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 ha ve been extensively documented: (i) activation of the alternative path way (AP) by the polysaccharide region, and (ii) activation of the clas sical pathway (CP) by the lipid A region. Here we demonstrate that LPS from the Klebsiella pneumoniae I-145 strain activates the AP by a mec hanism dependent on the acylation of the lipid A region. Cleavage of C 3 by K. pneumoniae LPS in EGTA was blocked by polymyxin B. Two 34 kDa derivatives were prepared from a membrane extract of this K. pneumonia e strain: (i) an acyl-poly (1,3) galactoside containing two galactosam ine-bound ester-linked and two amide-linked fatty acids (EFA-APG), and (ii) an acyl-poly (1,3) galactoside devoid of ester-linked fatty acid s (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 a ctivated by EFA-APG but not by APG nor by the isolated polygalactose c hain GC-APG, indicating a critical role for ester-linked fatty acids i n 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 decre ased by polymyxin B. Furthermore, APG formed aggregates of similar siz e although it was not able to activate AP. Therefore the role of lipid A acylation in triggering AP activation is not exclusively mediated b y aggregation of the molecule. LPS from the rough strain of Salmonella minnesota (Sm Re LPS) directly activates the CP but not the AP. Howev er, when mixed with the polygalactose chain GC-APG, Sm Re LPS activate d the AP. The data demonstrate a cooperation between the lipid A regio n and the polysaccharidic chain in activation of the AP. Similar coope ration may occur with other LPS molecules.