Endotoxin-like properties of a rhamnolipid exotoxin from Burkholderia (Pseudomonas) plantarii:: immune cell stimulation and biophysical characterization

Here we report on the purification, structural characterization, and biological activity of a glycolipid, 2-O-alpha-L-rhamnopyranosyl-alpha-L- rhamnopyranosyl-alpha( R)- 3-hydroxytetradecanoyl( R)-3-hydroxytetradecanoate (RL-2,2(14)) produced by Burkholderia (Pseudomonas) plantarii. RL2,2(14) is structurally very similar to a rhamnolipid exotoxin from Pseudomonas aeruginosa and identical to the rhamnolipid of Burkholderia pseudomallei, the causative agent of melioidosis. Interestingly, RL-2,2(14) exhibits strong stimulatory activity on human mononuclear cells to produce tumor necrosis factor a, the overproduction of which is known to cause sepsis and the septic shock syndrome. Such a property has not been noted so far for rhamnolipid exotoxins, only for bacterial endotoxins ( lipopolysaccharide, LPS). Consequently, we analyzed RL2,2(14) with respect to its pathophysiological activities as a heat-stable extracellular toxin. Like LPS, the cell-stimulating activity of the rhamnolipid could be inhibited by incubation with polymyxin B. However, immune cell activation by RL-2,2(14) does nor occur via receptors that are involved in LPS (TLR4) or lipopeptide signaling (TLR2). Despite its completely different chemical structure, RL2,2(14) exhibits a variety of endotoxin-related physicochemical characteristics, such as a cubic-inverted supramolecular structure. These data are in good agreement with our conformational concept of endotoxicity: intercalation of naturally originating virulence factors into the immune cell membrane leads to strong mechanical stress on integral proteins, eventually causing cell activation.