Human eotaxin-induces α₄ and β₂ integrin-dependent eosinophil accumulation in rat skin in vivo: Delayed generation of eotaxin in response to IL-4

The CC chemokine eotaxin, originally purified from bronchoalveolar lavage fluid of sensitized guinea pigs following allergen challenge, is a potent eosinophill-selective chemoattractant. In the present study, we have used ¹¹¹In-eosinophils and human eotaxin to characterize the profile of chemokine-induced eosinophil accumulation in vivo in rat skin. Intradermally injected eotaxin caused a dose-dependent accumulation of ¹¹¹In- eosinophils. Time course studies indicated that the response was rapid, since all the accumulation occurred within the first 1 to 2 h of eotaxin injection. The i.v. administration of anti-intercellular adhesion molecule-1, anti- vascular, cell adhesion molecule-1, or anti-α4 integrin mAbs significantly inhibited the eosinophil accumulation induced by 100 pmol of human eotaxin by 73, 43, and 67%, respectively. Further, when ¹¹¹In-eosinophils were pretreated in vitro with anti-α₄ integrin or anti-β₂ integrin mAbs, or with a combination of both mAbs, eotaxin-induced response in vivo were reduced by 52, 49, and 68%, respectively. Eosinophil accumulation induced by intradermal IL-4, but not that induced by TNF-α or leukotriene B₄, appeared to be mediated in part by endogenously generated eotaxin. Anti-eotaxin Abs significantly inhibited (54%) the later phase (24-28 h) but not the early phase (0-4 h) of the response to IL-4. This was consistent with eotaxin mRNA expression peaking at 18 h after IL-4 injection. Our findings show that human eotaxin is a potent inducer of eosinophil accumulation in vivo, this response being dependent on α₄, integrin/vascular cell adhesion molecule-1 and β₂ integrin/intercellular adhesion molecule-1 adhesion pathways. Further, the eosinophil accumulation in response to IL-4 is partly mediated by endogenously generated eotaxin.