Use of immunoglobulin-loaded protein A-bearing staphylococci as a primary solid phase immunoadsorbent in radioimmunoassay.

Protein A-bearing formalinized heat-inactivated Staphylococcus aureus bind rabbit 125I-IgG with high capacity of IgG for antigen. The affinity of immobilized IgG for antigen is equal to the affinity of soluble antibody, and the capacity for antigen approximates the capacity of soluble antibody for antigen. 125I-IgG bound to high affinity sites on bacteria is not substantially displaced in the presence of human serum after 4 h at 4 degrees C, but rabbit immunoglobulin can displace bound rabbit IgG. Protein A-bearing bacteria which have adsorbed IgG free from protease activity in antiserum provide a stable, sensitive, primary solid phase adsorbent with unique features for radioimmunoassay. Sedimentation characteristics of bacteria permit facile separation of bound from free ligand by centrifugation of the primary antibody. Immunoglobulin-loaded staphylococci can adsorb ligand from serum, simultaneously purifying and concentrating ligands for measurement in a single subsequent step. Immunoglobulin-bearing bacteria can be used directly in biological fluids containing immunoglobulin, such as serum and culture media. Improved economy of staphylococci, low nonspecific binding, high functional capacity, stability, and unique characteristics of radioimmunoassay using immunoglobulin-loaded staphylococci compare favorably with conventional solid phase adsorbents.