Effects of dialyzer membrane on interleukin-1β (IL-1β) and IL-1β-converting enzyme in mononuclear cells

Abstract

Background. In vitro stimulation of mononuclear cells (peripheral blood mononuclear cells; PBMCs) with an endotoxin (lipopolysaccharide; LPS) revealed elevated cell-associated levels of interleukin-1β (IL-1β) in end-stage renal disease (ESRD) patients on Cuprophan hemodialysis (HD), suggesting a defect in the process of IL-1β's release from activated cells. IL-1β is initially synthesized as an inactive precursor called proIL-1β. ProIL-1β is processed into the biologically active mature form of IL-1β (mIL-1β) requiring the specific IL-1β-converting enzyme (ICE). Methods. Using specific immunoassays (enzyme-linked immunosorbent assays), we measured cell-associated and extracellular proIL-1β as well as mIL-1β in LPS-stimulated PBMCs to test whether ICE-dependent processing of proIL-1β and/or secretion of mIL-1β was impaired in ESRD patients compared with healthy controls. PBMCs of healthy controls (N = 9), of ESRD patients on peritoneal dialysis (PD, N = 10), and of those patients on intermittent HD (N = 8) were studied. The same HD patients were studied three times with low-flux Cuprophan (GFS 12), low-flux polysulfon (F6 HPS), and high-flux polysulfon (F60S) in randomized order. PBMCs were separated from whole blood by Ficoll-Hypaque centrifugation and incubated in vitro for 18 hours in the presence LPS (10 ng/mL). Extracellular (PBMC culture supernatants) and cell-associated (cell lysates) levels of proIL-1β and mIL-1β were measured. Results. The total production (cell-associated plus extracellular) of LPS-induced IL-1β (proIL-1β plus mIL-1β) was similar in healthy controls (25.96 ± 0.84 ng/2.5 × 106 PBMC), PD patients (29.53 ± 1.31 ng/2.5 × 106 PBMC), and in Cuprophan-treated HD patients (23.28 ± 1.24 ng/2.5 × 106 PBMC). In normal controls, 43.6% of the total IL-1β was processed into mIL-1β, which was significantly more than that in PD patients (27.3%, P < 0.02) but was similar to that in Cuprophan-treated HD patients (37.1%). Comparing cell-associated and extracellular concentrations of mIL-1β, PBMCs of normal controls secreted 82.2% of mIL-1β; this was significantly more than that in PD patients (59.4%, P < 0.01) and that in Cuprophan HD patients (54.2%, P < 0.01). When HD patients were switched from Cuprophan to F6 HPS or F60S, neither total IL-1β production nor processing of IL-1β changed. However, secretion of mIL-1β increased significantly with F6 HPS (80.6%, P < 0.01) as well as with F60S (76.6%, P < 0.02) compared with Cuprophan. Conclusion. We conclude that the ability of PBMCs to produce IL-1β in response to LPS is normal in PD patients as well as in HD patients. ICE-dependent processing of inactive proIL-1β into biologically active mIL-1β is reduced in PD patients, but not in HD patients. Secretion of mIL-1β is impaired in PD and HD patients treated with Cuprophan. This impaired ability to secrete active mIL-1β seems to be independent of ICE activity and is normalized when HD-patients are switched from Cuprophan to low- or high-flux polysulfon. Increased cell-associated levels of biologically active mIL-1β in circulating PBMCs represent a state of inflammation that may contribute to chronic inflammatory diseases such as β2-microglobulin amyloidosis. Replacement of Cuprophan by synthetic membranes normalizes PBMC function and reduces the state of inflammation in ESRD patients.