Human complement (C3b) receptors defined by a mouse monoclonal antibody

The aim of the present study was to prepare a monoclonal antibody against human C3 receptors. The monoclonal antibody termed C3RTo5, which is characterized in this study, inhibited the ligand binding of C3b receptors of human erythrocytes, neutrophils and lymphocytes, but did not block the ligand binding of C3bi and C3d receptors. C3RTo5 reacted only with cells that express C3b receptors. It did not react with cells that do not express C3 receptors or with cells that express C3 receptors other than C3b receptors. The reactivity of C3RT05 against C3b receptors of human erythrocytes, neutrophils, glomerular cells, tonsil cells, or spleen cells could be removed by absorption with human erythrocytes or tonsil cells, whereas absorption with human peripheral T cells or sheep erythrocytes had no effect. In immunoprecipitation studies, a glycoprotein with a mol. wt of 205,000 could be isolated with C3RTo5 from non-ionic detergent lysate of tritiated tonsil cells. A rabbit antiserum prepared against this glycoprotein was able to stain C3b receptor-positive cells, inhibit C3b receptor ligand-binding activity and, furthermore, to precipitate a 205,000 mol. wt component. The results of this study indicate that C3RTo5 is a monoclonal antibody with selective reactivity to C3b receptors and presumably to the binding sites within the receptor molecule. Using C3RTo5 further strong evidence was obtained that membrane-bound C3b receptors have a mol. wt of 205,000.