Introduction
Autoantibodies directed against myelin antigens have been a long-standing focus of interest in multiple sclerosis (MS) research, especially those binding to myelin oligodendrocyte glycoprotein (MOG). MOG is predominantly expressed in the CNS, and is exposed on the outermost lamellae of the myelin sheath thus readily available for a humoral immune attack [
1]. MOG induces demyelinating experimental allergic encephalomyelitis (EAE), the animal model of MS, in a variety of species both by active immunization and by passively transferred anti-MOG antibodies (reviewed by [
1,
2]). Only those anti-MOG antibodies directed against conformational epitopes, as opposed to linear epitopes, appear to be pathogenic in EAE [
3‐
5]. Recently, it was shown that the murine monoclonal antibody (mAb) 8.18.c5 specific for rat MOG, that confers demyelination, maps to a discontinuous epitope of the surface exposed FG loop of rat MOG, that is also exposed on murine and human MOG [
6].
To date, the measurement of serum anti-MOG antibodies using various techniques and different MOG preparations has resulted in inconsistent results and limited reproducibility (reviewed by [
7,
8]). This study was thus designed to assess the MOG epitope usage in humans employing a novel quantitative high-throughput ELISA. The serum anti-MOG antibody responses of 325 patients with MS, 69 patients with a first demyelinating event (clinically isolated syndrome, CIS) and 164 healthy controls (HC) were assayed. Three isoforms of recombinant MOG were generated and the differential exposure of immunodominant epitopes characterized by a panel of monoclonal anti-MOG antibodies. Restricted patterns of anti-MOG reactivity could be observed in samples with sustained anti-MOG reactivity at high serum dilutions, defining high-titer reactivity. In this cohort we find that anti-MOG antibody levels strongly correlate with disease severity.
Materials and methods
Antigens and Antibodies
Three recombinant human MOG isoforms were used for the study. The first, spanning the extracellular domain, amino acids 1-125, (rhMOG
125) was expressed and purified under physiological conditions as described previously [
9]. Secondly, a seven amino acids shorter rhMOG protein, spanning the amino acids 1-118 (rhMOG
118) was created by usage of a different 3'-end primer: 5'-ATCCATGAGATCTAGGATCTTCTACTTTCAATTCCATTGCTGCC-3', and was expressed and purified as above. Finally, recombinant rat MOG, amino acids 1-125 (ratMOG
125) was produced in E. coli and purified as described previously [
10]. Purity was confirmed to be > 95% by SDS-PAGE (additional file
1) and correct folding ascertained by circular dichroism (additional file
1) [
11,
12].
The murine monoclonal IgG 8.18C5 against native rat MOG was a gift of Dr. Chris Linington [
13]. The marmoset Fab-fragments (Fabs) designated M26, M3-24, and M3-8 derived from a ratMOG
125-immune animal were generated in our laboratory as described previously [
10].
Patients
325 MS patients meeting the diagnostic criteria for clinically definite MS [
14,
15], and 69 patients with a first demyelinating were recruited for this study [
15]. 36% of the MS patients were treated with either interferon beta or glatiramer acetate at the time of sampling. Patients treated with glucocorticoids within three months or on immunosuppressive therapy within six months of phlebotomy were excluded. 164 volunteers served as healthy controls (HC). Informed consent was obtained from all subjects, and the study was conducted in accordance with Institutional Review Board approval. The clinical characteristics of the patients and HC are summarized in the table contained in additional file
2. Blood was drawn by venipuncture and clotted serum stored at -40°C.
High-throughput ELISA
Optimal protein concentrations for coating were determined in preliminary experiments; 0.5 μg of rhMOG125 and rhMOG118 and 1.0 μg of ratMOG125 were coated in PBS overnight on a single 384-well microtiter assay plate (Maxisorb, Nunc, Rochester, NY). Control wells were coated with 1.0 μg BSA. To quantify the antibody reactivity, an IgG standard curve was created by coating human IgG (I4506, Sigma, St. Louis, MO) in two-fold dilutions. After washing, plates were blocked for 2 hours with 1% BSA in PBS supplemented with 0.05% Tween 20. Then, human sera were added at three dilutions starting at 1/200, and incubated for 90 min. A positive control known to be reactive to all three rMOG isoforms, and a negative control omitting serum were included. Bound antibodies were detected by an alkaline phosphatase-labeled anti-human IgG (A9544, Sigma) and the optical density (OD) read at 405 nm wavelength in a microplate reader (SpectraMax, Molecular Devices, Sunnyvale, CA) after 30 minute incubation with para-nitrophenyl phosphate (Moss, Pasadena, MD). All samples were tested coded in a blinded fashion in duplicates, standard curves in quadruplicates; incubation was at room temperature (RT), except coating which was at 4°C. Sample handling and ELISA procedures were performed by a robotic workstation (Biomek FX, Beckman Coulter, Fullerton, CA). MOG-ELISA for the monoclonal reagents (8.18c5 and Fab fragments) were performed in 96-well Maxisorb plates according to the protocol as outlined above. 8.18c5 was detected by an anti-mouse IgG (A9044, Sigma) and the Fabs by Protein L (Pierce). Both were peroxidase labeled and developed by 3,3',5,5'-tetramethylbenzidine (Pierce) and the OD read at 450 nm wavelength.
Denaturing anti-MOG ELISA
To further demonstrate, that conformational MOG-epitopes are conserved in regular ELISA assays as described above, we compared binding of anti-MOG antibodies to recombinant MOG antigens coated under native (previous section) and denaturing conditions. The rMOG isoforms were coated as outlined above in 96-well plates. After washing coated MOG was denatured by incubation with 8 M urea supplemented with 10 mM dithiothreitol (DTT) for 4 hours at RT followed by addition of 25 mM 2-iodoacetamide (IDAA) for fixation. After overnight incubation at RT and extensive washing wells were blocked as above, subjected to either 8.18c5 or anti-His mAb (Invitrogen, Carlsbad, CA), and binding was detected as above.
Data processing and statistical analysis
The ODs were corrected for the individual background binding and the amount of specific IgG bound to the well interpolated from the on-plate standard curve. Additionally, results were expressed as the signal-to-background binding ratio (BR), calculated as the ratio of OD signal/background where applicable, as were results for 8.18c5 and the Fabs.
For the assessment of high-titer reactivity, samples above to 95th percentile of IgG concentrations were tested in a separate experiment in serial dilutions, those with BRs ≥ 2 at 1/3,200 dilution (i.e. signals against MOG that were at least two-fold above the BSA signal) were identified and their mean BR at 1/800 dilution defined as the cut-off for high-titer reactivity. This procedure was done for all three MOG isoforms. This procedure is one of the rigorous methods accepted for defining subtype cut-offs with stringent criteria.
Statistical analysis was conducted using Prism 4.0 (GraphPad, San Diego, CA). Categorical variables were compared using the χ2-test, continuous variables using ANOVA and ordinal as well as not normally distributed continuous variables by Kruskal-Wallis testing. The Student-Newman-Keuls method and Dunn's test were used to determine differences in between groups.
Discussion
Numerous studies have attempted to assess the presence and clinical implications of serum antibodies against MOG in MS reporting a wide range for the prevalence of anti-MOG antibodies (0% - 78%) (reviewed in [
7,
8]). The reason for this variability are likely differences in patient populations, assay techniques (e.g., Western blotting, ELISA, RIA), and MOG antigen preparations used [
7,
16]. Furthermore, an important factor to be taken into consideration is the biophysical environment of MOG in the respective assay system. In particular, denaturing conditions may render proteins linearized and thus inaccessible to pathogenic antibodies targeting conformation-dependent epitopes.
The approach presented here addresses two critical criteria: the diversity of the conformational MOG epitope repertoire at the molecular level, and implementation of a fully quantitative assay of the MOG-directed antibody response. In a preliminary experiment we demonstrated that in our ELISA assay antigen coating does not denature MOG (Figure
1), but rather harsh chemical forces would be necessary to fully linearize MOG rendering it inaccessible for antibodies specific for conformational MOG epitopes. We are thus confident that potentially pathogenic conformational anti-MOG antibodies can be detected by our ELISA. We are therefore able to stratify our analyses according to parameters of potential pathological relevance, an essential step towards meaningful interpretation of serum antibodies. We and others have previously shown that single target antigen ELISA may not sufficiently discriminate between pathogenic anti-MOG antibodies, naturally occurring low-affinity autoantibodies or non-pathogenic antibodies directed against linear MOG epitopes [
3,
9,
17]. To further emphasize the importance of antibody epitope characterization, we have recently reported in CIS patients a high prevalence of antibodies against native, membrane-embedded MOG that do not fully cross-react with recombinant MOG preparations as demonstrated by cytometry and competition assays [
18].
The most relevant finding of this study was derived from 8.2% of samples that were identified as high-titer reactive. In these samples, we set out to further characterize the diversity of the human serum anti-MOG antibody repertoire with respect to binding to conformational epitopes that was otherwise masked in the entire cohort predominantly consisting of low-titer anti-MOG reactive antibodies (table
1 Figure
3). At least three immunodominant epitopes became apparent, distinctly exposed on three different recombinant MOG "isoforms". These isoforms display only minor differences in terms of amino acid sequence (table
1) and have no other purpose than to rigorously define specific epitopes. These findings of distinct epitope exposure were corroborated using a panel of differentially reactive anti-MOG monoclonal agents (Figure
2) [
10]. Importantly, we present data (Figure
1) and have previously shown that relevant conformational MOG epitopes are preserved in ELISA systems [
3,
10]. Only after isolating high-titer samples, a strong association of circulating anti-MOG IgG with disability in RR-MS patients could be unmasked (Figure
4). This was achieved by combining the antibody reactivity against all identified putative target epitopes. RatMOG
125 was included in our panel of MOG antigens in order to assess the reactivity against an ubiquitously exposed common epitope, and because of a previously demonstrated cross-reactivity between ratMOG
125 and human anti-MOG IgG [
10]. The proportion of high-titer samples appears low at first sight; it is however, quite consistent with recent findings of the frequencies of MOG-reactive T cells in the peripheral blood of MS patients, that ranged between approximately 2-4 percent of T cells [
19,
20]. Interestingly and complementing our data, the frequency of these specific T cells correlated to disease activity and disability [
19].
It was an unexpected finding that reactivity to epitopes conserved between species, such as those defined by 8.18c5 or M26 was not predominant in MS high-titer samples (Figure
2A+B, table
1). Rather, specific reactivity appeared to be exclusively directed against restricted epitopes of human MOG (table
1). In contrast, humoral responses in rodent EAE are predominantly directed against the 8.18c5 defined epitope [
6].
This diversity in epitope recognition may explain why previous studies employing different MOG preparations have generated such divergent results [
7,
16]. Indeed, only one previous study has reported a comparable clinical correlation in 262 patients with MS of which 14% were deemed anti-MOG positive by comparison to the HC samples [
21]. Most other studies have tested considerably smaller numbers of MS patients, and may have not achieved sufficient statistical power to reveal correlations with clinical parameters. To put in perspective, in Type I diabetes mellitus, large sample numbers of 1,300 to over 4,000 per study were needed to identify the prognostic value of autoantibodies in offspring of diabetic patients, with less than 5% of samples showing high-titer reactivity against the respective antigens [
22].
We are aware, that our study does not sufficiently address the issue of specificity that has lacked in previous assays too [
7,
8], as exemplified by similar overall frequencies of high-titer reactivity in HC compared to MS and CIS (Figure
3; table
1). However, it is striking, that high-titer anti-MOG reactivity in HC includes epitopes of rodent MOG, while that in MS and CIS patients does not. While our study was clearly not designed to prove pathogenicity of anti-MOG antibodies as shown previously by passive transfer experiments in animals [
3,
23], our findings do suggest, that potentially pathogenic anti-MOG antibody responses are highly specific for certain epitopes of human MOG. In this context it was recently shown that demyelination is augmented in an animal model of virally induced mild demyelination if the animals are engineered to produce anti-MOG IgG prior to infection [
24]. Thus, it is intriguing to speculate that anti-MOG antibodies in HC render the same pathogenic potential as in MS patients, but HC individuals have not encountered the pathogenetically relevant viral infection. Furthermore, we cannot rule out the possibility, that the high-titer anti-MOG response occurs secondary to acquired myelin destruction and may as such reflect the magnitude of clinical disability [
25].
In summary, we demonstrate for the first time that target epitopes of autoantibody responses are differentially exposed and that the immune response in humans is restricted to distinct epitopes but not identical in all patients and certainly not identical to rodent EAE. Only in a fraction of samples a strong and sustained anti-MOG response can be detected, deemed high-titer reactivity. We report that in RR-MS patients with such reactivity the amount of anti-MOG antibodies are correlated with disability. Our findings bear significant clinical relevance that may have been overlooked in previous studies due to smaller sample sizes. This study supports the concept that establishing certain serum autoantibodies as predictive biomarkers in MS may be possible.
Acknowledgements
T.M. and P.H.L. were postdoctoral research fellows of the National Multiple Sclerosis Society.
We are indebted to Ishita Barman for recombinant MOG preparation and to Drew Dover and Lev Igoudin for performing the assay. Dr. Win Than helped with the denaturing ELISA assay. Lars Lüers oversaw the CD-spectroscopy at the biophysical research core facility in Düsseldorf.
We thank the UCSF MS Center staff and neurologists, and Dr. Pablo Villoslada (University of Navarra, Spain) for sample collection and extended clinical examination.
This work has been supported by grants from the National Institutes of Health (NS4678-01 to CPG); the National Multiple Sclerosis Society (RG3370-A-3 and 3438-A-7 to CPG); EMD-Serono; the Cure MS Now fund, the Lunardi fund and the Nancy Davis Center Without Walls.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
TM designed the study, performed the experiments, analyzed data, wrote the paper. PHL analyzed data, wrote the paper and performed experiments. H-CvB performed experiments, wrote the paper and reviewed the manuscript. CPG wrote the paper and reviewed the manuscript. TM and CPG had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. All authors read and approved the final manuscript.