In a recent study published in the journal Immunity, researchers in Brazil describe the crucial function of gut dysbiosis during pregnancy in promoting excessive macrophage pyroptosis, which increases vulnerability to sepsis.
Study: Gut microbiota in pregnancy, please don’t change me now. Image Credit: wutzkohphoto / Shutterstock.com
Sepsis and pregnancy
Sepsis is characterized by a life-threatening dysfunction of several organs caused by a dysregulated inflammatory response of the host to infection. Men are more frequently affected by sepsis as compared to women. However, certain events like pregnancy increase the risk of women suffering from severe sepsis, which accounts for one of the main reasons for maternal mortality and morbidity across the globe.
Changes in the immunological response are likely responsible for the increased susceptibility and severity of infections that occur as pregnancy advances. However, the exact mechanisms responsible for the increased risk of sepsis during pregnancy remain uncertain.
About the study
In the present study, researchers explore how gut dysbiosis during pregnancy inhibits the macrophage-dependent immunological response of the host and subsequently leads to an increased risk of sepsis.
To this end, the researchers investigated the vulnerability of pregnant mice to induced sepsis using two distinct experimental models of experimental sepsis. These included polymicrobial peritonitis caused by pneumonia elicited by Pseudomonas aeruginosa intratracheal inoculation and cecal ligation and puncture (CLP) surgery.
The potential role of gut dysbiosis during pregnancy in amplifying sepsis was assessed by transplanting fecal microbiota obtained from pregnant women or mice into non-pregnant mice. The team also conducted 16S ribosomal RNA gene sequencing to determine how gut microbial dysbiosis impacts sepsis susceptibility during pregnancy. A proteomic study was also performed.
Pregnancy was found to worsen sepsis outcomes, characterized by an excessive inflammatory response, severe multiple organ injury, and poor bacterial clearance. Furthermore, fecal microbiota transplantation from pregnant mice increased the inflammatory response and risk of death among non-pregnant mice with sepsis. In contrast, transplantation of fecal microbiota from non-pregnant mice reduced sepsis mortality.
Thus, modifications in gut flora during pregnancy increase vulnerability to sepsis. Furthermore, intestinal dysbiosis was associated with immunological dysfunction, resulting in greater sepsis susceptibility in mice.
Metabolomic analysis of cecum contents obtained from pregnant mice indicated a notable alteration in the metabolic characteristics of their gut microbiota, such as reduced formononetin (FMN) levels. In addition, pregnant mice that had undergone CLP exhibited reduced Parabacteroides merdae levels in the peritoneal fluid lavage, which correlated with lower FMN concentrations.
These findings indicate that P. merdae may affect endogenous FMN bioavailability. Gut microbial b-galactosidases also catalyzed the hydrolysis of bioactive substances from food, which resulted in the production of aglycones of isoflavones, such as FMN.
Selective depletion of macrophages was observed in the peritoneal cavity of pregnant mice with sepsis. Transplantation of fecal microbiota from pregnant mice to non-pregnant mice also replicated this behavior. Macrophage depletion with clodronate liposomes exacerbated CLP-induced sepsis, thereby nullifying the protective effects of FMN therapy.
Pregnancy or fecal microbiota transplantation from pregnant mice increased peritoneal macrophage cell mortality rates in mice with sepsis. In contrast, FMN therapy reduced the death of peritoneal macrophages.
The higher frequency of peritoneal macrophages in pregnant septic mice administered with a caspase-1 inhibitor and not with other cell death inhibitors suggests that pyroptosis may be needed to increase macrophage death induced by sepsis during pregnancy.
FMN therapy inhibited gasdermin D (GSDMD) and caspase-1 p20 synthesis, as well as apoptosis-associated speck (ASC) oligomerization elicited by lipopolysaccharide (LPS)/adenosine triphosphate (ATP) in bone-marrow-derived macrophages (BMDM), thereby suppressing macrophage pyroptosis and interleukin 18 (IL-18) and IL-1b release.
Furthermore, FMN lowered Nlrp3 messenger ribonucleic acid (mRNA) transcript among LPS/ATP-activated BMDM. This provided a novel mechanism by which FMN may regulate pyroptosis.
Silencing heterogeneous nuclear ribonucleoprotein U-like protein 2 (hnRNPUL2) lowered macrophage pyroptosis genetically. Furthermore, comprehensive analyses demonstrated that FMN could directly interact with hnRNPUL2.
This facilitated its subsequent association with the chaperone heat shock cognate 70 (HSC70) protein, which interacted with misfolded or denatured proteins to ultimately promote lysosomal protein breakdown as a vital mechanism for cellular homeostasis. FMN also promoted hnRNPUL2 colocalization with lysosomes, thus reducing its nuclear accumulation within LPS/ATP-activated BMDM.
Reporter gene and chromatin immunoprecipitation (ChIP) assays showed that hnRNPUL2 directly interacted with the promoter area and stimulated Nlrp3 transcription in BMDMs. These results demonstrate that FMN inhibited macrophage pyroptosis through the reduction in Nlrp3 gene expression and hnRNPUL2 nuclear accumulation.
Overall, the study findings highlight the function of the hnRNPUL2- NLRP3 axis, which is mediated by P. merdae dysbiosis and reduced FMN metabolite levels, in the excessive stimulation of macrophage pyroptosis in sepsis-induced immunological dysfunction during pregnancy. Therefore, future studies should consider whether FMN medication or supplements could be utilized as a therapeutic approach to treat sepsis in pregnant women.
- Nascimento, D. C., & Alves-Filho, J. C. (2023). Gut microbiota in pregnancy, please don’t change me now. Immunity 56(2); 232-234. doi:10.1016/j.immuni.2023.01.017