A soluble deletion mutant of the human complement receptor type 1, which lacks the C4b binding site, is a selective
S M Scesney1, S C Makrides, M L Gosselin
1T Cell Sciences, Inc., Needham, MA 02194, USA.
Insights
A new soluble complement receptor type 1 (sCR1) mutant, sCR1[desLHR-A], selectively inhibits the alternative complement pathway. This protein retains C3b binding but lacks C4b binding, showing targeted complement inhibition in vitro.
Area of Science:
- Immunology
- Complement System
- Protein Engineering
Background:
- Human complement receptor type 1 (CR1, CD35) is a glycoprotein with repeating homologous domains (SCRs) forming long homologous repeats (LHRs).
- CR1 plays a crucial role in regulating the complement system, a key part of innate immunity.
- Soluble forms of CR1 (sCR1) have been developed to study and potentially modulate complement activity.
Purpose of the Study:
- To produce and characterize a soluble deletion mutant of CR1, sCR1[desLHR-A], lacking the N-terminal LHR-A domain.
- To compare the functional activities of sCR1[desLHR-A] with the full-length soluble CR1 (sCR1) in vitro.
- To determine if sCR1[desLHR-A] exhibits selective inhibition of complement pathways.
Main Methods:
- Production and characterization of a soluble CR1 deletion mutant, sCR1[desLHR-A].
- Quantitative in vitro assays comparing sCR1[desLHR-A] and sCR1.
- Assays included C3b binding, cofactor activity for factor I-mediated degradation, and inhibition of erythrocyte lysis and anaphylatoxin production via alternative and classical complement pathways.
Main Results:
- sCR1[desLHR-A] retained C3b binding sites found in LHR-B and -C but lacked the C4b binding site in LHR-A.
- sCR1[desLHR-A] and sCR1 showed equal C3b binding and similar cofactor activity for C3b and C4b degradation.
- sCR1[desLHR-A] effectively inhibited alternative pathway-mediated lysis and anaphylatoxin production, but was less effective than sCR1 in inhibiting classical pathway activation.
Conclusions:
- The deletion of the LHR-A domain in sCR1[desLHR-A] results in a protein that selectively inhibits the alternative complement pathway.
- sCR1[desLHR-A] represents a potential therapeutic agent for conditions driven by alternative complement pathway overactivation.
- These findings highlight the domain-specific functions of CR1 in complement regulation.
Abstract:
The human complement receptor type 1 (CR1, CD35), is a single-chain glycoprotein consisting of 30 repeating homologous protein domains known as short consensus repeats (SCR) followed by transmembrane and cytoplasmic domains. The SCR themselves, considered in groups of seven, form long homologous repeats (LHR) which have been designated LHR-A, -B, -C, and -D for the most common human allotype of CR1. A soluble deletion mutant of CR1 which lacks the first seven N-terminal SCR (LHR-A) as well as the transmembrane and cytoplasmic domains was produced and characterized. The resulting protein, designated sCR1[desLHR-A], lacks the C4b binding site found in LHR-A, but retains the two C3b binding sites found in LHR-B and -C, respectively. The functional activities of sCR1[desLHR-A] were quantitatively compared in vitro to those of soluble complement receptor type 1 (sCR1) which has been shown to retain all known functions of the native cell surface receptor. sCR1[desLHR-A] and sCR1 competed equally for the binding of dimeric C3b to erythrocyte CR1. sCR1[desLHR-A] and sCR1 were similar in their capacity to serve as a cofactor in the factor I-mediated degradation of the C3b and C4b alpha chains. sCR1[desLHR-A] and sCR1 were comparable in their capacity to inhibit erythrocyte lysis and anaphylatoxin production mediated by the alternative complement pathway. sCR1[desLHR-A], however, was significantly less effective an inhibitor of erythrocyte lysis and anaphylatoxin production than sCR1 under conditions which allow classical pathway activation. These results demonstrate sCR1[desLHR-A] to be a selective inhibitor of the alternative complement pathway in vitro.
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