Expression and purification of stable 33-kDa soluble human CD23 using the Drosophila S2 expression system.

CD23, a 45-kDa type II membrane glycoprotein present on B cells, monocytes, and other human immune cells, is a low-affinity receptor for IgE. The extracellular region of the membrane-bound human CD23 is processed into at least four soluble (s) CD23 forms, with apparent molecular masses of 37, 33, 29, and 25 kDa. High levels of sCD23 are found in patients with allergy, certain autoimmune diseases, or chronic lymphocytic leukemia. Therefore, inhibition of the processing of membrane-bound CD23 to control the cytokine-like effects of sCD23 offers a novel therapeutic opportunity. While the 37-, 29-, and 25-kDa forms of sCD23 have been expressed previously as recombinant proteins, the 33-kDa form has not been purified and characterized. To further investigate the multiple roles of sCD23 fragments and to devise assays to identify potent small-molecule inhibitors of CD23 processing, we have produced the 33-kDa form of sCD23 using Chinese hamster ovary (CHO) and Drosophila S2 cells. The CHO-expressed 33-kDa protein was found to undergo proteolytic degradation during cell growth and during storage of purified protein, resulting in accumulation of a 25-kDa form. The Drosophila system expressed the 33-kDa sCD23 in a stable form that was purified and demonstrated to be more active than the CHO-derived 25-kDa form in a monocyte TNFalpha release assay.

Neutralizing murine monoclonal antibodies to human IL-5 isolated from hybridomas and a filamentous phage Fab display library.

Conventional hybridomas and combinatorial Ab libraries were used to develop neutralizing murine mAbs to human IL-5. Mice were immunized with rIL-5. Spleens from two mice were used to generate hybridomas. Spleens from an additional three mice were used to construct a combinatorial library. In both instances, Abs were identified and selected by ELISA using 96-well plates coated with rIL-5. These Abs were tested for the ability to block binding of iodinated rIL-5 to the alpha-chain of the human IL-5 receptor (IL-5R alpha) and to inhibit proliferation of IL-5-dependent cells. By hybridoma technology, 16 mAbs were obtained, 11 of which blocked binding to IL-5R alpha, including three that inhibited proliferation. Quantitative binding assays and sequence analysis revealed that these latter three mAbs were closely related. Combinatorial cloning and selection by phage display was used to isolate 24 bacterial colonies secreting Fabs that bound to 125I-rIL-5 and to rIL-5-coated plates. Sequencing of 10 of the Fabs indicated that four unique Abs were obtained, comprising one predominant VH paired with one of two different VL. The sequence of the Fabs was distinct from the sequences of the neutralizing mAbs. In contrast to the mAbs, none of the Fabs blocked binding of 125I-IL-5 to IL-5R alpha or neutralized the biologic activity of IL-5. The inability to identify neutralizing Fabs was shown not to result from their monovalency, because a Fab derived from one of the neutralizing mAbs, by cloning and expression of its Fd and kappa light chains, retained neutralizing activity. By chain shuffling, pairing of the Fd fragment of the heavy chain of one of the neutralizing mAbs (2B6), with the light chain library derived from the IL-5-immunized mice, neutralizing Fabs were obtained. These Fabs contained light chain sequences closely related to the original light chain of 2B6. Hence, chain shuffling allowed detection of a light chain sequence that was not evident upon two-chain combinatorial selection. The results reveal differences in the Abs obtained from a combinatorial library vs hybridomas and demonstrate how these approaches can be used in concert to select mAbs with neutralizing activity.

Analysis of the interaction between human interleukin-5 and the soluble domain of its receptor using a surface plasmon resonance biosensor.

A surface plasmon resonance (SPR) biosensor was used to study the interaction of human interleukin-5 (hIL5) with its receptor. IL5 is a major growth factor in the production and activation of eosinophils. The receptor for IL5 is composed of two subunits, alpha and beta. The alpha subunit provides the specificity for IL5 and consists of an extracellular soluble domain, a single transmembrane region and a cytoplasmic tail. We expressed the soluble domain of the human IL5 receptor alpha subunit (shIL5R alpha) and human IL5 (hIL5) in Drosophila. Both hIL5 and shIL5R alpha were immobilized separately through amine groups onto the carboxylated dextran layer of sensor chips of the BIAcore (Pharmacia) SPR biosensor after N-hydroxysuccinimide/carbodiimide activation of the chip surface. Interactions were measured for the complementary macromolecule, either shIL5R alpha or hIL5, in solution. Kinetics of binding of soluble analyte to immobilized ligand were measured and from this the association rate constant, dissociation rate constant and equilibrium dissociation constant (Kd) were derived. With immobilized shIL5R alpha and soluble hIL5, the measured Kd was 2 nM. A similar value was obtained by titration calorimetry. The Kd for Drosophila expressed receptor and IL5 is higher than the values reported for proteins expressed in different systems, likely due to differences in the methods of interaction analysis used or differences in protein glycosylation. Receptor-IL5 binding was relatively pH independent between pH 6.5 and 9.5. Outside this range, the dissociation rate increased with comparatively little increase in association rate.(ABSTRACT TRUNCATED AT 250 WORDS)