Abstract
The expression of the CC chemokine receptor-7 (CCR7) by cancers, including melanoma, augments lymph node (LN) metastasis, but little is known about its role in lymphangiogenesis and anti-tumor immunity. We injected control B16 murine melanoma cells (pLNCX2-B16) and CCR7-overexpressing B16 cells (CCR7-B16) in murine footpads and compared resulting tumors at the protein and mRNA level using immunostaining, Affymetrix gene microarray and quantitative reverse-transcriptase PCR. Although control and CCR7-B16 primary tumors were of similar size, LN metastasis was dramatically enhanced in CCR7-B16 tumors. Microarray analysis of leukocyte-depleted pLNCX2-B16 and CCR7-B16 tumor cell suspensions showed that three major groups of genes linked to interferon (IFN)-γ signaling pathways (for example, STAT1, CXCR 9-11, CCL5 and CXCL10, major histocompatibility complex (MHC) I and MHC II) were downregulated in the CCR7-B16 tumor microenvironment, suggesting activation through CCR7 can downregulate pathways critical for host anti-tumor immunity. In addition, mRNA expression of the lymphatic marker podoplanin was upregulated in CCR7-B16 tumors by 3.35-fold versus control tumors. Anti-podoplanin monoclonal antibody staining revealed a three-fold increase in intratumoral CCL21-expressing lymphatic vessels, as well as a two-fold increase in the number of invading tumor cells per lymphatic vessel in CCR7-B16 versus control tumors. Enhanced anti-vascular endothelial growth factor C (VEGF-C) staining was present in CCR7-B16 versus control tumors, suggesting that VEGF-C may have a role in the CCR7-mediated lymphangiogenesis. In summary, CCR7-B16 tumors show a striking decrease in IFN-γ-mediated inflammatory gene expression in contrast to increased expression of VEGF-C, CCL21 and podoplanin by lymphatic vessels. Enhanced lymphangiogenesis may contribute to the dramatic increase in LN metastasis that is observed in the CCR7-expressing tumors.