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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Arthritis Rheum. Author manuscript; available in PMC Nov 10, 2009.
Published in final edited form as:
PMCID: PMC2775083
NIHMSID: NIHMS145401

Plasmin Immunization Preferentially Induces IgG-Anticardiolipin Antibodies That Are Potentially Prothrombotic in MRL/MpJ Mice

Abstract

Objective

To test the hypothesis, utilizing 2 experimental mouse models, that plasmin is an important autoantigen that drives the production of certain IgG–anticardiolipin (aCL) antibodies in patients with the antiphospholipid syndrome.

Methods

BALB/cJ and MRL/MpJ mice were immunized with Freund’s complete adjuvant in the presence or absence of human plasmin. The mouse sera were analyzed for production of IgG-antiplasmin, IgG-aCL, and IgG–anti–β2-glycoprotein I (anti-β2GPI) antibodies. IgG monoclonal antibodies (mAb) were generated from the plasmin-immunized MRL/MpJ mice with high titers of aCL, and these 10 mAb were studied for their binding properties and functional activity in vitro.

Results

Plasmin-immunized BALB/cJ mice produced high titers of IgG-antiplasmin only, while plasmin-immunized MRL/MpJ mice produced high titers of IgG-antiplasmin, IgG-aCL, and IgG–anti-β2GPI. Both strains of mice immunized with the adjuvant alone did not develop IgG-antiplasmin or IgG-aCL. All 10 of the IgG mAb bound to human plasmin and cardiolipin, while 4 of 10 bound to β2GPI, 3 of 10 bound to thrombin, and 4 of 10 bound to the activated coagulation factor X (FXa). Functionally, 4 of the 10 IgG mAb inhibited plasmin activity, 1 of 10 hindered inactivation of thrombin by antithrombin III (AT), and 2 of 10 inhibited inactivation of FXa by AT.

Conclusion

Plasmin immunization leads to production of the IgG mAb antiplasmin, aCL, and anti-β2GPI in MRL/MpJ mice, but leads to production of only IgG-antiplasmin in BALB/cJ mice. IgG mAb generated from the plasmin-immunized MRL/MpJ mice bind to various antigens and exhibit procoagulant activity in vitro. These results suggest that plasmin may drive the potentially prothrombotic activities of aCL in genetically susceptible individuals.

The antiphospholipid syndrome (APS) is characterized by clinical manifestations of vascular thrombosis and pregnancy loss associated with the presence of persistently and significantly increased titers of antiphospholipid antibodies (aPL) (16). The antigenic specificities of aPL have been the subject of a number of studies, and these studies have shown that aPL represent a heterogeneous group of immunologically and functionally distinct antibodies that recognize various phospholipids, phospholipid-binding plasma proteins, and phospholipid–protein complexes (1,3,7,8). These plasma proteins include β2-glycoprotein I (β2GPI) and various factors involved in hemostasis, such as prothrombin, protein C, and protein S (7,8). Although aPL have been shown to promote thrombosis and miscarriage in animal studies, the etiology and pathogenic mechanisms remain unclear.

To characterize pathogenic aPL in APS, we previously generated 7 monoclonal IgG–anticardiolipin (aCL) antibodies from 2 patients with APS (9,10). Of these monoclonal antibodies (mAb), 5 were prothrombotic in an in vivo pinch–induced thrombosis model in mice (11). Importantly, we found that 4 of these 5 aCL directly bind to the key enzymes involved in hemostasis, namely, thrombin, activated protein C, tissue-type plasminogen activator, and plasmin (1215). These enzymes belong to the trypsin family and are homologous in their enzymatic domains (1619). Interestingly, these enzyme-reactive aCL bind to plasmin with relative Kd values in the range of 10−7M (14), which are 30–100-fold higher than the affinities of known IgG-aCL toward β2GPI, the major autoantigen in APS (20). These findings, in combination, suggest that plasmin may be an important autoantigen that drives the activities of certain IgG-aCL in some patients with APS.

Indeed, Chen et al, in a study in China, found that plasmin could induce IgG-aCL in immunized BALB/cJ mice, and that one of the mAb generated from these mice, IgG1-aCL, displayed lupus anticoagulant activity and induced fetal loss when injected into pregnant mice (21). However, the titers and kinetics of the plasmin-induced IgG-aCL were not given; the IgG-aCL values were only expressed as the fold change (in SD) above the mean value for control mice. Furthermore, although 2 of the mAb inhibited plasmin activity, the effects of the mAb on other cross-reacting target proteases (such as thrombin) were not explored.

To address these issues, we immunized BALB/cJ mice with human plasmin, which resulted in only transient and very low titers of IgG-aCL. Therefore, in addition to BALB/cJ mice, we also immunized MRL/MpJ mice with plasmin and analyzed the immune sera for IgG-antiplasmin antibodies and IgG-aCL. The MRL/MpJ strain was chosen because mild immunologic defects (i.e., the presence of low-titer anti–double-stranded DNA autoantibodies and low levels of glomerulonephritis) have been observed in older mice (>1 year of age) in this strain, and MRL/MpJ are the parent and control strain for the well-studied spontaneous lupus model in MRL/lpr mice. The results showed that immunized MRL/MpJ mice, as compared with control BALB/cJ mice, produced high titers of both IgG-antiplasmin antibodies and IgG-aCL. Moreover, the immunized MRL/MpJ mice also produced high titers of IgG–anti-β2GPI antibodies. Furthermore, when mAb were generated from the serum of the MRL/MpJ mice with high titers of IgG-antiplasmin and IgG-aCL, these mAb were found to bind human plasmin, cardiolipin, β2GPI, thrombin, and the activated coagulation factor X (FXa), to varying degrees. Importantly, some of the mAb inhibited plasmin activity and also hindered the inactivation of thrombin and FXa by antithrombin III (AT). Thus, these findings show that plasmin may serve as a driving autoantigen for certain prothrombotic IgG-aCL that have functional significance in vitro.

MATERIALS AND METHODS

Immunization of mice

All mouse studies were performed following protocols in accordance with our institutional guidelines. Six-to-seven-week-old female BALB/cJ and MRL/MpJ mice (The Jackson Laboratory, Bar Harbor, ME) were immunized subcutaneously with either 30 µg of human Lysplasmin (Haematologic Technologies, Essex Junction, VT) in Freund’s complete adjuvant (CFA; Sigma, St. Louis, MO) or CFA alone. The mice received booster immunizations subcutaneously with the same amount of plasmin in Freund’s incomplete adjuvant (Sigma) at month 1 and month 3 after the first immunization. Serum samples were obtained from the mice before immunization on day 0 and then monthly thereafter.

Analysis of sera for IgG antibodies against human plasmin, cardiolipin, and β2GPI

IgG-antiplasmin antibodies were analyzed as described previously (14). Briefly, high-binding plates were coated with 5 µg/ml of human plasmin in phosphate buffered saline (PBS) (pH 7.4). After incubating the plates overnight at 4°C, the cultures were blocked with 0.25% gelatin/PBS. Serum samples at 1:100 dilution in 0.1% gelatin/PBS were distributed to separate wells and incubated for 1.5 hours at room temperature (RT). Bound IgG antibodies were then measured in the sera, and those samples considered positive for IgG-antiplasmin were analyzed further at a series of 1:10 dilutions, up to the 1:105 dilution, to determine the antibody titers. IgG-aCL were analyzed in a similar manner, using a method as described previously (22), except that the microtiter plates were coated with cardiolipin at 50 µg/ml in ethanol and blocked with 10% bovine serum in PBS. Finally, IgG–anti-β2GPI antibodies were analyzed in a similar manner, as described previously (9), except that high-binding plates were coated with β2GPI at 5 µg/ml in PBS. In all of these experiments, a murine serum sample with high reactivity for each of the respective antigens (antiplasmin, aCL, or anti-β2GPI) was used repeatedly in all of the enzyme-linked immunosorbent assay (ELISA) experiments as the reference standard to determine antibody titers or reference units (RU).

Generation of monoclonal IgG-antiplasmin/IgG-aCL from the immunized mice

Mice with high titers of both IgG-antiplasmin antibodies and IgG-aCL received a booster immunization peritoneally with 30 µg of plasmin 3–4 days before fusion. The spleen cells were fused with the P3X63-AG8.653 mouse myeloma cell line (American Type Culture Collection, Manassas, VA). The culture supernatants were then screened for IgG-antiplasmin and IgG-aCL, and the hybridomas from the double-positive wells were subcloned twice at 1 cell/well. Of note, all of the IgG-aCL–positive wells were also positive for IgG-antiplasmin. The mAb were affinity-purified using a HiTrap Protein G–HP column (GE Life-sciences, Piscataway, NJ) and used for all studies. All mAb were then isotyped using the Mouse-Typer Isotyping ELISA panel (Bio-Rad, Hercules, CA), and all were determined to be of the IgG1 subtype (results not shown).

Analyses of mAb for reactivity with plasmin, cardiolipin, and β2GPI, and for cross-reactivity with thrombin and FXa

Purified mAb were analyzed for reactivity against human plasmin, cardiolipin, and β2GPI by ELISA in a manner similar to that described above, except that polyclonal mouse IgG and isotype mouse IgG1 (Chemicon, Temecula, CA) were used as negative controls, and all IgG were used at a concentration of 10 µg/ml. The mAb were also tested for thrombin reactivity as described previously (12). Briefly, high-binding ELISA plates were coated with 5 µg/ml of human α-thrombin (Haematologic Technologies) in Tris buffered saline (TBS; 0.05M Tris HCl and 0.15M NaCl, pH 7.5). After incubating the plates overnight at 4°C, the cultures were blocked with TBS containing 0.3% gelatin. The ELISA to test cross-reactivity with human FXa was conducted similarly, except that the plates were coated with 5 µg/ml of human FXa (Haematologic Technologies).

Effects of mAb on plasmin activity

The effects of the mAb on plasmin activity were studied using the chromogenic substrate S-2251 (Sigma) as described previously (21). Briefly, plasmin (20 mM) in 50 µl of HEPES buffer was incubated with 50 µl of mAb (200 µg/ml) for 1 hour at RT in microtiter wells. S-2251 at a concentration of 100 µl (250 µg/ml final concentration) was then added, and the plate was incubated in a humidified environment for 1 hour. The optical density (OD) value at 405 nm was measured.

Effects of mAb on the inactivation of thrombin by AT

The effects of the thrombin-reactive mAb on the inactivation of thrombin by AT were studied in a functional assay for the thrombin activity in the presence of AT and heparin, as described previously (12). Briefly, 25 µl of thrombin was incubated with 25 µl of either test mAb, normal polyclonal mouse IgG, or the isotype control monoclonal mouse IgG1, in duplicate for 1 hour at RT. Fifty microliters of AT (Enzyme Research Laboratories, South Bend, IN) was then added to each reaction mixture in buffer containing heparin, and finally, 200 µl of the chromogenic substrate S-2238 (150 µM; Chromogenix, Molndal, Sweden) was added, and after ~1 minute of incubation, the OD at 405 nm was measured. The percentage of thrombin inactivation by AT was calculated as (1 − [residual thrombin activity with AT]/[initial thrombin activity without AT]) × 100.

Effects of mAb on the inactivation of FXa by AT

A functional assay for the effects of the FXa-reactive mAb on FXa inactivation by AT was done using a method as previously described (23). The assay used was similar to the thrombin functional assay described above, except that 25 µl of FXa was incubated separately with 25 µl of test mAb or control IgG for 1 hour at RT. Fifty microliters of AT was then added in the buffer containing heparin, and subsequently, 50 µl of the chromogenic substrate S-2765 (660 µM; DiaPharma) was added, and the OD at 405 nm was measured over time. The percentage of FXa inactivation by AT was calculated as (1 − [residual FXa activity with AT]/[initial FXa activity without AT]) × 100.

Statistical analysis

Differences in the titers of IgG-antiplasmin antibodies and IgG-aCL between the 2 immunization groups (the control group and plasmin-immunized group) were analyzed with the Kruskal-Wallis test followed by the Dunn’s multiple comparison test. Of note, when a data point in a group was outside of the mean ± 3 SD of the remaining data points of the same group, it was considered an outlier and was excluded from all analyses. Differences in reactivity to β2GPI (OD values) in the sera between the 2 groups were analyzed with the unpaired t-test. Relationships between 2 of the variables (aCL titers versus anti-β2GPI antibody titers) were assessed by Pearson’s correlation analyses using log-transformed values of aCL and anti-β2 GPI titers (both expressed in RU × 103). Differences in the plasmin activity and percentages of either thrombin or FXa inactivation by AT between the mAb-treated and untreated control groups were determined by one-way analysis of variance followed by Dunnett’s multiple comparison test.

RESULTS

Induction of mainly IgG-antiplasmin antibodies in BALB/cJ mice, but both IgG-antiplasmin and IgG-aCL in MRL/MpJ mice, by plasmin immunization

When BALB/cJ and MRL/MpJ mice were immunized with human plasmin, both strains produced high titers of IgG-antiplasmin antibodies (Figures 1A and B). The IgG-antiplasmin antibody titers peaked at month 2 and remained high up to month 5 in both strains of mice.

Figure 1
Effects of plasmin immunization on production of IgG-antiplasmin antibodies (A and B) and IgG–anticardiolipin (aCL) antibodies (C and D) in BALB/cJ and MRL/MpJ mice. Mice were immunized (denoted by arrows) with either 30 µg human plasmin ...

Importantly, immunized MRL/MpJ mice also displayed significantly raised titers of IgG-aCL. From month 2 to month 5, the titers of IgG-aCL in the plasmin-immunized mice were significantly higher than those in the corresponding control mice (Figure 1D). Of note, MRL/MpJ sera were also analyzed for the presence of aCL of both the IgA and IgM isotypes, but no significant differences in the levels of IgA- or IgM-aCL were found in comparison with control mice (results not shown).

Immunized BALB/cJ mice also developed titers of IgG-aCL that were significantly higher (P < 0.05) than those in the control mice at month 2, although the significance of this difference was not sustained throughout the remainder of the experiment (Figure 1C). However, the levels of IgG-aCL in the BALB/cJ mice were much lower than in their MRL/MpJ counterparts (Figure 1C). The failure of induction of aCL in plasmin-immunized BALB/cJ mice might imply that plasmin can only drive aCL production in autoimmune-prone individuals.

The immunization experiments were repeated separately on 2 occasions for the BALB/cJ mice, and 3 times for the MRL/MpJ mice. Analysis of the sera was completed on all samples, and the results reported in Figure 1 are representative data from 1 experiment. The human Lys-plasmin used in these experiments was tested by Western blotting for contamination with β2GPI, and was found to have <0.25% β2GPI, which suggests that there was little to no contamination (results not shown).

The induced IgG-aCL did not react with cardiolipin in the absence of bovine serum (results not shown), suggesting that these murine mAb have the characteristics of IgG-aCL in APS. Taken together, these results showed that plasmin could drive IgG-aCL production, and suggested that genetic factors could play a significant role in the production of the plasmin-driven IgG-aCL.

Induction of IgG–anti-β2GPI antibodies in MRL/MpJ mice, but not in BALB/cJ mice

β2GPI is recognized as the major autoantigen in APS, and the reactivity of antibodies against β2GPI and its complexes with cardiolipin may account for most of the positive findings on aCL activity in APS (24). Therefore, it is of interest to investigate whether the plasmin-induced aCL could also react with β2GPI in these murine models. To this end, the immune sera that displayed peak titers of IgG-aCL (i.e., at the fifth month in MRL/MpJ mice) were analyzed for reactivity with β2GPI.

As shown in Figure 2B, the immune MRL/MpJ sera displayed significantly higher titers of IgG–anti-β2GPI antibodies as compared with those in control MRL/MpJ sera (P < 0.001). Moreover, the corresponding immune sera from BALB/cJ mice had only minimal levels of IgG–anti-β2GPI antibodies (Figure 2A). Thus, these results confirm that the plasmin-induced aCL in MRL/MpJ mice mimic the characteristics of aCL in APS.

Figure 2
Serologic analyses of immune sera for IgG–anti–β2-glycoprotein I (anti-β2GPI) antibodies in BALB/cJ and MRL/MpJ mice (A and B) and analyses of association of anticardiolipin (aCL) and anti-β2GPI antibodies with ...

Association of IgG-aCL and IgG–anti-β2GPI antibodies with IgG-antiplasmin antibodies

To further test our hypothesis that plasmin is a driving autoantigen for some IgG-aCL, we studied the association of the production of IgG-aCL with that of IgG-antiplasmin antibodies in the immune sera of mice obtained at month 5 after first immunization. As can be seen in Figure 2C, the levels of IgG-aCL were highly correlated with the levels of antiplasmin antibodies (Pearson’s r = 0.64, P = 0.026).

Moreover, we analyzed the relationship between anti-β2GPI antibody production and IgG-antiplasmin antibody production. As is evident in Figure 2D, the levels of anti-β2GPI antibodies were highly correlated with the levels of antiplasmin antibodies (Pearson’s r = 0.61, P = 0.048).

Reactivity of some plasmin-induced murine monoclonal IgG-aCL with human β2GPI

To further characterize the plasmin-induced IgG-aCL, we initiated efforts to generate and analyze monoclonal IgG-aCL from the immunized MRL/MpJ mice with high titers of IgG-aCL. The hybridomas were screened for IgG-antiplasmin antibodies and IgG-aCL, and only the double-positive hybridomas were utilized, resulting in generation of 10 test mAb. These mAb clones were designated M2-11, M4-0, M9-6, M9-12, M9-14, M9-15, M9-16, M9-17, M9-43, and M20-48 (Figures 3A and B).

Figure 3
Binding of plasmin-induced murine monoclonal anticardiolipin (aCL) antibodies to different antigens, including human plasmin (A), cardiolipin (B), β2-glycoprotein I (β2GPI) (C), thrombin (D), and activated coagulation factor X (FXa) ( ...

As noted in the preceding experiments, the plasmin-induced aCL were observed to bind to β2GPI and were thus shown to mimic the major characteristic of aCL in APS. To explore this finding further, we analyzed the test mAb for reactivity with β2GPI. As shown in Figure 3C, 4 (40%) of the 10 mAb (i.e., M2-11, M4-0, M9-6, and M9-12) bound to β2GPI, providing further support that a subset of plasmin-induced aCL mimic the main characteristic of aCL in APS.

Reactivity of some plasmin-induced murine monoclonal IgG-aCL with the serine protease thrombin and FXa

To test our hypothesis that some of the plasmin-driven aCL cross-react with certain serine proteases that share homologous enzymatic domains with plasmin, we analyzed our mAb for cross-reactivity with thrombin and FXa. Figure 3D demonstrates that several of the mAb bound to thrombin, including M2-11, M4-0, M9-12, and M9-43. In addition, as shown in Figure 3E, some of the mAb also bound to FXa, namely, M2-11, M4-0, and M9-43. Taken together, these results indicate that a significant subset of the plasmin-induced aCL cross-react with other serine proteases.

Murine mAb inhibition of the amidolytic activity of plasmin

To assess the pathologic significance of the plasmin-driven aCL, we first studied the effects of the mAb on the amidolytic activity of plasmin using the chromogenic substrate S-2251. The results showed that 4 mAb (M2-11, M9-16, M9-17, and M20-48) significantly inhibited the amidolytic activity of plasmin, with the extent of inhibition between 18% and 39% (P < 0.01 versus untreated controls) (Figures 4A and B). These results indicate that the plasmin-driven aCL may promote thrombosis by directly interfering with the fibrinolytic function of plasmin, similar to the previously described effects of patient-derived CL15 monoclonal IgG-aCL (14).

Figure 4
Effects of the 10 murine IgG-aCL mAb on plasmin amidolytic activity, using the chromogenic substrate S-2251. A, Plasmin amidolytic activity was assessed at the 60-minute time point using substrate S-2251 in the absence or presence of test mAb, polyclonal ...

Murine mAb inhibition of the inactivation of thrombin by AT

To further characterize the thrombin-reactive mAb, we investigated the ability of these mAb to interfere with AT inactivation of thrombin using a functional assay containing 0.1 units/ml of heparin. The addition of heparin is needed to approximate the in vivo inactivation of thrombin by AT, since AT often binds to heparin-like glycosaminoglycans (including heparan sulfates on the endothelial cell surface) (2527).

Under these conditions, AT inactivated 98% of thrombin activity, and the degrees of thrombin inactivation by AT were not changed by the presence of either polyclonal mouse IgG or a monoclonal mouse IgG1 isotype control (Figure 5A). In contrast, the mAb M2-11 reduced the degree of thrombin inactivation to 29% (SEM 1.4 [n = 2 experiments]; P < 0.01 versus untreated controls), whereas the remaining 2 mAb (M4-0 and M9-43) did not significantly affect AT inactivation of thrombin.

Figure 5
Inhibition of thrombin inactivation by antithrombin III (AT) with M2-11, one of the murine IgG-aCL mAb. A, Thrombin was preincubated with the test mAb, polyclonal mouse IgG (mIgG), or monoclonal mouse IgG1 isotype control (mIgG1), and AT was then added ...

Because M2-11 could reduce the thrombin inactivation from 98% in buffer alone to 29% in cultures with the M2-11 mAb, the resultant increase in thrombin activity over time could present a significant procoagulant effect, since the residual thrombin may continue to convert fibrinogen into fibrin at a constant rate. To visualize this rapid cumulative effect over time, overall conversion of a thrombin substrate in the absence or presence of test mAb, polyclonal mouse IgG, or an isotype control mAb was measured over a period of 5 minutes. As shown in Figure 5B, accumulated substrate conversion in the presence of M2-11 increased dramatically over that in the absence of M2-11 in the span of 5 minutes.

Murine mAb inhibition of the inactivation of FXa by AT

In a similar manner, we used a functional assay to study the FXa–cross-reactive mAb for their ability to interfere with the FXa inactivation by AT. The final concentrations of FXa, AT, and IgG were 1.25 nM, 12.5 nM, and 33.3 µg/ml, respectively. Under these conditions, AT inactivated 92% of FXa activity (in buffer alone), and the degrees of FXa inactivation by AT were not significantly changed by the presence of either polyclonal mouse IgG control or monoclonal mouse IgG1 isotype control (Figure 6A). In contrast, the mAb M2-11 and M4-0 reduced the degree of FXa inactivation to a mean ± SEM 3 ± 0.7% and 38 ± 4.2%, respectively (each n = 2 experiments; P < 0.01 versus untreated controls).

Figure 6
Reduction of antithrombin III (AT) inactivation of activated coagulation factor X (FXa) with M2-11 and M4-0, two of the IgG-aCL mAb. A, FXa was preincubated separately with the test mAb (each at a concentration of 33.3 µg/ml), polyclonal mouse ...

As noted, these experiments were done using IgG at a concentration of 33.3 µg/ml (222 nM) and FXa at a concentration of 1.25 nM, resulting in an IgG:FXa molar ratio of 178:1. Assuming that the plasma concentration of total IgG is 10 mg/ml and that IgG–anti-FXa antibodies in a patient with APS account for 1% of the possible total IgG, the concentration of antibodies used in this assay was ~30% of the possible total IgG–anti-FXa antibodies. Therefore, we studied the concentration-dependent effects of anti-FXa antibodies on AT inactivation of FXa. The mAb, polyclonal mouse IgG, and mouse monoclonal IgG1 isotype were analyzed in a graded series of 2-fold lower concentrations (from 3.13 µg/ml up to 50 µg/ml). The results showed that at a lower concentration of 12.5 µg/ml (resulting in an IgG: FXa molar ratio of 67:1), M2-11 and M4-0 reduced the degree of FXa inactivation to 26% and 74%, respectively (Figure 6B). Of note, increasing the concentration of M2-11 almost completely inhibited the anticoagulant function of AT on FXa (Figure 6B).

DISCUSSION

Previously, Chen et al showed that plasmin could induce both IgG-antiplasmin antibodies and limited IgG-aCL in BALB/cJ mice, and that one monoclonal aCL had lupus anticoagulant activity and induced fetal loss in mice (21). To characterize further the plasmin-driven IgG-aCL, we used plasmin to immunize BALB/cJ mice and MRL/MpJ mice. We found that plasmin immunization in MRL/MpJ mice induced IgG-aCL and IgG-antiplasmin antibodies, whereas only persistent IgG-antiplasmin antibodies were induced in BALB/cJ mice (Figure 1). Moreover, the antibodies produced in the plasmin-immunized MRL/MpJ mice reacted with human β2GPI (Figure 2B), and the IgG-antiplasmin antibody levels were highly correlated with those of IgG-aCL and IgG–anti-β2GPI (Figures 2C and D), demonstrating that the plasmin-driven aCL displayed a major characteristic of the diagnostic aCL in APS patients. Of note, to rule out the possibility that the observed anti-β2GPI activity might be due to β2GPI contamination in plasmin, we used a Western blotting analysis with 40 µg plasmin; the results (not shown) indicated that there was no detectable β2GPI contamination.

Taken together, these results suggest that aCL are tightly regulated in normal mice with a normal immune system, but are more ready to escape immune regulation and persist in autoimmune-prone mice that probably have deficient/defective immune regulation. Moreover, these findings imply that plasmin could drive aCL production more readily in certain genetically susceptible individuals who are prone to autoimmune disorders.

Subsequently, we generated 10 monoclonal IgG-aCL from the plasmin-immunized MRL/MpJ mice. Importantly, 4 (40%) of these monoclonal IgG-aCL reacted with β2GPI (Figure 3C), and some of monoclonal IgG-aCL cross-reacted with other human serine proteases, namely, thrombin and FXa (Figures 3D and E). Of note, the sera from the sixth blood withdrawal (i.e., 2 months after the third plasmin immunization) were analyzed for reactivity with thrombin and FXa, and these sera were found to contain antithrombin activity but not anti-FXa activity (P = 0.02 and P = 0.16, respectively, versus control mice immunized with adjuvants only) (results not shown).

Finally, the pathologic significance of the plasmin-induced IgG-aCL was assessed by studying the effects of the mAb on the function and regulation of the reactive target proteases. The results showed that 4 of the mAb (M2-11, M9-16, M9-17, and M20-48) could directly inhibit the amidolytic activity of plasmin, with the extent of inhibition ranging from 18% to 39% (Figure 4). Interestingly, the strength of binding of the mAb to plasmin (Figure 3A) did not directly correlate with the profiles of plasmin inhibition (Figure 4), which is likely a reflection of the fact that the mAb bind to different regions on plasmin, and that only the mAb that bind to or near the active site of plasmin may inhibit the activity of plasmin.

In addition, the thrombin-reactive monoclonal IgG-aCL M2-11 was also found to reduce the inactivation of thrombin by AT down to 29% (Figure 5), and 2 mAb, M2-11 and M4-0, reduced the inactivation of FXa by AT to 3% and 38%, respectively (Figure 6). These latter data suggest that ~20% of the plasmin-driven IgG-aCL may interfere with feedback regulation of FXa, resulting in unchecked activation of FXa and a proco-agulant state. Taken together, these data indicate that the plasmin-driven IgG-aCL may promote thrombosis from 2 ends, via the unregulated conversion of fibrinogen to fibrin and the reduced rate of fibrin resolution.

As reported in previous studies, 5 of 7 monoclonal IgG-aCL derived from 2 patients with APS reacted with plasmin (9,10,14). Furthermore, these 5 plasmin-reactive aCL were observed to bind to plasmin with relative Kd values in the range of 10−7 (14), which are 30–100-fold higher than the affinities of known IgG-aCL toward β2GPI, the major autoantigen in APS (20). Taken together with the present findings, these data strongly suggest that plasmin is an important autoantigen that drives the production of certain prothrombotic IgG-aCL in APS patients.

In this context, it is interesting to note that many bacteria use human plasmin to dissolve the surrounding fibrin clots that function as a host defense mechanism to prevent spreading of bacteria via the circulation (28). For example, group A streptococcus (GAS) uses its plasminogen receptors to recruit and activate human plasminogen to plasmin on the bacterial surface (28,29). Consequently, plasmin is presented to the host together with streptococci, and thus may induce the immune response to plasmin, according to the “danger signal” hypothesis (30,31). In addition to GAS, several other common pathogens, including Staphylococcus aureus and Yersinia pestis, use the human plasminogen system for survival (28). Although the majority of aCL generated in the postinfection period are transient in nature (32), it is possible that repeated exposure to common pathogenic bacteria may allow the development of antiplasmin antibodies in genetically susceptible individuals, which may, in turn, lead to the development of aCL, anti-β2GPI antibodies, and APS. Future studies are warranted to test the above hypothesis.

Alternatively, we recently reported that some aPL in APS patients recognize conformational epitopes shared by β2GPI and the homologous enzymatic domains of several serine proteases, including plasmin (33). Specifically, 2 patient-derived IgG anti-β2GPI mAb bound to thrombin and plasmin, and 1 antithrombin mAb reacted with β2GPI. In addition, the binding of a cross-reactive mAb to β2GPI was inhibited by α-thrombin, which contains only the catalytic domain of thrombin. Taken together with the results of the present study, these findings suggest that plasmin may induce and/or drive the production of aCL and anti-β2GPI antibodies in genetically susceptible individuals via epitope spreading.

Thus, the present study shows that plasmin immunization induces significantly and persistently raised titers of disease-relevant IgG-aCL in MRL/MpJ mice, but not in BALB/cJ mice, suggesting that plasmin-driven aCL production is under genetic control. Therefore, plasmin is more likely to serve as a driving autoantigen in certain genetically susceptible individuals.

Supplementary Material

Supplemental Data

ACKNOWLEDGMENTS

We thank Dr. Brian Skaggs for teaching Dr. M. Wu and providing reagents for the Western blot analyses.

Supported by NIH grant AR-42506. Dr. Ede is recipient of a fellowship grant from the Southern California Chapter of the Arthritis Foundation. Dr. Chen-Ching Wu’s work was supported by a Faculty Development award from Kaohsiung Medical University, Taiwan. Dr. Yang’s work was supported by a Faculty Development award from National Taiwan University Hospital, Taiwan. Dr. Lin’s work was supported by a Research Training grant from the Taiwanese government.

Footnotes

AUTHOR CONTRIBUTIONS

All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published. Dr. Ede had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study conception and design. Ede, Hwang, Tsao, P. P. Chen.

Acquisition of data. Ede, Hwang, C.-C. Wu, M. Wu, Yang, Lin, Chien, P.-C. Chen, McCurdy, P. P. Chen.

Analysis and interpretation of data. Ede, Hwang, C.-C. Wu, M. Wu, Lin, P.-C. Chen, Tsao, P. P. Chen.

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