Semiquantitative determination of Cc and C3 receptors on human lymphocytes by isotope-labelled marker cells.

The number of Fc and C3 receptors on normal B lymphocytes, isolated from blood, was assessed semiquantitatively by a new method in which the marker cells (pretreated sheep erythrocytes) for Fc and C3 receptors are labelled by 99Tc and 51Cr respectively. The mean number of bound marker cells per rosette was assessed simultaneously by measurements of radioactivity. Control studies showed that a measuring system using 1/4 agglutinating titre of antibody on the marker cells is particularly advantageous: (1) Fc and C3 receptors are not demonstrable on T lymphocytes; thus, B lymphocyte fractions with Fc and/or C3 receptors are well-defined. (2) The binding affinity of the marker cells for normal lymphocytes is so weak that the number of bound marker cells per rosette rarely reaches the maximum which was calculated and measured as about 30 marker cells per rosette; thus, the measuring system is sensitive to changes in the number of receptors. (3) By binding of marker cells to lymphocytes bearing Fc as well as C3 receptors it has been demonstrated that interference phenomena between the two marker cells are not operative. Studies of blood lymphocytes from normal human subjects showed B lymphocytes carrying either Fc and C3 receptors as well as a fraction of B lymphocytes having both Fc and C3 receptors. The latter maker up about 6% of the lymphocyte mass. Comparative assessment of the quantitative distribution of these receptors indicates a developmental sequence, B lymphocyte with Fc receptors evidently developing, via B lymphocytes with Fc as well as C3 receptors, into B lymphocytes having C3 receptors only.