Background: Patients on long-term dialysis eventually develop amyloid deposits with β2-microglobulin as a predominant component. Although several studies have suggested that high-flux membranes reduce β2-microglobulin in plasma compared with low-flux dialyzers, the mechanisms underlying this observation are still discussed. Methods: We revisited this important subject and measured β2-microglobulin in the plasma of healthy individuals (n = 8), and patients undergoing hemodialysis (n = 20) who for assigned periods of time were either treated with a low-flux membrane (cuprophan) or high-flux (polyamide) dialyzer with an ELISA. The number of blood cells was determined by FACS. β2-Microglobulin was also measured on the surface of granulocytes, lymphocytes, and monocytes before, directly after, and 4 h after hemodialysis. Expression of β2-microglobulin, c-fos, tumor necrosis factor-α (TNF-α), and interleukin-1 mRNA was determined in whole blood samples with quantitative RT-PCR using an internal standard in parallel. In the second part of the study, patients were assigned in a two-group cross-over design either to low- or high-flux dialyzers (n = 9 in each group), and dialyzer membranes were changed every 4 weeks for two consecutive periods. Serum β2-microglobulin concentrations were measured at the end of each period. Results: Healthy controls had a low plasma β2-microglobulin level of 1.2 ± 0.3 mg/l. Before hemodialysis, patients on low-flux dialyzers had a plasma β2-microglobulin level of 42.0 ± 14.0 mg/l, patients treated with high-flux dialyzers 21.5 ± 10.8 mg/l (p < 0.05 vs. low-flux dialyzers). In contrast, there was no significant difference in plasma concentrations of active transforming growth factor-β1 with the two different membrane types. The difference in serum β2-microglobulin between low- and high-flux membranes was more prominent directly after hemodialysis as well as 4 h after hemodialysis compared with the values directly before the start of treatment. At all studied time-points, leukocytes and platelets were significantly higher in patients on low-flux membranes. Healthy control persons exhibited a significantly higher amount of β2-microglobulin bound to granulocytes, lymphocytes, and monocytes compared with dialysis patients. Interestingly, β2-microglobulin bound to granulocytes, lymphocytes, and monocytes was significantly increased in patients treated with high-flux membranes compared with low-flux filters. Quantitative RT-PCR revealed no significant difference in β2-microglobulin expression in whole blood before hemodialysis, directly after hemodialysis, and 4 h after hemodialysis. However, TNF-α and c-fos transcripts were significantly higher in whole blood obtained from patients treated with low-flux membranes compared to high-flux dialyzers. The two-group cross-over study over three periods of 4 weeks revealed that switching from low-flux to high-flux dialyzers significantly reduced serum β2-microglobulin levels. Conclusion: Patients treated with a polyamide high-flux membranehad lower β2-microglobulin concentrations compared with those patients on low-flux dialyzers. This difference might not be mediated by an increase in β2-microglobulin mRNA, but may be caused by less β2-microglobulin released from the blood cells in patients treated with high-flux dialyzers, in addition to a better β2-microglobulin clearance.

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