High frequency of mononuclear myeloid-derived suppressor cells is associated with exacerbation of inflammatory bowel disease.

Exacerbation and relapse of inflammatory bowel disease (IBD) is associated with reduced antibacterial immunity and increased immune regulatory activity, but the source of increased immune regulation during episodes of disease activity is unclear. Myeloid-derived suppressor cells (MDSCs) are a cell type with a well-recognized role in limiting immune reactions. MDSC function in IBD and its relation to disease activity, however, remains unexplored. Here we show that patients with either ulcerative colitis (UC) or Crohn's disease (CD) have high peripheral blood levels of mononuclear MDSCs. Especially exacerbation of disease is associated with higher levels of mononuclear MDSC counts compared with those in remission of disease. Interestingly, chronic experimental colitis in mice coincides with increased MDCS mobilization. Thus, our results suggest that mononuclear MDCS are endogenous antagonists of immune system functionality in mucosal inflammation and the depression of antibacterial immunity associated with exacerbation of disease might involve increased activity of the MDSC compartment.