A temperature-dependent inhibitory activity of serum on the capacity of Saccharomyces cerevisiae-derived hepatitis B surface antigen to bind to monocytes

Summary. Hepatitis B surface antigen, when produced in yeast (rHBsAg), is capable of binding to cells that express the lipopolysaccharide coreceptor CD14. This interaction is enhanced by a serum protein, the lipopolysaccharide binding protein (LBP). Here we report that most of the rHBsAg particles that attached to monocytes at 0°C, were not endocytosed but were released back into the serum-containing binding buffer at 37°C. Additionally, serum-dependent binding at 37°C was weak when compared to the serum-dependent attachment at 0°C. Pre-incubation at 37°C of cells together with serum did not abolish binding of freshly added rHBsAg at 0°C. However, pre-incubation of rHBsAg with serum at 37°C reduced attachment to cells following incubation at 0°C. Soluble CD14 and LBP, two serum proteins which can act as phospholipid transfer molecules, were shown not to be responsible for the inhibitory effect. Pre-incubation at 37°C of rHBsAg in serum-free hepatoma cell line-conditioned media resulted in a pronounced reduction in subsequent binding to cells at 0°C. These observations suggest that the temperature-dependent inhibitory effect is caused by serum factors that are probably secreted by hepatocytes.