CXCL9: evidence and contradictions for its role in tumor progression
Different immune effectors control distinct steps in tumor metastasis; T cells inhibit the growth of primary and metastatic tumors, while NK control tumor cells in transit. Vaccination with DNA encoding Her-2 which is expressed on primary and metastasized breast cancer cells induced both humoral and cellular immunity to inhibit tumor growth. Vaccination efficacy can be amplified by depleting CD4;+CD25;+Foxp3;+ regulatory T cells (Treg), but the risk of inducing autoimmunity warrants new strategies to selectively amplify anti-tumor immunity when modulating Treg. In the tumors, the major cyclooxygenase (COX)-2 product is prostaglandin E2(PGE2) which suppresses T and NK cells while amplifying Treg. These cellular responses to PGE2 are mediated through four E prostanoid (EP) receptors. Cox inhibitors and EP antagonists enhance NK activity to inhibit tumor metastasis; but they may down regulate MHC class I expression. Since T and NK cells have opposite requirements for MHC class I expression, their relative contribution to cancer control may vary with the stage of the disease. To enhance tumor infiltration by immune effectors, the role of CXCL9 is discussed. The complex nature of tumor metastasis necessitates a comprehensive approach to achieve successful immune intervention.