Acylation of the lipid A region of a Klebsiella pneumoniae LPS controls the alternative pathway activation of human complement.

Two mechanisms of direct activation of the complement system by LPS have been extensively documented: (i) activation of the alternative pathway (AP) by the polysaccharide region, and (ii) activation of the classical pathway (CP) by the lipid A region. Here we demonstrate that LPS from the Klebsiella pneumoniae I-145 strain activates the AP by a mechanism dependent on the acylation of the lipid A region. Cleavage of C3 by K. pneumoniae LPS in EGTA was blocked by polymyxin B. Two 34 kDa derivatives were prepared from a membrane extract of this K. pneumoniae strain: (i) an acyl-poly (1,3) galactoside containing two galactosamine-bound ester-linked and two amide-linked fatty acids (EFA-APG), and (ii) an acyl-poly (1,3) galactoside devoid of ester-linked fatty acids (APG). APG and EFA-APG share the structure of LPS molecules, with a long polysaccharidic chain, a core, and a lipid A region. The AP was activated by EFA-APG but not by APG nor by the isolated polygalactose chain GC-APG, indicating a critical role for ester-linked fatty acids in AP activation. Polymyxin B which binds to the lipid A region of LPS completely inhibited AP activation by EFA-APG. A small part of EFA-APG was shown to form aggregates in saline, but aggregation was not decreased by polymyxin B. Furthermore, APG formed aggregates of similar size although it was not able to activate AP. Therefore the role of lipid A acylation in triggering AP activation is not exclusively mediated by aggregation of the molecule. LPS from the rough strain of Salmonella minnesota (Sm Re LPS) directly activates the CP but not the AP. However, when mixed with the polygalactose chain GC-APG, Sm Re LPS activated the AP. The data demonstrate a cooperation between the lipid A region and the polysaccharidic chain in activation of the AP. Similar cooperation may occur with other LPS molecules.