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Published online before print October 23, 2006
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,1
* IZKF Research Group 2, Nikolaus-Fiebiger-Center of Molecular Medicine, University of Erlangen-Nürnberg, Erlangen, Germany;
Department Chemistry, Technical University Munich, Munich, Germany; and
Department of Internal Medicine 3, University Hospital Erlangen, Erlangen, Germany
1Correspondence: IZKF Research Group 2, Nikolaus-Fiebiger-Center, Glückstrasse 6, 91054 Erlangen, Germany. E-mail: rvoll{at}molmed.uni-erlangen.de
ABSTRACT
High mobility group box 1 protein (HMGB1) is a ubiquitously expressed architectural chromosomal protein. Recently, it has become obvious that HMGB1 can also act as a proinflammatory mediator when actively secreted during cell activation or passively released from necrotic cells. HMGB1 appears to play an important role in the pathogenesis of diseases, including sepsis and rheumatoid arthritis. However, easy, sensitive, and reliable detection systems are required to investigate the clinical significance of HMGB1 in clinical samples for diagnosis and prognosis of diseases. Here, we describe sensitive ELISAs for the detection of HMGB1 in cell culture medium and cell lysates. However, these assays failed to reliably quantitate HMGB1 in serum and plasma when compared with immunoblot analysis. We found that serum/plasma components bind to HMGB1 and interfere with its detection by ELISA systems. In most serum/plasma samples investigated, including those from healthy individuals, we detected IgG antibodies binding to HMGB1. The titers of these antibodies correlated with the capacity of sera to interfere with the detection of recombinant HMGB1 by ELISA. Furthermore, HMGB1 coimmunoprecipitated with several proteins including IgG1, as identified by mass spectrometry. These HMGB1 interacting proteins are currently characterized and may contribute to complex formation, masking, and possibly, modulation of cytokine activity of HMGB1.
Key Words: autoantibodies • ELISA • systemic lupus erythematosus • rheumatoid arthritis
INTRODUCTION
High mobility group box 1 protein (HMGB1) is a ubiquitously expressed, highly conserved chromosomal protein. It consists of two DNA-binding domains, the HMG box A and B and a highly, negatively charged C-terminal domain, containing 30 repetitive aspartic and glutamic acid residues [1 ]. As a nuclear protein, HMGB1 binds to double-stranded, single-stranded, and distorted DNA without sequence specificity [2 , 3 ]. It stabilizes the nucleosomal structure and mediates bending of the DNA, thereby facilitating binding of certain transcription factors including steroid hormone receptors [4 ].
Recent studies of endotoxemia and sepsis identified extracellular HMGB1 as a potent, proinflammatory cytokine. HMGB1 is actively secreted from LPS- or TNF-activated macrophages/monocytes, pituicytes, and other cells [5 6 7 8 ]. In addition, HMGB1 is released passively by damaged or necrotic cells.
Extracellular HMGB1 can bind to cell surface receptors such as the receptor for advanced glycation end products, TLR2 and -4, and possibly, to as-yet unknown receptors [4
, 9
]. Released HMGB1 by itself acts as a strong mediator of macrophage activation and induces secretion of proinflammatory cytokines such as TNF-
, IL-1ß, IL-6, IL-8, MIP-1, and MIP-2ß. In addition, HMGB1, released from necrotic cells, induces up-regulation of activation markers (HLA-DR, CD83, CD80, and CD86) on dendritic cells [10
]. In contrast, apoptotic cells retain HMGB1, which is attached tightly to hypoacetylated chromatin. Therefore, HMGB1 is not released from apoptotic cells and does not induce inflammation [11
].
Extracellular HMGB1 has been detected in various pathological conditions. HMGB1 levels are increased markedly during severe sepsis in the blood of humans and animals. It is important that administration of neutralizing HMGB1-specific antibodies prevented lethality from established sepsis in animals [5 ]. Moreover, serum HMGB1 levels were significantly higher in septic patients who did not survive [5 ]. Hemorrhagic shock is also associated with significantly increased serum levels of HMGB1, even in the absence of infection and endotoxemia [12 ]. Intratracheal administration of HMGB1 into mice induces acute lung inflammation with accumulation of neutrophils, edema, and production of proinflammatory cytokines [13 ]. Furthermore, HMGB1 can be detected in the serum and alveolar fluid after pulmonal injury of humans and mice. Expression of HMGB1 is up-regulated in almost every tumor type and is particularly increased in epithelial neoplasms. Increased serum levels of HMGB1 in cancer patients are associated with a worse prognosis [14 ].
HMGB1 is also a mediator of chronic inflammatory diseases. The presence of cytoplasmic and extracellular HMGB1 has been reported in experimental arthritis models as well as in human rheumatoid arthritis (RA). Elevated levels of HMGB1 are also present in synovial fluid from RA patients [15 , 16 ]. Systemic application of an antagonistic A box domain or neutralizing HMGB1-specific antibodies ameliorated collagen-induced arthritis in rodents [15 ].
Anti-HMGB1 antibodies were detected in plasma of patients with several autoimmune diseases such as systemic sclerosis, juvenile idiopathic arthritis, ulcerative colitis, and systemic lupus erythematosus (SLE); however, the clinical relevance of these findings remains to be elucidated [17 18 19 ].
All these data indicate that HMGB1 might be a diagnostic marker, a potent mediator, and a therapeutic target in several inflammatory, autoimmune, and malignant diseases. However, so far, detection of HMGB1 is usually performed by time-consuming, semiquantitative Western blotting. An easy, sensitive, and reliable assay for quantification of HMGB1 in different biological fluids would be extremely helpful for further clinical investigations. Here, we describe serum/plasma factors that interfere with the detection of HMGB1 by our sandwich ELISAs, precluding the quantification of circulating HMGB1.
MATERIALS AND METHODS
Cell culture
Cell culture medium and supplements were obtained from Invitrogen Life Technologies (Karlsruhe, Germany). All cells were grown in a 5% saturated CO2 atmosphere at 37°C. The human T cell lymphoblast-like cell line Jurkat (TIB-152) was grown in complete RPMI medium (RPMI 1640 containing 10% FBS, 2 mM L-glutamine, 20 mM Hepes, 1 mM sodium pyruvate, 100 U/ml penicillin, and 100 µg/ml streptomycin). PBMCs were isolated from heparinized peripheral blood from healthy donors by Ficoll density gradient centrifugation (Lymphoflot, Biotest, Germany). After three washes with PBS, the cells were resuspended at 2 x 106 cells/ml in complete DMEM, and 1 ml of the cell suspension was added per well of 48-well plates. After incubation for 2 h, the cell layer was washed twice with PBS to remove nonadherent PBL. The adherent monocytes were cultured in DMEM containing 20% autologous serum plus 100 U/ml penicillin, 100 µg/ml streptomycin, and 2 mM L-glutamine to generate human monocyte-derived macrophages (HMDM). After 7 days, medium was replaced with serum-free OPTI-MEM. HMDM were stimulated with different concentrations of LPS or IFN-
. Cell culture supernatants were harvested after 24 h and were kept at –20°C or analyzed directly by ELISA.
Blood samples
Venous blood was collected from healthy donors and patients with SLE or RA into EDTA-containing tubes (Monovette, Sarstedt, Nümbrecht, Germany) for collecting plasma or into tubes without anticoagulants for serum (Monovette). Centrifugation was carried out at 2000 g for 30 min at 4°C. All samples were stored at –20°C.
Preparation of cell lysates
Cell lysates were prepared by homogenization of 1 x 107 Jurkat cells in 500 µl radioimmunoprecipitation assay (RIPA) buffer (10 mM sodium phosphate, 150 mM NaCl, 2 mM EDTA, 0.1% SDS, 1% sodium deoxycholate, 1% Nonidet P-40, 1 mM NaF, pH 7.5). Cell debris was removed by centrifugation. Protein concentrations were determined with a bicinchoninic acid protein assay (Pierce, Rockford, IL). Whole cell lysates were stored at –20°C or used directly for ELISA and Western blot.
Sandwich ELISA for HMGB1
To detect HMGB1 in cell culture medium and serum/plasma samples, an ELISA was performed. Maxisorp polystyrene 96-well plates (Nunc, Roskilde, Denmark) were coated with one of the following anti-HMGB1 capture antibodies: 0.75 µg/ml antibody (Cat. No. 556528, BD PharMingen, San Diego, CA; supply has been discontinued), 1.5 µg/ml antibody (Cat. No. 07-584, Upstate Biotechnology, Lake Placid, NY), 1.5 µg/ml antibody (Cat. No. sc-12523, Santa Cruz Biotechnologies, CA), and 0.75 µg/ml mAb (Cat. No. ab12029-100, Abcam, Cambridge, UK), diluted in PBS overnight at 4°C. The wells were blocked for 2 h with 1% BSA (Roth, Karlsruhe, Germany) in PBS at room temperature. Prior to adding the samples, plates were washed with PBS containing 0.05% Tween 20 (the same washing buffer was used for each washing step). Plasma/serum samples from SLE patients and healthy donors (1:2 or 1:4 dilutions in PBS) and HMGB1 standards were added to each well (100 µl/well) in duplicates. To establish the standard curve, recombinant (r)HMGB1 (Sigma Chemical Co., St. Louis, MO) was serially diluted in blocking buffer (1% BSA in PBS) to the following concentrations: 300, 100, 33.33, 11.11, 3.70, 1.23, and 0.41 ng/ml. After overnight incubation, the plates were washed, and 100 µl biotinylated anti-HMGB1 antibody (Cat. No. MAB1690, R&D Systems, Minneapolis, MN), diluted to the final concentration of 0.75 µg/ml in blocking buffer, was added and incubated for 1 h. After washing, 1:2000 dilution of streptavidin-alkaline phosphatase conjugate (Amersham Biosciences, Freiburg, Germany) in blocking buffer was added and incubated for 45 min. After a final washing, detection was achieved through addition of substrate solution: 3 mM 4-nitrophenyl phosphate (Serva Electrophoresis, Heidelberg, Germany) in substrate buffer (9.7% diethanolamine/HCl, pH 9.8, 0.1% sodium azide, 5 mM magnesium chloride). The absorbance at 405 nm was measured using a microplate spectrophotometer SPECTRA max 190 (Molecular Devices Corp., Boston, MA). Concentrations of HMGB1 in the samples were calculated using Softmax software.
Biotinylation of the detecting antibody was performed using EZ-Link Sulfo-NHS-LC-biotin reagent (Pierce) following the manufacturer’s instructions.
ELISA for anti-HMGB1 antibodies
Autoantibodies directed to HMGB1 were assayed by ELISA as described by Sobajima et al. [19
]. Briefly, Maxisorp polystyrene 96-well plates were coated with 50 µl per well rHMGB1 at 1 µg/ml in PBS and incubated overnight at 4°C. The plates were blocked with 5% BSA in PBS for 2 h. Serum samples, diluted 1:50 in blocking buffer, were added in duplicates (100 µl/well) and incubated for 2 h at room temperature. After 5 washes, 100 µl HRP-conjugated goat anti-human IgG (heavy+light chains; Caltag Laboratories, Burlingame, CA), diluted at 1:3000, was added to each well and incubated for 1 h at room temperature. After washing, antibodies were detected using O-phenylendiamine. The reaction was stopped with 2 M sulfuric acid. The absorbance at 490 nm was measured using a microplate spectrophotometer (SPECTRA max 190, Molecular Devices). For calculation of anti-HMGB1 antibody levels, the mean OD of three wells coated with human serum albumin was subtracted from the OD of each sample as background value (nonspecific binding); this OD, corrected for individual background, is termed corrected sample cOD. Antibody titers were expressed as units, calculated based on two reference sera/plasmas with high (S1) and low (S2) ELISA reactions against HMGB1. One unit was defined by subtracting the cOD value of S2 from the cOD value of S1 and dividing the result by 100 [1 unit=(cODS1–cODS2)/100]. These reference sera/plasmas were used as internal standards in each assay.
SDS-PAGE and Western blot analyses
Plasma/serum (3 µl) from healthy controls or patients with SLE and RA were diluted with 72 µl RIPA buffer and heated at 95°C for 5 min in SDS-loading buffer. For immunodetection, the proteins were separated by 12% SDS-PAGE and transferred onto an Immobilon polyvinylidene difluoride membrane (Millipore, Bedford, MA). The membrane was blocked at room temperature for 1 h with 25 mM Tris-HCl, 150 mM NaCl, pH 7.4, and 0.05% Tween-20 (TBS-T) containing 3% BSA. The primary anti-HMGB1 antibody (0.25 µg/ml, Medical and Biological Laboratories, Nagoya, Japan) was applied for 2 h at room temperature, followed by four 15-min washes with TBS-T. Blots were then incubated at room temperature for 1 h with a HRP-conjugated, anti-mouse IgG antiserum (Jackson Immunoresearch, West Grove, PA). After washing, proteins were detected using ECL reagents (Amersham Biosciences).
Coimmunoprecipitation
Coimmunoprecipitations were performed using Seize X mammalian immunoprecipitation kit (Pierce), according to the manufacturer’s instructions with minor modifications. Briefly, 7.5 µg anti-HMGB1 (R&D Systems) and 7.5 µg isotype control antibodies (murine IgG2b, Sigma Chemical Co.) were immobilized to 25 µl protein G-sepharose support using cross-linker disuccinimidyl suberate. Serum/plasma (100 µl) and 400 µl 0.1 M Tris/HCl, pH 7.5, 150 mM NaCl, and 0.05% Tween 20 (TNT buffer) or 100 ng rHMGB1 preincubated with 1 mg/ml -globulin Cohn Fraction II (Sigma Chemical Co.) in PBS were added to the immobilized antibodies and mixed gently for 4 h at 4°C. The samples were washed five times with ice-cold TNT buffer by centrifugation using HandeeTM Spin Cup columns, and the precipitated proteins were eluted by adding 80 µl elution buffer. Finally, the precipitates were heated at 95°C for 5 min in SDS loading buffer and size-separated by 7.5% SDS-PAGE, and proteins were visualized by colloidal Coomassie blue staining.
Mass spectrometry (MS)
For identification, bands were excised and digested following the protocol of Schafer et al. [20
]. Sample preparation for MALDI-MS was performed using ZipTips (Qiagen, Hilden, Germany) following the manufacturer’s protocol. For MALDI-MS and MALDI MS/MS analysis, an Ultraflex I ToF/ToF MS (Bruker Daltonik, Bremen, Germany) was used. Data analysis was performed using the BioTools (Bruker Daltonik) and Mascot (Matrix Science, London, UK) software packages.
RESULTS
Detection of recombinant intracellular and extracellular/secreted forms of HMGB1 by sandwich ELISA
To develop a specific and sensitive ELISA for detection of HMGB1, we have tested seven different immunoassay formats. We used six commercially available mAb and polyclonal antibodies against HMGB1. First, these antibodies were tested for their specific recognition of human HMGB1 in cell lysates by immunoblotting (Fig. 1A
and data not shown). Next, we investigated which antibodies also recognize the nondenatured protein by ELISA using prokaryotically expressed human rHMGB1 as standard. Four antibody combinations, which detect rHMGB1 in a sandwich ELISA, were established. As capture antibodies, polyclonal HMGB1 antibodies from Upstate Biotechnology, Santa Cruz Biotechnologies, and BD PharMingen and a mAb from Abcam could be used. As detection antibody, the HMGB1 mAb from R&D Systems worked best with all capture antibodies. Then we optimized various parameters, including concentrations of capture and biotinylated detection antibodies, sample dilution, incubation time, and temperature. Using human rHMGB1 for the standard dilutions, ELISAs, based on capture antibodies from BD PharMingen (Fig. 1B)
and Upstate (Fig. 1C)
, resulted in good correlation coefficients for standard curves, ranging from 0.4 ng/ml to 300 ng/ml, according to linear regression analysis. The detection limit for rHMGB1 was slightly below 0.4 ng/ml using the HMGB1 antisera from BD PharMingen or from Upstate Biotechnology as capture antibodies and the monoclonal anti-HMGB1 from R&D Systems as detection antibody.
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or LPS. After 24 h incubation, cell culture supernatants were collected, and HMGB1 concentrations were assessed. In the absence of activating stimuli, HMGB1 was barely detectable in the culture medium, whereas stimulation with IFN- or LPS increased the secretion of HMGB1 in a dose-dependent manner (Fig. 1D)
. Then we analyzed if the ELISAs can detect intracellular HMGB1 in lysates from human Jurkat cells. HMGB1 was quantified successfully in sequential dilutions of total cell lysates as shown in Figure 1E .
HMGB1 is passively released from primary necrotic cells [11 ] and as a complex with nucleosomes, from apoptotic cells undergoing secondary necrosis (data not shown). Therefore, we induced primary necrosis in Jurkat cells by heat and incubated the dead cells at 37°C for an additional hour. Apoptosis was induced by staurosporine treatment for 48 h, at which time, virtually all cells had undergone secondary necrosis. Released HMGB1 could be detected in cell culture supernatants of primary and secondary necrotic cells as shown in Figure 1F . Our results demonstrate that sandwich ELISAs based on these antibodies are useful to detect actively secreted, intracellular, and passively released human HMGB1 in cell culture medium and cell lysates. It is important that the ELISAs could also detect complexed HMGB1 released from secondary necrotic cells after prolonged culture. Under these conditions, most of the HMGB1 remains tightly attached to nucleosomes (data not shown).
Assessment of HMGB1 in serum/plasma samples: poor correlation of ELISA with immunoblot
HMGB1 has been shown to be elevated and to play an important pathophysiological role in conditions such as septic shock and arthritis. However, there is a need for easy, sensitive, and reliable detection systems. Hence, we tested if our ELISAs can be used to detect HMGB1 in serum and plasma of healthy individuals and patients with SLE and RA. The samples were analyzed in parallel by three ELISAs (capture antibodies from BD PharMingen, Upstate Biotechnology, and Santa Cruz Biotechnologies) and by Western blotting for the presence of HMGB1.
It is surprising that we found HMGB1, not only in patients with inflammatory diseases but also in a few healthy subjects. From several of these healthy subjects, we could obtain follow-up samples, which also contained detectable amounts of HMGB1 (Fig. 2A ). For most healthy subjects, the positive HMGB1 ELISA results could be confirmed by immunoblot analysis (data not shown). Clinical evaluation did not indicate infections or other inflammatory conditions.
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To ensure specificity of the ELISA systems, we identified sera (e.g., SLE5) negative for HMGB1 by Western blotting using three different antibodies (Fig. 2C and data not shown). This control serum was also negative when tested by the HMGB1 ELISA (Fig. 2C) . HMGB1 concentrations of sera/plasmas determined by ELISA and immunoblot showed a poor correlation, whereas both detection methods showed congruent results for cell culture supernatants and cell lysates. Especially HMGB1 concentrations determined by ELISA were often lower than those determined by immunoblot. Selected serum samples exemplifying congruent (SLE1, SLE5, SLE7, NHD2) and divergent (SLE3, SLE4, SLE6, SLE8) HMGB1 detections by immunoblot versus ELISA are presented in Figure 2C .
Serum/plasma components can interfere with rHMGB1 detection by ELISA
As we showed above, HMGB1 concentrations determined by ELISA did not correlate well with detection of HMGB1 by immunoblot. Often, we obtained false low or negative results by ELISA. Therefore, we asked whether serum components may interfere with the detection of HMGB1 by ELISA. To test this hypothesis, we added increasing amounts of rHMGB1 to serum/plasma from healthy individuals and SLE patients, which were all tested to be negative for endogenous HMGB1 by immunoblot. Analyses of rHMGB1-spiked serum/plasma samples by ELISA showed inadequate, low readings (Fig. 3A
and 3B
). HMGB1 added to samples at a final concentration of 10 ng/ml was completely undetectable in all serum/plasma samples investigated (not shown). Increasing concentrations of HMGB1 could largely overcome the sequestration by serum, indicating a saturation effect (Fig. 3B)
. The HMGB1-masking capacity differed in individual sera, and sera with weak and strong masking effect were observed among healthy subjects and patients with SLE.
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Identification of serum components binding to HMGB1
It has been demonstrated that HMGB1, as a "sticky" protein, binds to many membrane molecules, such as proteoglycans (syndecan-1, sulphoglycolipids), thrombomodulin, and phospholipids [21
, 22
]. Our previous results suggested that HMGB1 may also interact with serum/plasma proteins. When investigating different serum fractions, we found that the component(s) interfering with HMGB1 detection were present mainly within the -globulin fraction (data not shown).
To identify binding partners of HMGB1, we subjected HMGB1 containing human plasma samples to coimmunoprecipitations using mouse anti-human HMGB1 antibodies covalently linked to protein G sepharose. In parallel, we performed coimmunoprecipitations with -globulins from Cohn Fraction II preincubated with human rHMGB1, as this fraction displayed the strongest HMGB1 masking effect in ELISA. In both experimental settings, protein bands with an apparent molecular mass of 130 kDa and slightly more were detected (Fig. 3C)
. To identify these proteins, trypsin digestion followed by MS and fingerprint analysis identified several of these visible bands as diverse Ig chains of IgG-type antibodies. Confirming these results, the sequencing of a characteristic peptide repeatedly found in the digests revealed a fragment of the constant region of IgG1-type heavy chains (data not shown). Also, Western blot analysis revealed the presence of human IgG, predominantly, but not exclusively, of the IgG1 isotype (data not shown). Coimmunoprecipitations also revealed other, less-prominent bands of lower molecular weight, which are currently under investigation.
These findings imply that HMGB1-specific antibodies contribute to the interference in ELISA detection. Therefore, we analyzed multiple plasma/sera samples for the presence of antibodies specific for HMGB1. The majority of samples from healthy individuals and SLE patients showed marked levels of HMGB1 antibodies. However, sera from SLE patients displayed significantly higher levels of anti-HMGB1 compared with healthy individuals (Fig. 4A ). To evaluate the affinity of these antibodies, we serially diluted sera derived from three patients with SLE and three healthy controls and analyzed them further by ELISA. The titration curves revealed a relatively high binding affinity of IgG antibodies from several individuals including healthy donors (Fig. 4B) . It is most interesting that there was a correlation of anti-HMGB1 IgG antibody levels with masking activity of individual sera/plasma samples, further indicating a role of IgG antibodies for interference with ELISA detection of HMGB1 (Fig. 4C) . Furthermore, we isolated IgG antibodies from a serum with high HMGB1-neutralizing activity using protein G affinity purification. We added these antibodies to HMGB1-spiked samples in a concentration equivalent to serum IgG concentrations. The purified IgG antibodies resulted in a marked inhibition of HMGB1 detection by ELISA, further confirming the role of anti-HMGB1 antibodies for interfering with HMGB1 detection. In contrast, antibodies isolated from a donor with a relatively low HMGB1-neutralizing activity displayed low inhibition of HMGB1 detection by ELISA (data not shown).
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Here, we describe sensitive ELISAs for the detection of HMGB1 in cell culture supernatants and cell lysates. Using the polyclonal anti-HMGB1 antiserum from BD PharMingen or from Upstate Biotechnology, the detection limit of the ELISA was 0.4 ng/ml, which is lower than the detection limit of a HMGB1-ELISA described previously [23 ]. Testing serum and plasma samples, we often noticed that concentrations detected by ELISA were lower than expected from immunoblotting results.
Investigating serum components binding to HMGB1, we identified IgG antibodies complexed with HMGB1 within serum/plasma. Therefore, we tested sera and plasma samples for the presence of anti-HMGB1 antibodies. It is interesting that IgG and IgM antibodies binding to HMGB1 were found in most sera and plasma samples investigated, including those from healthy individuals. Moreover, the titers of antibodies against HMGB1 in serum correlated inversely with the ELISA detection of rHMGB1 added to the serum samples. These data imply a role of autoantibodies to HMGB1 for interfering with HMGB1 detection by ELISA.
To reliably detect HMGB1 by ELISA within serum/plasma samples, we investigated several methods to dissociate preformed HMGB1 complexes. Acidification (pH 2.5) with subsequent neutralization, high salt, or addition of detergents such as SDS and Tween 20 improved detection but still did not result in exact quantification of HMGB1 serum concentrations by ELISA. Currently, we further optimize the ELISA system for HMGB1 quantification in serum/plasma.
Previous reports described a high prevalence of autoantibodies against HMGB1 in patients with rheumatic diseases including SLE and RA [24 , 25 ]. However, the existence of IgG and IgM antibodies binding to HMGB1 in a high percentage of healthy subjects has not been reported yet. Although titers of these antibodies are relatively low, and HMGB1 may not represent the primary antigen inducing these antibodies, HMGB1-binding antibodies might serve important biological functions, possibly limiting overwhelming cytokine release as a result of ample tissue damage or sepsis. Currently, we investigate if naturally occurring or disease-associated anti-HMGB1 antibodies modulate the proinflammatory response induced by HMGB1. In fact, preliminary data indicate that anti-HMGB1 antibodies in human sera inhibit cytokine release from HMDM stimulated with a combination of IL-1ß and HMGB1. The almost universal detection of HMGB1-binding antibodies in healthy subjects is remarkable and might be a result of cross-reactivity and/or the sticky nature of HMGB1 [26 ]. The role of these antibodies for masking their target protein or modulating its biological activity needs further investigation.
Also, other candidate proteins binding to HMGB1 were found by coimmunoprecipitations. These proteins are currently being characterized and may very well contribute to complex formation, masking, and potentially, modulation of cytokine activity of HMGB1.
To our knowledge, this is the first description of serum components that bind to HMGB1 and interfere with its detection by ELISA. Complex formation may be crucially involved in the modulation of the cytokine activity of HMGB1. Detailed knowledge of the factors binding to HMGB1 and interfering with ELISA assays may help to develop new strategies for reliable quantification of this important proinflammatory mediator in research and clinical routine laboratory testing.
ACKNOWLEDGEMENTS
This work is supported by the Interdisciplinary Center for Clinical Research (IZKF, Project N2) and the German Research Society (Collaborative Research Centers SFB 643, Project B3). We thank Dr. Silke Meister, Kirsten Neubert, Benjamin Frey, and Damian Maseda for critical reading of the manuscript and their help during preparation of the manuscript.
Received April 21, 2006; revised September 1, 2006; accepted September 4, 2006.
REFERENCES
induces high mobility group box 1 protein release partly through a TNF-dependent mechanism J. Immunol. 170,3890-3897This article has been cited by other articles:
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