The function of the soluble IL-6 receptor in vivo

Interleukin-6 (IL-6) is considered an important mediator of acute inflammatory responses. Moreover, IL-6 functions as a differentiation and growth factor of hematopoietic precursor cells, B-cells, T-cells, keratinocytes, neuronal cells, osteoclasts and endothelial cells. IL-6 exhibits its action via a receptor complex consisting of a specific IL-6 receptor (IL-6R) and a signal-transducing subunit (gp130). Soluble forms of both receptor components are generated by shedding and are found in patients with various diseases such as AIDS, rheumatoid arthritis and others. The function of the soluble IL-6R in vivo is unknown. To discriminate between the biologic function of hIL-6 alone and that of the hIL-6/hsIL-6R complex, mice transgenic for human IL-6, for the human soluble IL-6R and for both, human IL-6 and the human soluble IL-6R were analyzed and compared with nontransgenic littermates. While IL-6 transgenic mice exhibit elevated acute phase protein levels and develop plasmacytomas, hsIL-6R single transgenic mice are hypersensitized towards human IL-6, mounting an acute phase protein gene induction at significantly lower IL-6 dosages compared to control animals. Furthermore, in hsIL-6R transgenic mice, the acute phase response persists for a longer period of time and the IL-6 plasma half life was markedly prolonged. IL-6/sI1-6R mice, however, develop massive hepatosplenomegaly caused by extramedullary hematopoisis in these organs. In IL-6- and IL-6R-single transgenic mice, no such effects were observed. Our study discloses a novel biologic effect of the hIL-6/hsIL-6R complex, which is clearly distinct from that of hIL-6 alone. We provide evidence that the activation of the gp130 signal transducer represents a major stimulation of growth and differentiation of hematopoietic progenitor cells.