Complement fixation by model immune complexes free in solution and bound onto cell surfaces

Abstract

Immunoglobulin (Ig)G molecules with anti-2,4-dinitrophenyl activity were covalently cross-linked by using 3 cross-linking reagents. The resulting oligomers were separated into monomer, dimer, trimer and heavy fractions. These stable assemblages of IgG molecules were incapable of fixing dilute whole guinea pig complement [C] in solution. When oligomers were further aggregated noncovalently into larger complexes, all were able to fix C. Radioiodinated oligomers were attached to 2,4,6-trinitrophenylsulfonic acid treated sheep red blood cells (N3ph-SRBC) and the number of bound molecules was determined from the cell-associated radioactivity. C-mediated lysis of N3ph-SRBC was then assayed over a range of levels of bound protein and at increasing concentrations of C. The lytic efficiencies of all oligomers increased with the number of bound molecules, with C concentration, with hapten density on N3ph-SRBC and with oligomer size. Two adjacent IgG molecules may not serve as a unit signal for triggering the C cascade; instead, initiation occurs with increasing efficiency as the size of cell-bound IgG clusters increases.