Apoptosis is a natural stimulus of IL6R shedding and contributes to the proinflammatory trans-signaling function of neutrophils

Abstract

Interleukin 6 (IL6) trans-signaling has emerged as a prominent regulator of immune responses during both innate and acquired immunity. Regulation of IL6 trans-signaling is reliant upon the release of soluble IL6 receptor (sIL6R), which binds IL6 to create an agonistic IL6/sIL6R complex capable of activating cell types that would not normally respond to IL6 itself. Here we show that intrinsic and extrinsic apoptotic stimulation by DNA damage, cytokine deprivation, and Fas stimulation promotes shedding of sIL6R. Apoptosis-induced shedding of the IL6R was caspase dependent but PKC independent, with inhibition of ADAM17 preventing IL6R shedding. Such insight is relevant to the control of acute inflammation, where transition from the initial neutrophil infiltration to a more sustained population of mononuclear cells is essential for the resolution of the inflammatory process. This transitional event is governed by IL6 trans-signaling. This study demonstrates that IL6R is shed from apoptotic human neutrophils. In vivo studies in a murine inflammation model showed that neutrophil depletion resulted in reduced local sIL6R levels and a concomitant decrease in mononuclear cells, suggesting that apoptosis-induced IL6R shedding from neutrophils promotes IL6 trans-signaling and regulates the attraction of monocytic cells involved in the clearance of apoptotic neutrophils.

Transgenic blockade of interleukin 6 transsignaling abrogates inflammation

“…Animals that have been depleted of neutrophils showed no increase in sIL6R levels in air pouches, and these changes were associated with a marked reduction in the number of infiltrating mononuclear cells. 27 We suggest that sIL6R liberated from the infiltrating neutrophil population is required to drive the recruitment of inflammatory mononuclear cells to the site of inflammation. 27 Because we were especially interested in the influence of IL6 transsignaling on monocyte recruitment during the late phase of acute inflammation, we next evaluated the inflammatory regulation of leukocytes in the opt_sgp130Fc transgenic mice.…”

“…27 We suggest that sIL6R liberated from the infiltrating neutrophil population is required to drive the recruitment of inflammatory mononuclear cells to the site of inflammation. 27 Because we were especially interested in the influence of IL6 transsignaling on monocyte recruitment during the late phase of acute inflammation, we next evaluated the inflammatory regulation of leukocytes in the opt_sgp130Fc transgenic mice.In vitro experiments revealed that at least a 10-fold molar excess of sgp130Fc to IL6/sIL6R is sufficient to completely block IL6-transsignaling. 24 Air pouches from transgenic animals contained sgp130Fc protein in the range of 3 g, which is a 100-fold molar excess of sgp130Fc to IL6/sIL6R and therefore should be sufficient to block IL6 transsignaling-mediated processes in vivo during acute inflammation ( Figure 4A).…”

“…Immune complexes were precipitated with 30 L protein A-Sepharose (50% slurry, CL-4B, Amersham Biosciences) for 2 hours at 4°C and separated by 10% SDS-PAGE, and subjected to Western blot analysis using a mAB against hsgp130 (BP-4, Milenia Biotec, Bad Nauheim, Germany) or hslL6R (clone 14-18). 27 …”

“…31,36 The studies outlined herein highlight that regulation of IL6 transsignaling relies primarily on the initial influx of neutrophils, which has been shown in vitro to shed IL6R in response to inflammatory chemokines, lipid mediators, complement components, C-reactive protein, and neutrophil apoptosis. 13,27,36,[45][46][47] The subsequent appraisal of leukocyte recruitment and inflammatory chemokine expression in opt_sgp130Fc transgenic mice showed that IL6 transsignaling was responsible for triggering the chemokine-mediated recruitment of mononuclear phagocytes. Indeed, the over-expression of sgp130Fc led to impaired expression of the inflammatory chemokine MCP-1/ CCL2.…”

Skeletal muscle cells: from local inflammatory response to active immunity

“…55 In vivo trans-signalling dramatically increases the number of potentially IL-6 responsive cells, which are otherwise limited by missing complete IL-6R expression: during acute inflammatory events, infiltrating neutrophils can become a source of sIL-6R through its shedding from the membranes of both living and apoptosing cells. 56 In an in vivo experimental model of myosininduced myositis, IL-6 À/À mice were disease free, with no clinical sign of muscle weakness, or any microscopic evidence of neuromuscular infiltration. 57 In polymyositis biopsies, only those myofibres that were invaded and destroyed by mononuclear cells express high levels of IL-6, MCP-1 and ICAM-1.…”