Interleukin 4 (IL-4) is a pleiotropic cytokine produced primarily by activated T lymphocytes, mast cells and basophils (1-3).

The cDNA sequence of rat IL-4 predicts a 147 amino acid (aa) residue precursor protein containing a 24 aa residue signal peptide that is cleaved to generate the 123 aa residue mature protein containing four potential N-linked glycosylation sites and seven cysteine residues (4-6). Mature rat IL-4 shares 60% and 44% amino acid sequence similarity with mouse and human IL-4, respectively (5,6).

IL-4 has multiple immune response-modulating activities on a variety of cell types. It is an important regulator of isotype switching, inducing IgE production in B lymphocytes. It is an important modulator of the differentiation of precursor T helper cells to the Th2 subset that mediates humoral immunity and modulates antibody production. In addition, IL-4 has also been shown to have anti-tumor activity both in vivo and in vitro (1-3).

The biological effects of IL-4 are mediated by specific cell surface receptor complexes. Although IL-4 R does not bind IL-13 directly, it has been shown to complex with the low-affinity IL-13 R to form the functional high-affinity receptor complex for IL-13 (7, 8). In addition to the membrane-bound form of IL-4 R, a naturally occurring soluble form of IL-4 R has been identified in human and mouse biological fluids and in mouse cell culture supernates (9-11). Soluble IL-4 R has been to shown to bind IL-4 with high affinity in solution.