Reactivity of sized thermal aggregates of immunoglobulin G with IgM rheumatoid factor

Solutions of purified pooled human IgG heated for varying times and temperatures between 55 degrees and 65 degrees yielded soluble turbid non-immune polydispersed aggregate mixtures. Gel filtration of IgG aggregate mixtures on calibrated Biorad A-15, A-50 or A-150 columns resulted in continuous size distributions from dimers to particles with mol. wts. as high as 5 X 10(7) (300-mers) with no particular size predominant. Chromatographically reproducible cuts of relatively narrow size heterogeneity were obtained by 15 min time fractionation. The reactivity of sized aggregates was studied with pooled purified IgM rheumatoid factor as well as with various individual IgM RF sera. The inhibition of agglutination method employing IgG-coated tanned sheep cells was used to assay relative reactivities. With all RF preparations studied, reactivity of sized IgG was primarily a function of aggregate size. Reactivity increased from virtually zero with monomeric IgG to maximal values with aggregates in the 10(6) size range. Larger aggregates (mol. wt. 10(7) or greater) exhibited diminishing and ultimately negligible reactivity. Reactivity of aggregates of given size was independent of heating time but increased with decrease in heating temperature. The maximal reactivity of certain sizes of IgG aggregates with RF suggests that at least two factors are operative. Enhanced reactivity may be due to unblocking of potential reactive sites on IgG during denaturation that precedes aggregation or to the more favourable energetics of binding with aggregates as compared to monomer. Subsequent loss of reactivity may be due to steric hindrance in large aggregates and/or to destruction of binding sites as a result of continued denaturation.