BIOCHEMICAL AND FUNCTIONAL-CHARACTERIZATION OF SOLUBLE FORM OF IL-5 RECEPTOR-ALPHA (SIL-5R-ALPHA) - DEVELOPMENT OF ELISA SYSTEM FOR DETECTION OF SIL-5R-ALPHA

Interleukin-5 (IL-5) mediates pleiotropic functions in various types of cells through its specific receptor (IL-SR) which is composed of two distinct subunits, alpha and beta. In mice, the alpha subunit (IL-5R alpha) specifically binds IL-5 with low affinity. The beta subunit (IL-SRP) does not bind IL-5 by itself, but constructs the high affinity receptor with IL-5R alpha. We have isolated cDNA encoding the soluble form of IL-5R alpha (sIL-5R alpha). To elucidate the biochemical and functional properties of sIL-5R alpha, we developed an expression system for sIL-5R alpha cDNA in insect cell line Sf21 using baculovirus expression vector and obtained conditioned medium containing large quantities of mouse sIL-5R alpha. Mouse siL-5R alpha was purified from the conditioned medium by using anti-IL-5R alpha mAb-coupled beads. Immunoaffinity-purified sIL-5R alpha with an approximate molecular mass of 42 kDa inhibited the binding of I-125-labeled IL-5 to IL-SR. By using purified sIL-5R alpha, we prepared rabbit anti-sIL-5R alpha antibody and developed a sandwich ELISA for detection of sIL-5R alpha. Significant amounts of sIL-5R alpha were detected in sera and ascitic fluids of mice bearing tumors (BCL(1) and MOPC104E) that responded to IL-5 for DNA synthesis, but not in sera of normal mice. Interestingly, elevated levels of serum sIL-5R alpha were observed in NZB and NZW mice. The sIL-5R alpha may, therefore, have an immunoregulatory role in vivo.