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Blocking of the Hemolytic Expression of the Classical Complement Sequence by Products of Complement Activation via the Alternate Pathway¹

Materials Responsible for the Blocking Phenomenon and Their Proposed Mechanism of Action

From the Departments of Medicine and Dermatology, Harvard Medical School, and the Department of Medicine, Robert B. Brigham Hospital, Boston, Massachusetts 02120

Abstract

Activation of the complement sequence through the alternate pathway with zymosan or the low molecular weight cobra venom factor (CoVF) generates products which block the lysis of hemolytic cellular intermediates of the classical complement sequence. The euglobulin fraction of the eluate of zymosan and products in CoVF-treated serum bind to EAC14_lim^oxy2 and EAC14_lim^oxy23 to prevent effective hemolytic interaction with C3 and C5, respectively. Analyses of fractions from gel filtration of both the zymosan eluate and CoVF-treated serum revealed blocking in the IgM region of EAC14_lim^oxy2, EAC14_lim^oxy23, and EACl-7 while materials eluting in the trough between the IgM and IgG peaks blocked only EAC14_lim^oxy2 and EAC14_lim^oxy23. Radiolabeled C5 in CoVF-treated serum was shown to be present in the material eluting in both regions representing a shift from the usual position of C5. Only the highest molecular weight material destroyed C9. It is proposed that the blocking of the cellular intermediates is due to complexes ranging in composition from C5 and C6 to C5, C6, C7 and C8, which are hemolytically inactive while maintaining certain functions, such as binding to the appropriate cellular intermediate or to one or more later components in the complement reaction sequence.

Footnotes

¹ Supported by Grants AI-07722 and RR-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 United States Public Health Service Research Career Development Award AM-46409.