RAT INTERLEUKIN-2 IMMUNOGLOBULIN-M FUSION PROTEINS ARE CYTOTOXIC IN-VITRO FOR CELLS EXPRESSING THE IL-2 RECEPTOR AND CAN ABOLISH CELL-MEDIATIED IMMUNITY IN-VIVO

A hybrid cDNA coding for a fusion protein between rat interleukin 2 (I L-2) and a truncated heavy chain from rat immunoglobulin M (IgM) was c onstructed. The rat IL-2 and rat IgM CH2-3-4 hybrid gene was subcloned into a vector (PKCR6) for expression of the fusion molecule in Chines e hamster ovary (CHO) cells. Cells transfected with the hybrid cDNA se crete multimeric forms of the fusion protein (IL-2-Mu). Size analysis of the construct revealed that the majority (95%) of the secreted prot eins have a high mw (>500 kDa). The IL-2-Mu construct bind specificall y to cells bearing the IL-2 receptors (IL-2R) with a binding affinity around 5 nM. The specific binding to IL-2R leads to T cell proliferati on or, if rabbit complement is added, to T cell lysis. Multimeric form s (>500 kDa) of the fusion protein mediate complement-dependent lysis but trigger only weak proliferation when compared with the low-mw form s (<500 kDa). In contrast, the latter only efficiently mediate T cell proliferation without inducing complement-dependent lysis. After intra venous administration of CHO supernatant containing IL-2-Mu, or purifi ed IL-2-Mu proteins into rats, the fusion proteins disappeared from th e circulation with a t(1/2) of 1 hr. The circulating IL-2-Mu construct s in the rat serum retained their capacity to induce complement-depend ent lysis of IL-2R-bearing T cells in vitro. Furthermore, the IL-2-Mu construct was able to suppress the delayed type hypersensitivity (DTH) reaction (an IL-2R, T helper cell-dependent event) in mice. A weak im mune response (antirat IL-2-Mu antibodies) was observed when rats rece ived multiple daily injections of the construct.