First printed in R&D Systems' 1999 Catalog.
Although many cells contribute to the general process known as inflammation, eosinophils in particular are noted for their contribution to late phase allergic-type disorders. Eosinophils make up less than 10% of the circulating leukocyte population, yet they are now known to be extremely important in the inflammatory response to allergic and parasitic challenges.1 In tissues and when activated, eosinophils release highly basic, preformed mediators such as eosinophil peroxidase and major basic protein. These substances are both toxic to parasites and damaging to surrounding tissue, inducing smooth muscle constriction, increased vascular permeability and superoxide-mediated tissue destruction. While eosinophils have been associated with these inflammatory processes, the soluble mediators that influence eosinophil availability and function have only recently been identified. Interleukin 5 (IL-5), along with the chemokine eotaxin, is a key player in the coordination and orchestration of eosinophil-based inflammatory processes.1-3 (See references 3-7 for reviews on IL-5).
Human IL-5 is a 134 amino acid (aa) polypeptide with a predicted mass of 12.5 kDa. It is secreted by a restricted number of mesenchymal cell types.8, 9 In its native state, mature IL-5 is synthesized as a 115 aa, highly glycosylated 22 kDa monomer that forms a 40-50 kDa disulfide-linked homodimer.8, 10 Although the content of carbohydrate is high, carbohydrate is not needed for bioactivity.10 Monomeric IL-5 has no activity; a homodimer is required for function.6 This is in contrast to the receptor-related cytokines IL-3 and GM-CSF, which exist only as monomers.13, 14 Just as one IL-3 and GM-CSF monomer binds to one receptor, one IL-5 homodimer is able to engage only one IL-5 receptor.7 It has been suggested that IL-5 (as a dimer) undergoes a general conformational change after binding to one receptor molecule, and this change precludes binding to a second receptor.15 Human and mouse mature IL-5 are 71% identical at the aa level.8, 16 While mouse IL-5 is highly active on human cells,6 human IL-5 is only marginally active on mouse cells.10, 17
|Figure 1. IL-5 first mobilizes existing bone marrow eosinophils and induces eosinophil production. These existing and developing eosinophils then respond to eotaxin, resulting in their exit from the marrow and entry into the tissue. Very early eosinophil progenitors also respond to eotaxin, and when they reach the tissues, mature into eosinophils in situ.48,49 While in tissues, IL-5 also maintains eosinophil levels by inhibiting normal eosinophil apoptosis.50|
The receptor for IL-5 consists of a ligand binding alpha subunit and a non-ligand binding (common) signal transducing beta subunit that is shared by the receptors for IL-3 and GM-CSF.18
The human IL-5 R alpha subunit is a 60 kDa, 400 aa type I transmembrane glycoprotein that binds IL-5 with low affinity.19, 20 In the human, the Kd for IL-5 binding to IL-5 R alpha is reported to be anywhere from 600-2000 pM,19, 21, 22 while the Kd for mouse IL-5 binding to mouse IL-5 Ralpha is 2-10 nM.23 Consistent with other cytokine receptors, the IL-5 R alpha has two tandem barrel structures (N- and C-domain) that are made of seven anti-parallel beta-strands. Within the N-domain are four conserved cysteine residues, and within the C-domain is the cytokine-associated WSXWS motif; N-terminal to both barrel structures is a fibronectin type III domain.19, 24 Although the a-chain has been described as a non-signal transducing subunit, evidence suggests that the membrane-proximal portion of the 55 aa cytoplasmic region either signals, or significantly contributes to signaling.25-28 Soluble, 50 kDa forms of IL-5 R alpha have been identified20-23, 29 and when present are inhibitory to IL-5 activity.21 Mouse IL-5 R alpha is 70% identical to human IL-5 R alpha include CD5+ B cells, eosinophils, mast cells, CD34+ stem cells, and basophils.
The human IL-5 R beta-subunit (beta c) is a 120-130 kDa, type I transmembrane glycoprotein that serves as the major signal transducing subunit for the IL-3, IL-5 and GM-CSF receptors.18, 30, 31 In this role, it is analogous to gp130 and IL-2 R gamma.32 The human molecule is 871 aa long, and is divided into a 422 aa extracellular region, a 27 aa transmembrane segment, and a 432 aa cytoplasmic domain.30 Like IL-5 R alpha, beta c contains N- and C-domains that are duplicated; unlike IL-5 R alpha, there is no N-terminal fibronectin domain.18, 24, 32 Mouse beta c (also known as AIC2B) is 55% identical to human beta c.30, 33 Cells known to express bc include basophils, eosinophils, neutrophils, monocytes and dendritic cells, endothelial cells, mast cells and macrophages, CD5+ B cells, and CD34+ stem cells.
The kinetics of IL-5 binding are still under investigation. Assuming equality with the IL-3 system, (homodimeric) IL-5 binds noncovalently to one IL-5 R alpha subunit, which then noncovalently recruits one beta c subunit, forming a temporary noncovalently-linked trimer.32, 35, 36 At this point, a second, newly generated IL-5 R complex can be engaged, with the IL-5 R alpha subunit from complex #1 forming a disulfide bond with beta c of complex #2. This is followed by disulfide bonding of the two remaining unlinked receptor components. It is likely that the two beta c subunits also become disulfide-linked, creating a functional signal transducing complex, with a stoichiometry of 2:2:2.32, 36 A newly formed IL-5 R trimer also may link with naturally preformed GM-CSF R alpha/beta c complexes to form hybrid IL-5 R/GM-CSF R complexes.37 It is unknown what function these complexes may play in IL-5 activity.
As with many cytokines and growth factors, IL-5 has an approximately 15 aa NLS in the body of the molecule.38 IL-5 with its receptor can be transported into the nucleus following its binding on the cell surface.39, 40 It is suggested that STATs, which are associated with the receptor, actually enter the receptor via the IL-5 NLS.41
Cells known to express IL-5 include eosinophils, NK cells, TC2 CD8+ T cells, mast cells, CD45+ CD4+ T cells, gamma delta T cells and IL-1 beta activated endothelial cells. IL-5 is best known for its activity on B cells and eosinophils. Relative to B cells, IL-5 appears to induce the differentiation of activated conventional B-2 cells into Ig-secreting cells. In addition, it induces the growth of B-1 progenitors as well as IgM production by B-1 cells.42 In mice, IL-5 promotes production of IgA, IgE and IgG1.42
IL-5 appears to perform a number of functions on eosinophils. These include the down modulation of Mac-1,43 the upregulation of receptors for IgA and IgG,44 the stimulation of lipid mediator (leukotriene C4 and PAF) secretion45, 46 and the induction of granule release.47 IL-5 also promotes the growth and differentiation of eosinophils.1 The exact role that IL-5 plays, however, is unclear; it may act in an adjunct fashion.48 Finally, there is a great deal of interest in the interaction between IL-5 and the CC chemokine eotaxin. Studies suggest that inflammatory-induced, and locally-produced, IL-5 and eotaxin may act on the bone marrow in a cooperative manner (see Figure 1).