Hydrophobic Amino Acid Residues Are Critical for the Immunodominant Epitope of the Goodpasture Autoantigen

A MOLECULAR BASIS FOR THE CRYPTIC NATURE OF THE EPITOPE*

Michelle DavidDagger §, Dorin-Bogdan BorzaDagger §, Anu LeinonenDagger , John M. Belmont, and Billy G. HudsonDagger ||

From the Departments of Dagger  Biochemistry and Molecular Biology and  Pediatrics, University of Kansas Medical Center, Kansas City, Kansas 66160

Received for publication, October 2, 2000, and in revised form, November 17, 2000



    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES

Goodpasture (GP) autoimmune disease is caused by autoantibodies to type IV collagen that bind to the glomerular basement membrane, causing rapidly progressing glomerulonephritis. The immunodominant GPA autoepitope is encompassed by residues 17-31 (the EA region) within the noncollagenous (NC1) domain of the alpha 3(IV) chain. The GP epitope is cryptic in the NC1 hexamer complex that occurs in the type IV collagen network found in tissues and inaccessible to autoantibodies unless the hexamer dissociates. In contrast, the epitope for the Mab3 monoclonal antibody is also located within the EA region, but is fully accessible in the hexamer complex. In this study, the identity of residues that compose the GPA autoepitope was determined, and the molecular basis of its cryptic nature was explored. This was achieved using site-directed mutagenesis to exchange the alpha 3(IV) residues in the EA region with the corresponding residues of the homologous but non-immunoreactive alpha 1(IV) NC1 domain and then comparing the reactivity of the mutated chimeras with GPA and Mab3 antibodies. It was shown that three hydrophobic residues (Ala18, Ile19, and Val27) and Pro28 are critical for the GPA autoepitope, whereas two hydrophilic residues (Ser21 and Ser31) along with Pro28 are critical for the Mab3 epitope. These results suggest that the cryptic nature of the GPA autoepitope is the result of quaternary interactions of the alpha 3, alpha 4, and alpha 5 NC1 domains of the hexamer complex that bury the one or more hydrophobic residues. These findings provide critical information for understanding the etiology and pathogenesis of the disease as well as for designing drugs that would mimic the epitope and thus block the binding of GP autoantibodies to autoantigen.



    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES

Goodpasture (GP)1 disease is defined by rapidly progressive glomerulonephritis, with or without lung hemorrhage, which is caused by autoantibodies targeted to type IV collagen of the glomerular and alveolar basement membranes. Left untreated, the disease is potentially lethal. If diagnosed early, GP patients can be treated by immunosuppression and plasma exchange to remove the toxic autoantibodies. This therapy has side effects, including a weakening of the natural defenses; thus, a more specific therapy is highly desirable. A detailed knowledge of the epitope would facilitate the development of therapies that selectively remove, neutralize, or prevent synthesis of the pathogenic autoantibodies and provide a foundation for studies to determine the etiology of the disease.

The GP autoantigen is the alpha 3 chain of type IV collagen (1, 2), one of the six chains (alpha 1-alpha 6) that compose type IV collagen (3). The GP autoepitopes are conformational and reside within the 232-residue long noncollagenous (NC1) domain at the C-terminus of the alpha 3(IV) chain. Three chains of type IV collagen assemble into triple-helical protomers (molecule) that further interact with each other at the amino and carboxyl termini to form supramolecular networks. In the glomerular basement membrane, which is the main target of autoantibodies, the alpha 3(IV) chain associates with the alpha 4(IV) and alpha 5(IV) chains to form a cross-linked alpha alpha alpha 5(IV) network (4). The GP epitopes are cryptic in the alpha alpha alpha 5 NC1 hexamer complex formed by the interaction of two triple-helical protomers through the NC1 domains (Fig. 1, top). As a result, the epitopes are inaccessible for binding of autoantibodies unless the hexamer dissociates (5-7). Unmasking of previously hidden GP epitopes is thought to be of fundamental importance for understanding the etiology and pathogenesis of GP disease.



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Fig. 1.   Model representing the location within the native NC1 hexamer complex of the cryptic GP autoepitopes and of the exposed Mab3 epitope of the alpha 3(IV) NC1 domain. In the type IV collagen networks found in vivo, two triple-helical collagen protomers interact through their carboxyl-terminal ends, forming an NC1 hexamer complex (top). Two conformational GP epitopes, designated GPA and GPB, have been localized to the EA and EB regions of the alpha 3(IV) NC1 domain. These regions jointly form the epitope for Mab3. The GP epitopes (diagonal lines) are cryptic in the NC1 hexamer complex and inaccessible for binding of autoantibody (shown for GPA antibodies; right), but they are exposed upon dissociation of the hexamer into subunits, allowing binding of the autoantibody. In contrast, the Mab3 epitope (solid black) is accessible in both the hexamer and dissociated (monomer) form of the alpha 3(IV)NC1 domain (left). Hence, in the NC1 hexamer complex, the EA and EB regions contain certain inaccessible residues that are critical for the GP epitopes and other accessible residues that are critical for the Mab3 epitope (7).

Two conformational GP autoepitopes have recently been mapped to two regions of the alpha 3(IV) NC1 domain, designated EA and EB, by homolog-scanning mutagenesis using chimeric alpha 1/alpha 3 NC1 domains in which the non-immunoreactive alpha 1 NC1 domain was used as a scaffold for exchanging short homologous alpha 1 sequences with alpha 3 NC1 segments to ensure correct folding of the epitope. The immunodominant autoepitope, designated GPA (7), has been localized to the N-terminal third of the alpha 3(IV) NC1 domain (8, 9) and specifically to residues 17-31, designated the EA region (7, 10, 11). Autoantibodies specific for the EA region, designated GPA, are believed to play an important role in the pathogenesis of GP disease because they are the predominant subpopulation (~60-65%) in all sera and have high affinity for autoantigen (7). Moreover, high titers of GPA antibodies are correlated with an unfavorable disease outcome (9). A second autoepitope, designated GPB (7), was also identified in the central portion of the alpha 3(IV) NC1 domain (9) and was further mapped to residues 127-141, designated the EB region (7, 10).

In contrast to the cryptic GP autoepitopes, the epitope for Mab3 monoclonal antibody, which is also localized to the EA and EB regions, is fully accessible even in the NC1 hexamer (7). These observations led to the hypothesis that only certain amino acid residues of the EA and EB regions constitute the GPA and GPB autoepitopes and that their cryptic nature in the NC1 hexamer complex is a result of either direct interactions with or close proximity to other NC1 domains in the hexamer, which prevents the access of autoantibodies. Moreover, these critical residues must be distinctly different from those that constitute the Mab3 epitope, which are accessible on the surface of the hexamer complex.

The aim of this study was to identify which residues compose the GPA autoepitope and to explore the molecular basis of its cryptic nature. This was accomplished using homolog-scanning mutagenesis to change the alpha 3(IV)-specific residues within the EA region of an alpha 1/alpha 3 chimera, one at a time, to the corresponding residues from the homologous but non-immunoreactive alpha 1(IV) NC1 domain. The reactivity of mutated NC1 domains with GPA and Mab3 antibodies was then compared. It was shown that three hydrophobic residues (Ala18, Ile19, and Val27) and Pro28 are critical for the GPA autoepitope, whereas two hydrophilic residues (Ser21 and Ser31) along with Pro28 are critical for the Mab3 epitope. These results suggest that the cryptic nature of the GPA autoepitope is the result of quaternary interactions among the alpha 3(IV), alpha 4(IV), and alpha 5(IV) NC1 domains of the alpha alpha alpha 5 hexamer complex that bury the one or more hydrophobic residues. These findings provide critical information for understanding the etiology and pathogenesis of the disease as well as for designing drugs that would mimic the epitope and thus block the binding of GP autoantibodies to autoantigen in vivo.


    EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES

Site-directed Mutagenesis and Expression of Mutated NC1 Chimeras-- Eight chimeric constructs with individual alpha 3(IV) to alpha 1(IV) amino acid substitutions in the EA epitope region, designated M1-M8 (see Fig. 2), were constructed using the GeneEditorTM in vitro site-directed mutagenesis system (Promega, Madison, WI) according to the manufacturer's protocol. Substitutions from alpha 3(IV) to alpha 1(IV) amino acids were introduced by specific primers (Table I). The previously described C2·6 construct (10), containing the GP epitope regions EA and EB from the alpha 3(IV) NC1 domain substituted into the alpha 1(IV) NC1 domain scaffolding, was used as a template. The clones thus obtained were digested with NheI and SacII restriction enzymes (New England Biolabs Inc., Beverly, MA) and subcloned into the pRcX expression vector (10, 12), which contains the BM-40 signal peptide and the FLAG peptide (DYKDDDDK) sequences downstream of the cytomegalovirus promoter. All constructs were sequenced to confirm the substitutions.


                              
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Table I
Primers used for homolog-scanning mutagenesis (alpha 3 right-arrow alpha 1)

Human embryonic kidney 293 cells (ATCC 1573 CRL) were transfected with ~5 µg of plasmid DNA using the calcium phosphate precipitation method, and G418-resistant cells were screened for expression of recombinant protein from the culture medium by Western blotting with anti-FLAG monoclonal antibody M2 (Sigma) as previously described (10). Recombinant proteins were purified by affinity chromatography on an anti-FLAG M2 affinity column (Sigma) according to the manufacturer's instructions. The concentration of the purified NC1 domains and chimeras was determined spectrophotometrically using 1.6 A280 = 1 mg/ml (10).

Monoclonal Antibodies-- Several monoclonal antibodies to the alpha 3(IV) NC1 domain were used, the epitopes of which have been previously localized (7). Mab3 was purchased from Wieslab AB (Lund, Sweden). M3/1, raised against an amino-terminal synthetic peptide of the human alpha 3(IV) NC1 domain, and monoclonal antibody 175, raised against randomly folded recombinant human alpha 3(IV) NC1 domain expressed in Escherichia coli, were previously described (13).

GP Patient Sera-- The serum or the plasmapheresis fluid from 13 patients diagnosed with Goodpasture disease (GP1-13) was used. The titer of GP antibodies was measured by ELISA in microtiter plates coated with alpha 3(IV) NC1 domain (100 ng/well); and for further analyses, sera were appropriately diluted to yield approximately equal reactivity. The relative autoantibody reactivity to the EA and EB regions was measured by ELISA using the C2 and C6 chimeras, respectively (10). GP sera that reacted predominantly with EA (GPA autoantibodies) were used without further purification. GP sera that showed significant reactivity against the EB region required further purification by absorption of GPB antibodies to a column with immobilized C6 as previously described (7). The unbound fraction, consisting of GPA antibodies, was used in the subsequent immunoassays for mapping the epitope location. The bound GPB fraction was eluted from the C6 affinity column with 3 M guanidinium chloride and used as a control to establish the correct folding of the mutated chimeras.

Western Blots-- The proteins (300 ng), separated by SDS-10% polyacrylamide gel electrophoresis under nonreducing conditions, were transferred onto nitrocellulose membranes and immunoblotted with anti-FLAG and Mab3 monoclonal antibodies as well as with GP sera as previously described (10).

Direct and Inhibition Immunoassays-- The alpha 1(IV) NC1 domain and chimera C6 (negative controls), chimera C2·6 and the alpha 3(IV) NC1 domain (positive controls), and mutated chimeras M1-M8 were coated in duplicates onto MaxisorpTM ELISA plates (Nunc) at 100 ng/well in 50 mM carbonate buffer, pH 9.6. Binding of GP autoantibodies or Mab3 was determined by direct ELISA as previously described (7). Detection was performed with alkaline phosphatase-conjugated secondary antibodies followed by p-nitrophenol phosphate, and color development was monitored at 410 nm with a Dynatech MR4000 plate reader. Each serum was analyzed at least four times, with the median of the results being taken as the serum's representative value. For inhibition ELISA, the GP sera or purified GP antibody fractions were incubated overnight at room temperature with various amounts of recombinant NC1 domains or chimeras prior to addition to plates coated with the C2·6 chimera.

Statistical Analysis-- The relative optical density of the mutated protein chimeras with respect to the binding of GP autoantibodies was calculated as the ratio of the absorption upon ELISA of each mutated chimera to that of the template C2·6 chimera. Results are reported as the mean ± S.D. of the serum's median values. Statistics were calculated using the SPSS software package (Version 9.0). The overall significance of differences in relative binding for M1-M8 was analyzed by repeated measures-analysis of variance. Reduced binding to individual chimeras, relative to the template C2·6 chimera, was tested by one-sample t tests. Possible serum subgroups based on nonparallel binding profiles for M1-M8 were revealed by hierarchical cluster analysis and followed up with independent-sample t tests and Pearson correlations to identify the specific chimeras responsible for significantly divergent profiles. All statistical tests were two-tailed. Significance was inferred when p was <0.05.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES

Homolog-scanning Mutagenesis of the Immunodominant GP Epitope Region EA-- A 15-residue region of alpha 3(IV) NC1, designated EA (residues 17-31), was sufficient to confer reactivity for the immunodominant population of GP autoantibodies (GPA) when substituted into the non-immunoreactive scaffold of the alpha 1(IV) NC1 domain (7, 10). Thus, the EA region encompasses the autoepitope for GPA antibodies. The EA region also contains residues that constitute the epitope for the Mab3 monoclonal antibody, along with other critical residues from the EB region (7). Although GPA and Mab3 antibodies bind to the same EA region, their respective epitopes are distinct because of dissimilar accessibility for antibody binding in the NC1 hexamer complex; this suggests that the EA region comprises buried (or sterically hindered) residues that compose the GPA autoepitope as well as surface-exposed residues that compose the Mab3 epitope (7). Here, homolog-scanning mutagenesis was used to identify which of the eight alpha 3-specific residues within the EA region constitute the epitopes for GPA and Mab3 antibodies.

The C2·6 chimera, which contains the EA and EB regions of the alpha 3(IV) NC1 domain in a scaffold of the alpha 1(IV) NC1 domain (Fig. 2, top), was used as a template for mutagenesis. The EB region was included in the chimera because it, along with the EA region, is required for binding of the Mab3 antibody (7). Moreover, the EB region encompasses residues that constitute the conformational epitope for GPB autoantibodies. Thus, the binding of GPB antibodies can serve as a control for assessment of overall conformation of proteins mutated in the EA region (see below). In this study, each of the eight alpha 3-specific residues of the EA region was substituted with the corresponding alpha 1 residue. The mutated chimeras, designated M1-M8 (Fig. 2, bottom), were then analyzed for their ability to bind GPA and Mab3 antibodies as well as the control GPB antibodies.



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Fig. 2.   Homolog-scanning mutagenesis strategy. The C2·6 chimera (top) is a chimeric alpha 1/alpha 3(IV) NC1 domain that contains the conformational epitopes of the GPA and GPB autoantibodies as well as of the Mab3 monoclonal antibody. It consists of two alpha 3(IV) sequences, the EA region, and the EB region (filled circles) substituted onto the scaffold of the non-immunoreactive alpha 1(IV) NC1 domain (open circles). To investigate the role of the eight alpha 3-specific residues within the EA region, each was mutated to the corresponding alpha 1(IV) residue, and the eight point mutants (designated M1-M8) were analyzed for their ability to bind GPA, GPB, and Mab3 antibodies.

Expression and Correct Folding of the M1-M8 Mutated Chimeras-- The mutated chimeras were expressed in human embryonic kidney 293 cells. After purification from the culture medium by affinity chromatography on an anti-FLAG column, the M1-M8 mutants appeared as a single band on SDS-polyacrylamide gel at the predicted molecular mass of ~25 kDa (Fig. 3A) and showed reactivity with anti-FLAG antibodies on Western blots (Fig. 3B). All mutated chimeras reacted with GPB antibodies as well as or better than the template C2·6 chimera (Fig. 3C), indicating that the mutations did not cause protein misfolding. Moreover, certain chimeras reacted with GPA antibodies, but not with Mab3 antibodies and vice versa (as described in detail below), further establishing that the loss of reactivity after mutagenesis was due to removal of a critical binding site and not to protein misfolding. The epitopes of GPA, GPB, and Mab3 antibodies are conformational and present only in correctly folded protein, as shown by the contrast between the recombinant alpha 3(IV) NC1 domain expressed in human embryonic kidney 293 cells, which is correctly folded, and that expressed in E. coli, which is misfolded (10). The misfolded protein did not react with GPA, GPB, and Mab3 antibodies, although it reacted with monoclonal antibodies that recognize linear epitopes, M3/1 and 175 (Fig. 3D).



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Fig. 3.   Expression and purification of mutated chimeras M1-M8 and assessment of their correct folding. After affinity chromatography on the anti-FLAG column, mutated chimeras M1-M8 were >95% pure and had the expected size (~25 kDa) as judged by SDS-polyacrylamide gel electrophoresis under nonreducing conditions, followed by Coomassie Blue staining (A) or by Western blotting with anti-FLAG antibody M2 (B). Reactivity of the M1-M8 mutants with the GPB autoantibodies, measured by ELISA (100 ng/well), was as high as or higher than that of the template C2·6 chimera (assigned a value of 1.0, indicated by the dotted line), indicating that the mutants were not misfolded (C). The GPA, GPB, and Mab3 epitopes are conformational and recognized by the respective antibodies only on the correctly folded alpha 3(IV) NC1 domain produced in human embryonic kidney 293 cells (black bars), but not on the misfolded protein produced in E. coli (hatched bars) (D). In contrast, monoclonal antibodies M3/1 and 175 recognize linear epitopes and thus bind to both the correctly folded protein and the misfolded protein.

Immunoreactivity of Mutants M1-M8 with GPA Autoantibodies-- To determine which amino acids of the EA region are critical for the GPA autoepitope, the reactivity of the M1-M8 chimeras with sera from 13 GP patients was analyzed by ELISA (Fig. 4). To allow comparison of data across multiple experiments, the reactivity of the mutants was expressed relative to that of the template C2·6 chimera, which constitutes the positive control because it contains the complete EA regions of alpha 3 along with the EB region. Chimera C6, containing the EB but not the EA region of the alpha 3(IV) NC1 domain, was used as a negative control. At least four measurements were performed for each serum. To exclude the interference of EB in the binding studies, EB-depleted GP sera and GP sera with little or no EB reactivity were used.



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Fig. 4.   Immunoreactivity of the M1-M8 mutated chimeras with GPA autoantibodies in direct ELISA. The reactivity with 13 GP sera of the M1-M8 mutated chimeras, relative to that of the template C2·6 chimera (assigned a value of 1.0, indicated by the dotted line), was determined by direct ELISA in microtiter plates coated with 100 ng/well protein. The aggregate binding data are shown as the mean reactivity ± S.D. (A). Among individual sera, there were large variations in the binding to the M3 mutant, which also showed a negative correlation with binding to the M6 mutant (B). The sera were divided into two groups based on the reactivity with the M3 mutant. Group A contained eight sera that had large reductions in reactivity with the M2, M6, and M7 mutants (C). Group B contained five sera that had large reductions in reactivity with the M2, M3, and M7 mutants (D). The reduction in binding was significant at p < 0.05 (*) or p < 0.001 (**).

For the overall group of 13 sera (Fig. 4A), the binding profile for mutated chimeras was characterized by large reductions for M2 (57%; p < 0.001), M6 (44%; p < 0.001), and M7 (67%; p < 0.001) and a small reduction in binding for M1 (16%; p = 0.035). The notably large variability in binding associated with M3 (Fig. 4, A and B) suggested potentially important differences among the binding profiles of individual sera. This was confirmed by a cluster analysis, which revealed two independent serum groups (p = 0.008). The eight sera in group A (Fig. 4C) had large reductions in reactivity with the M2, M6, and M7 mutants (by 56, 57, and 58%, respectively; p < 0.001). Corrected for reactivity of the negative control (the C6 chimera), the respective reductions with M2, M6, and M7 were 83, 84, and 85%. In contrast, the five sera in group B (Fig. 4D) had large reductions in reactivity with mutants M2 (60%), M3 (70%), and M7 (81%) (p < 0.001), along with small reductions with M1 (30%) and M6 (23%) (p < 0.05). Corrected for the reactivity of the negative control, in group B, the respective reductions with M2, M3, and M7 were 75, 85, and 96%. Group A differed significantly from group B regarding M3, M6, and M7 (p < 0.01). That M3 and M6 were singularly responsible for distinguishing the serum subgroups was shown by the large negative correlation between M3 and M6 in the group of 13 sera (r = -0.766; p = 0.002) (Fig. 4B). The reduced reactivity of M2, M3, M6, and M7 was confirmed by inhibition ELISA (Fig. 5). These findings indicate that Ala18, Ile19, Val27, and Pro28 constitute the critical residues of the GPA autoepitope.



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Fig. 5.   Immunoreactivity of the M1-M8 mutated chimeras with GPA autoantibodies in inhibition ELISA. Appropriately diluted GP sera were incubated with mutated (M1-M8) and control NC1 chimeras at concentrations varying between 0.01 and 10 µg/ml. The binding of sera containing inhibitor to plates coated with the C2·6 chimera (100 ng/well) was determined by ELISA and compared with the binding in the absence of inhibitor (considered as 100%). The data are shown for one GP serum from group A (A) and another serum from group B (B).

Immunoreactivity of Mutants M1-M8 with Mab3-- To determine which amino acids in the EA region are critical for the Mab3 epitope, the reactivity of the M1-M8 mutants with Mab3 was determined by Western blotting and ELISA (Fig. 6). Three of the eight mutants, M4, M7, and M8, showed no reactivity by Western blotting and significantly decreased reactivity by ELISA (by 97, 85, and 62%, respectively, relative to the reactivity of the C2·6 chimera; p < 0.005). This indicates that Ser21, Pro28, and Ser31 are critical residues for the Mab3 epitope. With the exception of Pro28, this subset of amino acids is different from that composing the GPA autoepitope, confirming the supposition that the Mab3 and GPA epitopes are different, but colocalized within the 15-residue EA region. Consistent with the location of the Mab3 epitope on the surface of the NC1 hexamer, the two serine residues important for the binding of Mab3 are hydrophilic and thus predicted to be solvent-accessible in folded proteins.



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Fig. 6.   Reactivity of the M1-M8 mutants with the Mab3 monoclonal antibody. The binding of Mab3 to the M1-M8 mutated chimeras was measured by Western blotting (A) and by direct ELISA (B) and was compared with the binding of Mab3 to the template C2·6 chimera. Significant reduction in reactivity (p < 0.001) was observed for mutants M4, M7, and M8 (*).

This result also provided additional evidence that the mutated chimeras were correctly folded, all showing reactivity with GPB and either GPA or Mab3 antibodies, which bind to conformational epitopes. Mutation of Pro18 abolished the interaction with both Mab3 and GPA, suggesting that this proline may be shared by both epitopes. Alternatively, Pro18 could play a structural role, being required for the correct conformation of the EA region within the alpha 3(IV) NC1 domain. The mutation P28I did not seem to affect the overall secondary structure of the NC1 domain because the far-UV circular dichroism spectrum of the M7 mutant was similar to that of the parent C2·6 chimera and to that of the recombinant alpha 3(IV) NC1 domain (data not shown). Moreover, if the P28I mutation had any effect on the conformation, this would be limited to the EA region, as binding of GPB autoantibodies to a neighboring EB epitope was not affected.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES

The inaccessibility of the GPA autoepitope, contrasting with the full accessibility of the Mab3 epitope, afforded an experimental strategy to identify critical residues of these conformational epitopes and to explore the molecular basis of the cryptic nature of the GPA autoepitope. To this end, the 15-residue EA region of the alpha 3(IV) NC1 domain was mutated at eight alpha 3-specific residues, and the resulting chimeras (M1-M8) were assessed for their ability to bind GPA and Mab3 antibodies. The M2 (A18D), M3 (I19D), M6 (V27K), and M7 (P28I) chimeras had greatly decreased binding to GPA antibodies, whereas the M4 (S21Q), M7 (P28I), and M8 (S31H) chimeras had greatly decreased binding to Mab3 antibodies. The loss of binding of GPA antibodies to one set of chimeras and of Mab3 to another, along with binding of GPB antibodies to all chimeras, indicated that the overall conformation of the chimeras did not differ from that of the control (template) C2·6 chimera. Hence, a decrease in antibody binding to a specific chimera was due to removal of a critical epitope residue rather than to misfolding and/or mispairing of disulfide bonds. Thus, the binding profiles indicate that Ala18 and Pro28, along with either Ile19 in certain GP patients or Val27 in other GP patients, are critical for the GPA autoepitope, whereas Ser21, Ser31, and Pro28 are critical for the Mab3 epitope, as depicted in Fig. 7.



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Fig. 7.   Schematic representation of the GPA autoepitope, highlighting quaternary interactions in the NC1 hexamer as the molecular basis of its cryptic nature. Within the EA region of the alpha 3(IV) NC1 domain, hydrophobic residues Ala18, Ile19, Val27, and Pro28 are critical for the GPA epitope (diagonal lines). One or more of these GPA residues are buried within the NC1 hexamer by hydrophobic interactions with residues of other NC1 domains, rendering the epitope cryptic and inaccessible for antibody binding (left). Upon hexamer dissociation, the buried residues are exposed and become accessible for binding (right). In contrast, hydrophilic residues Ser21 and Ser31 as well as Pro28 are critical for the Mab3 epitope (solid black), which also includes some residues from the EB region. These residues are exposed on the surface of the NC1 hexamers and monomers, thus accessible for Mab3 binding. Pro28 is critical for both GPA and Mab3 epitopes, and it is possibly important for the native conformation of the EA region.

Interestingly, three of the four GPA residues (Ala18, Ile19, and Val27) are hydrophobic (14, 15) and have a high propensity to be buried (16). This suggests that the cryptic nature of the GPA autoepitope is because one or more GPA residues participate in hydrophobic interactions with other NC1 domains in the hexamer complex, thus burying the epitope and rendering it inaccessible for binding of autoantibodies (Fig. 7). In contrast, the Mab3 epitope is accessible because it contains critical residues that have a high propensity to be located on the hexamer surface (16): two hydrophilic serine residues and one proline residue, which is most often found in beta -turns and other loops on the protein surface. Interestingly, Pro28 is critical for both GPA and Mab3 epitopes; thus, it may be important in defining the conformation of the EA region in the alpha 3(IV) NC1 domain.

The hydrophobic character of the GPA residues represents a departure from the conventional features of epitopes, which consist predominately of charged or polar residues and tend to be located in flexible turns or loops on protein surfaces (17, 18). Based on these features, numerous empirical methods have been developed that successfully predict the major antigenic sites of native proteins (19). The alpha 3(IV) NC1 sequence was analyzed with the program Epiplot, which calculates and plots flexibility, hydrophilicity, and antigenicity profiles using 13 different scales, chosen as those yielding the best predictions on proteins whose antigenic structures are known (20). None of these methods predicted the location of the GPA autoepitope or the Mab3 epitope, which reside within the amphipathic EA region of the alpha 3(IV) NC1 domain, as illustrated by the popular Hopp-Woods hydrophilicity plot in Fig. 8 (upper) (21). Thus, other factors must govern the immunogenicity of the EA region, eliciting the production of autoantibodies to GPA residues on one hand and of antibodies to Mab3 on the other.



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Fig. 8.   Structural features of the EA region. Upper, the hydrophilicity of the alpha 3(IV) NC1 domain was calculated and plotted according to the Hopp-Woods scale, using a window of six residues. The peaks indicate the most hydrophilic regions of a protein, located on the surface and predicted to be the major antigenic sites. The residues that compose the cryptic GPA epitope are located in an amphipathic region. Lower, comparison of the amino acid sequences of the six alpha (IV) NC1 domains in the EA region. The alpha 3 amino acids critical for binding of GPA autoantibodies are boxed, and those critical for Mab3 binding are encircled. Notice the high sequence divergence among the six chains at these positions, including numerous non-conservative substitutions. The amino acids conserved between alpha 3 and the other chains (shaded) are likely to play a structural role.

In particular, the lack of immune tolerance to the GPA epitope may be explained by its cryptic nature, which renders it an immunologically privileged site. To avoid autoreactivity, the B cell clones directed to self-antigens are edited out early in development, establishing self-tolerance. Because the GPA epitope is buried in the NC1 hexamer complex under normal physiological conditions, it is sequestered from the immune system. Therefore, if pathogenic factors induce hexamer dissociation, the newly exposed GPA residues would then be perceived as "foreign" by the immune system, eliciting an autoimmune response. What factors trigger this process in vivo remain unclear. Hydrocarbons or viral infections have been suggested as causative agents (22). A recent study provides evidence that reactive oxygen species may act as the physiological mediator for epitope exposure (23).

Certain of the GPA residues identified herein differ from those found in two recent studies by Wieslander and co-workers (11, 24), who did not identify the overall hydrophobic character of the epitope. The first study qualitatively investigated the role of 14 alpha 3(IV) NC1 domain residues, including six of the eight residues in the EA region (11). Similar to our findings, Ile19 and Pro28, but not Thr17, were found important for binding. However, Ala18 was reported to be not essential, and the role of Glu24 and Val27 was not addressed. In addition, Ser21 and Ser31 were reported to be critical for binding GP antibodies, whereas we found these residues to be important only for the binding of the Mab3 antibody. Since a quantitative data analysis was not reported in that study, it is difficult to directly compare their findings with ours. A very recent paper from the same group (24) quantitatively analyzed the role of four alpha 3 residues in the EA region and found Val27 to be critical for GP antibody binding (similar to our findings), whereas Thr17, Ala18, and Glu24 had a moderate effect. Some of the discrepancies may be explained by the different alpha 1/alpha 3 chimeras used as template for the homolog-scanning mutagenesis. Their template chimera contained only the EA region, whereas ours contained both the EA and EB regions of the alpha 3(IV) NC1 domain. Because the EA and EB regions are in close proximity in the natively folded alpha 3(IV) NC1 domain, mutants M1-M8 used in our work are likely to reproduce the native GP epitopes more closely. In addition, our study relied on a mutagenesis strategy that allowed (a) a verification of the conformation of the mutants by using three different antibodies that recognize conformational epitopes and (b) a comparison between the binding of GPA and Mab3 antibodies, a priori inferred to bind to different residues in the EA region.

The identification of the critical residues of the GPA epitope allows the determination of the structural features that selectively target GPA antibodies to the alpha 3(IV) NC1 domain, among the six homologous NC1 domains of type IV collagen. As revealed by the comparison in Fig. 8 (lower), all four GPA residues (Ala18, Ile19, Val27, and Pro28) occur at the respective position in the alpha 3 sequence only. Intriguingly, three of the GPA residues are hydrophobic in the alpha 3 sequence, but the homologous alpha 1 residues are hydrophilic, charged residues (Asp, Asp, and Lys, respectively). Analysis of data from experimentally determined antigenic sites on proteins has revealed that hydrophobic residues are more likely to be a part of antigenic sites, if they occur on the surface of a protein (25), as GPA residues do in the alpha 3(IV) NC1 monomer. No other chain besides alpha 3(IV) had a constellation of three hydrophobic residues at positions 18, 19, and 27. Moreover, Pro28 occurs only in alpha 3, but not in any of the other five NC1 domains. Therefore, three hydrophobic residues at positions 18, 19, and 27, together with a proline at position 28, in a distinct conformation distinguish alpha 3 among the six NC1 domains, conferring binding of GPA antibodies selectively to the alpha 3(IV) NC1 domain.

The EA region of the alpha 3(IV) NC1 domain emerges as a prime candidate for a molecular recognition site that specifies the chain-specific assembly of the alpha alpha alpha 5 network of type IV collagen. Recently, we showed that the NC1 monomers contain recognition sequences for selection of chains and protomers that are sufficient to encode the specificity of assembly of the alpha alpha 2 and alpha alpha alpha 5(IV) networks of the glomerular basement membrane (26), but their identity is unknown. That the EA region is a site of interaction between alpha 3 and the other NC1 domains in the alpha alpha alpha 5 hexamer is deduced from its cryptic nature of the GPA epitope. That the EA region may also be responsible for the specificity of interaction is suggested by the high sequence divergence of this region among the six NC1 domains (27). The EA region is also distinguished by the highest number of non-conservative amino acid substitutions. Hence, the pattern of hydrophobic and hydrophilic residues within the EA region is unique for each of the six NC1 domains and is likely to confer chain-specific conformations and interactions. In particular, the EA region of all six NC1 domains may contribute to the discriminatory interactions that result in specific assembly of chain-specific networks of type IV collagen.


    FOOTNOTES

* This work was supported by Grant DK18381 from the National Institutes of Health (to B. G. H.); by Grant 9920539Z from the American Heart Association, Kansas Affiliate (to D.-B. B.); and by the David Flinchbaugh Memorial Fund.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

§ The first two authors contributed equally to this work.

|| To whom correspondence should be addressed. Tel.: 913-588-7008; Fax: 913-588-7035; E-mail: bhudson@kumc.edu.

Published, JBC Papers in Press, November 29, 2000, DOI 10.1074/jbc.M008956200


    ABBREVIATIONS

The abbreviations used are: GP, Goodpasture; ELISA, enzyme-linked immunosorbent assay.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES


1. Butkowski, R. J., Langeveld, J. P., Wieslander, J., Hamilton, J., and Hudson, B. G. (1987) J. Biol. Chem. 262, 7874-7877[Abstract/Free Full Text]
2. Saus, J., Wieslander, J., Langeveld, J. P., Quinones, S., and Hudson, B. G. (1988) J. Biol. Chem. 263, 13374-13380[Abstract/Free Full Text]
3. Hudson, B. G., Reeders, S. T., and Tryggvason, K. (1993) J. Biol. Chem. 268, 26033-26036[Free Full Text]
4. Gunwar, S., Ballester, F., Noelken, M. E., Sado, Y., Ninomiya, Y., and Hudson, B. G. (1998) J. Biol. Chem. 273, 8767-8775[Abstract/Free Full Text]
5. Wieslander, J., Langeveld, J., Butkowski, R., Jodlowski, M., Noelken, M., and Hudson, B. G. (1985) J. Biol. Chem. 260, 8564-8570[Abstract/Free Full Text]
6. Kalluri, R., Sun, M. J., Hudson, B. G., and Neilson, E. G. (1996) J. Biol. Chem. 271, 9062-9068[Abstract/Free Full Text]
7. Borza, D.-B., Netzer, K. O., Leinonen, A., Todd, P., Cervera, J., Saus, J., and Hudson, B. G. (2000) J. Biol. Chem. 275, 6030-6037[Abstract/Free Full Text]
8. Ryan, J. J., Mason, P. J., Pusey, C. D., and Turner, N. (1998) Clin. Exp. Immunol. 113, 17-27[CrossRef][Medline] [Order article via Infotrieve]
9. Hellmark, T., Segelmark, M., Unger, C., Burkhardt, H., Saus, J., and Wieslander, J. (1999) Kidney Int. 55, 936-944[CrossRef][Medline] [Order article via Infotrieve]
10. Netzer, K. O., Leinonen, A., Boutaud, A., Borza, D.-B., Todd, P., Gunwar, S., Langeveld, J. P., and Hudson, B. G. (1999) J. Biol. Chem. 274, 11267-11274[Abstract/Free Full Text]
11. Hellmark, T., Burkhardt, H., and Wieslander, J. (1999) J. Biol. Chem. 274, 25862-25868[Abstract/Free Full Text]
12. Leinonen, A., Netzer, K. O., Boutaud, A., Gunwar, S., and Hudson, B. G. (1999) Kidney Int. 55, 926-935[CrossRef][Medline] [Order article via Infotrieve]
13. Penades, J. R., Bernal, D., Revert, F., Johansson, C., Fresquet, V. J., Cervera, J., Wieslander, J., Quinones, S., and Saus, J. (1995) Eur. J. Biochem. 229, 754-760[Abstract]
14. Kyte, J., and Doolittle, R. F. (1982) J. Mol. Biol. 157, 105-132[Medline] [Order article via Infotrieve]
15. Rose, G. D., Geselowitz, A. R., Lesser, G. J., Lee, R. H., and Zehfus, M. H. (1985) Science 229, 834-838[Medline] [Order article via Infotrieve]
16. Lesser, G. J., and Rose, G. D. (1990) Proteins Struct. Funct. Genet. 8, 6-13[Medline] [Order article via Infotrieve]
17. Barlow, D. J., Edwards, M. S., and Thornton, J. M. (1986) Nature 322, 747-748[Medline] [Order article via Infotrieve]
18. Hopp, T. P. (1993) Pept. Res. 6, 183-190[Medline] [Order article via Infotrieve]
19. Stern, P. S. (1991) Trends Biotechnol. 9, 163-169[Medline] [Order article via Infotrieve]
20. Menendez-Arias, L., and Rodriguez, R. (1990) Comput. Appl. Biosci. 6, 101-105[Abstract]
21. Hopp, T. P., and Woods, K. R. (1981) Proc. Natl. Acad. Sci. U. S. A. 78, 3824-3828[Abstract]
22. Wilson, C. B., Borza, D.-B., and Hudson, B. G. (2000) in The Molecular Pathology of Autoimmune Diseases (Theofilopoulos, A. N. , and Bona, A. C., eds), 2nd Ed. , pp. 981-1010, Gordon and Breach Science Publishers, Inc./Harwood Academic Publishers, Newark, NJ
23. Kalluri, R., Cantley, L. G., Kerjaschki, D., and Neilson, E. G. (2000) J. Biol. Chem. 275, 20027-20032[Abstract/Free Full Text]
24. Gunnarsson, A., Hellmark, T., and Wieslander, J. (2000) J. Biol. Chem. 275, 30844-30848[Abstract/Free Full Text]
25. Kolaskar, A. S., and Tongaonkar, P. C. (1990) FEBS Lett. 276, 172-174[CrossRef][Medline] [Order article via Infotrieve]
26. Boutaud, A., Borza, D.-B., Bondar, O., Gunwar, S., Netzer, K. O., Singh, N., Ninomiya, Y., Sado, Y., Noelken, M. E., and Hudson, B. G. (2000) J. Biol. Chem. 275, 30716-30724[Abstract/Free Full Text]
27. Netzer, K. O., Suzuki, K., Itoh, Y., Hudson, B. G., and Khalifah, R. G. (1998) Protein Sci. 7, 1340-1351[Abstract/Free Full Text]


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