Activator-specific requirement of properdin in the initiation and amplification of the alternative pathway complement

Abstract

Properdin is a positive regulator of alternative pathway (AP) complement. The current understanding of properdin function is that it facilitates AP complement activation by stabilizing the C3 convertase C3bBb. Properdin-deficient patients are susceptible to lethal meningococcal infection, but the mechanism of this selective predisposition is not fully understood. By gene targeting in the mouse, we show here that properdin is essential for AP complement activation induced by bacterial lipopolysacharride (LPS) and lipooligosacharride (LOS) and other, but not all, AP complement activators. LPS- and LOS-induced AP complement activation was abolished in properdin-/- mouse serum, and properdin-/- mice were unable to clear Crry-deficient erythrocytes, which are known to be susceptible to AP complement-mediated extravascular hemolysis. In contrast, zymosan- and cobra venom factor-induced AP complement activation, and classical pathway-triggered AP complement amplification were only partially or minimally affected in properdin-/- mice. We further show that the ability of human properdin to restore LPS-dependent AP complement activity in properdin-/- mouse serum correlated with the human properdin-binding affinity of the LPS. These results reveal a novel role of properdin in AP complement initiation and have implications for understanding the selective predisposition of properdin-deficient patients to meningococcal infection.

Figure 1

Generation of properdin^−/− mice. (A) Schematic representation of the mouse properdin gene locus. Vertical columns symbolize exon (E) locations. Horizontal rectangle box indicates the location of cDNA probe used for ES cell screening. (B) Targeting vector. Big arrowheads represent LoxP sites and small arrowheads represent FRT sites. Neo indicates neomycin; and DT, diphtheria toxin. (C) Actual recombinant properdin gene locus. (D) Expected restriction fragment lengths of wild-type and recombinant alleles. (E) Representative Southern blot screening result of ES cells after HincII and ScaI digestion. (F) Northern blot analysis of properdin mRNA in wild-type (WT) and properdin knockout (P^−/−) mouse tissues. (G) Immunodiffusion analysis of properdin in plasma. Antihuman properdin antibody was placed in the center well and mouse (10 μL) and human (5 μL) plasma or purified human properdin (0.5 μg) was placed in the peripheral wells. A precipitation line between the center and a peripheral well indicates the presence of properdin in the testing sample. (H) Western blot analysis showing the lack of properdin protein (P, →) in properdin^−/− mouse serum. The band below properdin represents goat IgG heavy chain used in immunoprecipitation. Purified human properdin (hP) was used as a positive control on the right lane. The size (in kDa) and position of molecular weight markers was shown on the left.

Figure 2

Rescue of properdin gene knockout by NEO deletion. (A) Schematic diagram showing expected recombinant properdin gene locus after FLPe-mediated NEO deletion. (B) PCR genotyping of 7 mice derived from properdin^−/− × FLPe-transgenic mouse crossing. Using LoxP or FLPe-specific primers, 2 mice (numbers 1 and 5) were identified as having recombinant properdin gene and 4 mice (numbers 1-4) were FLPe transgenic. As expected, the FLPe-negative, LoxP-positive mouse (number 5) contained NEO, whereas the FLPe-positive, LoxP-positive mouse (number 1) did not contain NEO. (C) Immunodiffusion analysis of plasma properdin showing that no properdin was present in mouse 5 (properdin^−/−), whereas properdin was detected in mouse 1 (knockout rescued). Antihuman properdin antibodies were placed in the center well, and plasma samples for mice 1, 2, 5, and 6 (B) were placed in the peripheral wells.

Figure 3

ELISAs of LPS-induced AP complement activation. (A) ELISA detection of LPS on LPS-coated plates. (B) AP complement activation by plate-bound S typhosa LPS in wild-type (WT) or properdin knockout (P^−/−) mouse serum. To reconstitute AP activity in properdin^−/− mouse serum, C3^−/− serum or purified human properdin (hP) was premixed with properdin^−/− mouse serum. Alternatively, LPS-coated plates were incubated with hP and washed (hP coat) before exposure to properdin^−/− serum. Similar assays were performed with plate-bound LPS from S minnesota (S) (C) or E coli (D). (E,F) ELISAs of hP interaction with plate-bound LPS. Plates were first coated with different concentrations of LPS and then incubated with a fixed concentration of purified hP (62.5 ng/well; E). (F) Plates were first coated with a fixed concentration of LPS (5 μg/mL) and then incubated with increasing concentrations of purified hP. After washing, the amount of plate-bound properdin was detected by antiproperdin antibodies. OD, optical density.

Figure 4

Crry^−/− erythrocytes– and zymosaninduced AP complement activation. (A) Survival of biotin–labeled Crry^−/− mouse erythrocytes (10⁹) in wild-type (WT) or properdin^−/− mice. The percentage of Crry^−/− erythrocytes in the recipient mouse 5 minutes after transfusion was determined by FACS and taken as 100%. (B) Representative FACS analysis of C3 deposition on zymosan after incubation with WT, properdin^−/−, or factor B knockout (fB^−/−) mouse serum in Mg²⁺-EGTA. (C) Quantitation of C3 deposition on zymosan. Experiments were performed with 2 serum dilutions (1:10, 1:20) and 2 zymosan concentrations (0.025 mg/mL, 0.125 mg/mL). N = 3 mice per group. Error bars represent standard deviations. MFI indicates mean fluorescence intensity. P values refer to Student t test.

Figure 5

CVF–induced AP and anti-OVA/OVA–induced classic pathway complement activation. Western blot analysis of C3 activation in wild-type (A) or properdin^−/− (B) mouse serum. Cleavage product of the C3 α-chain was detected in serum treated with CVF in Mg²⁺-EGTA but not in untreated serum or serum treated with CVF in EDTA. (C) Densitometry of cleaved and intact C3 α-chain in panels A and B. (D) ELISA plate assays of anti-OVA/OVA–induced classic pathway complement activation in wild-type (WT), properdin^−/−, and factor B knockout (fB^−/−) mouse serum or in properdin^−/− serum treated with an antihuman fB antibody.

Figure 6

LOS- and LPS-induced complement activation in vivo and in vitro. ELISAs of plasma C3 activation products in wild-type (WT) and properdin^−/− mice 1 hour after LOS (A) or LPS (B) treatment. LOS or LPS was given at 20 mg/kg (intraperitoneally) and phosphate-buffered saline was used as a vehicle control. N = 3 mice per group. Error bars represent standard deviations. A wild-type mouse plasma sample treated with CVF in vitro was used as a reference for C3 activation (100%). ELISA of LOS– (C) or LPS–induced (D) total complement activation in wild-type (WT), properdin^−/−, or factor B knockout (fB^−/−) mouse serum in GVB²⁺ buffer.