Potential role of myeloid-derived suppressor cells in pulmonary fibrosis

Abstract Idiopathic pulmonary fibrosis (IPF) is a progressive fibrotic lung disease without effective curative therapy. Recruitment of bone marrow-derived myeloid cells is implicated in lung fibrosis by promoting fibrosis via paracrine mechanisms. Recent evidence shows increased circulating myeloid-derived suppressor cells (MDSC) in IPF patients. More recently, the importance of MDSC is suggested by the observation of MDSC accumulation in IPF lungs. The objective of this study was to assess the accumulation/expansion of MDSC in bleomycin-induced mouse model of pulmonary fibrosis and human IPF, and investigate their potential role in lung myofibroblast differentiation. Flow cytometry analysis showed that MDSCs in lung were increased in a mouse model of lung fibrosis, as well as in peripheral blood samples from IPF patients. The increase of the monocytic subtype of MDSC (M-MDSC) was greater than that for the granulocytic subtype (G-MDSC) in the peripheral blood samples from patients with IPF. In vitro co-culture of normal mouse lung fibroblasts with sorted bone marrow (BM)-derived MDSCs using trans-well inserts revealed paracrine activation of lung fibroblasts by activated M-MDSC, but not GMDSC, as manifested by significantly elevated mRNA levels of α-smooth muscle actin and TGFβ mRNAs. Both quiescent M-MDSC and G-MDSC failed to stimulate fibroblast activation or myofibroblast differentiation. Interestingly, the level of TGFβ mRNA in M-MDSC was higher than G-MDSC. These findings indicated that in pulmonary fibrosis, BM-derived MDSCs were mobilized and recruited to the lung, and the recruited M-MDSC subpopulation subsequently played a paracrine role by activating resident lung fibroblasts and myofibroblast differentiation.