C3b receptors on human erythrocytes, peripheral blood lymphocytes, and cultured lymphoid cells were shown to be sensitive to treatment with dithiothreitol, 2-mercaptoethanol, 2-amino-ethylisothiouronium bromide hydrobromide (AET), reduced glutathione, and l-cysteine (in order of decreasing effectiveness). Sensitivity was dependent on temperature, time, dose, and pH of the solution applied. Loss of C3b receptor activity became apparent at AET concentrations which caused an increase in erythrocytes' susceptibility to lysis. Raji cells which had lost C3b receptor activity due to AET treatment, started to re-express this receptor function after a lag period of 8 to 10 hr. AET treatment had no detectable effect on the expression of other surface markers, e.g., those detected on erythrocytes by concanavalin A and anti-AB blood grouping serum, or on cultured human lymphoid cells by HL-A alloantisera. However, changes of erythrocyte morphology could be seen in the scanning electron microscope. The sensitivity of C3b receptors to reducing agents suggests that disulfide bridges are important to this receptor's functioning. On the other hand, free sulfhydryl groups appear not to be involved because α-iodoacetamide did not alter the cell surface expression of the C3b complement receptor. Treatment of C3b receptor carrying cells with sulfhydryl compounds can be used as a tool to study the participation of the receptors in the cells' biologic functions.
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