Complement receptors CR1 on human peripheral nerve fibres.

Receptors for C3 and C4 in human peripheral nerve tissue were studied by examining the adherence of complement (C) coated erythrocytes (E) and by using monoclonal antibodies against epitopes on the receptors for C3b (CR1), C3d (CR2) and C3bi (CR3). E (erythrocyte)-bearing C3b or C4b adhered to sections of myelinated peripheral nerves and the binding was inhibited only by anti-CR1 antibodies. By immunofluorescence, anti-CR1 antibodies stained the nerve fibres, whereas anti-CR2 and anti-CR3 antibodies did not. The staining was apparently localized to the Schwann cell membrane. E-bearing C3bi or C3d did not adhere to myelinated or unmyelinated nerves. CR1 are therefore the only C3 receptors expressed in human peripheral nerves. E-bearing C3b or C4b did not adhere to unmyelinated nerves from adults or to nerves from fetuses at a gestational age of approximately 21 weeks, whereas monoclonal anti-CR1 antibodies stained myelinated, unmyelinated and fetal nerves equally well. The results indicate that CR1 in unmyelinated and fetal nerves are either functionally inactive or express a lower affinity for C3b/C4b than CR1 in myelinated nerves. There were no significant differences in the binding of E-bearing C3b or C4b to myelinated peripheral nerves from 50 individuals, indicating that CR1 activity is not distributed phenotypically.