1. Introduction
The exponential growth of the world economy and its accompanying exploitation of resources and industrial production have caused an unprecedented transfer of contaminants to the environment, with multiple impacts on health. Because these human-induced impacts on earth systems are so extensive, a new definition of this geological era has been proposed, the Anthropocene. Although the last few decades have been characterised by marked technological innovation that aims at the concept of sustainability as a way to improve the quality of the environment, anthropogenic activities continue to have a profound effect on human health, mainly due to poor remediation actions [
1]. Humans are regularly exposed to many environmental chemicals that may have potentially toxic effects on health. These chemicals can enter the human body through various routes: ingestion of chemicals via the consumption of contaminated food or water, inhalation of airborne pollutants through the respiratory system, as well as dermal absorption through the skin.
The most recent WHO environmental burden of disease estimates that, every year, 13% of deaths in the 28 European Member States are attributable to environmental stressors [
2]. In terms of the absolute number of deaths attributable to the environment, the European Environment Agency concluded that 90% of deaths attributable to the environment result from non-communicable diseases (NCDs), including cancers, cardiovascular disease, diabetes and chronic lung illnesses (EEA Report No 21/2019). The interplay between the immune system and non-communicable diseases is complex and multifaceted. Chronic inflammation, immune dysregulation, and lifestyle factors can significantly impact the prevention and management of NCDs. Understanding this relationship can provide insights into the prevention and management of NCDs.
The exposome concept was introduced in the field of epidemiology by Wild in 2005 to encompass “the totality of human environmental exposures from conception onwards, complementing the genome” [
3]. Therefore, the exposome concept provides a description of lifelong (from the prenatal period) exposure history. It is now known that the exposome poses new challenges in assessing the relationship between exposure to environmental factors (pollution, climate change, lifestyle, and diet) and health. Indeed, there is increasing evidence that genetic variants explain a limited fraction of the variability in the risk of chronic diseases, as shown by studies in monozygotic twins, leaving a potentially large role for environmental exposures and interaction between environmental and genetic factors [
4]. The molecular processes underlying human health and disease are highly complex. Often, genetic and environmental factors contribute to a given disease or phenotype in a non-additive manner, yielding a gene–environment interaction [
5]. Epigenetic mechanisms, including DNA methylation, histone modification and non-coding RNA, can modulate gene expression levels without changing the underlying DNA sequence. Moreover, many epigenetic modifications are dynamic, reflecting cumulative environmental exposures throughout the lifespan and correlating with ageing related diseases and outcomes [
6].
In 2012, Wild has enhanced his approach, describing three overlapping domains within the exposome that refer to different factors, such as: i) the general external environment (including factors such as the urban–rural environment, climate factors, social capital and education); ii) the specific external environment (including diet, physical activity, tobacco, infections, occupation, etc.); iii) the internal environment including internal biological factors, such as metabolic factors, gut microbioflora, inflammation, oxidative stress and ageing [
7]. This, together with the results of studies on animal models, would suggest a pathological mechanism of “multiple hits'' [
8]. According to this model, the rearrangement of gene expression through epigenetic mechanisms can involve different organ systems and is caused by a combination of genetic predisposition and prenatal injury (the "first-hit"), which may not be enough to change the adult phenotype on its own. However, tissue imbalances resulting from the perinatal insults and/or adverse stressors/exposures during postnatal life may serve as a "second-hit", which might reveal or accentuate the underlying abnormalities leading to disease states.
All chemicals found in the environment are referred to as pollutants or xenobiotics. Many of such chemicals can persist in the environment over long periods of time and, for this reason, are called persistent organic pollutants (POPs). POPs are characterised by ubiquity and persistence in the environment, and it is known that the derived products can interact with the environment and undergo biotransformation and bioaccumulation processes [
9]. Among the anthropogenic environmental pollutants, in addition to certain natural compounds like heavy metals, pesticides , drugs, and synthetic chemical compounds generated during production processes (hydrocarbons, dioxins, polychlorinated diphenyls, polybromodiphenyl ethers, etc.) can contribute to the pathogenesis of several diseases in living organisms.
The immune system is an ancient defence system formed by all metazoans while struggling with various internal and external factors, whose perturbation may lead to increased susceptibility to pathogens and diseases. Inflammation is a common response to a variety of stressors, including xenobiotics. Under normal conditions, inflammation is a healthy and adaptive process that both combats infection and aids in repairing damage to tissues. However, xenobiotics can elicit prolonged, severe, and/or inappropriate inflammatory responses that play a causal role in the progression of biological events, linking a molecular initiating event to an adverse outcome pathway (AOP) [
10]. Ordering relevant and causally linked events in the response cascade is critical for applying this information to Environmental Risk Assessment, which is why the AOP framework was created [
11].
In this view, the immune system has a recognized central role in many processes involving chronic diseases, and the frontiers between communicable and non-communicable diseases also require additional exploration [
12]. The recognition of altered immune system function in many chronic disease states has proven to be pivotal in non-communicable diseases that are characterised by a chronic low level of inflammation, including autoimmune [
13] and neurodegenerative conditions [
14], cardiovascular diseases [
15], cancer [
16], diabetes mellitus and Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD) [
17,
18]. Therefore, compared with other toxicity endpoints (i.e., genotoxicity, endocrine toxicity or developmental toxicity), there are still many challenges for the introduction of immunotoxicity endpoint evaluation [
19].
So far, there is still insufficient knowledge about the effects of pollutants on the different cell populations of the immune system, and the knowledge of related toxicological mechanisms remains elusive. Many findings have suggested that a subset of environmental pollutants can bind the receptors involved in metabolism, oxidative stress, immune response and inflammation, such as the aryl hydrocarbon receptors (AhR) [
20]. The complexity of AhR signalling arises from multiple factors, including the diverse ligands that activate the receptor, the expression level of AhR itself, and its interaction with the AhR nuclear translocator [
21]. Indeed, AhR engages in crosstalk with the AhR repressor or other transcription factors and signalling pathways and, in this way, it can also mediate non-genomic effects [
22].
In recent decades, AhR has been increasingly recognized as an important modulator of disease because of AhR’s role in the regulation of the redox system and of immune and inflammatory responses [
23]. AhR has significant effects on the control of adaptive immunity, modulating T cell differentiation and function directly and indirectly through its effects on antigen-presenting cells. For instance, 2,3,7,8-tetraclorodibenzo-p-dioxin (TCCD) is a potent xenobiotic ligand of AhR and, from this, it was found to inhibit immune responses [
24], showing an effect linked to the induction of CD4
+ T cells with a regulatory phenotype [
25]. Indeed, the ability to activate AhR has been demonstrated for other contaminants, such as polycyclic aromatic hydrocarbons (PAHs) [
26] and polybrominated diphenyl ethers (PBDEs) [
27]. The production of reactive oxygen species (ROS) and organic metabolites due to the dysregulation of this receptor can cause serious consequences on biological macromolecules such as proteins, lipids and DNA [
28]. Indeed, oxidative stress, inflammation and apoptosis can compromise immune response, with the possible development of infections, tumours and lung diseases [
29].
In this review, we analyse how environmental pollutants can modulate immune response, reviewing the biological effects of the most common environmental pollutants on different immune cells in
in vitro, ex vivo and in vivo studies. Indeed, we will devote special attention to pregnancy, describing the effects of pollutants on immune response during different windows of prenatal life (
Figure 1).
3. Developmental Immunotoxicology
The term developmental immunotoxicology (DIT) has been used to identify a new approach to understanding the effects of immune development perturbation on adverse outcomes [
132]. DIT addresses the impact of xenobiotics on immune responses during the early stages of immune system development/maturation compared to adults. Immuno toxicology studies have defined some critical windows of vulnerability during development [
133] where xenobiotics can exert their effects , but sometimes stressors may be applied during rather broad periods which often cover the entire immune developmental period, such as in highly polluted areas where environmental stressors can be relevant for a large period of prenatal and post-natal life. Furthermore, considering the accumulation of evidence supporting the hypothesis of the Developmental Origin of Health and Disease (DOHaD) [
134,
135], disturbances of the immune system during foetal development are of particular interest for possible long-term consequences. According to epidemiological studies, adverse effects of in utero exposures on the immune system have been associated with higher rates of chronic immune disorders in adults, including autoimmunity, immune deficiency, inflammation and allergic reactions [
136].
Although these associations are of considerable importance, examining only outcomes directly linked to immune system elements fails to capture the complete range of health risks associated with DIT. In fact, from the early stages of life, immune cells are found in all tissues and organs where they serve as homeostatic regulators of the physiological function of the tissue and as sentinels to defend it. In the liver, for example, the functional state of hepatocytes (the majority of cell populations) can be significantly altered by a small number of resident immune cells (i.e., Kupffer cells). This occurs similarly between microglia and astrocytes in the central nervous system and many other organs [
137]. For these reasons, the involvement of DIT events underlying behavioural disorders and metabolic dysfunction has been suggested [
138].
During pregnancy, major adaptations occur in the maternal immune system to protect the mother and her future baby from pathogens; therefore, the perturbation of this balance may have detrimental immune responses against the allogeneic foetus. Pro- and anti-inflammatory stimuli alternate during gestation, and their regulation is the basis of the success of numerous key steps of the pregnancy. For example, the implantation of the embryo on the uterine wall is facilitated by a period of active inflammation, essential for the remodelling of the maternal uterus [
139]. On the contrary, the phases of active foetal growth and development are accompanied by a relative inflammatory quiescence [
140]. Failure to induce these systemic changes predisposes women to adverse pregnancy outcomes, such as recurrent miscarriage [
141], preeclampsia [
142] and preterm birth [
143]. Additionally, depending on the duration and severity, this inappropriate activation of the maternal immune response may have transgenerational consequences. For instance, the activation of the mother's immune/inflammatory response has been linked to negative neurobehavioral outcomes in offspring [
144,
145,
146,
147]. These findings are noteworthy from an immunological standpoint, as the physiological development of the foetal brain involves a variety of immunological factors at the maternal-foetal interface [
148]. For example, in the case of extremely preterm infants, placental methylation of genes related to inflammation has been observed to be associated with a decreased risk of cognitive impairment and decreased neonatal systemic inflammation [
149].
Evidence suggests that prenatal exposure to xenobiotics may play a role in altering the epigenetic regulation of immune-related genes that influence the development of regulatory T cell (Treg) networks or the ratio of Th1 and Th2 cells. Because Th1/Th2 immunity is closely linked to disease, epigenetic changes caused by an adverse intrauterine environment could explain the occurrence of disease susceptibility later in life [
150]. In parallel with the direct modulation of epigenetic regulation, numerous toxic substances can impact the endocrine system. Given its close collaboration with the immune system in guiding development from gestation to early childhood, endocrine disruptor chemicals may also trigger DIT [
151].
In this section, we will overview the effects of the most relevant xenobiotics during pregnancy with specific reference to the immune dysregulation.
Arsenic toxicity has been linked to many possible pathways, including oxidative stress, epigenetic modification, interference with DNA repair mechanisms and alterations of the immune system [
152,
153]. Exposure to As during pregnancy can result in DIT interacting directly and indirectly with the foetal environment. In both cases, the interference is strongly related to the placental function that is permeable to As while also being particularly susceptible to its detrimental consequences, including oxidative stress, inflammation and immune system perturbation [
33]. In two separate cohort studies from Bangladesh and United States, prenatal As exposure was associated with lower percentages of CD4
+ T cells and, in particular, activated memory CD4
+ T helper cells in cord blood [
154,
155]. Although a direct comparison of exposure levels between the two populations is not possible, the effects of As exposure appear to be consistent. In the study on the Bangladeshi population, As exposure is determined from the levels found in drinking water samples, whereas in the U.S. study, exposure is assessed from maternal toenail samples. Evidence of the similarity in findings showed that both cohorts demonstrated relatively higher susceptibility to infectious diseases at different stages of childhood, which was associated with prenatal arsenic (As) exposure [
156,
157]. This suggests that prenatal As exposure causes overall immunosuppression in offspring. Among the mechanisms of action that could explain the negative effects of As exposure on immune system cells, the most widely accepted appears to be the induction of oxidative stress. Arsenic exposure has been shown to trigger oxidative stress in peripheral blood, in both adults and children [
158,
159]. Studies conducted on populations exposed to high levels of As have demonstrated that prenatal exposure to arsenic increases the expression of numerous markers of inflammatory and oxidative stress in umbilical cord blood and placenta [
33,
160].
The long-term effects on the immune system of arsenic exposure during pregnancy may be mediated by epigenetic mechanisms. Specifically, exposure to arsenic has been shown to induce alterations in DNA methylation patterns, which can influence gene expression and immune cell function [
161,
162]. Studies on mice have suggested that one possible mechanism linking prenatal As exposure to health effects later in life may be epigenome alteration. Cytosine methylation (e.g., CpG methylation) has been suggested as a valuable candidate for As-related health effects [
163].
The toxic effects related to Hg exposure, in particular to methylmercury (MetHg), mainly affect the nervous system [
164]. However, as previously mentioned, in vitro experimental evidence showed possible associations between Hg exposure and immunotoxicity [
61,
65,
67]. Mercury can be transferred from mother to foetus through the placenta [
164], and in utero exposure to Hg has been associated with a higher risk of respiratory infection in infants during the first year of life [
165]. Mercury levels during pregnancy and postpartum have been linked to the frequency of natural Treg and NKT cells as shown in the New Hampshire Birth Cohort Study. Higher natural Treg counts have also been associated with an increased risk of Hg exposure as measured by seafood consumption. Elevated Tregs may represent a counterbalancing mechanism in response to the increased autoimmune pressure associated with low-level Hg exposure [
166]. In a study focused on a highly exposed population from the Faroe Islands, Oulhote and colleagues reported a correlation between prenatal methylmercury (MetHg) exposure and a decreased total white blood cell count, specifically affecting monocytes, basophils, CD3
+ T and CD4
+ T lymphocytes, and CD19
+ B lymphocytes at the age of 5 [
167]. It could be hypothesized that such an alteration in the balance of white blood cells could affect inflammatory cytokine levels. In this view, a multicentric cohort study conducted in five European countries (France, Greece, Norway, Spain and the United Kingdom) showed that in utero exposure to Hg was correlated with poorer childhood metabolic status and higher levels of TNF-α, IL-6 and IL-1 at 6 and 12 years. In this report, in fact, the authors suggested that changes in key inflammatory cytokines and the metabolic profile are strictly interconnected [
168]. In an Amazonian Brazil population exposed to high levels of mercury, a positive association between mercury levels and serum concentration of antinuclear antibodies was found, suggesting a possible role of mercury in autoimmune disease [
169]. In a previous study conducted in the same population, Pinheiro and colleagues reported elevated glutathione levels and decreased catalase activity in response to Hg exposure [
170]. Mercury's capacity to modulate the synthesis and activity of enzymes within the endogenous antioxidant system may underlie its effects on immune cells. By disrupting the normal function of these enzymes, mercury can impair the antioxidant defences of immune cells, resulting in altered immune responses and increased vulnerability to infections and diseases.
Studies on the effects of Pb during pregnancy suggest that exposure to the metal can lead to adverse outcomes, such as an increase of enzymes involved in oxidative stress, tissue damage and the dysregulation of inflammatory pathways [
171], and a polarization towards a Th2-type response with elevated serum IgE levels [
51,
172]. Indeed, lead exposure can affect thymic function, altering the development and maturation of immune cells [
173] . This can cause an imbalance in immune functionality, shifting towards a more pro-inflammatory response and increasing the susceptibility to allergies and autoimmune diseases. In utero Pb exposure has been identified as a risk factor for childhood asthma later in life [
47]. Indeed, Pb-related immunomodulating effects have also been highlighted by cross-sectional studies that revealed associations between blood Pb levels and biomarkers linked to allergy and infectious diseases in children [
174,
175].
Cadmium exposure induces oxidative stress by generating ROS, through which it exerts its toxicity. Cd exposure can occur through the consumption of contaminated food as well as through the inhalation of cigarette smoke. Smoking pregnant women have considerably greater amounts of Cd in their blood, placenta and umbilical cord blood. The concentration of Cd was associated with a higher expression of miRNA in cord plasma [
176]. Cd exposure was associated with respiratory symptoms in adolescents. One proposed mechanism of action is increased vulnerability to acute respiratory infections in the early years of life [
177,
178]. The accumulation of Cd in the placenta has been proven in both in vitro and in epidemiological studies [
179,
180]. Cadmium concentrations have been analysed in cord blood, maternal blood and placental tissue, with demonstrated Cd-induced oxidative stress that adversely affects birth outcomes [
181]. Sanders and co-workers found a significant association between in utero Cd exposure and DNA methylation patterns in the leukocytes of newborns and their mothers. It is worth noting that the methylation pattern was Cd specific; in fact, when compared to those produced using cotinine rather than Cd levels, the methylation patterns were non-overlapping [
182]. Cadmium levels in the urine of pregnant women were associated with a lower absolute number of CD3
+ and CD4
+ lymphocytes and lower levels of IL-4 and IL-6 in girls, while an inverse association with the absolute count of CD3
+ and CD8
+ was only seen in males, according to a recent cohort study done in Wuhan, China [
183]. Similarly, a sex-related effect was noted in a study conducted by Nygaard and colleagues. They reported that the concentration of Cd in maternal nails, sampled immediately after the delivery, was associated with a decreased number of T helper memory cells in cord blood [
155]. Moreover, studies in rats have demonstrated that the sex-specific effects of cadmium on immune system cells might result from its interaction with sexual hormones, particularly 17β-estradiol [
184].
Exposure to PFAS and PFOA has been associated with reduced immunogenicity of vaccines in offspring in two independent cohort studies from Norway and the Faroe Islands [
185,
186,
187]. In the cohort of Faroese children, prenatal exposure to MetHg, PCBs and PFASs was also associated with higher levels of autoantibodies [
188]. However, the authors concluded that more comprehensive cohort studies are needed to determine whether the presence and concentrations of these autoantibodies can predict the occurrence and severity of autoimmune diseases, or whether their presence should be considered merely a consequence of tissue damage.
PFAS exposure during pregnancy has also been linked to increased risks of asthma and respiratory syncytial virus infection during childhood [
189]. On the same line, a study conducted among the National Health and Nutrition Examination Survey (NHANES) survey reported an increased risk of asthma in adolescent exposed to high concentration of PFOA [
190,
191]. Moreover, Wang and colleagues found a positive relationship between maternal levels of PFOA and PFOS and IgE levels in the cord blood. Interestingly, the authors do not report any association between exposure levels and asthma symptoms when models are corrected for other possible confounders [
192]. However, it is relevant to report that several other studies demonstrated an inverse association between PFAS exposure and the risk of Eczema and Rhinitis [
193].
Epidemiological studies in birth cohorts have shown that newborns exposed to high doses of PCBs in maternal blood, or children exposed to higher levels of PCBs in early childhood, have reduced thymus size, increased onset of respiratory disease and reduced immune response following vaccination [
194,
195]. PCB exposure during pregnancy has been linked to abnormal cellular immunity development at 6 and 16 months after birth: subjects most exposed to PCBs had significantly higher expression of CD3
+ T-lymphocytes, B-lymphocytes, and activated B-lymphocytes, while NK cells were less expressed. In the same study, altered serum immunoglobulins were found in subjects exposed to higher levels of PCBs in utero compared to those less exposed [
196].TCDD exposure during pregnancy has been observed to impact the miRNA profile of the foetal thymus, affecting the regulation of a wide number of genes that may impair immune system development [
197]. TCDD also has a harmful effect on reproductive health, inducing epigenetic modifications in both human germlines, such as DNA methylation, suggesting that, if TCDD exposure happens during the initial germ cell development, the alteration can be transmitted to subsequent generations [
198]. Few studies have focused on TCDD’s multigenerational and transgenerational effects on human reproductive health, despite the quantity of evidence of such effects on male and female reproductive health in animal models. These studies show that paternal ancestral TCDD exposure substantially contributed to pregnancy outcome and foetal health, although pregnancy outcome was considered tightly related to the woman's health [
198].
Cases of occupational occurrence of allergic contact dermatitis in workers exposed to plastic based on BPA, and in some cases to bis-phenol F (BPF), have been reported, and for this reason such compounds are classified as highly allergenic [
199]. Several human birth cohort studies have reported an association between prenatal BPA exposure and allergy symptoms such as asthma and wheezing in children of different ages [
200,
201]. Although many articles mention allergenic effects as an additional way to demonstrate the immunotoxicity of BPA, there are currently few studies investigating the sensitization effects of BPA analogues (i.e., BPF and bisphenol S [BPS]). Therefore, Mendyet and coworkers analysed the NHANES data and found a positive correlation between urinary BPF levels and current asthma and hay fever, while BPS was associated with a higher likelihood of asthma in men [
202].
In the United States, blood PBDE levels range from 30-100 ng/g of lipids in adults, but the alarming health concern was mainly based on children who showed blood PBDE levels 3 to 9-fold higher than adults. PBDEs disrupt endocrine, immune, reproductive and nervous systems. Studies performed on the Boston Birth Cohort provided evidence that in utero exposure to PBDEs may epigenetically reprogramme the offspring's immunological response through promoter methylation of a proinflammatory gene [
203]. These data agree with the in vitro results using macrophage-like cell lines, where cytokine production and miRNA expression were modulated by BDE-47 treatment [
97,
98,
99].
It is becoming clear that DIT and prenatal programming may have a crucial role in raising the risk of infectious and noncommunicable disease incidence; thus, greater efforts are needed to understand the events leading to DIT in order to effectively counteract them.
An initial line of intervention could be to evaluate populations exposed to various concentration ranges by standardizing exposure variables and identifying priority outcomes for multicenter studies. Most of the studies mentioned in this paragraph examine the association between exposure and immune system outcomes in highly exposed populations. While these studies provide valuable insights into the effects of high levels of exposure, they do not fully reflect potential health risks. The immune system responses and health outcomes observed at high exposure levels might differ significantly from those at lower levels, potentially leading to an underestimation or overestimation of risks. For example, focusing on highly exposed populations might cause studies to miss subtler effects that occur at lower, more common exposure levels. Conversely, highly exposed populations also represent an important focus for studying co-exposures.
For instance, in a cohort of pregnant women living in a highly contaminated area, Longo and coworkers studied the effects of simultaneous exposure to a multi-pollutant mixture using a WQS regression model, demonstrating the association between the expression levels of immune relevant miRNAs and levels of a suite of inorganic and organic elements in the sera of pregnant women. The study emphasized the concurrent assessment of essential elements, recognizing their indispensable role in maintaining physiological balance. This approach allowed the interplay between exposure to environmental pollutants and the availability of essential elements to be unravelled, shedding light on potential synergistic or antagonistic effects, with particular reference to epigenetic alterations inherent in the maintenance of the redox state and cellular homeostasis during pregnancy [
204].