Circadian Clock Disruption Suppresses PDL1⁺ Intraepithelial B Cells in Experimental Colitis and Colitis-Associated...

The circadian clock is crucial for physiological homeostasis including gut homeostasis. Disorder of the circadian clock may contribute to many diseases including inflammatory bowel disease (IBD). However, the role and the mechanisms of circadian clock involvement in IBD still are unclear.

Disorder of the circadian clock including chronic social jet lag and circadian clock gene deficiency mice (Bmal1^-/-, and Per1^-/-Per2^-/-) were established. Dextran sulfate sodium (DSS) and/or azoxymethane were used to induce mouse models of colitis and its associated colorectal cancer. Flow cytometry, immunohistochemistry, immunofluorescence, Western blot, and reverse-transcription quantitative polymerase chain reaction were used to analyze the characteristics of immune cells and their related molecules.

Mice with disorders of the circadian clock including chronic social jet lag and circadian clock gene deficiency were susceptible to colitis. Functionally, regulatory B (Breg) cells highly expressing Programmed cell death 1 ligand 1 (PDL1) in intestinal intraepithelial lymphocytes (IELs) helped to alleviate the severity of colitis after DSS treatment and was dysregulated in DSS-treated Bmal1^-/- mice. Notably, interleukin 33 in the intestinal microenvironment was key for Bmal1-regulated PDL1⁺ Breg cells and interleukin 33 was a target of Bmal1 transcriptionally. Dysregulated PDL1⁺ B cells induced cell death of activated CD4⁺ T cells in DSS-treated Bmal1^-/- mice. Consequently, circadian clock disorder was characterized as decreased numbers of Breg⁺ PDL1⁺ cells in IELs and dysfunction of CD4⁺ T cells promoted colitis-associated colorectal cancer (CRC) in mice. In clinical samples from CRC patients, low expression of Bmal1 gene in paracancerous tissues and center area of tumor was associated closely with a poorer prognosis of CRC patients.

Our study uncovers the importance of the circadian clock regulating PDL1⁺ Breg⁺ cells of IELs in IBD and IBD-associated CRC.