Reactivity of low molecular weight material in cellular immune complex assays

A major difference in molecular size of material reactive in the C1q binding assay and two cellular assays (Raji and L1210) for immune complexes, is reported. Elevated C1q binding of pathological sera was associated with material in the range 7-19S, as determined by Sepharose 6B chromatography of sera from patients with chronic inflammatory and neoplastic lung diseases. By contrast, reactivity of identical sera in the Raji and L1210 assays was linked predominantly with material of molecular size 7S. Dissociation of immune complexes on storage and/or in consequence of the chromatographic procedure was effectively discounted. Furthermore, differential binding of 7S IgG fractions tested at a standard concentration indicated that reactivity in either test was not attributable to non-specific binding of IgG. In a previous study, saturation of FcR (on L1210 and Raji) and C3R (on Raji only) by heat-aggregated IgG failed to distinguish whether binding directly involved these receptors or other cell surface components. In the present investigation, no firm correlations emerged between reactivity in the two tests and possible candidate antibodies reactive with cell surface components such as anti-lymphocyte and anti-nuclear antibodies. It is therefore suggested that low molecular weight binding may be attributable to more than one factor including 7S IgG (immune complex dissociated, or otherwise), autoantibodies, IgG-C3 complexes and possibly very small immune complexes (Ag1, Ab1). The assumption that Raji and L1210 and possibly other cellular assays detect only high molecular weight immune complexes is questionable and the need for further characterization of other reactive material is emphasized.