First printed in R&D Systems' 1997 Catalog.
While considerable data has accumulated relative to eosinophils, and their pharmocologic mediators, one area that still shows considerable uncertainty is that involving eosinophil migration and activation.1 With the discovery of the first chemokine (IL-8) in 1987, and the subsequent expansion of the chemokine family into multiple subfamilies, molecules were discovered that explained, in part, the migratory and activation behavior(s) of eosinophils in allergic inflammation and parasitic infestation. Among these were RANTES, MIP-1 alpha and MCP-3.1 Questions remained, however, particularly with respect to the apparent non-specificity of many subfamily chemokines towards multiple peripheral blood leukocytes.1, 2 The recent discovery of eotaxin, the CCR-3 receptor-specific, eosinophil-selective chemokine, shows that specificity does exist in some cases among chemokine ligands for specific receptors.3 It is hoped that this specificity will reveal something about the role that eosinophils play both in homeostasis and pathology.
|Figure 1. Eotaxin and eosinophil migration.|
Human eotaxin (Eot) is an 8.4 kDa, 74 amino acid (aa) residue polypeptide that is produced by a number of normal cells and cell lines.3, 4 Molecules in this group are characterized by the presence of four cysteines, the first two of which are immediately adjacent. To date, human members of this group include MCP-1, MCP-2, MCP-3, MIP-1 alpha, MIP-1 beta, RANTES and I-309,8 and Eot.
Although a C-C subfamily member, the Eot sequence, when optimally aligned against all other C-C members, shows several unique features. First, Eot shows a two aa residue deletion at positions 5 and 6 when compared to the MCPs and other beta chemokines. Second, in contrast to all other 6 Third, although all other MCPs possess an N-terminal glutamine, Eot does not. When compared to other subfamily members, Eot shows 60-64% aa sequence identity to the MCPs,4 Cells known to produce Eot include pseudostratified ciliated columnar epithelium,10 lymphocytes, macrophages and eosinophils,5, 11 and simple squamous epithelium.11 Cell lines known to express Eot include HUVEC, U937 monocytes, CACO-2 colonic and BEAS-2B respiratory epithelial cells,6 and 3T3 NIH fibroblasts plus END-2 brain endothelial cells.11
A receptor for human Eot has been identified and found to be the third numbered receptor in the C-C chemokine subfamily of receptors (CCR-3).12 Compared to other human C-C chemokine subfamily receptors, CCR-3 exhibits 63% aa residue identity with CCR-1,15 Collectively, or among all five C-C chemokine subfamily receptors, there is an overall 21% aa residue identity across non-transmembrane segments. Within these segments, the greatest identity (67%) exists in the intracellular segment connecting transmembrane domains 3 and 4 and the lowest regions of identity are found in the intracellular C-terminal segment (18%) and the extracellular N-terminal segment (9%).15 Finally, the mouse counterpart to human CCR-3 has also been isolated. It is 359 aa residues in length and shares 68% aa residue identity with human CCR-3. Notably, it binds both mouse and human Eot with equal affinity.16, 17
Eot is unique among known C-C chemokines in that it binds to only one receptor, CCR-3 (Kd=0.1-1.5 nM).12, 13 The receptor is apparently less selective in its ligand binding properties. Although some investigators have reportes that only Eot bound to CCR-3,4, 18 others have reported binding of Eot plus RANTES and MCP-3 to CCR-3.12, 13 Preliminary reports also suggest that MCP-4 may be a ligand for the receptor.8 Cells known to express CCR-3 are limited to eosinophils in the human,12, 13 and eosiniphils plus neutrophils and macrophages in mice.17 Eot has no known activity on neutrophils or macrophages.17
As suggested by the restricted expression of its receptor, Eot activity is limited to eosinophils. For eosinophils, Eot has been proposed to be both an activator and chemoattractor. As an activator, Eot initiates actin polymerization intracellularly, an event that is a prerequisite to chemotaxis, and induces the production of reactive oxygen species, thereby playing a role as a mediator of inflammation.19, 20 It appears then that Eot alone has the ability to selectively prime eosinophils for chemotaxis, to direct their migration/chemotaxis, and to activate inflammatory activity in the cells attracted.
While this suggests an extensive range of activities for Eot, it is still insufficient to explain fully the in vivo process of eosinophil accumulation. For example, in an in vivo model of lung eosinophilia, increased levels of Eot, when combined with a decrease in IL-5, fail to induce eosinophil migration in response to helminth infection.11, 21, 22 One possible explanation for this result could involve an IL-5/Eot cooperative interaction. In one scenario, a number of resident cell types could be activated at the site of an antigenic challenge. Two cell types that are candidates for mediating such an early or initial response are mast cells, a known source of IL-4 and TNF-alpha,25 Depending upon the local levels of cytokines (such as IL-4) and the type(s) of antigen present, resident but uncommitted CD429
In parallel with the locally-developing Th2-type response, mast cell and macrophage-produced TNF-alpha and IL-1 alpha could also act on adjacent endothelial cells. Both TNF-alpha and IL-1 alpha are known to induce Eot production by endothelial cells, and this would define a "site" for future eosinophil migration.6 As locally-produced levels of IL-5 rise, significant quantities of IL-5 could enter the circulation. This circulating IL-5 could induce the release of mature (and immature?) eosinophils from bone marrow, resulting in transient peripheral eosinophilia. In support of this suggestion: 1) serum IL-5 levels are positively correlated with blood eosinophil numbers;6