Air pollution is a prominent global environmental and public health issue. Epidemiological studies have revealed a negative correlation between PM2.5 exposure and immune response ability (Che W et al., 2009). PM2.5, which enters alveolar epithelial cells through respiration, can activate the monocyte macrophage system and other immune systems in lung tissue, triggering immune and inflammatory responses (Alessandrini et al., 2013). Some immune regulatory factors, inflammatory cytokines, etc. can enter the bloodstream through the microcirculation of lung tissue, leading to a decrease in the body's immune capacity.
To explore the mechanism of PM2.5 and its components inducing cellular inflammation, Jurkat T cells were used as an immune cell model. They were stimulated by different concentrations of TRPM2.5, WSI, and OE. We found that TRPM2.5 and WSI could exacerbate inflammation by diminishing the proportion of Treg cells, causing an imbalance of immune cells. In contrast, the expressions of Treg were increased after treatment with AG490 (Fig. 1). It reminds us that Treg cells differentiation was partly regulated by JAK2. As we know, Treg cells can play critical roles in inhibiting of inflammation. TRPM2.5 and its constituents gave rise to severe inflammation by decreasing Treg cells. This is consistent with previous research, which proved that PM2.5 impaired the differentiation of Treg cells and aggravated asthma in an AhR-dependent manner (Shi Y et al., 2011). Wang (Wang Ca et al., 2021) found that TRPM2.5 and its water-soluble and organic components can exacerbate cellular inflammation by increasing the proportion of Th17 cells and reducing the proportion of Treg cells, causing an imbalance of Th17/Treg cells. Besides, asthma rats were exposed to PM2.5 and its water-soluble and organic components (Wang J, 2019), it clearly led to an imbalance in Treg cells, especially in high-dose water-soluble components. The differentiation of Treg cells may be influenced by PM2.5 and its components, leading to a decrease in the body's immune function.
A prior study indicated that exposure to PM2.5 decreased the expression of IL-10 and TGF-β in Jurkat T cells, which led to inflammation exacerbation through impaired Treg cells function and T-helper type (Th1/Th2) cells imbalance (Zhang X et al., 2015). Many epidemiological studies have displayed that air pollution, especially TRPM2.5, is more closely affiliated with the incidence of diseases associated with a variety of immune cells (Wu Y et al., 2022; Wan Q et al., 2019). Our study showed that the levels of IL-10 and TGF-β in cell supernatant decreased after treatment with TRPM2.5, WSI, or OE, whereas the expressions of IL-10 and TGF-β were increased after treatment with AG490. Similarly, the reduced expression of cytokines IL-10 and TGF-β promotes the production of other inflammatory cytokines, inhibits the formation of allergen tolerance, and ultimately promotes the development of allergic disease (McGuirk et al., 2010; Raphael et al., 2015). These results were in line with other research (Wang Ca et al., 2021). It has been found that exposure to TRPM2.5, WSI, or OE of asthmatic rats promoted pulmonary inflammatory response, inhibited the differentiation of Treg cells, and reduced the secretion of IL-10 and TGF-β, thus aggravating asthma. Xu et al (Xu Z et al., 2020) showed that the level of IL-10 was significantly reduced in the lung injury induced by PM2.5 in asthmatic mice. Zhao et al (Zhao H et al., 2019) proved that the percentage of Treg cells and the level of TGF-β in the peripheral blood of asthmatic rats were significantly lower than that of the control group. Recently, Wang et al (Wang Cb et al., 2022) reported that after exposure to PM2.5, the levels of treg-related cytokines TGF-β and IL-10 in the alveolar lavage fluid of allergic airways in rats showed a decreasing trend. The above research indicated that PM2.5 and its water-soluble and organic components can promote cellular inflammation by inhibiting the secretion of TGF-β and IL-10.
Numerous studies have shown that the activation of JAK/STAT signaling pathway regulates immune cell imbalance and inflammatory response induced by PM2.5 (Hu H et al., 2016). In addition, the JAK/STAT signaling pathway is also associated with Treg cell differentiation (Shi et al., 2017). JAK2/STAT5 is an important pathway member of the JAK/STAT family, which can mediate various cytokine signal transduction mechanisms and generate biological effects through the activation of target genes (Huynh et al., 2017). Studies have shown that the JAK2 inhibitor, AG490 can regulate related cell balance through JAK2/STAT5 signaling pathway by inhibiting inflammation and immune response (Sheng W et al., 2014). Xu (Xu Z et al., 2020) proved that AG490 supplementation could reduce the activation of JAK2/STAT3 signaling pathway, attenuate the expression and release of cytokines in bronchial epithelial cells, and thereby alleviate inflammation. Our data showed that Jurkat T cells treated with TRPM2.5, WSI, and OE could significantly reduce the levels of STAT5, p-STAT5, and Foxp3, while significantly improving the expression of JAK2 and p-JAK2. After AG490 treatment, the expressions of JAK2 and p-JAK2 were reduced, while the terms of STAT5, p-STAT5, and Foxp3 were enhanced. This is consistent with the research conclusion proposed by Wang that the STAT5 and Foxp3 protein levels of PHA-activated Jurkat T cells were reduced after PM2.5 treatment (Wang Ca et al., 2021). Moreover, in the early stage of our team's in vivo experiments (Wang Cc et al., 2020), it was found that PM2.5 and its water-soluble and organic components can increase the expression of STAT3 and RORγt proteins, reduce the expression of STAT5 and Foxp3 proteins, promote Th17 cell differentiation, reduce the proportion of Treg cells in peripheral blood, and exacerbate asthma attacks in asthmatic rats; It was also found that low and high-dose exposure to water-soluble components of PM2.5 significantly reduced the expression levels of STAT5 and Foxp3 proteins in the lung tissue of asthmatic mice compared to the control group (Wang J, 2019).
STAT5, as an important member of the STAT family, can directly bind to the gene of Foxp3, and initiate the differentiation program of Treg (Cohen and Nadeau, 2006). Studies have shown that STAT5, as a transcription factor involved in gene induction, its lack of expression can inhibit the expression of CD25, leading to T lymphocyte proliferation failure (Shatrova et al., 2016). Another study found that during the maturation process of T lymphocytes, they undergo rearrangements of related gene loci, while the deletion of STAT5 can disrupt TCR γ rearrangement and survival of CD8+T lymphocytes affect the differentiation and functional maintenance of peripheral blood Treg cells (Soeharso et al., 1992). Our current study revealed that STAT5 was closely involved in the function of Jurkat T cells. To determine the effect of differentially expressed STAT5 on the part of Jurkat T cells, we transfected the overexpressed plasmid to up-regulate STAT5 expression. We found that the expression of Foxp3 was also significantly increased. Increased activation of STAT5 promotes its binding to the Foxp3 promoter, thereby activating Foxp3 and reducing the secretion of inflammatory factors. These results suggest that STAT5 expression plays a regulatory role in developing immune inflammation response.
In conclusion, traffic-related PM2.5 and its water-soluble and organic components can mediate JAK2/STAT5/Foxp3 signaling pathway, inhibit the proliferation and differentiation of Treg cells in Jurkat T cells, and reduce the secretion of IL-10 and TGF-β cytokines and further exacerbate the inflammatory response (Diagram of mechanism see Fig. 8). It suggests that this signaling pathway can be blocked to alleviate the inhibitory effects of PM2.5 and its components on the immune system, providing therapeutic targets for clinical treatment of related diseases.