Effect of Ions on Agitation- and Temperature-Induced Aggregation Reactions of Antibodies

The impact of ions on protein aggregation remains poorly understood. We explored the role of ionic strength and ion identity on the temperature- and agitation-induced aggregation of antibodies. Stability studies were used to determine the influence of monovalent Hofmeister anions and cations on aggregation propensity of three IgG(2) mAbs. The C(H)2 domain melting temperature (T (m1)) and reduced valence (z*) of the mAbs were measured. Agitation led to increased solution turbidity, consistent with the formation of insoluble aggregates, while soluble aggregates were formed during high temperature storage. The degree of aggregation increased with anion size (F- < Cl- < Br- < I- < SCN- similar to ClO4 (-)) and correlated with a decrease in T (m1) and z*. The aggregation propensity induced by the anions increased with the chaotropic nature of anion. The cation identity (Li+, Na+, K+, Rb+, or Cs+) had no effect on T (m1), z* or aggregation upon agitation. The results indicate that anion binding mediates aggregation by lowering mAb conformational stability and reduced valence. Our observations support an agitation-induced particulation model in which anions enhance the partitioning and unfolding of mAbs at the air/water interface. Aggregation predominantly occurs at this interface; refreshing of the surface during agitation releases the insoluble aggregates into bulk solution.