild-type, C57BL/6J, mice following 60-day drinking water exposures to 1 mg/L (ppm) As^III. The evaluation of hematological indicators of anemia revealed significant reductions in red blood cell counts, hemoglobin levels, and hematocrit in As^III-exposed wild-type mice as compared to unexposed controls. No such changes in the blood of As3mt-KO mice were detected. Compared with unexposed controls, the percentages of mature RBCs in the bone marrow and spleen (measured by flow cytometry) were significantly reduced in the bone marrow of As^III-exposed wild-type, but not As3mt-KO mice. This was accompanied by increased levels of mature RBCS in the spleen and elevated levels of circulating erythropoietin in the serum of As^III-exposed wild-type, but not As3mt-KO mice. Taken together, the findings from the present study suggest that As3MT-dependent biotransformation has an essential role in mediating the hematotoxicity of As^III following drinking water exposures.

Chronic Community Exposure to Environmental Metal Mixtures Is Associated with Selected Cytokines in the Navajo Birth Cohort Study (NBCS)

Many tribal populations are characterized by health disparities, including higher rates of infection, metabolic syndrome, and cancer-all of which are mediated by the immune system. Members of the Navajo Nation have suffered chronic low-level exposure to metal mixtures from uranium mine wastes for decades. We suspect that such metal and metalloid exposures lead to adverse health effects via their modulation of immune system function. We examined the relationships between nine key metal and metalloid exposures (in blood and urine) with 11 circulating biomarkers (cytokines and CRP in serum) in 231 pregnant Navajo women participating in the Navajo Birth Cohort Study. Biomonitored levels of uranium and arsenic species were considerably higher in participants than NHANES averages. Each biomarker was associated with a unique set of exposures, and arsenic species were generally immunosuppressive (decreased cellular and humoral stimulating cytokines). Overall, our results suggest that environmental metal and metalloid exposures modulate immune status in pregnant Navajo women, which may impact long-term health outcomes in mothers and their children.

Arsenic as an immunotoxicant

Arsenic is world-wide contaminant to which millions of people are exposed. The health consequences of arsenic exposure are varied, including cancer, cardiometabolic disease, and respiratory disorders. Arsenic is also toxic to the immune system, which may link many of the pathologies associated with arsenic exposure. The immune system can be classified into two interconnected arms: the innate and the adaptive immune responses. Herein, we discuss the effects of arsenic on key cell types within each of these arms, highlighting both in vitro and in vivo responses. These cells include macrophages, neutrophils, dendritic cells, and both B and T lymphocytes. Furthermore, we will explore data from human populations where altered immune status is implicated in disease and identify several data gaps where research is needed to complete our understanding of the immunotoxic effects of arsenic.

Arsenite and monomethylarsonous acid disrupt erythropoiesis through combined effects on differentiation and survival pathways in early erythroid progenitors

Strong epidemiological evidence demonstrates an association between chronic arsenic exposure and anemia. We recently found that As+3 impairs erythropoiesis by disrupting the function of GATA-1; however the downstream pathways impacted by the loss of GATA-1 function have not been evaluated. Additionally, our previous findings indicate that the predominant arsenical in the bone marrow of mice exposed to As+3 in their drinking water for 30 days was MMA+3, but the impacts of this arsenical on erythorpoisis also remain largely unknown. The goal of this study was to address these critical knowledge gaps by evaluating the comparative effects of arsenite (As+3) and the As+3 metabolite, monomethyarsonous acid (MMA+3) on two critical regulatory pathways that control the differentiation and survival of early erythroid progenitor cells. We found that 500 nM As+3 and 100 and 500 nM MMA+3 suppress erythropoiesis by impairing the differentiation of early stage erythroid progenitors. The suppression of early erythroid progenitor cell development was attributed to combined effects on differentiation and survival pathways mediated by disruption of GATA-1 and STAT5. Our results show that As+3 primarily disrupted GATA-1 function; whereas, MMA+3 suppressed both GATA-1 and STAT5 activity. Collectively, these findings provide novel mechanistic insights into arsenic-induced dyserythropoiesis and suggest that MMA+3 may be more toxic than As+3 to early developing erythroid cells.

Arsenic methylation - Lessons from three decades of research

Between 1990 and 2020, our understanding of the significance of arsenic biomethylation changed in remarkable ways. At the beginning of this period, the conversion of inorganic arsenic into mono- and di-methylated metabolites was viewed primarily as a process that altered the kinetic behavior of arsenic. By increasing the rate of clearance of arsenic, the formation of methylated metabolites reduced exposure to this toxin; that is, methylation was detoxification. By 2020, it was clear that at least some of the toxic effects associated with As exposure depended on formation of methylated metabolites containing trivalent arsenic. Because the trivalent oxidation state of arsenic is associated with increased potency as a cytotoxin and clastogen, these findings were consistent with methylation-related changes in the dynamic behavior of arsenic. That is, methylation was activation. Our current understanding of the role of methylation as a modifier of kinetic and dynamic behaviors of arsenic is the product of research at molecular, cellular, organismic, and population levels. This information provides a basis for refining our estimates of risk associated with long term exposure to inorganic arsenic in environmental media, food, and water. This report summarizes the growth of our knowledge of enzymatically catalyzed methylation of arsenic over this period and considers the prospects for new discoveries.

Origins, fate, and actions of methylated trivalent metabolites of inorganic arsenic: progress and prospects

The toxic metalloid inorganic arsenic (iAs) is widely distributed in the environment. Chronic exposure to iAs from environmental sources has been linked to a variety of human diseases. Methylation of iAs is the primary pathway for metabolism of iAs. In humans, methylation of iAs is catalyzed by arsenic (+ 3 oxidation state) methyltransferase (AS3MT). Conversion of iAs to mono- and di-methylated species (MAs and DMAs) detoxifies iAs by increasing the rate of whole body clearance of arsenic. Interindividual differences in iAs metabolism play key roles in pathogenesis of and susceptibility to a range of disease outcomes associated with iAs exposure. These adverse health effects are in part associated with the production of methylated trivalent arsenic species, methylarsonous acid (MAs^III) and dimethylarsinous acid (DMAs^III), during AS3MT-catalyzed methylation of iAs. The formation of these metabolites activates iAs to unique forms that cause disease initiation and progression. Taken together, the current evidence suggests that methylation of iAs is a pathway for detoxification and for activation of the metalloid. Beyond this general understanding of the consequences of iAs methylation, many questions remain unanswered. Our knowledge of metabolic targets for MAs^III and DMAs^III in human cells and mechanisms for interactions between these arsenicals and targets is incomplete. Development of novel analytical methods for quantitation of MAs^III and DMAs^III in biological samples promises to address some of these gaps. Here, we summarize current knowledge of the enzymatic basis of MAs^III and DMAs^III formation, the toxic actions of these metabolites, and methods available for their detection and quantification in biomatrices. Major knowledge gaps and future research directions are also discussed.

Gestational arsenic exposure induces site-specific DNA hypomethylation in active retrotransposon subfamilies in offspring sperm in mice

Background

Environmental impacts on a fetus can disrupt germ cell development leading to epimutations in mature germ cells. Paternal inheritance of adverse health effects through sperm epigenomes, including DNA methylomes, has been recognized in human and animal studies. However, the impacts of gestational exposure to a variety of environmental factors on the germ cell epigenomes are not fully investigated. Arsenic, a naturally occurring contaminant, is one of the most concerning environmental chemicals, that is causing serious health problems, including an increase in cancer, in highly contaminated areas worldwide. We previously showed that gestational arsenic exposure of pregnant C3H mice paternally induces hepatic tumor increase in the second generation (F2). In the present study, we have investigated the F1 sperm DNA methylomes genome-widely by one-base resolution analysis using a reduced representation bisulfite sequencing (RRBS) method.

Results

We have clarified that gestational arsenic exposure increases hypomethylated cytosines in all the chromosomes and they are significantly overrepresented in the retrotransposon LINEs and LTRs, predominantly in the intergenic regions. Closer analyses of detailed annotated DNA sequences showed that hypomethylated cytosines are especially accumulated in the promoter regions of the active full-length L1MdA subfamily in LINEs, and 5′LTRs of the active IAPE subfamily in LTRs. This is the first report that has identified the specific positions of methylomes altered in the retrotransposon elements by environmental exposure, by genome-wide methylome analysis.

Conclusion

Lowered DNA methylation potentially enhances L1MdA retrotransposition and cryptic promoter activity of 5′LTR for coding genes and non-coding RNAs. The present study has illuminated the environmental impacts on sperm DNA methylome establishment that can lead to augmented retrotransposon activities in germ cells and can cause harmful effects in the following generation.

Changes in human peripheral blood mononuclear cell (HPBMC) populations and T-cell subsets associated with arsenic and polycyclic aromatic hydrocarbon exposures in a Bangladesh cohort

Exposures to environmental arsenic (As) and polycyclic aromatic hydrocarbons (PAH) have been shown to independently cause dysregulation of immune function. Little data exists on the associations between combined exposures to As and PAH with immunotoxicity in humans. In this work we examined associations between As and PAH exposures with lymphoid cell populations in human peripheral blood mononuclear cells (PBMC), as well as alterations in differentiation and activation of B and T cells. Two hundred men, participating in the Health Effects of Arsenic Longitudinal Study (HEALS) in Bangladesh, were selected for the present study based on their exposure to As from drinking water and their cigarette smoking status. Blood and urine samples were collected from study participants. We utilized multiparameter flow cytometry in PBMC to identify immune cells (B, T, monocytes, NK) as well as the T-helper (Th) cell subsets (Th1, Th2, Th17, and Tregs) following ex vivo activation. We did not find evidence of interactions between As and PAH exposures. However, individual exposures (As or PAH) were associated with changes to immune cell populations, including Th cell subsets. Arsenic exposure was associated with an increase in the percentage of Th cells, and dose dependent changes in monocytes, NKT cells and a monocyte subset. Within the Th cell subset we found that Arsenic exposure was also associated with a significant increase in the percentage of circulating proinflammatory Th17 cells. PAH exposure was associated with changes in T cells, monocytes and T memory (Tmem) cells and with changes in Th, Th1, Th2 and Th17 subsets all of which were non-monotonic (dose dependent). Alterations of immune cell populations caused by environmental exposures to As and PAH may result in adverse health outcomes, such as changes in systemic inflammation, immune suppression, or autoimmunity.

Toxicity of environmentally-relevant concentrations of arsenic on developing T lymphocyte

Arsenic is a ubiquitous environmental contaminant that exists in many inorganic and organic forms. In particular, arsenite is known to induce immunotoxicity in humans and animals. There are still major gaps in our understanding of the mechanism(s) of the immunotoxicity induced by arsenic at environmentally-relevant concentrations. T cells are an essential part of the immune system required for host resistance to infections and protection from cancer. Developing T cells in the thymus have been shown to be particularly prone to arsenite-induced toxicity at low concentrations. Suppression of DNA repair proteins and oxidative stress have been identified as a mechanism of genotoxicity that occurs at low to moderate concentrations. Inhibition of the IL-7 signaling pathway was thought to be responsible for the non-genotoxicity induced by low to moderate doses of arsenic. Interestingly, T cells at different stages of their development had distinct sensitivities to arsenite, which was regulated by arsenite exporters. The current evidence strongly suggests that low to moderate doses of arsenic induces toxic effects in the developing T cells and accumulates to highest levels in the early cells that are least capable to pump out arsenic, which may be the mechanism of the high arsenic sensitivity. Therefore, quantification of the exposure levels should be encouraged in future arsenic toxicity studies.

Arsenic and Immune Response to Infection During Pregnancy and Early Life

PURPOSE OF REVIEW

Arsenic, a known carcinogen and developmental toxicant, is a major threat to global health. While the contribution of arsenic exposure to chronic diseases and adverse pregnancy and birth outcomes is recognized, its ability to impair critical functions of humoral and cell-mediated immunity-including the specific mechanisms in humans-is not well understood. Arsenic has been shown to increase risk of infectious diseases that have significant health implications during pregnancy and early life. Here, we review the latest research on the mechanisms of arsenic-related immune response alterations that could underlie arsenic-associated increased risk of infection during the vulnerable periods of pregnancy and early life.

RECENT FINDINGS

The latest evidence points to alteration of antibody production and transplacental transfer as well as failure of T helper cells to produce IL-2 and proliferate. Critical areas for future research include the effects of arsenic exposure during pregnancy and early life on immune responses to natural infection and the immunogenicity and efficacy of vaccines.

Genotoxicity induced by monomethylarsonous acid (MMA+3) in mouse thymic developing T cells

Drinking water exposure to arsenic is known to cause immunotoxicity. Our previous studies demonstrated that monomethylarsonous acid (MMA⁺³) was the major arsenical species presented in mouse thymus cells after a 30 d drinking water exposure to arsenite (As⁺³). MMA⁺³ was also showed to be ten times more toxic than As⁺³ on the suppression of IL-7/STAT5 signaling in the double negative (DN) thymic T cells. In order to examine the genotoxicity induced by low to moderate doses of MMA⁺³, isolated mouse thymus cells were treated with 5, 50 and 500nMMMA⁺³ for 18h in vitro. MMA⁺³ suppressed the proliferation of thymus cells in a dose dependent manner. MMA⁺³ at 5nM induced DNA damage in DN not double positive (DP) cells. Differential sensitivity to double strand breaks and reactive oxygen species generation was noticed between DN and DP cells at 50nM, but the effects were not seen at the high dose (500nM). A stronger apoptotic effect induced by MMA⁺³ was noticed in DN cells than DP cells at low doses (5 and 50nM), which was negated by the strong apoptosis induction at the high dose (500nM). Analysis of intracellular MMA⁺³ concentrations in DN and DP cells, revealed that more MMA⁺³ accumulated in the DN cells after the in vitro treatment. Collectively, these results suggested that MMA⁺³ could directly induce strong genotoxicity in the early developing T cells in the thymus. The DN cells were much more sensitive to MMA⁺³ induced genotoxicity and apoptosis than DP cells, probably due to the higher intracellular levels of MMA⁺³.

Efflux Transporters Regulate Arsenite-Induced Genotoxicity in Double Negative and Double Positive T Cells

Arsenite (As+3) exposure is known to cause immunotoxicity in human and animal models. Our previous studies demonstrated that As+3 at 50-500 nM concentrations induced both genotoxicity and nongenotoxicity in mouse thymus cells. Developing T cells at CD4-CD8- double negative (DN) stage, the first stage after early T cells are transported from bone marrow to thymus, were found to be more sensitive to As+3 toxicity than the T cells at CD4 + CD8 + double positive (DP) stage in vitro. Induction of Mdr1 (Abcb1) and Mrp1 (Abcc1), 2 multidrug resistance transporters and exporters of As+3, was associated with the reversal of As+3-induced double strand breaks and DNA damage. In order to confirm that the thymus cell populations have different sensitivity to As+3in vivo, male C57BL/6J mice were exposed to 0, 100, and 500 ppb As+3 in drinking water for 30 d. A significant decrease in DN cell percentage was observed with exposure to 500 ppb As+3. Low to moderate concentrations of As+3 were shown to induce higher genotoxicity in sorted DN than DP cells in vitro. Calcein AM uptake and Mdr1/Mrp1 mRNA quantification results revealed that DN cells not only had limited As+3 exporter activity, but also lacked the ability to activate these exporters with As+3 treatments, resulting in a higher accumulation of intracellular As+3. Knockdown study of As+3 exporters in the DN thymic cell line, D1 using siRNA, demonstrated that Mdr1 and Mrp1 regulate intracellular As+3 accumulation and genotoxicity. Taken together, the results indicate that transporter regulation is an important mechanism for differential genotoxicity induced by As+3 in thymocytes at different developmental stages.

Low level arsenite exposures suppress the development of bone marrow erythroid progenitors and result in anemia in adult male mice

Epidemiological studies report an association between chronic arsenic (As) exposure and anemia in men, and women who are predisposed to anemia. The purpose of these studies was to determine whether a 60 d drinking water exposure of adult male C57BL/6J mice to 0, 100, and 500ppb arsenite (As⁺³) results in anemia due to alterations in erythroid progenitor cell development in the bone marrow. Exposure to 500ppb As⁺³ for 60 d resulted in a reduction of mean corpuscular hemoglobin (MCH) levels, but did not significantly alter red blood cell (RBC) counts, hemoglobin (Hgb) levels, mean corpuscular Hgb concentrations (MCHC), or mean corpuscular volumes (MCV). Attenuation of burst-forming unit-erythroid (BFU-E) colony formation was observed in bone marrow cells of mice exposed to 500ppb As⁺³. The differentiation of late-stage bone marrow erythroblasts as defined by CD71 and Ter119 surface marker expression was reduced with the 500ppb As⁺³ exposure. Mice exposed to 500ppb As⁺³ also had elevated serum levels of erythropoietin (EPO). Collectively, these results show that exposure to low levels of As⁺³ attenuate the development of early BFU-E cells and reduce the differentiation of late-stage erythroblasts. This suppression of bone marrow erythropoiesis may be a contributing factor to the mild hypochromic anemia observed in 500ppb As⁺³ exposed mice.

Differential sensitivities of bone marrow, spleen and thymus to genotoxicity induced by environmentally relevant concentrations of arsenite

It is known in humans and mouse models, that drinking water exposures to arsenite (As⁺³) leads to immunotoxicity. Previously, our group showed that certain types of immune cells are extremely sensitive to arsenic induced genotoxicity. In order to see if cells from different immune organs have differential sensitivities to As⁺³, and if the sensitivities correlate with the intracellular concentrations of arsenic species, male C57BL/6J mice were dosed with 0, 100 and 500ppb As⁺³via drinking water for 30d. Oxidation State Specific Hydride Generation- Cryotrapping- Inductively Coupled Plasma- Mass Spectrometry (HG- CT- ICP- MS) was applied to analyze the intracellular arsenic species and concentrations in bone marrow, spleen and thymus cells isolated from the exposed mice. A dose-dependent increase in intracellular monomethylarsonous acid (MMA⁺³) was observed in both bone marrow and thymus cells, but not spleen cells. The total arsenic and MMA⁺³ levels were correlated with an increase in DNA damage in bone marrow and thymus cells. An in vitro treatment of 5, 50 and 500nM As⁺³ and MMA⁺³ revealed that bone marrow cells are most sensitive to As⁺³ treatment, and MMA⁺³ is more genotoxic than As⁺³. These results suggest that the differential sensitivities of the three immune organs to As⁺³ exposure are due to the different intracellular arsenic species and concentrations, and that MMA⁺³ may play a critical role in immunotoxicity.

Environmentally relevant concentrations of arsenite and monomethylarsonous acid inhibit IL-7/STAT5 cytokine signaling pathways in mouse CD3+CD4-CD8- double negative thymus cells

Environmental arsenic exposure is a public health issue. Immunotoxicity induced by arsenic has been reported in humans and animal models. The purpose of this study was to evaluate mechanisms of As(+3) and MMA(+3) toxicity in mouse thymus cells. Because we know that MMA(+3) inhibits IL-7 signaling in mouse bone marrow pre-B cells, we studied the influence of As(+3) and MMA(+3) on T cell development in the thymus at the earliest stage of T cell development (CD4-CD8-, double negative, DN) which requires IL-7 dependent signaling. We found in a DN thymus cell line (D1) that a low concentration of MMA(+3) (50 nM) suppressed IL-7 dependent JAK1, 3 and STAT5 signaling. As(+3) suppressed STAT5 and JAK3 at higher concentrations (500 nM). Cell surface expression of the IL-7 receptor (CD127) was also suppressed by 50 nM MMA(+)3, but was increased by 500 NM As(+3), indicating possible differences in the mechanisms of action of these agents. A decrease in cyclin D1 protein expression was observed in D1 cells exposed to As(+3) at 500 nM and MMA(+3) starting at 50 nM, suggesting that arsenic at these environmentally-relevant doses suppresses early T cell development through the inhibition of IL-7 signaling pathway.