Studies of the mechanism of bacterial resistance to complement-mediated killing. V. IgG and F(ab')₂ mediate killing of E. coli 0111B4 by the alternative complement pathway without increasing C5b-9 deposition

The mechanism of antibody-dependent complement-(c) mediated killing of Escherichia coli 0111B4, strain 12015 (12015), was examined. 12015 was resistant to serum killing when incubated in hypogammaglobulinemic serum (HγS) or pooled normal human serum (NHS) that had been previously adsorbed to remove specific antibody (Abs NHS). Presensitization with immune rabbit serum or purified immune rabbit IgG resulted in 1 to 3 log killing when 5 x 10⁸ colony forming units (CFU)/ml were incubated in 10 to 40% Abs NHS. Binding of ¹²⁵I-C3 and the nonpresensitized strain was quantitated when these organisms were incubated in 10, 20, and 40% Abs NHS. Stable binding of up to 3.0 x 10⁶ molecules of C3 and 8.0 x 10⁴ molecules of C9 to presensitized and nonpresensitized isolates occurred in the highest concentration of serum, but there was no killing without presensitizaton. Similar results were found when Abs NHS was chelated with ethylene bis glycoltetraacetic acid containing 2 mM MgCl₂ (Mg EGTA) to block classical pathway activation, indicating that antibody mediated the bactericidal reaction through the alternative pathway. Deposition of C3 and C9 and killing of 12015 in 10% Abs NHS or 10% HγS was measured after presensitization with increasing amounts of IgG, F(ab')₂, or Fab'. There was a dose-dependent increase in C3 deposition and killing, but only minimal change in C9 binding when 1.0 x 10³ to 3.2 x 10⁴ IgG or F(ab')₂/CFU were bound to the bacterial surface. In contrast, there was no increase in C3 or C9 binding and no bacterial killing when 1 x 10³ to 3.4 x 10⁴ molecules Fab'/CFU were bound to the bacterial surface. These experiments show that immune IgG and F(ab')₂ can mediate killing of E. Coli 0111B4 by the alternative pathway without changing the extent of terminal C component attachment to the bacterial surface.