The pro-inflammatory cytokine interleukin (IL)-6 (refs. 1-5) can bind to cells lacking the IL-6 receptor (IL-6R) when it forms a complex with the soluble IL-6R (sIL-6R) (trans signaling). Here, we have assessed the contribution of this system to the increased resistance of mucosal T cells against apoptosis in Crohn disease (CD), a chronic inflammatory disease of the gastrointestinal tract. A neutralizing antibody against IL-6R suppressed established experimental colitis in various animal models of CD mediated by type 1 T-helper cells, by inducing apoptosis of lamina propria T cells. Similarly, specific neutralization of sIL-6R in vivo by a newly designed gp130-Fc fusion protein caused suppression of colitis activity and induction of apoptosis, indicating that sIL-6R prevents mucosal T-cell apoptosis. In patients with CD, mucosal T cells showed strong evidence for IL-6 trans signaling, with activation of signal transducer and activator of transcription 3, bcl-2 and bcl-xl. Blockade of IL-6 trans signaling caused T-cell apoptosis, indicating that the IL-6-sIL-6R system mediates the resistance of T cells to apoptosis in CD. These data indicate that a pathway of T-cell activation driven by IL-6-sIL-6R contributes to the perpetuation of chronic intestinal inflammation. Specific targeting of this pathway may be a promising new approach for the treatment of CD.

Signal transduction in response to interleukin‐6 (IL‐6) requires binding of the cytokine to its receptor (IL‐6R) and subsequent homodimerization of the signal transducer gp130. The complex of IL‐6 and soluble IL‐6R (sIL‐6R) triggers dimerization of gp130 and induces responses on cells that do not express membrane bound IL‐6R. Naturally occurring soluble gp130 (sgp130) can be found in a ternary complex with IL‐6 and sIL‐6R. We created recombinant sgp130 proteins that showed binding to IL‐6 in complex with sIL‐6R and inhibited IL‐6/sIL‐6R induced proliferation of BAF/3 cells expressing gp130. Surprisingly, sgp130 proteins did not affect IL‐6 stimulated proliferation of BAF/3 cells expressing gp130 and membrane bound IL‐6R, indicating that sgp130 did not interfere with IL‐6 bound to IL‐6R on the cell surface. Additionally, sgp130 partially inhibited proliferation induced by leukemia inhibitory factor (LIF) and oncostatin M (OSM) albeit at higher concentrations. Recombinant sgp130 protein could be used to block the anti‐apoptotic effect of sIL‐6R on lamina propria cells from Crohn disease patients. We conclude that sgp130 is the natural inhibitor of IL‐6 responses dependent on sIL‐6R. Furthermore, recombinant sgp130 is expected to be a valuable therapeutic tool to specifically block disease states in which sIL‐6R transsignaling responses exist, e.g. in morbus Crohn disease.

The human cytomegalovirus glycoprotein, UL16, binds to two members of a novel family of molecules, the ULBPs, and to the MHC class I homolog, MICB. The ULBPs are GPI-linked glycoproteins belonging to the extended MHC class I family but are only distantly related to MICB. The ULBP and MICB molecules are ligands for the activating receptor, NKG2D/DAP10, and this interaction is blocked by a soluble form of UL16. The ULBPs stimulate cytokine and chemokine production from NK cells, and expression of ULBPs in NK cell-resistant target cells confers susceptibility to NK cell cytotoxicity. Masking of NK cell recognition of ULBP or MIC antigens by UL16 provides a potential mechanism by which human cytomegalovirus-infected cells might evade attack by the immune system.

The ligand-binding subunit (gp80) of the human interleukin-6 receptor (IL-6R) was transiently expressed in COS-7 cells. The metabolically labeled protein was shown to be quantitatively released from the membrane within 20 h. We identified the protein released from the transfected COS-7 cells after purification to homogeneity and N-terminal sequencing as a soluble form of the gp80/IL-6R. Shedding of the gp80 protein was strongly induced by 4 beta-phorbol-12-myristate-13-acetate, indicating that the process was regulated by protein kinase C (PKC). This was further corroborated by the finding that co-transfection of a PKC expression plasmid led to enhanced shedding of the gp80 protein. Since shedding of gp80 could not be prevented by treatment of the cells with inhibitors of all known classes of proteases, a novel protease seems to be involved. As a control, an unrelated membrane protein (vesicular stomatitis virus glycoprotein) was transfected into COS-7 cells and analyzed for shedding. Since the turnover of this protein was not mediated by shedding, we conclude that the release of gp80 from COS-7 cells is a specific process. The shed gp80 protein specifically binds IL-6, and this complex shows biological activity on human hepatoma cells. Human peripheral blood monocytes released a soluble form of the gp80 protein into the culture medium upon PMA treatment indicating that PKC-regulated shedding is the physiological mechanism of generation of the soluble IL-6R.

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 acquired immune deficiency syndrome, rheumatoid arthritis, and others. The function of the soluble (s)IL-6R in vivo is unknown. Since human (h)IL-6 acts on human and murine target cells, but murine IL-6 on murine cells only, we constructed transgenic mice expressing the hsIL-6R. We report here that in the presence of hsIL-6R, mice are hypersensitized towards hIL-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 detected acute phase response persists for a longer period of time. The IL-6/IL-6R complex prolongs markedly the Il-6 plasma half-life. Our results reinforce the role of the hsIL-6R as an agonistic protein, help to understand the function of the hsIL-6R in vivo, and highlight the significance of the receptor in the induction of the acute phase response.