Peptide inhibitor of complement c1, a novel suppressor of classical pathway activation: mechanistic studies and clinical potential

Abstract

The classical pathway of complement plays multiple physiological roles including modulating immunological effectors initiated by adaptive immune responses and an essential homeostatic role in the clearance of damaged self-antigens. However, dysregulated classical pathway activation is associated with antibody-initiated, inflammatory diseases processes like cold agglutinin disease, acute intravascular hemolytic transfusion reaction (AIHTR), and acute/hyperacute transplantation rejection. To date, only one putative classical pathway inhibitor, C1 esterase inhibitor (C1-INH), is currently commercially available and its only approved indication is for replacement treatment in hereditary angioedema, which is predominantly a kinin pathway disease. Given the variety of disease conditions in which the classical pathway is implicated, development of therapeutics that specifically inhibits complement initiation represents a major unmet medical need. Our laboratory has identified a peptide that specifically inhibits the classical and lectin pathways of complement. In vitro studies have demonstrated that these peptide inhibitors of complement C1 (PIC1) bind to the collagen-like region of the initiator molecule of the classical pathway, C1q. PIC1 binding to C1q blocks activation of the associated serine proteases (C1s-C1r-C1r-C1s) and subsequent downstream complement activation. Rational design optimization of PIC1 has resulted in the generation of a highly potent derivative of 15 amino acids. PIC1 inhibits classical pathway mediated complement activation in ABO incompatibility in vitro and inhibiting classical pathway activation in vivo in rats. This review will focus on the pre-clinical development of PIC1 and discuss its potential as a therapeutic in antibody-mediated classical pathway disease, specifically AIHTR.

Keywords: ABO incompatibility; AIHTR; C1q; MBL; classical pathway; complement; inhibitor; peptide.

Figure 1

ABO incompatible destruction of RBCs by the classical pathway of complement. In this example, transfused cells expressing A antigen have been exposed to incompatible recipient serum containing anti-A IgM. The bound IgM molecule is then recognized by serum complement C1 complex. Upon binding to IgM, C1 activates the classical pathway, resulting in complement deposition on the donor cell and eventual clearance or lysis of host RBCs by the membrane attack complex (MAC). For clarity, only C1 binding and MAC formation are illustrated here.

Figure 2

Alignment of CP with the HNP-1 molecule using ClustalW analysis. Homologous residues are identified as follows: identical (*), conserved (:), and semi-conserved (.) residues. CPP1 and CPP2 were synthesized based upon this alignment.

Figure 3

A model of PIC1 interfering with the normal association of C1q/MBL with their cognate serine proteases (C1s–C1r–C1r–C1s/MASPs). Our data suggest PIC1 functionally disrupts the C1s–C1r–C1r– C1s/MASPs interaction with collagen-like region (CLR) of C1q/MBL, respectively. By virtue of PIC1 binding to this region of the CLR, the serine proteases are hypothesized to be in suboptimal conformation for autoactivation and initiation of the classical and/or lectin pathways of complement.

Figure 4

Peptide inhibitors of complement C1 derivative PA. (A) The 15 residue PA molecule is made up of 9 hydrophobic (red), 3 polar/charged (blue), and 3 intermediary (green) amino acid residues. (B) A linear version of the PA molecule illustrating the amino acid side chains. (C) A structural model of PA. (B,C) Created in MarvinView.

Figure 5

Recognition of RBC antigens on recipient cells by host derived natural antibodies initiating classical complement via C1 leading to opsonization of host cells with C3b, anaphylatoxin C5a recruitment and activation of neutrophils, and cell lysis by membrane attack complex (MAC) pores. PIC1 blocks C1 activation at the first step of the amplification cascade.