Influence of endotoxin on production of leucocyte migration inhibition factor. Part I

Evidence for Endotoxin Contamination in Plastic Na+-Heparin Blood Collection Tube Lots

BACKGROUND

Biomarker assays are often conducted on whole blood samples in the course of drug development studies. Because bacterial lipopolysaccharide (LPS) (endotoxin) contamination is known to cause spontaneous cytokine production by monocytes, contamination of blood collection tubes may interfere with biomarker assay results.

METHODS

Whole blood from healthy donors was collected into plastic or glass sodium (Na+)-heparin Vacutainer™ blood collection tubes and heparinized syringes. Samples were analyzed for phosphoprotein response, cytokine production, and RNA expression. Tubes were tested for endotoxin contamination by use of the limulus amoebocyte lysate assay.

RESULTS

Results of phospho-flow cytometry, branched DNA (bDNA), and ELISA assays indicated that a specific lot (#5339582) of plastic Na+-heparin Vacutainer tubes was highly contaminated with an endotoxinlike substance, and contamination was confirmed by the limulus amoebocyte lysate assay. Analysis of multiple-analyte panels revealed that analytes whose changed expression was predictive of LPS stimulation were increased when whole blood was incubated in contaminated tubes for 6 or 18 h. Two additional lots of plastic tubes tested had detectable amounts of endotoxin sufficient to strongly alter phospho-flow cytometry analyses, as determined by the fold change in phosphorylation of p38 mitogen-activated protein kinase in response to tumor necrosis factor α and LPS. In contrast, 3 lots of glass tubes had substantially lower levels of spontaneous blood activation.

CONCLUSIONS

Endotoxin contamination associated with tubes from 3 lots of a particular type of plastic Na+-heparin Vacutainer tube dramatically affected biomarker assay measurements. Prescreening these tubes is suggested before their use in clinical sample analysis.

Removal of endotoxin from culture media by a polymyxin B sepharose column. The activity of contaminating endotoxin in culture media measured by the interleukin 1 inducing effect on human monocyte cultures and by the Limulus test

The in vitro study of monocytes (Mo) poses several problems. Minor contamination with endotoxin (ET) of media and utensils as well as adherence to glass or plastic surfaces may activate the cells and cause pronounced production of monokines. Many commercially liquid culture media were found to contain ET in concentrations above 25 X 10(-12) g/ml. A simple system for the removal of ET from media and solutions was established by use of a commercially available Polymyxin B Sepharose gel. To measure the lipopolysaccharide (LPS) binding capacity of the gel, known concentrations of LPS were added to culture media, which were passed through a column consisting of the Polymyxin B Sepharose gel. The content of ET and added LPS in media was measured by the Limulus amoebocyte lysate (LAL) test before and after passage of the column. The LPS-binding capacity of the gel was approximately 2.4 X 10(-6) g/10 ml. The biological activity of contaminating ET and added LPS in media, before and after passage of the column, was also characterized by the capacity of the media to induce interleukin 1 (IL-1) secretion in human Mo cultures. The content of IL-1 in Mo culture supernatants was determined by the mouse thymocyte costimulatory (LAF) assay. By comparison of the activity of ET in these different biological systems, it was demonstrated that 15-20 X 10(-12) g/ml of ET stimulate human Mo cultures to IL-1 secretion.

Estimation of variability in the assay of human interleukin-1

The variability in measurements of the interleukin-1 (IL-1) responsiveness of human monocyte (M phi)-depleted mononuclear cells (MDC) and in IL-1 production of human M phi has been estimated. Subjects were grouped so that inter- and intra-donor biological variability were evaluated, as well as day-to-day and basic intrinsic test-to-test methodological variability. IL-1 responsiveness was measured by the co-mitogenic effect of unstimulated and LPS-stimulated IL-1 standards on the PHA-induced proliferation of MDC. IL-1 production was measured by the enhancing effect of unstimulated and LPS-stimulated M phi-culture supernatants on PHA-stimulated allogeneic MDC. The variability of the IL-1 responsiveness of MDC was influenced mainly by biological factors, and this variability was effectively reduced by calculating the difference between the response of unstimulated and stimulated cultures, or by using cryopreserved MDC originating from a single stock of cells. In contrast, IL-1 production from both unstimulated and LPS-stimulated M phi was subjected to less biological and methodological variability, and this variability was not reduced by the above mentioned procedures. The results indicate that when assaying IL-1 production in clinical investigations involving longitudinal or comparative studies, a test system consisting of cryopreserved MDC from one stock of cells should be used. Alternatively the difference between the IL-1 activity from unstimulated and LPS-stimulated cultures should be calculated, since these procedures enhance the reproducibility of the measurements.

Mechanisms of pancreatic islet cell destruction. Dose-dependent cytotoxic effect of soluble blood mononuclear cell mediators on isolated islets of Langerhans

Supernatants of peripheral blood mononuclear cells from healthy human donors stimulated with recall antigen (purified protein derivative of tuberculin) or lectin (phytohaemagglutinin) markedly inhibited the insulin release from isolated human and rat islets of Langerhans, and decreased rat islet contents of insulin and glucagon in a dose-dependent manner. A maximal effect on islet function was obtained with supernatant concentrations down to 5%. Supernatants of mononuclear cells stimulated with tuberculin were more potent than supernatants produced by lectin stimulation. Culture medium reconstituted with tuberculin or phytohaemagglutinin did not impair islet function. Electron microscopy demonstrated that supernatants were cytotoxic to islet cells. The cytotoxic mononuclear cell mediator(s) was non-dialysable, sensitive to heating to 56 degrees C, labile even when stored at -70 degrees C, but stable when lyophilised.

The indirect leucocyte migration inhibition assay--an endotoxin-sensitive chemokinetic assay. Part II

Depletion of agarose for endotoxins resulted in a low spontaneous migration of polymorphnuclear cells (PMNC). Re-addition of endotoxin, in casu lipopolysaccharide from E. coli 026:B6 (LPS), enhanced the spontaneous PMNC migration in a two-phased dose-response pattern, reaching maximum migration with LPS 1 x 10(-7) g/ml. Thus, the migration of PMNC under agarose seems to be a chemokinesis. Leucocyte migration inhibition factor (LIF), induced by PPD 50 micrograms/ml at endotoxin-free conditions, significantly reduced the PMNC migration compared to supernatants from control cultures, however not compared to the conventional limit of significance, MI = 0.80. With increasing PMNC migration there was an insignificant decrease in the MI. Addition of LPS, 1 x 10(-9) g/ml, during LIF induction caused a significant increase in LIF production, an effect which overshadowed the effect of PPD. Thus, the application of the conventional limit of significance, MI = 0.80, may result in false-negative or false-positive conclusions, depending upon the endotoxin contamination. A standardization of the endotoxin content in both steps of the indirect leucocyte migration inhibition assay seems mandatory in order to obtain a reliable and reproducible bioassay.

Phorbol myristate acetate--fate and usability in a human interleukin-1 assay

Phorbol myristate acetate (PMA) has been widely used to produce interleukin-1 (IL-1) from monocytes (Mø) of various species, including man. Supernatants from cultures of human Mø, stimulated with PMA, 2 X 10(-6) M, contained IL-1-like activity, as judged by their high co-mitogenic effect on mitogen-stimulated human T-cells. However, a solution of PMA in culture medium was equally as active as the supernatants. By way of 3H-PMA it was demonstrated that the IL-1-like effect of PMA-induced supernatants was non-dialyzable, was coupled to proteins with a molecular weight greater than 70,000 Dalton and could be dependent mainly upon the PMA content. Thus, previous reports dealing with basic immunological as well as immunopharmacological aspects of IL-1, must be interpreted with caution if the conclusions have been based on results obtained with crude or high molecular weight forms of PMA-induced IL-1. It could be that PMA can be used as an IL-1 inducer. However, it must first be demonstrated that PMA does not interfere with the detection of IL-1 or, if so, that PMA has been removed sufficiently from the test material to avoid interference. Secondly, the specificity of PMA with respect to IL-1 inducing effect must be demonstrated under endotoxin-free culture conditions.

Cytokines cause functional and structural damage to isolated islets of Langerhans

Cytokines are soluble, antigen non-specific, non-immunoglobulin mediators produced and secreted by blood mononuclear cells interacting in the cellular immune-response. To test the possibility that cytokines participate in the autoimmune destruction of the pancreatic beta-cells leading to insulin-dependent diabetes mellitus, isolated human or rat islets of Langerhans were incubated for 7 days with cytokine-rich, cell-free supernatants of blood mononuclear cells from healthy human donors stimulated with or without purified protein derivative of tuberculin or phytohaemagglutinin. Glucose stimulated insulin-release, and contents of insulin and glucagon in islets incubated with cytokine-rich supernatants were markedly reduced. This impairment of islet function was due to a cytotoxic effect of cytokine-rich supernatants as judged by disintegration of normal light-microscopic morphology.