BACKGROUND AND AIMS

Myeloid-derived suppressor cells (MDSCs) encompass a novel population of suppressor cells and a potential candidate for cell-based therapies in inflammatory diseases. Herein, we investigated their immunomodulatory properties in experimental inflammatory colitis and T cell-mediated immune responses in inflammatory bowel disease (IBD) patients.

METHODS

MDSCs (defined as CD14HLADRCD33CD15) numbers were determined in peripheral blood (PB) from IBD patients. PB MDSC function was assessed in vitro. Experimental colitis was induced upon 2,4,6-trinitrobenzene sulfonic acid (TNBS) treatment and MDSCs were characterized by flow cytometry. The in vivo suppressive potential of bone marrow (BM)-derived MDSCs (BM-MDSCs) was tested by using both depleting and adoptive transfer strategies.

RESULTS

MDSCs were enriched in the periphery of IBD patients during active disease. TNBS colitis induced amplification of MDSCs, particularly of the granulocytic (Ly6G) subset during the effector phase of disease. Of interest, BM-MDSCs potently suppressed CD4T cell responses under steady state but failed to control colitis-associated immune responses in vivo. Mechanistically, under the colonic inflammatory milieu MDSCs switched phenotype (decreased proportion of Gr1 and increased numbers of Gr1) and downregulated CCAAT/enhancer-binding protein beta (CEBPβ) expression, a critical transcription factor for the suppressive function of MDSCs. In accordance with the murine data, human CD33  CD15MDSCs from peripheral blood of IBD patients not only failed to suppress autologous T cell responses but instead enhanced T cell proliferation in vitro.

CONCLUSIONS

Our findings demonstrate an aberrant function of MDSCs in experimental inflammatory colitis and in IBD-associated immune responses in vitro. Delineation of the mechanisms that underlie the loss of MDSCs function in IBD may provide novel therapeutic targets.