Two conformational forms of target-bound iC3b that distinctively bind complement receptors 1 and 2 and two specific monoclonal antibodies

Abstract

Introduction. The complement system is an essential part of the immune system of vertebrates. The central event of the complement activation cascade is the sequential proteolytic activation of C3, which is associated with profound alterations in the molecule's structure and conformation and is responsible for triggering most of the biological effects of complement.

Material and methods. Here, we have studied the conformation of C3 fragments deposited onto an IgG-coated surface from human serum during complement activation, using a set of unique monoclonal antibodies (mAbs) that are all specific for the C3dg portion of bound iC3b.

Results. We were able to identify two conformational forms of target-bound iC3b: the first recognized by mAb 7D18.1, and the second by mAb 7D323.1. The first species of iC3b bound recombinant complement receptor 1 (CR1), while the second bound CR2. Since CR1 and CR2 are expressed by different subsets of leukocytes, this difference in receptor-binding capacity implies that there is a biological difference between the two forms of surface-bound iC3b.

Conclusion. We propose that mAbs 7D18.1 and 7D323.1 can act as surrogate markers for CR1 and CR2, respectively, and that they may be useful tools for studying the immune complexes that are generated in various autoimmune diseases.

Key words::

• Complement C3
• complement receptors
• immunoglobulin G (IgG)
• monoclonal antibodies
• neo-epitopes

Acknowledgements

We are most grateful to Mrs Margita Nilsson and Mrs Kerstin Sandholm for their skillful technical assistance and to Dr Deborah McClellan for editorial assistance.

This work was supported by grants from the Swedish Research Council (VR) 2009-4675, 2009-4462, and grant # EB003968 from the National Institutes of Health (USA) and from faculty grants from the Linneaus University in Sweden.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.