Research reveals how the dietary flavonoid promotes regulatory T cell development and suppresses inflammation through key immune pathways.
Study: Kaempferol Exerts Anti-Inflammatory Effects by Accelerating Treg Development via Aryl Hydrocarbon Receptor-Mediated and PU.1/IRF4-Dependent Transactivation of the Aldh1a2/Raldh2 Gene in Dendritic Cells. Image Credit: Danijela Maksimovic/Shutterstock.com
In a recent article published in the Allergy, a group of researchers studied how kaempferol, a dietary flavonoid, promotes regulatory T cell (Treg) development by activating the Aldh1a2/Raldh2 gene in dendritic cells through the aryl hydrocarbon receptor (AhR) and PU.1/IRF4 pathways.
Background
Raldh2, encoded by the Aldh1a2 gene, is expressed in intestinal dendritic cells (DCs) and plays a critical role in immune regulation. It converts retinal into retinoic acid (RA), a key ligand for the nuclear receptor retinoic acid receptor (RAR). RA drives the development of Tregs by activating Forkhead box P3 (Foxp3), the master transcription factor for Treg differentiation.
Tregs are essential for maintaining immune tolerance and preventing inflammatory diseases. However, the regulatory mechanisms of Aldh1a2 expression and Raldh2 function remain unclear, necessitating further research to uncover pathways that enhance Treg development and immune homeostasis.
The study
The study began by screening dietary compounds for their ability to promote Treg development through Aldh1a2 activation in DCs. Kaempferol was identified as the most effective compound in increasing Aldh1a2 messenger Ribonucleic acid (mRNA) expression.
Experiments using bone marrow-derived dendritic cells (BMDCs) and migratory DCs (migDCs) from mesenteric lymph nodes (MLNs) confirmed that kaempferol treatment enhanced Raldh2 enzymatic activity.
To evaluate the functional impact of kaempferol-induced Raldh2 activity, DCs pretreated with kaempferol were co-cultured with naïve Cluster of Differentiation (CD)4+ T cells.
The results showed a significant increase in the frequency of Foxp3+ Tregs, demonstrating that kaempferol confers Treg-inducing potential to DCs. This effect was accompanied by suppression of T-cell proliferation, further indicating kaempferol's role in promoting immune tolerance.
The study next investigated the molecular mechanisms behind kaempferol-induced Aldh1a2 expression. The aryl hydrocarbon receptor (AhR), a transcription factor and known target of kaempferol, was identified as a key mediator. Kaempferol treatment increased both the mRNA and protein levels of AhR in BMDCs.
Knockdown of AhR using small interfering (si)RNA abolished the kaempferol-induced increase in Aldh1a2 mRNA and Raldh2 activity, confirming the dependence of this pathway on AhR. In contrast, treatment with 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), an AhR agonist, reduced Aldh1a2 expression, while an AhR antagonist, CH-223191, enhanced it.
These results suggest that kaempferol functions by antagonizing AhR, thereby relieving its inhibitory effect on Aldh1a2 expression.
The transcription factors PU.1 and IRF4 are known to transactivate Aldh1a2 by binding to its upstream enhancer regions in DCs. The study examined the role of these factors in kaempferol-induced Aldh1a2 expression.
Kaempferol treatment increased both PU.1 and IRF4 mRNA and protein levels in BMDCs. Chromatin immunoprecipitation (ChIP) assays further demonstrated that kaempferol enhanced PU.1 recruitment to the Aldh1a2 enhancer. Knockdown of Spi1 (PU.1-encoding gene) or Irf4 using siRNA abolished kaempferol-induced Aldh1a2 expression, confirming the requirement of these factors.
Interestingly, the study revealed that AhR negatively regulates PU.1 post-transcriptionally while also repressing IRF4 transcription. Kaempferol antagonizes AhR to relieve this repression, leading to increased PU.1 and IRF4 expression and subsequent Aldh1a2 activation. These findings highlight a complex regulatory interplay involving AhR, PU.1, and IRF4.
The study extended its findings to an in vivo mouse model. Intraperitoneal administration of an AhR antagonist in mice increased the frequency of DCs with Raldh2 activity in the mesenteric lymph nodes.
Furthermore, administration of kaempferol-treated BMDCs into mice enhanced the frequency of Foxp3+ Tregs in Peyer’s patches, demonstrating the physiological relevance of kaempferol’s effects on DCs.
To evaluate kaempferol's role in allergic inflammation, the study employed an ovalbumin (OVA)-induced food allergy model in mice. Kaempferol treatment during the sensitization phase significantly suppressed allergic diarrhea and reduced the rapid drop in body temperature observed after the OVA challenge.
These results suggest that kaempferol enhances immune tolerance by promoting Treg development and suppressing allergic responses.
Conclusions
To summarize, the study concludes that kaempferol enhances Treg development by upregulating Aldh1a2 expression and Raldh2 activity in DCs.
This effect is mediated through the antagonism of the AhR, which relieves its repression of the transcription factors PU.1 and IRF4.
The findings demonstrate that kaempferol confers Treg-inducing potential to DCs, promoting immune tolerance and suppressing allergic inflammation in vivo.
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