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Basophils: A Nonredundant Contributor to Host Immunity
Brandon M. Sullivan1 and Richard M. Locksley1,*
1UCSF School of Medicine and Howard Hughes Medical Institute, 513 Parnassus Avenue, S-1032B, University of California, San Francisco, San Francisco, CA 94143-0795, USA *Correspondence: DOI 10.1016/j.immuni.2008.12.006

The role of basophils, therarest of blood granulocytes, in host immunity has been a mystery. Long considered the poor relative of mast cells, basophils have received much recent attention because of the availability of new reagents and models that reveal unique properties of these cells. Basophils are known to have distinct roles in allergic hypersensitivity reactions and in the immune response to intestinal helminthes.In this review, we highlight these advances and summarize our current understanding of the repertoire of functions attributed to these cells. Despite these recent insights, we are likely only beginning to gain a full understanding of how and where these cells lend effector functions to vertebrate immunity. Advances are likely to come only with the development of specific reagents that enable thefiner study of basophil lineage and function. Although many fundamental aspects of basophil biology remain unanswered, the prospects remain bright for unmasking new contributions by these unusual cells.

Introduction Basophils make up less than 1% of circulating blood leukocytes and are present in all vertebrates. Recognized by Paul Ehrlich by their cytoplasmic granules that stain with basophilicdyes, basophils are typically grouped with mast cells based on their appearance in tissues during allergic and antihelminth immune responses and on their expression of high-affinity IgE receptor (Fc3RI) that renders both of these cell types responsive to activation by crosslinking IgE bound to the surface. Progress in basophil research was slowed for many years by the inability to follow these cells byother than morphologic criteria. Advances largely over the past 5 years, however, have enabled a more comprehensive look at the role of basophils in immunity and suggest that these cells may provide unique functions unmet by other hematopoietic cells, particularly during Th2-associated allergic and antihelminth responses. Here, we review new findings of basophil function that support a closer lookat this infrequently examined cell lineage. Basophil Lineage The appreciation of a role for basophils in immunity was hampered greatly by their obscurity. Until relatively recently, their existence in mice was actively questioned or these cells were assumed to be an evolutionary relic of the mast cell lineage. The initial descriptions of cells in the mouse that possessed the morphologicalcharacteristics, including electron microscopic features, of basophils appeared in the literature only in the early 1980s (Dvorak et al., 1982; Galli et al., 1982; Nagao et al., 1981; Urbina et al., 1981). Since then, a number of flow cytometry-based purification schemes and microarray analyses have established criteria for the recognition of basophils in the mouse, a process that was aided greatly by theestablishment of various interleukin-4 (IL-4) reporter mouse strains (see below), which, serendipitously at the time, labeled basophils (Min et al., 2004; Voehringer et al., 2004b). Use of these various markers has
12 Immunity 30, January 16, 2009 ª2009 Elsevier Inc.

enabled studies seeking to determine the lineage of these cells during hematopoietic differentiation. All cells of the immunesystem develop from rare bonemarrow-resident hematopoietic stem cells (HSC). Common lymphoid and myeloid progenitors (CLP and CMP, respectively) derived from the HSC represent the earliest progenitors committed to populating the cellular subsets within their respective lineages. Whereas CLPs exclusively give rise to T, B, and NK cells, CMPs are the antecedents of erythrocytes, monocytes, and...
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