Effect of exposure to arsenic on immune function and anemia status in children in a birth cohort study

Abstract

Environmental exposure to arsenic is a global health concern which is linked to a number of diseases causing adverse health effects. Chronic arsenic exposure affects an estimated number of 200 million people worldwide through contaminated groundwater and food. In Bangladesh, the arsenic problem is very devastating and related health problems have yet not been fully investigated. There is consistent evidence that chronic arsenic toxicity may cause cancer and numerous other pathological effects in humans as well as immune toxicity. Several cross-sectional studies have indicated that arsenic adversely affects the immune system in both children and adults. However, very few longitudinal studies in human have further indicated that the adverse effects of the toxicant start from in utero. Exposure to arsenic has also been found to increase risk of anemia by disrupting cell membranes and declining erythrocyte survival. However, findings from epidemiological studies are equivocal in support to this association. This thesis delineates the plausible effects of chronic arsenic exposure from in utero to pre- adolescent age and the detrimental effects on fetal and infant health and development, as well as undesirable health effects later in life. The overall objective of the present study has been focused to investigate the impact of prenatal and concurrent exposure to arsenic in 9 years old school-going children on anemia and immune function. The studies were nested into a large food and micronutrient supplementation trial conducted in a rural area of Bangladesh called Matlab. Mother-child pairs were followed from early gestation to 9 years. Elevated arsenic concentrations in groundwater are common in Matlab where 95% of the population uses hand-pumped tubewell water as their main source of drinking water with 43% of the wells exceeding the World Health

Organization (WHO) guideline value of 10 µg/L, and 34% exceeded the Bangladesh standard of 50 μg/L. Arsenic exposure in the present studies was assessed based on the concentrations of the sum of inorganic arsenic and its methylated metabolites (monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)) in maternal urine during pregnancy and child urine at 4.5 and 9 years of age (Urinary arsenic: U-As). Hemoglobin (Hb) was measured in whole blood in pregnant women at gestation week (GW) 14 and 30, and in children at 4.5 and 9 years of age. Other markers that were studied in 4.5 and 9 years old children were- anemia related biomarkers (soluble Transferrin Receptor (sTfR), ferritin, vitamin B12, folate, vitamin A, hepcidin, zinc), inflammatory markers (C-reactive protein (CRP)), oxidative stress marker (8-hydroxy-2'-deoxyguanosine (8- OHdG)) in plasma, and immune marker (signal joint T cell receptor excision circles (sjTRECs) and indicator of senescence (telomere length (TL)) in peripheral blood mononuclear cells (PBMC). No effects of arsenic exposure on Hb levels and anemia biomarkers was found in children. About 28% of the women were found to be anemic at GW14, 35% at GW30 and 23% at both time points. The prevalence of anemia was low in 4.5 years old children (5%) that somewhat increased in 9 years old (15%). Only 2.5% children had iron deficiency anemia according to sTfR cut-off at 9 years. The risk (Odds Ratio (OR)) of being anemic in 9 years of age was 1.8 and 2.3 fold higher if their mothers were anemic at either GW14 or GW30, respectively, compared to non-anemic mothers. The chances of having anemia at 9 years was 3 times higher if the mothers were anemic at both time points (OR=3.05). This risk increased about 6 times (OR=5.9) when the children were additionally anemic at 4.5 years. No impact of maternal anemia was observed at 4.5 years of age. Multiple Micronutrient (MM) supplementation significantly improved body mass index-for-age z- score (BAZ) and reduced markers of inflammation in 9 years old children.

In multivariable-adjusted spline regression analyses, both prenatal (U-As at GW8) and childhood arsenic exposure (U-As at 4.5 and 9 years of age) above U-As of 45 µg/L (spline knot) strongly reduced TL and sjTRECs at 9 years. However, concurrent U-As below the spline knot was significantly positively associated with TL and sjTRECs at 9 years of age. It is plausible that there is a critical threshold of arsenic exposure beyond which exposed cells either undergo telomere attrition or elongation of TL. In 9 years old children, fraction of MMA above spline knot 7% were significantly inversely associated with both TL and sjTRECs reflecting increased toxicity due to less-efficient arsenic metabolism. Prenatal and childhood arsenic exposure were positively associated with 8-OHdG at 9 years which in turn was inversely associated with sjTRECs at 9 years. However, adjustment with 8-OHdG did not change the estimated association of U-As with sjTRECs suggesting that the mechanisms of 8-OHdG-mediated oxidative damage of naïve T cells may be distinct from arsenic-induced oxidative damage with little overlap. We can, therefore, summarize that chronic arsenic exposure did not appear to have any impact on anemia status of children. Anemia during pregnancy and early childhood are important risk factors for the occurrence of anemia in school-age children. Maternal supplementation during pregnancy with MM had beneficial effects on child nutritional status and reduced markers of inflammation in the children. Chronic arsenic exposure from early life can result in TL attrition and lower production of naïve T cells potentially leading to immunosenescence and immunodeficiency. The adverse effects of arsenic on sjTRECs and TL may be mediated by pathways other than 8-OHdG-induced oxidative stress, e.g. impairment in DNA repair enzymes and synthesis.