The presence of eosinophils in the lung is often regarded as a defining feature of asthma. On allergen stimulation, numbers of eosinophils and their progenitors are increased in both the bone marrow and lungs. Eosinophil progenitors provide an ongoing supply of mature eosinophils. Here, we report that deficiency in the regulator of calcineurin 1 gene (Rcan1) leads to a near-complete absence of eosinophilia in ovalbumin-induced allergic asthma in mice. In the absence of Rcan1, bone marrow cells produce significantly fewer eosinophils in vivo and in vitro on interleukin-5 stimulation. Importantly, eosinophil progenitor populations are significantly reduced in both naive and ovalbumin-challenged Rcan1(-/-) mice. Bone marrow cells from Rcan1(-/-) mice are capable of developing into fully mature eosinophils, suggesting that Rcan1 is required for eosinophil progenitor production but may not be necessary for eosinophil maturation. Thus, Rcan1 represents a novel contributor in the development of eosinophilia in allergic asthma through regulation of eosinophil progenitor production.
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