Neutralizing monoclonal antibodies can potentiate IL-5 signaling

IL-5 is a major determinant in the survival, differentiation and effector-f unctions of eosinophils. It mediates its effect upon binding and activation of a membrane bound receptor (R), composed of a ligand-specific alpha -cha in and a beta -chain, shared with the receptors for IL-3 and granulocyte-ma crophage colony-stimulating factor. We have generated and mapped the epitop es of three monoclonal antibodies (mAb) directed against this cytokine: the strong neutralizing mAb 5A5 and 1E1, and the very weak neutralizing mAb H3 0. We found that H30 as well as 5A5 can increase proliferation above the le vel induced by human (h)IL-5 alone, in a JAK-2-dependent manner, and at eve ry sub-optimal hIL-5 concentration analyzed. This effect is dependent on mA b-mediated cross-linking of IL-5R complexes, and is only observed on cell l ines expressing a hybrid human/mouse IL-5R alpha -chain. We discuss these f indings in view of the stoichiometric and topological requirements for an a ctivated IL-5R. Since humanized anti-IL-5 mAb are currently in clinical tes ting, our findings imply that such mAb should be carefully evaluated for th eir potentiating effects.