Preface [Hot Topic: Chemokines (Guest Editor: Chang H. Kim)]

The chemokine network in our bodies is composed of ∼40 chemokines and 18 chemokine receptors. The primary function of chemokines is to attract and retain hematopoietic cells within tissue microenvironments, thereby facilitating cell-cell interaction necessary for hematopoiesis, development and effector function. Each tissue site expresses unique combinations of chemokines to ensure the recruitment of blood cell subsets expressing appropriate chemokine receptors. This journal issue contains seven review articles reviewing the up-to-date findings on the roles of chemokines and their receptors in the migration of blood cells at various developmental and functional stages from stem and progenitor cells in the bone marrow and the thymus to functionally mature effector cells at sites of inflammation. Hematopoiesis, which is to make all blood cells, occurs in the bone marrow in adults. Retention of stem and progenitor cells in the bone marrow and emigration of mature leukocytes out of the bone marrow are regulated by chemokines. SDF-1 / CXCL12 is the major player in this aspect. The article by Drs. Broxmeyer and Christopherson entitled “Stromal cell derived factor- 1 / CXCL12, CXCR4 and CD26 in the mobilization and homing of hematopoietic stem and progenitor cells” reviews the role and practical implications of the SDF-1 / CXCR4 axis in the homing, engraftment and mobilization of hematopoietic stem cells. Some stem and progenitor cells migrate to the thymus to generate functional T cells that protect our bodies from pathogens. T cells undergo maturation and selection processes in the thymus to become naïve T cells that recognize foreign, but not self, antigens. The review by Drs. Uehara, Farber and Love at NIH entitled “Migration of T cell Progenitors in the Thymus” reviews the recent progress in thymocyte migration into, within and out of the thymus. Recently, significant progress has been made in the characterization of unconventional T cells such as NKT cells, γδ T cells and CD4+CD25+ regulatory T cells. The next article, entitled “Trafficking Potentials of Unconventional T cells” by Robert Johnson and myself, reviews the recent progress in research in the migration of NKT cells, γδ T cells and regulatory T cells. These T cells are unconventional in their function, which are well reflected in their unconventional patterns of migration. Functional maturation of naïve T cells to become memory / effector cells occurs in secondary lymphoid tissues. Many of these memory / effector cells migrate to the liver, an organ that has peculiar characteristics in terms of lymphocyte migration. Drs. Sato, Thorlacius and Butcher at Stanford University School of Medicine review the recent progress in this area in their article entitled “Lymphocyte Migration to the Liver.” Although immune cells have been evolved to protect our bodies from pathogens, sometimes they recognize self antigens and attack our own tissues or organs. The review entitled “Chemokines and Autoimmune diseases” by Drs. Tomoya Katakai and Akira Shimizu discusses the onset and progression of autoimmune diseases, and the roles of chemokines in the migration of effector T cells and the development of ectopic lymphoid tissues in inflamed tissues of autoimmune diseases. While most autoimmune responses are Th1 diseases, there are some diseases that are of Th2 type and are mediated, in part, by a specialized group of chemokines. The review article entitled “Chemokines in Allergic Inflammation. Human Disease and Animal Models” by Dr Harm HogenEsch at Purdue University provides an extensive review on the involvement of chemokines in allergic and Th2 type diseases. In order to suppress the autoimmune diseases and allergic diseases, it is important to regulate the migration of self-reactive immune cells to lymphoid and non-lymphoid tissues. The article by Dr. Toshihiko Saeki entitled “Small-Molecule Chemokine Receptor Antagonists: Potential Targets for Inflammatory and Allergic Disorders” reviews the small molecule antagonists developed so far to block interactions between various chemokine receptors and their chemokine ligands. Some of these antagonists are in clinical trials for treatments of inflammatory and allergic disorders in animals and human. I would like to thank all the authors for their contribution of the interesting review articles, and thank the editorial board members and staffs for their help in publication of this special journal issue.