Kinetic studies of the association of complement components and IgG-coated latex particles incubated in normal or complement-activated human serum

Abstract

The complement components C1q, C1s, C4 and C3 were instantly associated with IgG-coated latex particles incubated in fresh but not heated (56°C, 30 min) NHS and were detectable as a particle agglutination induced by antibodies to these complement subcomponents. The agglutination reactions were studied kinetically utilizing aggregation equipment. After about 60 sec of incubation of the particles with NHS, no C1s antigenic determinants were detectable on the particles. C1q also became less particle-associated during a 2-min incubation as judged from the decline in particle agglutination. These findings suggest a complete dissociation of C1s and a partial dissociation of C1q from IgG complexes incubated in fresh NHS. The proposed discharge of C1q but not C1s was dependent on Mg⁺⁺, prevented by dilution of serum and not observed when IgG particles were incubated in serum previously exposed to inulin. Both C1q and C1s remained particle-associated when IgG particles were incubated in sera activated in vitro or in vivo according to the classical pathway. These findings may indicate that the normal behaviour of C1q, but not that of C1s, after their initial binding to IgG complexes, is dependent upon components of the alternative pathway. C4 and C3 remained particle-associated after their initial binding to the IgG particles. Although C4 was antigenically detectable on the particles after longer incubation with serum, the patterns of the C4-dependent agglutination changed with time, which may reflect changes in the molecular arrangement of C4 after its initial binding to IgG complexes. The changes in the C4-dependent agglutination reactions were dependent on the alternative pathway. No or reduced binding of C3 to IgG particles was found when IgG particles were incubated in sera containing C3 in a mainly activated form after complement activation according to either pathway. Neither became C4 particle-associated when completely converted. C5 antigenic determinants were not detectable on IgG particles incubated in fresh NHS but were found in NHS activated according to the classical but not the alternative pathway.