Opsonization of normal myelin by anti-myelin antibodies and normal serum

Fc receptor-dependent myelin phagocytosis has been proposed as a possible important effector mechanism in several immune-mediated demyelinating diseases. The present study was designed to determine whether myelin is opsonizable by anti-myelin antibodies. Thioglycolate-elicited mouse peritoneal macrophages were cultured with 125I-labelled bovine central myelin pretreated with normal or immune serum. Serum opsonic activity was determined by a kinetic study comparing macrophage uptake of opsonized and untreated 125I-myelin. Heat-stable and heat-labile myelin opsonins were detected in normal rabbit serum. Myelin was also opsonized by normal rabbit gamma globulin and by heat-inactivated normal mouse, human, and guinea pig serum. Increased opsonic activity was detected in rabbit anti-myelin antiserum and the gamma globulin fraction prepared from this serum, in anti-myelin basic protein and anti-galactocerebroside antiserum but not in anti-myelin-associated glycoprotein antiserum or in serum from rabbits injected with Freund's adjuvant alone. One out of three anti-sheep red blood cell antisera tested also showed increased myelin opsonic activity. It is concluded that anti-myelin antibodies can promote opsonic phagocytosis, and that normal serum and normal serum gamma globulin also opsonize myelin.