Lymph-absorbed Radix Astragali polysaccharide induces IgA production through TLR4-dependent DC-B axis to alleviate colitis

Background:
Immunoglobulin A (IgA) is essential for maintaining intestinal homeostasis and protecting against colitis, yet the mechanism by which natural polysaccharides enhance IgA responses remains unclear.

Objective:
This study aimed to elucidate how lymph-absorbed Radix Astragali polysaccharide (RAP) regulates anti-colitis IgA levels.

Methods:
RAP’s protective effects were evaluated in a DSS-induced colitis mouse model, and its cellular target within Peyer’s patches (PPs) was investigated by flow cytometry and confocal microscopy. In vitro DC-B culture, RNA sequencing, and functional assays were applied to elucidate RAP-induced IgA production and underlying signaling. TLR2- and/or TLR4-deficient models were used to verify their roles in vivo and in vitro.

Results:
RAP alleviated DSS-induced colitis in mice, with increased IgA expression in the PPs and colon. Mechanistically, RAP was found to target and activate dendritic cells (DCs) after entering PPs, stimulating cytokine secretion to mediate B cell activation and IgA production, without directly affecting B cells. TLR2/4 signaling (predominantly TLR4) was identified as the key player for RAP-induced DC activation and subsequent cytokine secretion, as verified in TLR2/4-deficient cells and mice. Although RAP-induced DC cytokines were sufficient for IgA production in vitro, additional mechanisms likely drive the rapid in vivo response. Interestingly, while TLR2/4 was indispensable for DC function, TLR2/4-deficient B cells retained IgA-producing capacity when driven by RAP-activated DCs.

Conclusion:
The lymph-absorbed RAP targets DCs to induce TLR4-dependent cytokine release, driving IgA production through the DC-B axis, thereby alleviating colitis. This DC-B axis provides valuable insights into the IgA-based anti-colitis mechanisms underlying lymph-absorbed natural polysaccharides.