Preface [Hot topic: Hemopoiesis and Allergy (Guest Editor: P. Xavier Elsas)]

Sometimes an opportunity arises to survey a rapidly developing field of specialized research just before it begins to attract more widespread attention. It is my feeling that this applies to the role of hemopoietic cells and hemopoietic factors in allergy and asthma, which constitutes the theme of this Hot Topic issue. For a decade now, this issue has been explored by a number of dedicated specialists, who have repeatedly drawn our attention to the fact that hemopoiesis continually provides allergic inflammation sites with novel cells, and thereby plays an essential role in the chronicity of allergic disease. Over the last few years, this view has been refined to include an ever growing awareness of the pathogenetic potential of hemopoietic factors which had been originally described in a context quite foreign to allergy and asthma, as is the case with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF), Interleukin-5 (IL-5) and Stem Cell Factor (SCF). As it turns out, hemopoietic factors do much more in allergy than command the replenishment of infiltrating leukocytes. The diverse roles of GM-CSF include its essential participation as an instructive signal in the initiation of allergic sensitization itself. It also operates as a critical interface between the organism and its environment, since it is induced by a variety of environmental influences known by their association with the explosive growth of allergic diseases in recent years. SCF has proven no less protean in its actions, for it not only orchestrates the development and function of mast cells but displays an amazing plasticity in regulating them, for it interacts with the diverse signals converging on mast cells to provide remarkably diverse outcomes. In both cases, the issue of the role of hemopoietic factors in allergy has been given an unexpected answer, as it appears that hemopoietic factors are important, but not in conventional sites of hemopoiesis. The research in this field is, in my view, approaching a new turning point, as it becomes clear that the hemopoietic cells found in sites of allergic inflammation differ in some important ways from those found in the bone-marrow. Several groups have now concentrated in the study of the various types of hemopoietic progenitor and precursor cells that accumulate in the circulation and tissues of allergic subjects. As discussed in this issue, these have turned out to have unusual and interesting properties. For at least one of those emerging targets - the hemopoietic cells that accumulate in murine lungs after allergen challenge of the sensitized airways - the pattern of response to glucocorticoids, which have obvious clinical relevance, differs strikingly from that of the corresponding cells in bone-marrow. Such findings suggest that the issue of the role of hemopoietic cells in allergy also has an unconventional answer, as the cells themselves differ from those found in conventional sites of hemopoiesis. In this context, novel targets and avenues for therapeutic intervention are certainly waiting for the right questions to be posed, and for the right experiments to be designed. Hopefully, this Hot topic issue will provide the reader with an appropriate perspective on this exciting field, and invite new questions and experiments. For attempting that, I am deeply indebted to our contributors, who represent some of the most active groups in this field.