Laboratory studies are crucial for learning more about the mechanisms by which the lipids exert their protective function. One of the first to study this phenomenon was Dr Sucharit Bhakdi from the Institute of Medical Microbiology, University of Giessen (Institut für Medizinsche Mikrobiologie, Justus-Liebig-Universität Gießen), Germany along with his team of researchers from various institutions in Germany and Denmark.15
Staphylococcus aureus α-toxin is the most toxic substance produced by strains of the disease-promoting bacteria called staphylococci. It is able to destroy a wide variety of human cells, including red blood cells. For instance, if minute amounts of the toxin are added to a test tube with red blood cells dissolved in 0.9 percent saline, the blood is hemolyzed, that is the membranes of the red blood cells burst and hemoglobin from the interior of the red blood cells leaks out into the solvent. Dr. Bhakdi and his team mixed purified α-toxin with human serum (the fluid in which the blood cells reside) and saw that 90 percent of its hemolyzing effect disappeared. By various complicated methods they identified the protective substance as LDL, the carrier of the so-called bad cholesterol. In accordance, no hemolysis occurred when they mixed α-toxin with purified human LDL, whereas HDL or other plasma constituents were ineffective in this respect.
Dr. Willy Flegel and his co-workers at the Department of Transfusion Medicine, University of Ulm, and the Institute of Immunology and Genetics at the German Cancer Research Center in Heidelberg, Germany (DRK-Blutspendezentrale und Abteilung für Transfusionsmedizin, Universität Ulm, und Deutsches Krebsforschungszentrum, Heidelberg) studied endotoxin in another way.16 As mentioned, one of the effects of endotoxin is that white blood cells are stimulated to produce cytokines. The German researchers found that the cytokine-stimulating effect of endotoxin on the white blood cells disappeared almost completely if the endotoxin was mixed with human serum for 24 hours before they added the white blood cells to the test tubes. In a subsequent study17 they found that purified LDL from patients with familial hypercholesterolemia had the same inhibitory effect as the serum.
LDL may not only bind and inactivate dangerous bacterial toxins; it seems to have a direct beneficial influence on the immune system also, possibly explaining the observed relationship between low cholesterol and various chronic diseases. This was the starting point for a study by Professor Matthew Muldoon and his team at the University of Pittsburgh, Pennsylvania. They studied healthy young and middle-aged men and found that the total number of white blood cells and the number of various types of white blood cells were significantly lower in the men with LDL-cholesterol below 160 mg/dl (mean 88.3 mg/l),than in men with LDL-cholesterol above 160 mg/l (mean 185.5 mg/l).18 The researchers cautiously concluded that there were immune system differences between men with low and high cholesterol, but that it was too early to state whether these differences had any importance for human health. Now, seven years later with many of the results discussed here, we are allowed to state that the immune-supporting properties of LDL-cholesterol do indeed play an important role in human health