Complement ring lesions on erythrocytes require later complement components beyond C5. Studies using complement-deficient sera show C5 binding alone doesn
Area of Science:
Immunology
Cell Biology
Background:
The formation of complement (C)-induced ring lesions on antibody-coated erythrocytes (EA) is a key indicator of complement system activation.
The precise step in the complement cascade where these 8- to 11-nm ring lesions appear has been a subject of ongoing debate.
Previous research has yielded conflicting conclusions regarding whether lesions form at the C5 step or require later components like C8 and C9.
Purpose of the Study:
To definitively determine the complement sequence step at which ring-shaped lesions form on antibody-coated erythrocytes.
To investigate the relationship between C5 binding and the ultrastructural formation of complement-induced ring lesions.
To clarify the complement requirements for ring lesion formation in relation to cell lysis.
Main Methods:
Utilized sheep antibody-coated erythrocytes (EA) and human sera genetically deficient in specific complement components (C5, C6, C7, or C8).
Examined negatively stained erythrocyte membranes using electron microscopy (x 220,000) in a blinded manner.
Correlated the observed ring lesion density with quantitative measurements of 125I-C5 binding to EA.
Main Results:
Rare ring lesions were observed on EA exposed to sera lacking C5, C6, C7, or C8, and heated normal serum, with minimal lesion density.
Fresh normal serum (NHS) induced significantly higher lesion densities (140–220/µm²).
While C5 binding varied in deficient sera, even enhanced C5 binding (with iodine treatment) did not increase lesion formation unless later complement components were present.
Conclusions:
Complement C5 binding to erythrocytes is insufficient for the formation of ultrastructural ring lesions in the absence of later-acting complement components (at least through C8).
The formation of complement-induced ring lesions requires the action of terminal complement components, similar to the requirements for erythrocyte lysis.
These findings resolve the controversy by demonstrating that ring lesions are not solely a C5-mediated event but depend on the complete complement cascade.