Detection of IgD-secreting Human Lymphocytes at the Single Cell Level by a Protein A Plaque Assay

By the use of Staphylococcus aureus protein A-coupled erythrocytes as target cells, and anti-Ig antibodies as a developing serum, immunoglobulin-secreting cells can be detected as plaque-forming cells in a complement-dependent haemolytic plaque assay. This method is based on the fact that the Fc portion of IgG will bind to protein A molecules. Using a rabbit anti-human IgD immunoglobulin as a developing serum we were able to detect secretion of IgD molecules from human cells. In a patient with an IgD myeloma, 6% of the bone marrow cells were found to form plaques. Bone marrow cells and blood lymphocytes from healthy donors as well as human tonsil cells were also examined for delta chain secretion. In some donors, a low increase in plaque numbers was obtained when anti-IgD Ig was used as a developing agent. In spite of the fact that the antisera used were highly specific we cannot rule out the possibility that the increase was due to cross-reactivity. However, these findings clearly demonstrate that the release of IgD from normal human lymphoid cells occurs at a much lower level as compared to secretion of IgM, IgG or IgA.