Modified peptide antagonists of interleukin 5 exhibit extended in vivo persistence but restricted species specificity

Cytokine recognition by human interleukin 5 receptor

The activation of interleukin 5 (IL-5) receptor is a dynamic process that depends on specific interaction of IL-5 with IL-5 receptor alpha, the formation of oligomeric receptor complexes with receptor beta, and the initiation of cytoplasmic phosphorylation events. These steps culminate in the triggering of a cellular response. Important advances have been made recently in understanding the molecular mechanisms of cytokine recognition, receptor assembly, and signal triggering. Cytokine recognition can be envisioned by relating structure to function in IL-5 and IL-5 receptor alpha. A pair of charge-complementary regions plays an essential role in the specific interaction between IL-5 receptor alpha and IL-5. Moreover, peptide library methodology has led to the discovery of IL-5 receptor alpha antagonists that mimic key elements in IL-5 receptor recognition. Because IL-5 has been implicated in the pathology of eosinophil-related inflammatory diseases, revealing the key recognition elements of IL-5, IL-5 mimetic peptides, and IL-5 receptor alpha could help drive the design of new compounds for therapeutic treatment against allergic inflammatory diseases such as asthma.

Functional analysis of the interleukin-5 receptor antagonist peptide, AF18748

AF18748 is a dimeric peptide of 2 x 19 amino acids that specifically binds to the alpha-chain of the human IL-5 receptor (IL-5R), preventing binding of IL-5 and acting as a receptor antagonist. However, the peptide acts by inducing alpha-chain dimerization, and we therefore set out to investigate whether this peptide would have any residual agonist activity. AF18748 was unable to induce activation of a number of signal transduction pathways, but was able to specifically block IL-5-dependent signal transduction. The peptide was unable to support the survival and proliferation of the cytokine dependent cell line, Ba/F3-IL-5R, and was unable to prime the respiratory burst, or induce adhesion, migration or survival of primary human eosinophils. In each case the AF18748 functioned as an antagonist. These data suggest that AF18748 may be useful to specifically modulate eosinophil function in vivo.

Cyclic peptide interleukin 5 antagonists mimic CD turn recognition epitope for receptor ?

The cyclic peptide AF17121 (Ac-VDECWRIIASHTWFCAEE) that inhibits interleukin 5 (IL-5) function and IL-5 receptor alpha-chain (IL-5Ralpha) binding has been derived from recombinant random peptide library screening and follow-up synthetic variation. To better understand the structural basis of its antagonist activity, AF17121 and a series of analogs of the parent peptide were prepared by solid phase peptide synthesis. Sequence variation was focused on the charged residues Asp(2), Glu(3), Arg(6), Glu(17), and Glu(18). Two of those residues, Glu(3) and Arg(6), form an EXXR motif that was found to be common among library-derived IL-5 antagonists. The E and R in the EXXR motif have a proximity similar to charged residues in a previously identified receptor alpha binding region, the beta-strand between the C- and D-helices of human IL-5. Optical biosensor interaction kinetics and cell proliferation assays were used to evaluate the antagonist activities of the purified synthetic peptides, by measuring competition with the highly active single chain IL-5. Analogs in which acidic residues (Asp(2), Glu(3), Glu(17), and Glu(18)) were replaced individually by Ala retained substantial competition activity, with multiple replacements in these residues leading to fractional loss of potency at most. In contrast, R6A analogs had strongly reduced competition activity. The results reveal that the arginine residue is crucial for the IL-5Ralpha binding of AF17121, while the acidic residues are not essential though likely complex-stabilizing particularly in the Asp(2)-Glu(3) region. By CD, AF17121 exhibited mostly disordered structure with evidence for a small beta-sheet content, and replacement of the arginine had no influence on the observed secondary structure of the peptides. The dominance of Arg(6) in AF17121 activity corresponds to previous findings of dominance of the positive charge balance in the antiparallel beta-sheet of IL-5 composed of (88)EERRR(92) in one strand of the CD turn region of IL-5 and with Arg(32) in the neighboring beta-strand. These results argue that AF17121 and related library-derived peptides function by mimicking the CD turn receptor alpha recognition epitope in IL-5 and open the way to small molecule antagonist design.