They are not only involved in tissue development and homeostasis but also perform various immune regulatory functions 1–4. They are efficient effector cells of the innate immune system as they have the capacity to respond to parasite, viral or bacterial infections. In addition, eosinophils have an important role in bridging innate and adaptive immunity. In particular, activated eosinophils are crucial in promoting TH2 responses by secreting TH-cell polarizing cytokines such as IL-4 and IL-13, and this release of IL-4 also promotes rapid differentiation of B cells into IgM plasma cells 5, 6. Thus, in T-cell-dependent immune responses eosinophils are required for the early protective IgM response

7, 8. In contrast, the generation of an antigen-specific IgG response seems not to be affected as in eosinophil-deficient ΔdblGATA-1 Ceritinib molecular weight mice B-cell maturation in germinal centers and their differentiation into memory B cells and plasma cells were shown to be normal 9. Eosinophils, however, are crucial for the long-term survival of plasma cells in the BM as in their absence the plasma cells quickly die by apoptosis. Thus, as a major source of the plasma cell survival factors APRIL and IL-6, eosinophils Sunitinib in vitro have an essential function in the

long-term maintenance of humoral immunity 9. Eosinophils produce and store a wide range of cytokines whose release is dependent on the nature of the activating stimulus 10–12. Eosinophils respond by piecemeal degranulation leading to a highly controlled secretion of specific mediators 13. Full activation of eosinophils may induce de-granulation and thus a rapid release of tissue-destructive cationic proteins. Activated eosinophils may also respond by the ejection of extracellular traps consisting of mitochondrial DNA and granule-derived mediators 14. Human eosinophils

have been shown to express constitutively not only the TH2-related cytokines IL-4, IL-13 and IL-10 but also IL-12 and IFN-γ, below which are characteristic of TH1 responses 11, 15. Upon immunization, IL-4 production by eosinophils is up-regulated, although similar effects were seen when animals were injected with aluminum potassium sulfate (alum) alone, an adjuvant commonly used in antigen priming 7, 8. To further investigate the activation of eosinophils and their expression of cytokines, BALB/c mice were immunized with the T-cell-dependent antigen 2-phenyl-oxazolone (phOx) precipitated in alum or emulsified with CFA. Eosinophil activation was monitored in the primary response and also after secondary challenge with soluble antigen. We found that only in the presence of antigen was a stable activation of eosinophils and continuous expression of plasma cell survival factors achieved. By contrast, injection of adjuvant alone only transiently enhanced cytokine production. Together, these data suggest that in immune responses, eosinophils are primed to become effector cells that prevent plasma cells from undergoing apoptosis.