Responsiveness of human monocytes to Ru 41.740 (Biostim). Influence of preincubation in vitro.

Biostim, which is a glucoprotein extract of Klebsiella pneumoniae, is known to trigger human monocytes to increased secretion of metabolites, which inhibit lymphocyte mitogenesis and which augment NK-activity of lymphocytes. Some aspects of this monocyte activation have been examined in this study. The conclusion of the present investigation is that preincubation of human monocytes for 6-24 h in serum-free medium at 37 degrees C renders them unresponsive to Biostim, as assessed by secretion of the above factors. Monocytes which were preincubated in 100% serum or in serum-free medium at 3 degrees C retained their responsiveness to Biostim. Loss of Biostim responsiveness could not be prevented by interferon alpha, beta, gamma or by granulocyte-monocyte and granulocyte colony stimulating factors. One interpretation of these results is that human monocytes possess distinct receptors for Biostim whose expression is lost under suboptimal culture conditions. Expression of these receptors is a prerequisite for responsiveness to the drug.