Differences in interaction between immune complexes and complement receptors in serum and cerebrospinal fluid

Antigen—antibody complexes (Ag—Ab), prepared from ¹²⁵I-radiolabelled bovine serum albumin (BSA) and guinea-pig antibody, were (1) pre-incubated at 37°C for 30 min with serum or cerebrospinal fluid (CSF) in different proportions and then reacted with cells, (2) incubated at 37°C directly with cells suspended in serum or CSF for different time periods, or (3) bound to cells (following incubation with serum in optimal proportions) and the cell-bound immune complexes (IC) incubated with serum or CSF at 37°C for different time periods. When Ag—Ab were pre-incubated with serum or CSF and reacted with unfractionated blood cells or mononuclear cells, binding decreased as serum to Ag—Ab proportion was increased above 1:16, but increased as CSF to Ag—Ab proportion was increased. When serum diluted 200-fold (to approximate the protein concentration of CSF) was used in place of undiluted serum, serum-mediated binding paralleled CSF-mediated binding. Inactivation of serum, CSF, and 1:200 serum in different ways and substitution of human red blood cells (RBC) (known to possess C3b receptors) or sheep RBC (known not to possess C3b receptors) demonstrated that binding was to C3b receptors. Addition of CSF to serum did not alter serum-mediated binding. When Ag—Ab were incubated directly with unfractionated blood cells suspended in serum or CSF, binding increased rapidly in serum, reaching a maximum within 2—4 min, and IC then rapidly dissociated, whereas binding increased gradually in CSF and IC remained associated with cells. When serum diluted approximately 100-fold was used in place of undiluted serum, kinetics of serum-mediated interaction approached that of CSF-mediated interaction. When IC were bound to Raji cells or human RBC and the cell-bound IC incubated in serum or CSF, > 85% of IC dissociated in serum after 30 min, but no dissociation occurred in CSF. Dilution of serum > 1:16 and > 1:64 abolished dissociation from the two cell types, respectively. These results indicate that CSF mediates binding of IC to complement receptors on cells but lacks the activities of serum which convert IC into a non-binding state.