Abstract 1533: Fibrocytes promote metastasis by recruiting Ly-6C+ myeloid cells

Abstract Background: Fibrocytes are circulating, hematopoietic cells characterized by their expression of CD45 and procollagen I. During wound repair, these cells migrate to areas of injury and differentiate into activated fibroblasts. We have found fibrocytes contribute to metastasis. Here, we show these cells promote metastasis by recruiting monocytes of the premetastatic niche. Methods: Fibrocytes were isolated from pulmonary mesenchymal cells using CD45+ immunomagnetic beads. 1x105 cells were intravenously injected into selected C57BL/6 or BALB/c mice. Changes induced by fibrocytes were studied by collecting tissue 24 hours after injection. Modeling metastases was performed by intravenously injecting 7.5x106 B16-F10 or 2x105 CT26 cells 24 hours after fibrocyte injection. Metastases were counted two weeks later. For wounding experiments, a 3 cm abdominal midline incision was made and closed in two layers. CT26 cells were injected after wounding on post-op days 3, 5, 8, and 13 in separate cohorts. Results: Wild type (WT) CD45+ fibrocytes increased B16-F10 metastasis in CCR5-/- mice when compared to WT CD45- fibroblasts (p<0.002) and CCR5-/- fibrocytes (p<0.01). WT fibrocytes failed to increase metastasis in CCR2-/- or CCL2-/- mice. Consistent with this observation, WT fibrocytes induced the expression of CLL2 in a cell extrinsic manner in CCR5-/- mice. Flow cytometric analysis demonstrated that WT fibrocytes generated a significant increase in Gr-1Int, CD11b+ monocytes (p<0.05). No such increase was seen in CCR2-/- mice or following the injection with CCR5-/- fibrocytes. Further analysis of the recruited cells revealed expression of Ly-6C, CD117, and CD45, which suggested the presence of premetastatic-niche monocytes. These Ly-6C++ cells were isolated after fibrocyte injection and adoptively transferred into CCR5-/- mice. These mice developed more metastasis than mice treated with other populations of cells including B cells, immature dendritic cells, and alveolar macrophages. Wounding is a more physiologic means of increasing circulating fibrocytes. For these experiments we used WT BALB/c mice since intravenous injection of WT fibrocytes increased metastasis of the CT26 cell line. Significant increases in metastasis were also noted when mice were injected 8 or 13 days after a wound but not after 3 or 5 days. This timeline correlated with the increase in circulating fibrocytes associated with wounding. Conclusions/Implications: These data add support to our hypothesis that fibrocytes contribute to premetastatic conditioning by recruiting Ly-6C++ monocytes in a process dependent on CCR5 and CCL2/CCR2. This work links conditions that mobilize fibrocytes to increased metastatic risk. Such conditions include inflammation and surgical wound repair. Therapeutically, these results provide a rationale for the use of drugs that inhibit fibrocyte migration including chemokine receptor inhibitors and mTOR inhibitors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1533. doi:1538-7445.AM2012-1533