Bifidobacterium infantis regulates the programmed cell death 1 pathway and immune response in mice with inflammatory...

Inflammatory bowel disease (IBD) is caused by an abnormal immune response. Programmed cell death 1 (PD-1) is an immunostimulatory molecule, which interacts with PD ligand (PD-L1) playing a prime important role among autoimmune diseases. Bifidobacterium infantis (B. infantis) can promote the differentiation of CD (cluster of differentiation) 4⁺ T cells into regulatory T cells (Tregs). Tregs participate in the development of IBD and may be related to disease activity. B. infantis amplify the expression level of PD-1, PD-L1 and Tregs' nuclear transcription factor forkhead box protein 3 (Foxp3). But the mechanism of B. infantis on PD-1/PD-L1 signaling remains unclear.

To explore the mechanism of B. infantis regulating the immune response in IBD.

Forty-eight-week-old BALB/c mice were randomly divided into five groups: The control group, dextran sulphate sodium (DSS) model group, DSS + B. infantis group, DSS + B. infantis + anti-PD-L1 group, and DSS + anti-PD-L1 group. The control group mice were given drinking water freely, the other four groups were given drinking water containing 5% DSS freely. The control group, DSS model group, and DSS + anti-PD-L1 group were given normal saline (NS) 400 μL daily by gastric lavage, and the DSS + B. infantis group and DSS + B. infantis + anti-PD-L1 group were given NS and 1 × 10⁹ colony-forming unit of B. infantis daily by gastric lavage. The DSS + B. infantis + anti-PD-L1 group and DSS + anti-PD-L1 group were given 200 μg of PD-L1 blocker intraperitoneally at days 0, 3, 5, and 7; the control group, DSS + anti-PD-L1 group, and DSS + B. infantis group were given an intraperitoneal injection of an equal volume of phosphate buffered saline (PBS). Changes in PD-L1, PD-1, Foxp3, interleukin (IL)-10, and transforming growth factor β (TGF-β) 1 protein and gene expression were observed. Flow cytometry was used to observe changes in CD4⁺, CD25⁺, Foxp3⁺ cell numbers in the blood and spleen.

Compared to the control group, the expression of PD-1, Foxp3, IL-10, and TGF-β1 was significantly decreased in the intestinal tract of the DSS mice (P < 0.05). Compared to the control group, the proportion of CD4⁺, CD25⁺, Foxp3⁺ cells in spleen and blood of DSS group was visibly katabatic (P < 0.05). B. infantis upgraded the express of PD-L1, PD-1, Foxp3, IL-10, and TGF-β1 (P < 0.05) and increased the proportion of CD4⁺, CD25⁺, Foxp3⁺ cells both in spleen and blood (P < 0.05). After blocking PD-L1, the increase in Foxp3, IL-10, and TGF-β1 protein and gene by B. infantis was inhibited (P < 0.05), and the proliferation of CD4⁺, CD25⁺, Foxp3⁺ cells in the spleen and blood was also inhibited (P < 0.05). After blocking PD-L1, the messenger ribonucleic acid and protein expression of PD-1 were invariant.

It is potential that B. infantis boost the proliferation of CD4⁺, CD25⁺, Foxp3⁺ T cells in both spleen and blood, as well as the expression of Foxp3 in the intestinal tract by activating the PD-1/PD-L1 pathway.