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Differentiation of Binding from Complete Activation by Use of Heterologous Components of Complement¹

From the Departments of Dermatology and Medicine, Harvard Medical School, and the Department of Medicine, Robert Brigham Hospital, Boston, Massachusetts

Abstract

C2^hu can bind to cellular intermediates of the complement system in three forms. Native C2 fixes to the EAC intermediates presumably without alteration such that it can transfer quantitatively to a compatible EAC^huhu recipient cell, or can be activated on the cell by the introduction of C. C2^hu interaction with a homologous EAC
cell forms a convertase capable of continuing the hemolytic sequence upon the addition of the terminal components. Finally C2^hu can be bound by EAC^gpgp to achieve an intermediate designated EAC^gpgp2h^hu; C2h^hu differs from native C2^hu by its failure to transfer and from fully activated C2^hu by its failure to decay or efficiently mediate the hemolytic sequence. An EAC^gpgp2h^hu intermediate will, however, bind C3^hu in a hemolytically inactive state designated C3h^hu. C3h^hu differs from native C3^hu in that it is inactivated by the C3b inactivator and from C3b in that it fails to participate in immune adherence. Addition of C2^gp to EAC^gpgp2h^hu3h^hu intermediate imparts hemolytic activity and the ability to react in immune adherence. The C2^gp presumably forms a homologous convertase on vacant SAC^gpgp and in this way activates a neighboring C3h^hu. C2h^hu will fulfill the C2 requirements for C5 activation in the presence of C3b even though C2h^hu does not fully activate C3^hu and cells containing both C2h^hu and C3h^hu apparently do not activate C5.

Footnotes

¹ Supported by grants AI-07722 and FR-05669 from the National Institutes of Health, and a grant from the John A. Hartford Foundation, Inc.

² Predoctoral trainee supported by United States Public Health Service Training Grant AI 00378-01.

³ Recipient of a Faculty Research Award from The American Cancer Society, Inc.