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The role of C1, C1-inactivator and C4 in modulating immune precipitation
Insights
The complement system
Area of Science:
- Immunology
- Biochemistry
- Complement System
Background:
- The classical complement pathway plays a crucial role in immune responses.
- Understanding the early steps of complement activation is vital for immune complex processing.
- Inhibition of immune precipitation by early complement components requires detailed mechanistic insight.
Purpose of the Study:
- To elucidate the mechanism by which early classical complement pathway components inhibit immune precipitation.
- To investigate the roles of C1, C1-inactivator (C1-In), and C4 in modulating immune complex aggregation.
Main Methods:
- Aggregation assays using radiolabeled BSA-anti-BSA antibody complexes.
- Inclusion of purified C1, C1-In, and C4.
- Use of p-nitrophenyl-p-guanidinobenzoate (NPGB) to inhibit C1 activation.
- Employing EDTA to chelate divalent cations.
Main Results:
- Purified C1 significantly delayed immune precipitation in a concentration-dependent manner.
- The anti-aggregational effect of C1 was independent of C1 activation but required C1 formation for C1-In to reverse it.
- C1-In and EDTA reversed C1-mediated inhibition, indicating C1's primary role.
- C4 binding to immune complexes was confirmed, but it did not significantly influence the precipitation delay.
Conclusions:
- C1 is the primary inhibitor of immune precipitation among the early classical complement components studied.
- C1 activation is not necessary for its inhibitory effect, but C1 itself is essential for C1-In's regulatory function.
- C4 binds to immune complexes but does not contribute to the delay in precipitation kinetics.
- Immune complex processing may differ in various classical complement deficiency states.
Abstract:
To clarify the mechanism of inhibition of immune precipitation by early components of the classical pathway of complement, aggregation of 125I-BSA-rabbit-anti-BSA antibody complexes was performed in the presence of purified C1, C1-inactivator (C1-In) and C4. C1 delayed the rate of immune precipitation in a concentration dependent manner. This phenomenon was not influenced by the presence of 0.3 mM p-nitrophenyl-p-guanidinobenzoate (NPGB) which inhibits C1 activation. The antiaggregational effect of C1 was reversed by 10 mM EDTA and by C1-In at a C1-In/C1 molar ratio of greater than or equal to 4/1. C1-In was not effective when the reaction was performed in the presence of NPGB. Thus, although the inhibitory effect of C1 on immune precipitation was not dependent upon C1 activation, the formation of C1 was required to observe the effect of C1-In. The addition of C4 to C1 did not modify the slow aggregation of complexes, even when a limiting concentration of C1 was used. C1-In and EDTA were both able to cause similar rapid precipitation of complexes prepared in the presence of C1 and C4, demonstrating that C4 did not play a significant role in delaying the precipitation reaction. However, soluble complexes prepared in the presence of C1 and C4 were specifically precipitated by the addition of excess anti-C4 antibody, attesting to the binding of C4 to immune complexes. These observations suggest that the processing of immune complexes in vivo may not be similar in different classical pathway complement deficiency states.