Storage of protein A-conjugated sheep erythrocytes for use in plaque-forming cell (PFC) assays

Sheep red blood cells conjugated with protein A from Staphylococcus aureus were frozen in liquid nitrogen. After thawing these cells were used as indicator cells in a plaque-forming cell assay visualizing human B lymphocytes secreting IgG, IgM or IgA. The thawed erythrocytes performed as well in the assay as erythrocytes freshly conjugated with protein A. Recovery of conjugated erythrocytes after freezing and thawing averaged 66%. By the method described indicator cells for a number of PFC assays may be prepared in bulk, frozen in suitable portions and thawed when needed. This prevents possible batch-to-batch variations and saves time.