Do cytokine-inducing substances penetrate through dialysis membranes and stimulate monocytes?

Does a relatively low concentration of endotoxin in dialysate, seen in our clinical dialysis, enhance cytokine production of monocytes across high-flux membranes? Several investigators using extremely high concentrations of endotoxin in dialysate maintain that it does. In vitro experiments in this study were conducted to clarify this. Peripheral blood monocytes were isolated from healthy volunteers and incubated for 20 hours. The incubation medium was made of back-filtrates obtained either from Pseudomonas-contaminated water with endotoxin concentration of 621 pg/ml or water with 10 ng/ml or 1 microgram/ml of lipopolysaccharide (LPS), by passing it through a high-flux membrane dialyzer. Endotoxin free water and addition of LPS (0.01 to 10 ng/ml) were used as a negative and positive control. Interleukin-1 beta (IL-1 beta), IL-6 and tumor necrosis factor alpha (TNF alpha) were measured. Back-filtrate from Pseudomonas-contaminated water did not enhance cytokine production, while 50 pg/ml of endotoxin in culture medium induced a significant cytokine production. Back-filtrate of 1 microgram/ml of endotoxin solution marginally increased IL-6 production, but not the other two cytokines. However, none of the cytokines was induced by the back-filtrate of 10 ng/ml of endotoxin. Monocytes isolated from blood following three hour extracorporeal recirculation did not alter the production of cytokines. These results cannot confirm the transfer of cytokine inducing substances across the membrane at a relatively low endotoxin concentration in dialysate. Further study should be made as to the minimal requirement of dialysate purification for preventing monocyte stimulation.