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In patients with asthma, eosinophils are primed and massively infiltrate lung tissues and migrate across epithelia into airways. Using blocking monoclonal antibodies, we found that eosinophil transmigration across a lung epithelial cell monolayer depended on the functions of αMβ2 integrin CD11b/CD18. To study the role of Ca2+ in eosinophil priming and transepithelial migration, we treated eosinophils with eotaxin or thapsigargin (TG), reagents that increase cytoplasmic free Ca2+ concentrations by receptor- or nonreceptor-mediated mechanisms, respectively. Pretreatment of eosinophils with TG enhanced CD11b/CD18-dependent transmigration across lung epithelium. Within minutes, TG time- and dose-dependently upregulated the expression of CD11b/CD18 but did not upregulate the expression of αL (CD11a) or β1 (CD29) integrin. The upregulation of CD11b/CD18 expression by eotaxin or TG was prevented when Ca2+ entry was blocked. The priming of eosinophil transmigration by TG was also abrogated by the blockade of Ca2+ entry. Our results indicate that induction of Ca2+ entry by the depletion of Ca2+ from intracellular stores upregulates CD11b/CD18 expression on eosinophils and primes eosinophil transmigration across lung epithelium. Both responses are therefore elicited by extracellular Ca2+. We suggest that, as an important priming signal for human eosinophil functional responses, store-operated Ca2+ entry may be one of the underlying mechanisms of eosinophilic inflammation in asthma.