IgG-protein A complexes modulate thymidine incorporation into DNA of antibody and complement-stimulated cells.

Incubation of protein A (PA) and molar excess of whole rabbit immune IgG in solution favored the formation of high m.w. complexes with the molecular formula of [(IgG)2(PA)]2, which enhanced the incorporation of [3H]thymidine ([3H]dThd) into DNA of L cells, especially in the presence of rabbit or human complement (C). In contrast, complexes with the molecular formula (IgG)(PA), formed by incubation of IgG with molar excess of PA, failed to enhance, or inhibited, the incorporation of [3H]dThd into cells treated with immune IgG alone or with C present, respectively. Concanavalin A (Con A), which is known to bind to critical exposed oligosaccharide structures present in the Fc region of IgG in the [(IgG)2(PA)]2 complexes, reversed the enhancement of [3H]dThd incorporation, and alpha-methyl mannoside, but not D-galactose, inhibited the Con A reversal. The [(IgG)2(PA)]2 complexes efficiently activated the C cascade, caused significantly higher levels of aggregation of cells, and increased the binding of immune IgG to the cells when compared to the (IgG)(PA) complexes. Both types of complexes, isolated by sucrose density ultracentrifugation, were precipitable in a radioimmunoassay with chicken anti-PA antibodies and were capable of binding to intact L cells.