Failure of Brucella abortus lipopolysaccharide (LPS) to activate the alternative pathway of complement

Bovine erythrocytes (E) coated with either crude or purified preparations of Brucella abortus LPS were not lysed by human complement (C) in the presence of the chelating agent ethyleneglycol-bis-N, N'-tetraacetic acid (EGTA). On the other hand, bovine red cells coated with Salmonella typhimurium LPS were lysed by human C in EGTA. B. abortus LPS preparations did not cause fluid phase human C consumption in the presence of calcium and magnesium ions. However, as expected, S. typhimurium LPS consumed C from human serum in a dose-dependent fashion. The results of these experiments indicate that B. abortus LPS differs from the Enterobacterial LPSs in that it cannot activate the alternative pathway of C in human serum. Furthermore, the failure of B. abortus LPS to consume C in the fluid phase in the presence of calcium and magnesium ions suggests that the LPS cannot cause antibody-independent activation of the classical pathway.