Reprogramming of Myeloid Compartments Supporting Tissue Repair During Dss-Induced Colitis Recovery

Myeloid-derived suppressor cells (MDSC), emerging during tumor growth or chronic inflammation play a critical role in regulating T cell function. However, mechanisms governing the generation of these cells remain unclear, and need to be further defined. Using a DSSinduced colitis and recovery model, we characterized the dynamic changes within myeloid compartments and the emergence of MDSC during active and resolution phases of inflammation. We show that the immature myeloid compartment expands in bone marrow (BM) specifically at the resolution phase of inflammation during colitis transition to recovery. Additionally, we found enhanced levels of IL-17 in the serum of colitis mice tightly correlates with expansion of the IMC compartment, and is likely the factor responsible for expansion of these cells. Our study also determined that the expanded population of myeloid cells underwent a functional reprogramming event. In particular, two major functional changes occurred when colitic mice were allowed to recover: 1) CD11bGr-1 myeloid cells in bone marrow and spleen acquired T cell suppressive functions, and 2) acquired the ability to enter into circulation from BM, confirming previously reported characteristics of MDSC. Additionally, we determined that acquired migratory capability in the low density myeloid cells isolated from resolution time points was due to enhanced surface expression of chemokine receptor CXCR2. Furthermore, we determined that after mobilization of MDSC from the bone marrow, these cells collected in the T cell-rich spleens, where they effectively functioned to suppress T cell proliferation. Through these acquired functions, our study determines a protective role for MDSC during the recovery phase of post-acute inflammation during persistent DSS-induced colitis. INDEX WORDS: MDSC, Colitis, IBD, Inflammation, Tissue Repair, Immunosuppression REPROGRAMMING OF MYELOID COMPARTMENTS SUPPORTING TISSUE REPAIR DURING DSS-INDUCED COLITIS RECOVERY