Myeloid cells are the most abundant and heterogeneous population of leukocytes. They are rapidly recruited from the blood to areas of inflammation and perform a number of important biological functions. Chronic inflammatory conditions contribute to generation of myeloid-derived suppressor cells (MDSCs). These pathologically activated cells are increasingly recognized as important players in cancer, transplantation, and autoimmunity for their abilities to modulate innate and adaptive immune responses.
Since clinical data on MDSC accumulation in human patients affected with inflammatory bowel diseases (IBD) are relatively scarce, most of the information described in this review came from studies using experimental mouse models of IBD.
In this review, we discuss possible roles of these cells in chronic immune-mediated disorders focusing on studies conducted in IBD. We will review the available evidence on how MDSCs are involved in modulating T cell responses and look into the complex relationship between Th1, Th17 cells, and myeloid cells. Finally, we will review some recent successes and failures resulted from therapies aimed at manipulating myeloid cell numbers and/or their function.
Although MDSCs have been described in animal models of experimental colitis and in patients with IBD, their exact role in IBD pathogenesis is unclear and needs to be studied further. Information obtained from these studies will be useful to better understand the cross talk between myeloid cells in T cells during chronic inflammation and may identify novel pathways to be targeted therapeutically.