Influence of the electric charge of the antigen and the immune complex (IC) lattice on the IC activation of human complement

Abstract

In order to understand the mechanism of complement (C) activation by immune complexes (ICs), the anti-complementary effect of ICs containing cationized antigens was compared in vitro to that using ICs formed by native antigens. ICs were prepared with affinity-purified rabbit polyclonal IgG antibovine serum albumin (BSA) antibody and either native BSA (isoelectric point 4.2) or BSA rendered cationic by treatment with ethylenediamine (isoelectric point 9.4). Native and cationized antigens were characterized by isoelectric focusing. ICs containing anti-BSA IgG or F(ab')2, formed either at equivalence or in excess of native or cationized antigen, were submitted to ultracentrifugation in a sucrose gradient for mesh size determination. The anti-complementary effect of ICs was evaluated by kinetic determination of haemolytic activity of human serum on haemolysin-sensitized sheep red blood cells. In conditions of antigen excess, the ICs formed by cationized BSA were significantly more efficient in activating human complement than those formed by native antigen. This higher activity was dependent on cationized antigen complexed with complete antibody molecules, as non-complexed cationized BSA or ICs prepared with F(ab')2 fragments were inactive under the same experimental conditions. Furthermore, this difference did not depend on the mesh size of the immune complexes. Our results suggest that the balance between antigen, antibody and C may be of importance in vivo for the onset and course of infections and other pathological processes involving IC formation. ICs containing cationized antigens should be proven of value in experimental models for studies on the regulation of C activation.

Figure 1

Immunoelectrophoretic analysis of cationized BSA (a) and native BSA (b). Antigen samples were applied in the two antigen wells, and electrophoresis was performed at 4 mA for 90 min. The antibody trough was filled with affinity-purified anti-BSA IgG antibody, and diffusion was left to occur for 18 h at 24 °C. After washing in a 0.9% NaCl solution, the slide was dried in air and stained for protein with Amido Black.

Figure 2

Ultracentrifugation in a sucrose gradient of ICs formed by native BSA (IC nat-BSA) or cationized BSA (IC cat-BSA). Samples of ICs (200 µL) formed at 5 times antigen excess were applied to a sucrose linear gradient (10–50%) and centrifuged at 80 000 × g for 18 h at room temperature. Sample fractions of 2.0 mL were collected and the absorbance recorded at 280 nm. The radioactivity of the ¹²⁵I radiolabelled ICs was measured by a gamma counter. Irrelevant IgM and IgG were used as sedimentation coefficient markers.

Figure 3

(a) Inhibition of complement activity by ICs formed by native BSA (IC nat-BSA) or cationized BSA (IC cat-BSA) at Ab/Ag equivalence. (b) Inhibition of complement activity by ICs prepared with fivefold Ag excess. Each line was obtained by linear regression of the data. (X) axe: amount of ICs (µg); (Y) axe: mean percent of inhibition of haemolytic activity in six individual experiments. The horizontal bar corresponds to the d50 of each IC preparation.