CD59 homologue regulates complement-dependent cytolysis of rat Schwann cells

Antibody (Ab) sensitized sciatic nerve Schwann cells (SchC) of 2-day-old rats (SchC/2d) were significantly more susceptible to cytolysis by both heterologous, guinea pig (GP), and homologous rat serum complement (40 ± 3.8% and 21.2 ± 3.1%, respectively) than SchC of 6-day-old rats (SchC/6d) (7.9 ± 5.9% and 2.6 ± 3.1%, respectively). To determine if resistance to complement (C)-mediated cytolysis correlated with expression of membrane proteins which regulate C activation, we used Western blot and FACS analysis. Binding of specific polyclonal Ab demonstrated similar concentrations of Crry, a regulator of C3 convertase formation, on plasma membranes of SchC 2d and 6d. During C activation, both C3b deposition and iC3b formation were greater on SchC/6d than on SchC/2d and the C3b deposition did not correlate with enhanced cytolysis. In contrast, 2.1-fold more rat CD59, a regulator of C8 and C9 incorporation into C5b-9, detected with Western blot on SchC/6d compared with SchC/2d was confirmed by FACS. Further, both rat and GP C8/C9 lysed SchC/2d expressing human C5b-7 (20.1 ± 3.7 and 21.6 ± 4.7%, respectively), while only GP C8/C9 caused cytolysis of 10.7 ± 4.3% SchC/6d expressing hu C5b-7 and rat C8/C9 did not (0.5 ± 0.5%). Preincubation of SchC/6d with an F(ab) 2 fragment of an mAb to rCD59 with blocking capacity, increased cytolysis mediated by rat serum C more than 6-fold to 16.7 ± 3.0% but only 1.7-fold (maximum cytolysis 37.4 ± 11.2%) in SchC/2d. Our data suggest that expression of rat CD59 on SchC increased almost two-fold between postnatal days 2 and 6, and this increased expression on more terminally differentiated SchC is a significant factor in regulating terminal complement complex formation and limiting cytolysis of rat SchC by homologous serum complement.