In our study, we found that the study populations were exposed to microcystins. The concentration range of serum microcystins was 0.06 ~ 0.66 µg/L, and the median was 0.21 µg/L. Our study site is located in Central China, with a subtropical monsoon humid climate, which is suitable for the growth and propagation of microcystins. Microcystins pollution is not a rare phenomenon in China.
In some areas of China, microcystin pollution is a serious phenomenon, especially in tropical and humid areas, among which Chao Lake is the region that is most seriously polluted by microcystins, where the serum level of MC-LR in fishermen can be up to 0.38 µg/L[19]. Many researchers have pointed out that MCs may affect human health in the following ways: contaminated water and aquatic products, dietary supplements, body contact, hemodialysis and inhalation[20–21]. Therefore, the reasons for the study populations exposure to microcystins may be as follows. First, water and aquatic products(such as fish, snails, soft-shelled turtles, and ricefield eel.) are contaminated by MCs. Second, residents lack knowledge of cyanobacteria and microcystins, so they ignore daily contact in everyday life. Finally, residents’ exposure to microcystins is also affected by their behavior and lifestyles, such as their preference for aquatic products and well water.
We conducted a case-control study in women of childbearing age and found that the average level of serum MC-LR in CPID control subjects was higher than that in case patients. Microcystins(0.06 ~ 0.66µg/L) were a protective factor for CPID risk. Previous animal experiments and clinical observations have reported that MCs can promote and inhibit inflammation, but CPID epidemiological studies are rare. The potential mechanism for the association between MCs and CPID is complicated. First, microcystins caused imbalance of pro-inflammatory and anti-inflammatory cytokines in vitro. According to previous studies, IL-1, TNF-α, IFN-g and GM-CSF are recognized as the main inflammatory cytokines and participate in resistance to xenoantigens, and their mRNA levels were inhibited by MC-LR in a dose dependent manner[22]. Several studies have shown that the expression of pro-inflammatory cytokines were suppressed and anti-inflammatory cytokines were promoted in blood with microcystins at higher concentrations[16–17, 23].The downregulation of inflammatory levels caused by MC-LR may be related to escaping immune monitoring and these changes may weaken the function of macrophages.
In contrast, the lower concentration of MCs promoted pro-inflammatory and suppressed anti-inflammatory in blood, and the imbalance of pro-inflammatory cytokines expression were more rapid than that of anti-inflammatory cytokines. This change might be related to the low concentration of microcystins inducing macrophages to activate the NF-κB and ERK1/2 pathways in autophagy[24–25]. This serve as a possible explanation for the low incidence rate of CPID with MCs at the higher concentration (≥ 0.25 µg/L) in this study, but it is notesd that, the low concentration levels in animal and cell experiments are much higher than those in human serum; for example, the incubation of leukocytes isolated from blood and head kidney with a high concentration of MC-LR (0.1 mg/ml) caused a significantly increased (P < 0.05) expression of IL-10 and TNF-α, while a low concentration (0.01 mg/ml) did not induce the expression of IL-10 and TNF-α[23]. In addition, the inhibition of MC-LR on inflammation is closely related to the time of action. For example, medium and high concentration of MC-LR (40 ~ 80 g/L) could promote the expression of the pro-inflammatory cytokines- TNF-α, IL-1β and IFN-γ after 48 hours, and inhibit the expression of the anti-inflammatory cytokines- IL-4, IL-10 and IL-13 after 72 hours[26]. The anti-inflammatory effect of microcystin is significantly different based on its concentration and time. Second, the ratios of Th, Tc and double-positive T cells were changed at certain concentrations of microcystins, which affected the adaptive immune response of the body. NK cells and gamma-delta T cells were increased at higher concentrations of microcystins, which affected the innate immune responses. NK cells and γδ T cells are the bridge between adaptive and innate immune response in vivo. Therefore, human body exposure to microcystins generates an adaptive immune response, stimulating the production of antibodies and inhibiting inflammation[27]. Third, MCs can easily cross the intestine-blood barrier and enter various cells through OATP transporters[28]. Some studies have shown that subchronic exposure to MC-LR increased the gut microbial diversity in the caecum, including Porphyromonadacese, Lachnospiraceae, Ruminococcaceae and Prevotellaceae, in both Bacteriodetes and Firmicutes. Firmicutes were absolutely predominant in caecum and colon[29]. Lactobacillus is a species of Firmicutes, and the increase in intestinal Lactobacillus is beneficial for maintaining the microecological balance, and preventing retrograde infection of intestinal flora, thus promoting the health of the body or treatment of certain diseases. Therefore, microcystins have a certain effect on changes in the microclimate in the genital tract. Finally, phycocyanin is produced by cyanobacteria, which are rare natural nutrients. Previous studies have proven that phycocyanin has anti-inflammatory and anti-tumor effects and increases the richness and diversity of the microbiota[30–32]. However, when phycocyanins is extracted, a large amount of microcystins may be released. Phycocyanin can enter the human body along with microcystins through the consumption of cyanobacteria-contaminated water and food and plays an anti-inflammatory role and modulates genital tract microbiota. Further research is required to determine the concentration of phycocyanin in the human body, and its role, and underlying mechanisms of phycocyanin in regulating genital tract microbiota.
Our study has several limitations. First, the sample size is small. Second, we did not measure all types of microcystins in serum. Third, we did not perform additional experiments to explore the possible mechanism of association between MCs and CPID. We will conduct related studies in the future.