Quasi-elastic light scattering of antigen-antibody complexes

Many biological properties of immune complexes (IC) depend upon their size. Quasi-elastic light scattering (QLS) was used to measure a mean equivalent hydrodynamic radius ( R h ) and variance of the distribution of model IC composed of bovine serum albumin (BSA) as antigen (Ag) and combinations of two or three well-characterized monoclonal antibodies (MAb) which bound non-competitively to unique epitopes on BSA. With the molar ratio ( X) of each MAb to Ag fixed, R h increased with concn. R h was maximal at equivalence ( X = 0.54) with two MAb and at slight MAb excess ( X = 0.67) with three MAb. The largest R h with two MAb was about 200 Å, and R h was uniformly different amongst the three combinations of two MAb IC. The largest hydrodynamic radius of individual complexes which formed with two MAb was estimated to be about 400 Å; even larger individual complexes were formed with three MAb. Size changes following alteration of solution concns were also followed with QLS. Kinetics of two MAb IC association were too fast to observe; dissociation following large dilution (40-fold) requried 5–10 min to attain a new steady state, much less at small dilution. With three MAb, R h dropped sharply in 5 min and became steady after 1–2 hr. These results suggest that conventional Chromatographic and ultracentrifugation techniques for studying IC size, involving large dilution and long measurement time, provide misleading results. Association of three MAb produced a rapid initial increase of R h in several min, followed by diverse behavior which depended upon concn. From high to low concn, these included (1) exponential growth of R h with time and appearance of visible macroscopic particles; (2) metastable states for several hr followed by slow growth to large size over several days, leading to formation of particles; and (3) rapid growth to steady state conditions with no visible particles. This heretofore unobserved equilibrium and kinetic behavior of model IC in solution may be reflected in the