CCL2 Promotes Colorectal Carcinogenesis by Enhancing Polymorphonuclear Myeloid-Derived Suppressor Cell Population and Function

Abstract

Our study reveals a non-canonical role for CCL2 in modulating non-macrophage, myeloid-derived suppressor cells (MDSCs) and shaping a tumor-permissive microenvironment during colon cancer development. We found that intratumoral CCL2 levels increased in patients with colitis-associated colorectal cancer (CRC), adenocarcinomas, and adenomas. Deletion of CCL2 blocked progression from dysplasia to adenocarcinoma and reduced the number of colonic MDSCs in a spontaneous mouse model of colitis-associated CRC. In a transplantable mouse model of adenocarcinoma and an APC-driven adenoma model, CCL2 fostered MDSC accumulation in evolving colonic tumors and enhanced polymorphonuclear (PMN)-MDSC immunosuppressive features. Mechanistically, CCL2 regulated T cell suppression of PMN-MDSCs in a STAT3-mediated manner. Furthermore, CCL2 neutralization decreased tumor numbers and MDSC accumulation and function. Collectively, our experiments support that perturbing CCL2 and targeting MDSCs may afford therapeutic opportunities for colon cancer interception and prevention.

Figure 1. CCL2 Levels Increase with Neoplastic Progression in Human and Mouse Colitis-Associated CRC

(A) CCL2 expression levels in 11 human colitis-associated colorectal dysplasia and adenocarcinoma (ACA) cases. Left panel: total CCL2⁺ area from five fields (objective 40×) plotted for each case. Areas for quantification were no colitis (normal), colitis (no dysplasia/ACA), dysplasia, and ACA. Symbols are color-coded to track samples. Right panel: representative images of CCL2 immunostaining in human colon samples. Yellow triangles, epithelial cells; red triangles, infiltrating immune cells. Scale bar, 50 μm. (B) CCL2 colonic protein levels from T-bet^−/− Rag2^−/− mice across the neoplastic continuum, quantitated by ELISA. Mean ± SEM with age and number of mice per group shown. (C) Prevalence of dysplasia and neoplasia in Ccl2^−/− T-bet^−/− Rag2^−/− and T-bet^−/− Rag2^−/− mice monitored by mouse age. Symbols represent data from individual mice, with color-coding indicating age in months. *p < 0.05, **p < 0.01, and ***p < 0.001 (unpaired, two-tailed Student's t test). See also Figure S1.

Figure 2. CCL2 Induces Colonic MDSC Accumulation and Alters MDSC Gene Expression in Colitis-Associated Pre-malignancy

Colonic myeloid cells were isolated from Ccl2^−/− T-bet^−/− Rag2^−/− and T-bet^−/− Rag2^−/− mice with or without low-grade dysplasia (LGD). (A) The gating strategy for myeloid cell subset identification. Following a leukocytes gate (CD45⁺), cells are further gated for CD11b⁺ myeloid cells only and CD11b⁺Gr-1⁺ MDSCs or CD11b⁺Gr-1⁻ macrophages (population I). Ly6C and Ly6G were used to distinguish Mo-MDSCs (population II) and PMN-MDSCs (population III). The F4/80 and MMR flow plot was used to identify tumor-promoting macrophages (TAMs and M2-like TAMs). (B) Flow cytometric analysis of populations I-III as gated in (A). Data are representative of at least three independent experiments. (C) Quantification of select colonic myeloid cells by flow cytometry. Symbols represent data from individual mice, and bars show the mean. (D) Colonic MDSC sorted from Ccl2^−/−T-bet^−/− Rag2^−/− and T-bet^−/− Rag2^−/− mice and Arginase1 and Il10 (C), S100A8, S100A9, gp91phox, and iNOS (D) expression levels were evaluated by qRT-PCR. Symbols represent data from individual mice. Data are from age-matched (4- to 5-month old) and littermate mice. *p < 0.05, **p < 0.01, and ***p < 0.001 (unpaired, two-tailed Student's t test). See also Figure S2.

Figure 3. CCL2 Levels Increase in Human Sporadic CRC and CCL2 Enhances Tumor MDSC Accumulation during Colonic Adenocarcinoma Growth

(A) CCL2 levels in human colon adenocarcinoma tissue microarrays (normal, n = 29; adenocarcinoma, n = 119). Representative images are shown. Scale bar, 500 μm. (B-F) Colon26^shGFP (shControl) or Colon26^shCCL2-2 (shCCL2) cells were subcutaneously injected into BALB/c mice and tumor growth evaluated at day14. (B) Representative images of tumor-bearing mice (upper panel) and tumor volumes (lower panel). (C) Representative immunofluorescence microscopy images. Tumors were stained with DAPI and for CD11b and Gr-1; single-channel and pseudocolored images. Scale bar, 50 μm. (D) Flow cytometry gating strategy and (E) analysis of intratumoral Mo-MDSCs, PMN-MDSCs, and CD11b+Gr-1⁻ myeloid cells. (F) Intratumoral MDSC, PMN-MDSC, and Mo-MDSC numbers from shControl versus shCCL2 tumor-bearing mice with cell numbers normalized by tumor weight. Symbols represent individual mice. (G and H) shCCL2 cells were subcutaneously injected into mice and recombinant CCL2 or PBS was intratumorally injected into shCCL2 tumor-bearing mice at day 5 and day 12. Tumor volumes at day 19 (G). Intratumoral MDSC, PMN-MDSC, and Mo-MDSC numbers normalized by tumor weight (H). (I) Intratumoral TAM and M2-like-TAM cell numbers. (J) shCCL2 cells were subcutaneously injected into mice, and then sorted TAMs or MDSCs from spleens of shControl tumor-bearing mice were intratumorally injected into shCCL2 tumor-bearing mice at day 5. Tumor volumes were evaluated at day 14. All data reflect at least three independent experiments. *p < 0.05, **p < 0.01, and ***p < 0.001 (unpaired, two-tailed Student's t test). See also Figure S3.

Figure 4. CCL2 Promotes T Cell Suppression via PMN-MDSCs

(A and B)T cell proliferation assays. Intratumoral PMN-MDSCs and Mo-MDSCs were sorted from shControl or shCCL2 tumor-bearing mice. (A) CellTrace-labeled splenic CD4⁺T cells from syngeneic mice were coincubated for 3 days with sorted intratumoral PMN-MDSCs or Mo-MDSCs. Representative flow cytometric analyses of activated or nonactivated CD4⁺ T cell proliferation and activated CD4⁺ T cell in the presence of sorted MDSCs are shown. Bar graph shows mean ± SEM of three independent experiments. (B) CellTrace-labeled splenic CD8⁺T cells were coincubated for 3 days with sorted intratumoral PMN-MDSCs or Mo-MDSCs. (C and D) Suppressive mechanisms of PMN-MDSCs or Mo-MDSCs on TCR components. ζ chain expression (C) and nitrotyrosine levels (D) of CD4⁺ T cells or CD8⁺ T cells coincubated with sorted intratumoral PMN-MDSCs or Mo-MDSCs were measured by flow cytometry. Data, shown as MFI ± SEM, reflect three independent experiments. *p < 0.05, **p < 0.01, and ***p < 0.001 (unpaired, two-tailed Student's t test). See also Figure S4.

Figure 5. CCL2 Enhances Function of PMN-MDSCs

(A) Representative images from shControl and shCCL2 tumors stained with DAPI, and for ROS (Hydro-Cy3), CD11b, and Gr-1. Single-channel pseudocolored and merged images (left panel). Scale bar, 20 μm. Microscopy-based quantitation of MFI of ROS per MDSC from shControl and shCCL2 tumors is shown (right panel). (B) ROS production levels in intratumoral PMN-MDSCs and Mo-MDSCs from shControl or shCCL2 tumor-bearing mice were detected by DCFDA staining and flow cytometry. Data are shown as MFI ± SEM. (C and D) Expression levels of gp91phox (C), S100A8 and S100A9 (D) in intratumoral PMN-MDSCs and Mo-MDSCs from shControl or shCCL2 tumors measured by RT-qPCR. Mean relative expression ± SEM are shown. (E) Expression of iNOS levels (MFI) in sorted intratumoral PMN-MDSCs or Mo-MDSCs from shControl or shCCL2 tumor-bearing mice measured by flow cytometry. (F) Expression of PD-L1 in sorted tumor-derived PMN-MDSCs or Mo-MDSCs from shControl or shCCL2 tumor-bearing mice measured by flow cytometry. Data are shown as MFI ± SEM. All data reflect three independent experiments. *p < 0.05, **p < 0.01, and ****p < 0.0001 (unpaired, two-tailed Student's t test). See also Figure S5.

Figure 6. CCL2, via STAT3, Regulates PMN-MDSC T-Cell-Suppressive Activity

(A) Phospho-STAT3 (p-STAT3) levels from PMN-MDSCs from shControl or shCCL2 tumors measured by flow cytometry. Mean ± SEM. **p < 0.001. Unpaired, two-tailed Student's t test. (B) Expression of C/ebpβ measured by qRT-PCR with data normalized to actin expression (mean relative expression ± SEM). (C–H) Analysis of sorted intratumoral PMN-MDSCs from the mice with shControl tumors followed by treatment with a STAT3 inhibitor (S3I-201) (50 μM) for 18 hr. Phospho-STAT3 (p-STAT3) levels (C) and ROS levels (DCFDA staining) (D), gp91phox (E) S100A8 (F), S100A9 (G) and iNOS (H) expression levels were measured. Data are shown as mean ± SEM. *p < 0.05, **p < 0.01 (paired, two-tailed Student's t test). (I–L) Sorted intratumoral PMN-MDSCs from shCCL2 tumor-bearing mice were treated with S31-201 and then coincubated with CD4⁺ T cells or CD8⁺ T cells. T cell proliferation assays: CD4⁺ T cells (I) and CD8⁺ T cells (J) are shown. Division index represents T cell divisions. *p < 0.05, **p < 0.01 (paired, two-tailed Student's t test). ζ chain expression of CD4⁺ T cells (K) and nitrotyrosine levels of CD8⁺ T cells (L) measured by flow cytometry. All data represent two or three independent experiments. *p < 0.05 (paired, two-tailed Student's t test). See also Figure S6.

Figure 7. CCL2 Affects Adenoma Number and MDSC Accumulation in Apc^Min/+ Mice

(A) CCL2 levels in human colon adenoma tissue microarrays (normal, n = 38; adenoma, n = 40). Representative images are shown. Scale bar, 500 μm. (B–E) Apc^Min/+ mice were injected with anti-CCL2 or isotype control mAb (10 μg/kg) from 6 to 10 weeks of age twice a week for 4 weeks. (B) Photograph of intestinal tumors from representative mice (left panel). Intestinal tumor counts from Apc^Min/+ mice (right panel). Symbols represent data from individual mice. (C) MDSC, PMN-MDSC, and Mo-MDSC numbers from intestinal tumors from Apc^Min/+ mice treated with anti-CCL2 or isotype control mAb, with cell numbers normalized by tumor weight. (D) ROS production levels (DCFDA staining) in intratumoral PMN-MDSCs from Apc^Min/+ mice treated with anti-CCL2 or isotype control mAb. (E) Expression of PD-L1 in intratumoral PMN-MDSCs from Apc^Min/+ mice treated with anti-CCL2 or isotype control mAb measured by flow cytometry. (F–H) Apc^Min/+ mice were injected with recombinant mouse CCL2 (20 μg/kg) from 6 to 10 weeks of age twice a week for 4 weeks or with PBS as a control. (F) Intestinal tumor counts. (G) Intestinal tumor MDSC, PMN-MDSC, and Mo-MDSC numbers. (H) PD-L1 expression in intratumoral PMN-MDSCs from Apc^Min/+ mice treated with recombinant CCL2 or PBS measured by flow cytometry. All data reflect at least three independent experiments. *p < 0.05 and **p < 0.01 (unpaired, two-tailed Student's t test).