The mechanism of cooperation between
IL-5 and eotaxin for the selective accumulation of eosinophils at sites of allergic
inflammation is unknown. In this investigation we have used
IL-5 deficient mice to define the relationship between this
cytokine and eotaxin in the regulation of blood
eosinophilia and eosinophil homing and tissue accumulation. Both
IL-5 and eotaxin could independently induce a rapid and pronounced blood
eosinophilia in wild type mice when administered systemically. In contrast, only eotaxin induced a pronounced blood
eosinophilia in
IL-5 deficient mice. The eosinophilic response induced by intravenous eotaxin in wild type mice did not correlate with a significant reduction in the level of bone marrow eosinophils, whereas intravenous
IL-5 resulted in depletion of this store. These results suggest the existence of two mechanisms by which eosinophils can be rapidly mobilized in response to intravenous eosinophil
chemoattractants; first, mobilization of an
IL-5 dependent bone marrow pool, and second, an eotaxin-induced sequestration of eosinophils from tissues into the blood.
Subcutaneous injection of eotaxin induced a local tissue
eosinophilia in wild type mice but not in
IL-5 deficient mice. Furthermore, tissue
eosinophilia in wild type mice, but not in
IL-5 deficient mice, was enhanced by adoptive transfer of eosinophils or the administration of intravenous
IL-5. However, pretreatment of
IL-5 deficient mice with intraperitoneal
IL-5 for 72 h restored eosinophil homing and tissue accumulation in response to subcutaneous eotaxin. We propose that eotaxin secreted from inflamed tissue may play an important role in initiating both blood and tissue
eosinophilia in the early phases of allergic
inflammation. Furthermore,
IL-5 is not only essential for mobilizing eosinophils from the bone marrow during allergic
inflammation, but also plays a critical role in regulating eosinophil homing and migration into tissues in response to eotaxin and possibly other specific chemotactic stimuli.