Murine Melanomas Expressing High Levels of SDF-1 Repel and Escape Control by Tumor Specific T-Cells.

Most antigenic tumors are able to evade the immune system despite T cell recognition of tumor specific antigens. The chemokine, stromal derived factor- 1 (SDF-1 or CXCL12) is constitutively expressed by both normal and neoplastic tissues and is involved in tumor growth, metastasis and modulation of the anti-tumor immune response. We have previously shown that human T cells are repelled by the chemokine SDF-1, in a concentration-dependent and CXCR4 receptor-mediated manner. We proposed that the repulsion of tumor-specific T cells from a tumor expressing SDF-1 may represent a novel mechanism of immune evasion. B16 melanoma cells stably expressing OVA were genetically modified to constitutively express SDF-1 using a bicistronic MSCV2.2 retroviral transfer vector encoding murine SDF-1α and EGFP. SDF-1 generated by transduced cells was 58 ± 28.5 ng/ml/1x10⁶ cells/24 hours as measured by ELISA. Immunization of mice with irradiated B16/OVA cells lead to destruction of B16/OVA.MSCV control tumors but not tumors expressing high levels of SDF-1 (p=0.0001). Failure to reject B16/OVA.SDF-1 tumors was associated with a reduction of T cell infiltration compared to control B16/OVA.MSCV (p=0.001). Similarly, early recruitment of adoptively-transferred OVA-specific CD8+ T cells into B16/OVA.SDF-1 tumors was significantly reduced in comparison to B16/OVA.MSCV control tumors as demonstrated by immunohistochemistry and real-time magnetic resonance imaging of nanoparticle labeled CTLs (p=0.018). Transferred memory OVA-specific CD8+ T-cells demonstrated antitumor activity against B16/OVA.MSCV control tumors but not against B16/OVA-SDF-1 tumors (p=0.04). Isolation and FACS analysis of tumor-infiltrating lymphocytes showed a 4-fold reduction of T cells in B16/OVA.SDF-1 compared to B16/OVA.MSCV control tumors (p=0.005). The in-vitro killing activity of tumor specific CD8+ T cells against B16/OVA.SDF-1 tumors was significantly reduced compared to B16/OVA.MSCV control tumors when measured in 51Cr release assay performed in a flat-bottom well system where cytotoxicity was dependent on effector cell migration (p=0.0004). Preincubation of OT-I CD8+ T cells with the CXCR4 antagonist AMD3100 restored their cytolytic activity against B16/OVA.SDF-1 cells (p=0.045). These data support the thesis that the active movement of tumor-specific T cells away from SDF-1, termed fugetaxis, may provide a novel mechanism by which a tumor evades challenge by immune effector cells and raises the possibility of the development of a novel immunotherapeutic approach to cancer.