What buffer system are you concentrating the protein in? What is the isoelectric point(pI) of the protein, compared to the pH of your buffer (generally proteins will be less stable at pH's near their pI). I would recommend a change in buffer- we have had good success staying away from PBS, and using 10 mM Histidine, pH 6.5, with additives to help stabilize the protein. Excipients like Polysorbate-80 (Tween-80) in low concentrations (0.03% w/v), sucrose and mannitol are all helpful in minimizing aggregation at higher concentrations.
David Bienvenue, Ph.D.
Senior Principal Scientist