Systemic immunoregulatory and pathogenic functions of homeostatic chemokine receptors.

The adoptive immune response relies on a precise temporal and spatial positioning of lymphocytes within lymphoid and nonlymphoid tissues. Chemokines, constitutively expressed or induced during inflammation provide a flexible navigation system directing lymphocytes into specific microcompartments. Precision and specificity in this process are achieved by varying patterns of chemokine receptors expressed on the cell surface of lymphocytes in the course of cell differentiation. The chemokine receptors CXCR5 and CCR7 are principal regulators for targeting T cells, B cells, and dendritic cells into secondary lymphoid organs. The analyses of knockout mice have been instrumental in exploring the crucial role of these receptors for the compartmentalization of secondary lymphoid organs into functionally separated T and B cell zones. Aside from the homeostatic recirculation of lymphocytes and inflammatory processes, chemokine receptors are also involved in malignancies such as lymphoproliferative diseases and cancer metastasis. Recent results from our laboratory present evidence for the involvement of CCR7 in the dissemination of neoplastic cells in classic Hodgkin disease. There is also accumulating evidence for the involvement of CXCR5 in the formation of ectopic follicles as observed in lymphomas or autoimmune diseases. In addition, CCR7 and CXCR5 have been identified as useful markers in the classification of functionally distinct subsets of T-helper cells, which will lead to a better understanding of T cell memory and T cell effector function in lymphoid system homeostasis and disease.