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

Nilsson UR, Funke L, Nilsson B, Ekdahl KN - Ups. J. Med. Sci. (2010)

Bottom Line: 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.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.

Affiliation: Division of Clinical Immunology, Rudbeck Laboratory C5, Uppsala University, Sweden.

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.

Sequential digestion of C3, generating C3b, iC3b, C3c, C3dg, and C3d. Activation of C3 leads to disruption of the thiol ester (TE circular), which then establishes covalent bonds (TE linear). The interchain disulfide bonds are indicated (S S).
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Figure 1: Sequential digestion of C3, generating C3b, iC3b, C3c, C3dg, and C3d. Activation of C3 leads to disruption of the thiol ester (TE circular), which then establishes covalent bonds (TE linear). The interchain disulfide bonds are indicated (S S).

Mentions: During its cleavage to C3b and iC3b, the C3 molecule undergoes several profound structural and conformational changes. These alterations in structure and conformation have recently been elucidated in a series of reports on the 3-D crystal structure of the molecule (1–5). After cleavage of C3 by the convertases, the so-called thiol ester domain (TED) is totally dislocated and moved into a position that allows covalent binding of the molecule to a target surface. Digestion of C3b to iC3b by factor I affects the molecule further by releasing the C3f fragment and displacing the C3c domain from the TED. Finally, factor I further cleaves iC3b, generating the fragments C3dg (TED) and fluid-phase C3c (Figure 1).

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

Nilsson UR, Funke L, Nilsson B, Ekdahl KN - Ups. J. Med. Sci. (2010)

Sequential digestion of C3, generating C3b, iC3b, C3c, C3dg, and C3d. Activation of C3 leads to disruption of the thiol ester (TE circular), which then establishes covalent bonds (TE linear). The interchain disulfide bonds are indicated (S S).
© Copyright Policy - open-access
Figure 1: Sequential digestion of C3, generating C3b, iC3b, C3c, C3dg, and C3d. Activation of C3 leads to disruption of the thiol ester (TE circular), which then establishes covalent bonds (TE linear). The interchain disulfide bonds are indicated (S S).
Mentions: During its cleavage to C3b and iC3b, the C3 molecule undergoes several profound structural and conformational changes. These alterations in structure and conformation have recently been elucidated in a series of reports on the 3-D crystal structure of the molecule (1–5). After cleavage of C3 by the convertases, the so-called thiol ester domain (TED) is totally dislocated and moved into a position that allows covalent binding of the molecule to a target surface. Digestion of C3b to iC3b by factor I affects the molecule further by releasing the C3f fragment and displacing the C3c domain from the TED. Finally, factor I further cleaves iC3b, generating the fragments C3dg (TED) and fluid-phase C3c (Figure 1).

Bottom Line: 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.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.

Affiliation: Division of Clinical Immunology, Rudbeck Laboratory C5, Uppsala University, Sweden.

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.

View Similar Images In: Results  - Collection
View Article: PubMed Central -  PubMed
Show All Figures - Show MeSH
getmorefigures.php?pmc=3039757&rFormat=json&query=null&req=5