Complement receptor 2-mediated targeting of complement inhibitors to sites of complement activation

Abstract

In a strategy to specifically target complement inhibitors to sites of complement activation and disease, recombinant fusion proteins consisting of a complement inhibitor linked to a C3 binding region of complement receptor (CR) 2 were prepared and characterized. Natural ligands for CR2 are C3 breakdown products deposited at sites of complement activation. Fusion proteins were prepared consisting of a human CR2 fragment linked to either the N terminus or C terminus of soluble forms of the membrane complement inhibitors decay accelerating factor (DAF) or CD59. The targeted complement inhibitors bound to C3-opsonized cells, and all were significantly more effective (up to 20-fold) than corresponding untargeted inhibitors at protecting target cells from complement. CR2 fusion proteins also inhibited CR3-dependent adhesion of U937 cells to C3 opsonized erythrocytes, indicating a second potential anti-inflammatory mechanism of CR2 fusion proteins, since CR3 is involved in endothelial adhesion and diapedesis of leukocytes at inflammatory sites. Finally, the in vivo validity of the targeting strategy was confirmed by the demonstration that CR2-DAF, but not soluble DAF, targets to the kidney in mouse models of lupus nephritis that are associated with renal complement deposition.

Figure 1

Diagram of the various CR2 complement-inhibitor fusion proteins.

Figure 2

SDS-PAGE and Western blot analysis of purified recombinant fusion proteins and soluble complement inhibitors. Gels (10% acrylamide) were stained with Coomassie blue. Western blots were developed using antibodies to complement inhibitors as the primary antibody. MW, molecular weight; stds, standards.

Figure 3

Binding of recombinant fusion proteins to C3-opsonized CHO cells. Antibody-sensitized CHO cells were incubated in C6-deficient human serum, washed, and incubated with soluble complement inhibitor (thin black trace) or fusion protein with CR2 at the N terminus (light gray trace) or C terminus (dark gray trace) at 20 μg/ml. Cell binding of recombinant proteins was detected by flow cytometry using anti-DAF or anti-CD59 mAbs followed by a FITC-labeled secondary antibody. Incubation of CHO cells with PBS instead of complement inhibitor is shown in the right-hand panel. Data are representative of three separate experiments.

Figure 4

Analysis of interaction between CR2 fusion proteins and C3dg by SPR. Solid lines indicate various concentrations of CR2 fusion proteins. Dashed lines show curves fitting to a 1:1 Langmuir binding model.

Figure 5

Inhibition of complement-mediated lysis by recombinant sDAF and DAF fusion proteins. Antibody-sensitized CHO cells (a) or SRBCs (b) were incubated with recombinant protein and 10% human serum (CHO cells) or 0.33% human serum (erythrocytes). These concentrations resulted in approximately 90% lysis of unprotected cells. Lysis was determined after 60 minutes at 37°C. Background lysis determined by incubating cells in heat-inactivated serum was less than 5% and was subtracted (mean ± SD, n = 4).

Figure 6

Inhibition of complement-mediated lysis by recombinant sCD59 and CD59 fusion proteins. Antibody-sensitized CHO cells (a) or SRBCs (b) were incubated with recombinant protein and 10% human serum (CHO cells) or 0.33% human serum (erythrocytes). These concentrations resulted in approximately 90% lysis of unprotected cells. Lysis was determined after 60 minutes at 37°C. Background lysis determined by incubating cells in heat-inactivated serum was less than 5% and was subtracted (mean ± SD, n = 4).

Figure 7

Effect of recombinant fusion proteins on U937 cell adhesion. SRBCs were sensitized with IgM antibody and incubated in C6-deficient serum. C3-opsonized erythrocytes were coincubated with U937 cells in the presence of 500 nM recombinant fusion protein or PBS. After incubation, the average number of U937 cells bound per erythrocyte was determined by microscopy (mean ± SD, n = 3).

Figure 8

Biodistribution of [¹²⁵I]CR2-DAF and [¹²⁵I]sDAF in nephritic 34-week-old and prenephretic 8-week-old NZB/W F1 mice. Radiolabeled proteins were injected into the tail vein, and biodistribution of radiolabel was determined after 24 hours or 48 hours. Each protein was injected into two mice.

Figure 9

Imaging of CR2-DAF bound to glomeruli of 24-week-old MRL/lpr mice. Glomerular binding of CR2-DAF (a) and sDAF (b) was analyzed 24 hours after tail-vein injection of each protein. The figure shows immunofluorescence staining of kidney sections.