Loss of SMAD4 Promotes Colorectal Cancer Progression by Accumulation of Myeloid-Derived Suppressor Cells through the CCL15-CCR1 Chemokine Axis.

PURPOSE

We previously reported that loss of SMAD4 promotes chemokine CCL15 expression to recruit CCR1(+) myeloid cells via the CCL15-CCR1 axis, which facilitates metastasis of colorectal cancer to the liver. The purposes of this study were to investigate whether essentially the same mechanism works in tumor invasion of the primary colorectal cancer and to evaluate the clinical importance of CCL15 expression and CCR1(+) cell accumulation.

EXPERIMENTAL DESIGN

Using human colorectal cancer cell lines with reduced expression of SMAD4 or CCL15, we investigated tumor growth activities in vivo. We used immunohistochemistry (IHC) to investigate expression of SMAD4, CCL15, and CCR1 with 333 clinical specimens of primary colorectal cancer. We next characterized the CCR1(+) cells using double immunofluorescence staining with several specific cell-type markers. Finally, we determined the serum CCL15 levels in 132 colorectal cancer patients.

RESULTS

In an orthotopic xenograft model, CCL15 secreted from SMAD4-deficient colorectal cancer cells recruited CCR1(+) cells, resulting in aggressive tumor growth. IHC indicated that loss of SMAD4 was significantly associated with CCL15 expression, and that CCL15-positive primary colorectal cancers recruited approximately 2.2 times more numbers of CCR1(+) cells at their invasion front than CCL15-negative colorectal cancers. Importantly, these CCR1(+) cells were of the myeloid-derived suppressor cell (MDSC) phenotype (CD11b(+), CD33(+), and HLA-DR(-)). Most CCR1(+) cells showed the granulocytic-MDSC phenotype (CD15(+)), whereas some showed the monocytic-MDSC phenotype (CD14(+)). Serum CCL15 levels in colorectal cancer patients were significantly higher than in controls.

CONCLUSIONS

Blocking the recruitment of CCR1(+) MDSCs may represent a novel molecular-targeted therapy, and serum CCL15 concentration can be a novel biomarker for colorectal cancer.