Activation of Complement by Endotoxin: A Role for γ2 Globulin, C1, C4 and C2 in the Consumption of Terminal Complement Components by Endotoxin-Coated Erythrocytes

Abstract

Abstract Sheep erythrocytes (E) coated with endotoxin (LPS) from Salmonella typhosa or Veillonella alcalescens (E-LPS) have been employed to define more precisely the nature of a pathway by which LPS activates the terminal components of complement (C). E-LPS consumed C in normal guinea pig serum in a manner identical to free LPS, that is, with marked consumption of C3 through C9 and little utilization of C1, C4 and C2. A γ2-migrating globulin, isolated from normal guinea pig serum by DEAE-cellulose chromatography, was required for C-mediated lysis of E-LPS in diluted guinea pig serum. Absorption experiments with E-LPS indicated that the heat stable γ2 globulin showed specificity for the LPS employed. Cobra venom factor, known to activate C by the alternate pathway (C3 proactivator system), was equally able to deplete C activity in normal serum or serum absorbed with E-LPS. E-LPS reacted with γ2 globulin, C1, C4, C2, followed by C-EDTA resulted in lysis of E-LPS. Deletion of γ2 globulin or any of the earlier acting C components from the reaction prevented lysis. The small amounts of the early acting C components employed, particularly C4 and C2, indicated that it would be difficult to measure their loss in whole serum after activation with LPS by conventional hemolytic assay. This could explain the finding of a relative sparing of C1, C4 and C2 during consumption of C3 through C9 by LPS in normal guinea pig serum. Thus, a classical C pathway involving a “natural” γ2 globulin and early components of C has been defined as a mechanism by which endotoxin can efficiently activate the terminal components of C.