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Reduced activity of DAF on complement enzymes bound to alternative pathway activators. Similarity with Factor H
1Department of Biochemistry, University of Texas Health Science Center, Tyler 75710.
Insights
Complement system regulators Decay-accelerating factor (DAF) and Factor H show reduced effectiveness on cells activating the alternative pathway. This dysfunction may explain previous observations of impaired DAF activity on certain cell types.
Area of Science:
- Immunology
- Complement System Biology
- Molecular Medicine
Background:
- The alternative pathway of complement is crucial for innate immunity but requires tight regulation.
- Factor H and Decay-accelerating factor (DAF) are key regulators that prevent complement-mediated damage to host cells.
- The interaction of complement activators with regulatory proteins can influence their efficacy.
Purpose of the Study:
- To investigate the regulatory activity of Factor H and DAF on complement C3 convertase (C3b,Bb) bound to complement activators.
- To compare the effectiveness of DAF and Factor H on various cell types, including activators and non-activators of the alternative pathway.
- To elucidate the impact of cell surface properties on the regulatory function of DAF and Factor H.
Main Methods:
- Assay of purified DAF and Factor H using radiolabelled Bb release from C3b,Bb.
- Quantification of regulatory activity on zymosan (activator) and various erythrocytes (activators and non-activators).
- Determination of the restriction index (RI) to measure the relative effectiveness of DAF and Factor H.
Main Results:
- Both DAF and Factor H exhibited significantly reduced regulatory activity on complement activators (zymosan, rabbit erythrocytes) compared to non-activators (sheep, bovine erythrocytes).
- The relative effectiveness (RI) for DAF and Factor H on activators like zymosan and rabbit erythrocytes was markedly lower than on non-activators.
- DAF and Factor H demonstrated quantitatively similar decreases in regulatory activity when bound to complement activators.
Conclusions:
- The regulatory efficacy of DAF is diminished when associated with cells that activate the alternative pathway of complement.
- This reduction in regulatory function may be linked to cellular transformation, viral infection, or surface alterations.
- Findings provide a potential explanation for previously reported instances of dysfunctional DAF on alternative pathway-activating cells.
Abstract:
Attachment of C3b to activators of the alternative pathway of complement results in a decrease in regulatory activity expressed by Factor H. Decay-accelerating factor (DAF) and Factor H were found to exhibit quantitatively similar decreases in regulatory activity toward the C3 convertase (C3b,Bb) bound to activators, such as zymosan (Zym) and rabbit erythrocytes (ER), compared to non-activators, such as sheep (ES) and bovine (EB) erythrocytes. Purified DAF and Factor H, in 0.1% NP-40, were assayed by measuring the amount required to release 50% of the radiolabelled Bb in 10 min from C3b,Bb on Zym or cross-linked erythrocytes. The relative effectiveness (i.e. the restriction index, RI) of DAF for accelerating the decay of C3b,Bb on the various particles was: ES (1.0), ER (0.04) and Zym (0.03). The RI for Factor H was: ES (1.0), ER (0.04) and Zym (0.07). The rate of decay of C3b,Bb induced by DAF and Factor H showed similar restriction. The results suggest that the regulatory properties of DAF are reduced if the cells on which it resides become activators of the alternative pathway as a result of transformation, virus infection or surface alteration. These findings may explain reports of dysfunctional DAF on alternative pathway-activating cells.