Circulating Fibrocytes Prepare the Lung for Cancer Metastasis by Recruiting Ly-6C+ Monocytes Via CCL2

Fibrocytes are circulating, hematopoietic cells that express CD45 and Col1a1. They contribute to wound healing and several fibrosing disorders by mechanisms that are poorly understood. In this report, we demonstrate that fibrocytes predispose the lung to B16-F10 metastasis by recruiting Ly-6C+ monocytes. To do so, we isolated fibrocytes expressing CD45, CD11b, CD13, and Col1a1 from the lungs of wild type (WT) and Ccr52/2 mice. WT but not Ccr52/2 fibrocytes increased the number of metastatic foci when injected into Ccr52/2 mice (73 6 2 versus 32 6 5; p < 0.001). This process was MMP9 dependent. Injection of WT enhanced GFP+ fibrocytes also increased the number of Gr-1Int, CD11b+, and enhanced GFP– monocytes. Like premetastaticniche monocytes, these recruited cells expressed Ly-6C, CD117, and CD45. The transfer of these cells into Ccr52/2 mice enhanced metastasis (90 6 8 foci) compared with B cells (27 6 2), immature dendritic cells (31 6 6), or alveolar macrophages (28 6 3; p <0.05). WTand Ccl22/2 fibrocytes also stimulated Ccl2 expression in the lung by 2.07 6 0.05- and 2.78 6 0.36-fold compared with Ccr52/2 fibrocytes (1.0 6 0.06; p < 0.05). Furthermore, WT fibrocytes did not increase Ly-6C+ monocytes in Ccr22/2 mice and did not promote metastasis in either Ccr22/2 or Ccl22/2 mice. These data support our hypothesis that fibrocytes contribute to premetastatic conditioning by recruiting Ly-6C+ monocytes in a chemokine-dependent process. This work links metastatic risk to conditions that mobilize fibrocytes, such as inflammation and wound repair.