Characterization of the interaction of staphylococcal enterotoxin B with CD1d expressed in human renal proximal tubule epithelial cells
© Hammamieh et al.; licensee BioMed Central . 2015
Received: 17 December 2014
Accepted: 13 January 2015
Published: 4 February 2015
Participation of renal cells in the pathogenesis of staphylococcal enterotoxin B (SEB) is critical for late cleansing and sequestration of the antigens facilitated by CD1d mediated antigen sensing and recognition. This is a noted deviation from the typical antigen recognition process that recruits the major histocompatibility complex class II (MHC II) molecules. The immunological importance of CD1d is underscored by its influences on the performances of natural killer T-cells and thereby mediates the innate and adaptive immune systems.
Using diffraction-based dotReady™ immunoassays, the present study showed that SEB directly and specifically conjugated to CD1d. The specificity of the conjugation between SEB and CD1d expressed on human renal proximal tubule epithelial cells (RPTEC) was further established by selective inhibition of CD1d prior to its exposure to SEB. We found that SEB induced the expression of CD1d on the cell surface prompting a rapid conjugation between them. The mRNA transcripts encoding CD1d remained elevated potentially after completing the antigen cleansing process.
Molecular targets associated with the delayed pathogenic response have essential therapeutic values. Particularly in the event of bioterrorism, the caregivers are typically able to intervene much later than the toxic exposures. Given circumstances mandate a paradigm shift from the conventional therapeutic strategy that counts on targeting the host markers responding to the early assault of pathogens. We demonstrated the role of CD1d in the late stage of pathogen recognition and cleansing, and thereby underscored its clinical potential in treating bioweaponizable antigens, such as Staphylococcal enterotoxin B (SEB).
SEB, a member of the exotoxin family produced by staphylococci [1,2], is a superantigen (sAg) capable of inducing toxic shock through intranasal or intravenous portals. Transmittable via air, food and water, the SEB-induced toxemia causes pyogenic damage that manifests as immunological irregularities, arthritis, and autoimmune disease cascading to multi-organ dysfunction and lethal consequences [1,3]. Continued clinical interest about staphylococcal enterotoxins (SEs) is attributed to a general inadequacy of its effective treatment [3,4]. Many incidences alleging SEB as the chief food poisoning contaminant  and its easy adaptability to bioweapons  further justify its continued clinical relevance.
Administrations of the anti-toxin and anti-inflammatory agents are the typical clinical strategies available nowadays [3,7]. Conventional strategies targeting early pathogenic markers have faced some serious deficiencies particularly when they intervene long after the pathogenic assault . This concern has been multiplied by the failure of the conventional therapeutic strategy in treating septic shock , which shares many patho-clinical similarities with toxic shock [10,11]. In the advent of chemical warfare, therapeutic interventions typically lag behind the toxic assault. Hence, the molecular signatures involved with the delayed response to bioweaponizable toxins, such as SEB, are of significant clinical interest.
Accumulating evidences suggest robust clinical efficacy in targeting the post-assault downstream candidates responsible for SEB pathogenesis [3,12]. In recent past, the focus of investigation for clinical targets has shifted from blood cells to the non-lymphoid cell types derived from kidney , spleen , lungs , and gut . Recent findings demonstrating the translocation of bacterial infection from skin  to organs, such as kidney and spleen without hematic invasion, further enhanced the interests on such peripheral organs in this context. It was also noted that many cell types [18,19] express atypical binding sites for SEB, which underscore the essentiality of cell/tissue-specific studies. In other words, CD1d (cluster of differentiation 1d glycoprotein) expressed on kidney cell surface may demonstrate a very unique response characteristics to SEB.
The critical role of the kidneys in exotoxin clearance and sequestration [20-25] was attested by identifying the renal excretion process as the primary route of removing staphylococcal enterotoxin A (SEA) from plasma . Supporting evidence includes the non-human primate study reporting ~70% accumulation of SEB in the renal proximal tubule epithelial cells (RPTECs) within 90 minutes of aerosol administration . SEB introduced through a cutaneous burn wound of rat was found traversed to and localized in the kidney 6 days post-burn , and a murine genomic study showed a delayed response of renal genes after lethal SEB shock . Further investigation of the renal response in SEB pathogenesis may help to identify an alternative avenue for clinical intervention.
The present study is focused on CD1d for two primary reasons. Firstly, CD1d selectively expressed in the renal epithelial tissues are recruited for antigen sensing, recognition and cleansing at the late stage of pathogenesis [13,28-31]. Unlike a typical host-mediated antigen recognition sequence, the intact sAgs are submitted to the antigen presenting cells (APC) by CD1d, a phylogenetic analog of major histocompatibility complex (MHC) class I and II molecules operating as an antigen-trafficking agent [32-34]. Secondly, CD1d controls the function of natural killer T-cells (NKT cells); the CD1d-NKT cell-mediated sAg recognition event rapidly triggers the release of many cytokines, and thereby systematically influences the hosts’ adaptive and innate immune systems [35,36].
The involvement of CD1d in pathogenesis has been investigated in the past, primarily focused on the lymphoid cells [37-41], while the knowledge gap exists in comprehending the CD1d-mediated pathogenesis in kidney cells and its role in the late stage of pathogenesis. To bridge the knowledge gap, our study sought to understand the renal response to SEB shock, focusing on the interaction of SEB with CD1d in RPTECs.
Cells and reagents
Human RPTECs were purchased from Lonza (Frederick, MD). SEB and biotinylated SEB (bt-SEB) (95% pure, vendor defined) were purchased from Toxin Technology, Inc. (Sarasota, FL). CD1d-(Immunoglobin) Ig recombinant protein, which has extracellular MHC class I-like domains of the human CD1d molecule fused with the VH regions of mouse IgG1, was purchased from BD Biosciences (San Jose, CA). Horse radish peroxidase (HRP)-linked anti-mouse (H + L) horse antibody (HRP-anti-mouse) was obtained from Cell Signaling Technology, Inc. (Danvers, MA). Mouse IgG (msIgG), a polyclonal antibody purified from pooled mouse serum by fractionation and ion-exchange chromatography, was from Axela, Inc. (Toronto, Ontario, Canada). Biotin-SP-AffiniPure Donkey Anti-Rabbit IgG (H + L) antibody (bt-Dar) was from Jackson ImmunoResearch Laboratories (West Grove, PA). 1-Component TMB (3,3',5,5'-Tetramethylbenzidine) Membrane Peroxidase Substrate (a precipitable form of TMB) was from KPL, Inc. (Gaithersburg, MD). All other antibodies used in this study were purchased from Abcam, Inc., Cambridge, MA.
Diffraction-based dotReady™ immunoassay
The assays were performed in biosensors with avidin surface chemistry (Axela, Inc.). All experiments were carried out with the dotLab® System (Axela, Inc.).
Principle of dot®
The dot® (diffraction optics technology) utilizes diffraction grating to analyze real-time protein-protein interactions. A capture reagent is immobilized on a specific pattern of lines on the surface of a prism-shaped dotLab® Sensor. The sensor surface forms the base of a 10 μL flow cell. A series of discrete diffraction beams are generated when illuminating the underside of each assay spot with a laser. The subsequent capture of the binding partner alone or bound to a detector antibody increases the average height of the surface pattern and causes an increase in the diffraction intensity (DI) signal that is recorded in real-time. The DI signal is directly related to the size and quantity of the bound complex [42,43]. The size of the bound complex is not limited to the addition of proteins, as previous studies  have shown that oxidization of TMB, a precipitable form of HRP substrate, can cause specific and localized precipitations on the sensor surface and result in a significant increase of DI signals.
The principle mechanism governing the assay performance is very similar to the other industry standards such as the BIAcore with a primary difference in their tethered ligand characters . Many studies reported comparable performance of dot® technology in comparing other platforms such as BIAcore, amperometric assay or other methods for detecting real-time bimolecular interactions [46-48].
The change in DI was recorded during each loading episode. Prior to the introduction of each reagent, the sensor was blocked with the BSA blocking buffer to establish the baseline. The delta (Δ) (the change of DI signals from the introduction of one reagent to the next one) indicated the extent of binding during each step. Each binding event was independent from each other and evaluated separately. The DI signals derived from each step of sequential addition were normalized against the delta of bt-SEB binding (as “normalized DI”), thus minimizing inter-sensor variations. Such ratio comparison is typical and has been reported previously [48,49].
The specificity of CD1d in binding SEB was evaluated by systematically replacing bt-SEB with bt-Dar (Additional file 1: Figure S1A) and CD1d:Ig with BSA (Additional file 2: Figure S1B), keeping the rest of the assay protocol unchanged. Furthermore, the efficacy of CD1d:Ig fusion protein was validated with two separate experiments. In one of them, the Ig tag uncoupled from CD1d was presented instead of CD1d:Ig fusion protein, keeping the rest of the protocol unaltered. In the second experiment, HRP was presented coupled with nonspecific mouse antibody, instead of the CD1d-specific mouse antibody. The normalized DIs reported from each of the scenarios were presented along with the DI derived from the standard assay protocol described above (Additional file 3: Figure S1C)
The assay was repeated with a range of concentrations of CD1d:Ig fusion protein (0, 0.05, 0.1, 0.5, and 1.0 μg/ml), keeping the rest of the assay conditions unchanged, and the corresponding DIs for each CD1d:Ig concentration were reported in Figure 1B. All data recorded in dotLab® Software were analyzed using GraphPad Prism® (GraphPad Software, Inc., San Diego, CA).
RPTEC culture and fluorescence-based reporting
For the SEB reporting assays (data not shown), the cells were treated with rabbit anti-SEB antibody for 30 m on ice, washed with 1× PBS, and incubated on ice with FITC-conjugated goat anti-rabbit antibodies for 5 m. Subsequently, the cells were washed and resuspended in 1 ml of formaldehyde and PBS mix (1:1). After incubation at room temperature for 15 m, it was washed with 1× PBS, resuspended again in 1× PBS and proceeded to imaging. The images were analyzed using a fluorescent microscope equipped with a digital camera (Olympus Optical Company). The control chamber slide had no SEB loading.
Immunoblotting to report time dependent binding efficiency of SEB to CD1d
Co-immunoprecipitation of CD1d-bound SEB was carried out using the Universal Magnetic Co-IP kit (Active-Motif, Carlsbad, CA) according to the manufacturer’s instructions. Briefly, RPTECs were exposed to SEB for 1 h and 2 h as per the protocol described earlier. An aliquot of 400 ng of whole cell extract and 2 μg of mouse anti-CD1d antibody was combined in a pre-chilled 1.5 ml tube and incubated for 2 h at 4°C on a rolling shaker. Samples were centrifuged for 30 s at 4000 rpm at 4°C, then 25 μl of Protein G Magnetic Beads was added to each tube. The mixture was incubated for 1 h at 4°C and centrifuged for 30 s at 4000 rpm at 4°C. Each tube was placed on a magnetic stand to pellet the beads, supernatants were discarded, and the pellets were resuspended and subsequently washed with 1× PBS. Immunoblotting was performed according to the protocol described elsewhere (Additional file 4: Figure S2) .
qPCR assay to assess CD1d expression
The protocol described earlier was followed to harvest RPTECs and to expose them to 100 μg/ml SEB for 1 h, 2 h and 24 h. Total RNA was isolated from RPTECs in a 125 ml flask using TRIzol reagent (Invitrogen, Carlsbad, CA) as per manufacturer’s protocol. RNA was quantified via spectrophotometry followed by analysis with a Bioanalyzer 2100 (Agilent Technologies, Santa Clara, CA).
Immunological inhibition of CD1d fluorescence-based reporting
Binding efficiency and specificity of SEB to CD1d
The dotLab® System with diffractive optics technology (dot®) was used to systematically investigate the binding specificity of SEB to CD1d. In this study, biotinylated SEB (bt-SEB) was immobilized on a sensor equipped with avidin surface chemistry. The binding as the result of avidin-biotin interaction was displayed as an elevated upward ramp of DI at the end of around 500 s episode of BSA incubation (Figure 1A). Sequential BSA washing and the loading of CD1d:Ig to bt-SEB (each episode of incubation lasted for approximately 500 s) did not register the detectable increment of DI. The conjugation of SEB and CD1d took place nonetheless, as demonstrated by presenting an HRP-linked detector antibody (HRP-anti-mouse) that was conjugated to the Ig domain of CD1d:Ig duplex. Subsequently, DI was ramped as we introduced TMB, a precipitating substrate of HRP.
Four negative control assays were carried out to test the specificity of the conjugation between SEB and CD1d. In a mock surface control, nonspecific antibody bt-Dar was immobilized on the surface. CD1d:Ig followed by HRP-linked anti-mouse antibody was sequentially introduced as described earlier. TMB administration however, failed to increase the DI signals (Additional file 1: Figure S1A) denying the chance of nonspecific binding of CD1d in this assay. In an analyte blank control, BSA blank instead of CD1d:Ig was introduced on the immobilized SEB surface. Following the same assay sequence, weak signal amplification was recorded after TMB loading (Additional file 2: Figure S1B). Henceforth the specificity of the HRP-linked antibodies to CD1d:Ig was established.
Furthermore, we repeated this experiment, replacing CD1d:Ig with either (i) the Ig tag uncoupled from CD1d or (ii) HRP linked to nonspecific mouse antibody (HRP-1A11), which can only bind to human cardiac troponin T (Additional file 3: Figure S1C). Presentation of Ig-only substrate feebly increased DI while HRP-1A11 failed to increase DI after 600 s incubation.
The assay was further repeated with serially increasing concentrations of CD1d:Ig from 0.05 μg/ml to 1.0 μg/ml (Figure 1B), showing concurrent increment of the corresponding fluorescence signals.
Longitudinal dynamics of the conjugation of SEB and CD1d on RPTECs’ surface
The longitudinal dynamics of RPTECs’ response to SEB exposures were evaluated by incubating the cells with SEB for increasing durations. To examine whether SEB treatment can affect the intracellular trafficking of CD1d, the distribution of CD1d expression on the RPTEC’s surface was monitored with and without SEB treatment. Compared to the untreated cells, the 1 h SEB treatment elevated CD1d expression on the RPTEC’s surface as reported by the Alexa 594-based fluorescence assay (Figure 2).
The outcome was further validated by the qPCR assay of the expression of CD1d after increased durations of SEB exposures. After 1 h SEB treatment, CD1d expression became significantly higher (p < 0.05) than that of the untreated control samples. Longer durations of SEB treatment retained the elevated level of CD1d mRNA transcriptomic expression (Figure 4).
The longitudinal dynamics of SEB binding to RPTEC were reported with accompanying CD1d expression on the cell surface (Figure 3). Sequential reporting of CD1d and SEB using antibodies with mutually exclusive binding epitopes showed co-localization of SEB and CD1d on the cell surface. After displaying a gradual increment until 1 h, the expression density gradually declined as reported after 2 h SEB exposure (Figure 3).
The reduction of SEB concentration conjugated to CD1d after 2 h incubation was verified with an immunoblotting study (Figure S2). Here, post-1 h and −2 h SEB exposure, the CD1d protein was surface-captured and SEB levels were reported by sequential addition of antibodies with mutually exclusive capturing epitopes. The concentration of SEB conjugated to CD1d was found diminished after 2 h SEB exposure.
Consequences of inhibiting CD1d of RPTECs before SEB exposure
RPTECs were pre-treated with polyclonal anti-CD1d antibody before regular SEB treatment. The SEB reporting profile demonstrated the consequences of inhibiting CD1d from conjugating with the antigens. In Figure 5, the first bar from the left (SEB + CD1d) displayed the intensity of SEB conjugated to CD1d in the absence of the anti-CD1d antibody. A significant reduction of signal was observed in presence of the anti-CD1d antibody (second bar plot; SEB + inhCD1d) that inhibited the CD1d ability to conjugate SEB. A faint signal was nevertheless recorded. The third bar showed a negative signal (inhCD1d), which potentially attested that the anti-SEB primary antibody (and the corresponding secondary antibody) shared no common epitope with anti-CD1d polyclonal antibody.
Multiple studies reported the critical protective role played by the kidney in excretion of SEB [20-26]. RPTECs selectively express CD1d, which play a significant (and MHC II-independent) role in antigen sensing and recognition process [13,29-31]. The present study sought to elucidate the renal pathogenesis of SEB with the primary focus on the toxin’s interaction with CD1d expressed on RPTECs.
Time sensitive co-localization of SEB with CD1d expressed on RPTECs’ surface (Figure 3) was found in accordance with others’ findings [12,32,33,55]. The renal localization of SEB began as the toxin conjugated to CD1d; in fact, the co-localization, which began merely 15 m post-exposure possibly explained the etiology behind the ‘jumpstart’ of host immunity after CD1d-NKT cell conjugation .
We also showed that SEB conjugated directly to CD1d using the dot® platform (Figure 1A), a technology enabling us to probe the protein-ligand interaction in a real-time format with proven success at per the other industry standards such as BIAcore [47,48]. The direct and specific binding of renal CD1d with SEB was further attested by the selective immunoinhibition of CD1d (Figure 5) that inhibited SEB to bind RPTECs at all. This information is rather important in context to the evidence that showed no direct involvement of CD1d in tuberculosis antigen presentation [55,56] and to other reports suggesting non-conventional sAg presenting sites [18,19].
We paid particular attention to minimize the risk of false positive result of dot® assay by conducting an array of validation experiments. These included the probing of (i) the blank capturing agents on the surface that attested the specificity of the SEB and CD1d binding (Additional file 1: Figure S1A). (ii) The analyte blank control for CD1d attested the specificity of the HRP-linked antibody to CD1d:Ig (Additional file 2: Figure S1B). (iii) The analyte quantity control investigated the dose titration of the CD1d amount and verified the stoichiometric dynamics between CD1d and SEB (Figure 1B). (iv)The analyte tag negative control replacing the Ig tag control of CD1d with msIgG reconfirmed the specificity of Ig to HRP-like antibody (Additional file 3: Figure S1C). Finally (v) the HRP-linked detector antibody was replaced by HRP-1A11 as a nonspecific detector for CD1d that revalidated the same (Additional file 3: Figure S1C). Together these assay results imparted maximum confidence on the present inference about the direct and specific conjugation of SEB to CD1d.
Present methodology, however was not able to empirically measure the affinity between SEB and CD1d. This could be due to a weak SEB-CD1d interaction, which was potentially unlikely in light of the past studies that demonstrated high affinity of CD1d for glycolipids ; in fact, our data displaying a short duration required to form SEB-CD1d conjugates further supports this report . As a more probable alternative, the requirement of a signal enhancing addendum (TMB, in this case) could be simply due to the fact that the reagents and the experimental conditions used in vitro in dot® experiments were different from what happens in the cell. A possible role of a secondary adjunct facilitating SEB CD1d conjugation in the cellular environment could not be ruled out; a more comprehensive study is required.
The fluorescence images obtained from RPTECs displayed the density of SEB conjugated with CD1d without the additional help from signal amplification agents. The result indicated that the SEB exposure prompted the elevation of CD1d expression on the cell surface (Figure 2).
As shown in Figure 3, the co-localization profile of SEB and CD1d displayed a gradual increment of their densities up to 1 h and concluded with a subsequent decline, which suggested the eventual trafficking of SEB. Also, Figure 2 suggested that the SEB challenges prompted CD1d expression on the cell surface after 1 h incubation, while the immunoblotting result (Additional file 4: Figure S2) validated the potential excretion of SEB. The enrichment of the cell-bound SEB depleted as the incubation period was extended to second hour. The present study was limited by not tracking the unbound SEB during the second hour of exposure.
Interestingly, the transcriptomic expression of CD1d remained elevated (Figure 4), possibly long after the SEB was sequestered. Of note, a previous study demonstrated a long half-life of CD1d . These outcomes, coupled with Figure 2 displaying the SEB-induced the cell surface expression of CD1d may indicate that SEB regulates CD1d expression on RPTEC’s surface. Further investigation of CD1d in RPTECs could have major clinical interest, particularly in the context of understanding a robust system that enables facilitation of a delayed antigenic cleansing program .
The essential role of renal CD1d in SEB-associated pathogenesis was further validated with a fluorescence-based assay demonstrating the sequential inhibition of CD1d and preventing its binding to SEB (Figure 5). In agreement, multiple studies observed diminished host defense as a result of the targeted CD1d deficiency [37,40,59,60]. It was, however, beyond the scope of the present study to suggest putative SEB-induced toxemia through CD1d-independent routes. Such possibilities, nonetheless, have been suggested by the recruitment of CD1d-restricted T-cells [40,41] and invariant Natural Killer cells .
In conclusion, the present study examined the renal pathogenesis of SEB, which revealed a potential functional role for CD1d in antigen recognition, cleansing and sequestration [12,22,26,27]. The binding specificity of CD1d and SEB was established. A rapid conjugation of CD1d with SEB implicated a possible high affinity between the two molecules, particularly in RPTECs. Such prompt interaction may facilitate the rapid surge of cytokines triggered by the crosstalk between CD1d and NKT cells .
The display of the co-localization of SEB and CD1d at the RPTEC’s surface and the decline of the SEB population on the cell surface after CD1d immunoinhibition suggested that RPTECs undertake an exclusive CD1d-mediated antigen presentation process. The transcriptomic elevation (and cell surface expression) of CD1d in RPTECs was concurrent with SEB exposure, which remained elevated potentially long after SEB trafficking. Together, a feedback-controlled conjugation of SEB and CD1d is suggested in carrying out the recognition, cleansing and sequestration of SEB. Further investigation is required to establish this CD1d-mediated mechanism as a potential downstream mechanism of SEB excretion and a prospective therapeutic target.
Authors like to acknowledge the insightful contribution from the internal editor Dr. Julia Scheerer. RH, NC, SM and MJ like to acknowledge the funding from DTRA: CB3293-Host Factors and Shock-Inducing Bioagents.
The views, opinions, and/or findings contained in this report are those of the authors and should not be construed as official Department of the Army position, policy, or decision, unless so designated by other official documentation.
Citations of commercial organizations or trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organizations.
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