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Here we report on the purification, structural characterization, and biological activity of a glycolipid, 2- O -α-L-rhamnopyranosyl-α-L-rhamnopyranosyl-α( R )-3-hydroxytetradecanoyl-( R )-3-hydroxytetradecanoate (RL-2,2 14 ) produced by  Burkholderia  ( Pseudomonas )  plantarii . RL-2,2 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 α, 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 RL-2,2 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, RL-2,2 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.

biological chemistry

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