Biologicals 42 (2014) 312e315 Contents lists available at ScienceDirect Biologicals journal homepage: www.elsevier.com/locate/biologicals A novel monoclonal antibody to Neisseria meningitidis serogroup X capsular polysaccharide and its potential use in quantitation of meningococcal vaccines tima Reyes a, b, *, Oscar Otero a, Frank Camacho a, b, Nevis Amin a, Fidel Ramírez a, Fa s a, Reynaldo Acevedo a, Luis García a, Daniel Cardoso a, Maribel Cuello a Yolanda Valde a b Research and Development Vicepresidency, Finlay Institute, La Habana, A.P. 16017, Cod. 11600, Cuba Department of Pharmacology, Faculty of Biological Sciences, University of Concepcion, Concepcion, P.O. Box 160C, Chile a r t i c l e i n f o a b s t r a c t Article history: Received 10 May 2014 Received in revised form 6 August 2014 Accepted 8 August 2014 Available online 11 September 2014 A novel murine hybridoma monoclonal antibody (MAb) was produced against the capsular polysaccharide (CP) of Neisseria meningitidis serogroup X (MenX) in order to develop a sandwich enzyme linked immunosorbent assay (ELISA) for the quantitation of the meningococcal polysaccharide. The MAb only reacted with the CP from MenX and did not react with CPs from N. meningitidis serogroups A, C, Y and W (MenA, MenC, MenY, MenW). The affinity constant (Ka) of the MAb measured by non-competitive ELISA was 7.25
107 M1. The application of this MAb in a sandwich ELISA was demonstrated by its ability to properly quantitate three lots of an experimental meningococcal CP-based vaccine. The MAb obtained in this work could be a valuable reagent for the detection and quantitation of future meningococcal vaccines containing MenX CP. © 2014 The International Alliance for Biological Standardization. Published by Elsevier Ltd. All rights reserved. Keywords: Monoclonal antibody Neisseria meningitidis serogroup X ELISA Quantitation 1. Introduction ́ Neisseria meningitidis is a major cause of bacterial meningitis worldwide, especially in the African meningitis belt, and has a high associated mortality [1]. Among the 13 distinct meningococcal serogroups, which are classified on the structure of their CP, serogroups A, B, C, Y, W, and X have commonly caused invasive infections [2]. Cases of meningococcal disease caused by serogroup X meningococci, for which no vaccine is available, have been reported in African countries [3e6]. Also, sporadic cases of meningococcal disease caused by MenX have been reported in industrialized countries [7]. There are two main types of vaccines used for protection against meningococcal infection: plain polysaccharide vaccines and protein/polysaccharide conjugate vaccines. Both are based on the CPs of the bacteria, which is a major virulence factor and is responsible for prevention of host-mediated bacterial killing [8]. * Corresponding author. Finlay Institute, Calle 27, No. 19805, La Lisa, A.P. 16017, Cod. 11600 Havana, Cuba. Tel.: þ53 7 2716911; fax: þ53 7 2731218. E-mail addresses: fatima8526@gmail.com, fatireyes@udec.cl (F. Reyes). Determination of total CP content is one of the product specí fications required for release of the final lot vaccine and should be determined by means of an appropriate assay. For this purpose, several methods for quantification of meningococcal polysaccharides have been used. Chemical methods like determination of total phosphorus content [9] and sialic acid content [10] has been used. Alternatively, other methods such as high performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) [11,12] and capillary zone electrophoresis (CZE) [13,14] have been reported. Recently, the development of a glycoconjugate vaccine candidate against MenX and preclinical data from its use in animal studies has been reported [1]. Although the development of a new vaccine through to licensure takes many years, suitable methods for the quality control of these products must be established before the commercial step. MAbs are antibodies with a defined specificity derived from cloned cells or organisms. They can be obtained from immortalized B lymphocytes that are cloned and expanded as continuous cell lines or from rDNA-engineered mammalian or bacterial cells (engineered monoclonal antibodies). On the other hand, MAbs have provided the means for developing a number of highly specific and reproducible immunological assays. http://dx.doi.org/10.1016/j.biologicals.2014.08.001 1045-1056/© 2014 The International Alliance for Biological Standardization. Published by Elsevier Ltd. All rights reserved. F. Reyes et al. / Biologicals 42 (2014) 312e315 In this study, we developed and characterized a MAb designated as 10B5F10 with appropriate specificity to the MenX CP, which was used either as the capture and peroxidase-conjugated MAb in a sandwich ELISA assay. Using the proposed ELISA, the quantitation of MenX CP in three lots of an experimental multivalent meningococcal CP-based vaccine was assayed. 2. Materials and methods 2.1. Reagents and buffers Unless otherwise SigmaeAldrich (USA). stated, reagents were obtained from  Buffers: Coating buffer (15 mM Na2CO3, 35 mM NaHCO3, pH 9.6), PBS (140 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, 1.8 mM KH2PO4, pH 7.4), blocking buffer (PBS, 3% non-fat dried milk), washing buffer (PBS, 0.05% (v/v) Tween 20, pH 7.4), substrate buffer (35 mM citric acid, 67 mM Na2HPO4, 0.012% (w/v) H2O2, pH 5.0).  MenA, MenC, MenW, MenX and MenY are purified CPs from N. meningitidis serogroups A, C, W, X and Y respectively used as internal controls (IC). These IC are supplied as lyophilized powders with 100 mg of each CP and were provided by Reference Materials Department of Finlay Institute, Cuba.  Tetanus toxoid (TT) (Finlay, Institute, Cuba).  MenX CP conjugated to TT (MenX-TT) provided by Biomolecular Chemistry Center, Cuba. 313 overnight with 100 mL per well of 10 mg/mL of TT or MenA, MenC, MenW and MenY CPs. Subsequent steps were performed as described in Section 2.2. 2.5. Calibrator, quality control and calibration curve construction In the absence of an International Standard, calibrator consisted of PXCI, which was quantitated by Chen colorimetric assay [9] using KH2PO4 as a standard. A two-fold serial dilution of the PXCI was used to generate an eight-point calibration curve with a range from 0.039 to 5 ng/mL. GraphPad Prism 5 software (GraphPad Software, San Diego, CA, USA) was used for plotting Abs492nm against the CP concentrations and four-parameter logistic equation (4 PL) was applied to obtain the function describing a sigmoid model. The quality control (QC) consisted of MenX CP powder (Finlay Institute, Cuba) which was weighted, dissolved in PBS and adjusted to a final concentration of 1.25 ng/mL (w/v). The calibrator and QC were stored as single ready-to-use aliquots at 20  C. 2.6. Description and preparation of samples Three lots of an experimental meningococcal vaccine were analysed. - Experimental pentavalent vaccine ACWXY (lots 101, 102, 103): meningococcal polysaccharide groups A, C, W, X and Y provided by Pharmaceutical Development Direction of Finlay Institute, Cuba. The vaccine has an expected amount of 50 mg of each CP. The vaccine was reconstituted as described by the manufacturer and was prepared to a final concentration of 1.25 ng/mL. 2.2. Production of anti-MenX CP monoclonal antibody 2.7. ELISA sandwich for serogroup quantitation For specific hybridomas against MenX CP, BALB/c mice were immunized subcutaneously with 10 mg/dose of MenX-TT. Five doses were administered in a 20-day interval. The fusion of spleen cells from immunized animals and SP2/0 myeloma cells was performed using the method of Kohler and Milstein [15]. Hybridoma screening was performed by indirect ELISA as described previously [16]. Briefly, microtiter plates (Maxisorp, Nunc, Denmark) were coated overnight with 10 mg/mL of MenX purified CP in PBS. After washing and blocking steps, supernatant from hybridomas were incubated 2 h at 37  C. Normal mouse serum was used as a negative control. Clones were considered positive when absorbance measured at 492 nm (Abs492nm) was twice Abs492nm of negative control. Cells from positive wells were cloned by the limiting dilution technique. The selected hybridoma 10B5F10 was injected intraperitoneally into pristane-primed mice. Ascitic fluids were collected 10 days after injections. Polystyrene microwell plates (Maxisorp, Nunc, USA) were coated with 10 mg/mL of MAb 10B5F10 in coating buffer (100 mL/well). After overnight incubation at 4  C, the coated wells were washed three times and blocked with blocking buffer for 1 h at 37  C. The calibrator, QC and samples were added (100 mL/well) and incubated 1 h at 37  C. Subsequently, the wells were washed three times and 10B5F10 peroxidase-conjugated MAb diluted to 1:25,000 in washing buffer containing 1% non-fat dried milk was added to the wells (100 mL/well) and incubated for 30 min at room temperature. After four washes, 0.4 mg of OPD per mL substrate buffer was added to the wells (100 mL/well) and following 15 min the color development was stopped with 1 M H2SO4. The Abs492nm was measured using a Microplate Reader (Thermo Electron Corporation, USA) and unknowns concentrations were calculated using CDC Software [19]. The calibrator, QC and samples were run in triplicates unless otherwise stated. Three independent experiments were performed and results were expressed as mean of CP content. Optimal dilutions of MAb and HRP conjugated MAb were determined by standard checkerboard titrations. 2.3. Purification and peroxidase labeling of 10B5F10 MAb Purification of 10B5F10 MAb from ascites fluid was performed by affinity chromatography using HiTrap Protein G (GE Healthcare, Germany) according to the manufacturer's procedure. In addition, 8 mg of purified (10B5F10) MAb were conjugated in-house to 4 mg of horseradish peroxidase type VI (HRP) as described by Wilson and Nakane [17]. 2.8. Limits of detection and quantification Lower limits of detection (LOD) and quantification (LOQ) were defined as the mean of blank values plus either 3 or 10 standard deviation (SD). 2.4. Characterization of MAb 3. Results and discussion MAb isotype was determined using a mouse isotyping kit (Pierce, USA) according to the manufacturer's protocol. The affinity constant (Ka) was determined using the method described by Beatty et al. [18]. Cross-reactivity of MAb was evaluated by indirect ELISA using a microtiter plate (Maxisorp, Nunc, Denmark) coated 3.1. Generation and characterization of anti-MenX CP monoclonal antibody A MAb of IgG1 isotype was obtained against MenX CP and designated as 10B5F10. Ka determined by non-competitive ELISA 314 F. Reyes et al. / Biologicals 42 (2014) 312e315 was 7.25
107 M1. Cross-reactivity assay showed that MAb 10B5F10 only reacts with PXCI (Fig. 1). Due to the similarity between the structures of MenA and MenX CPs on the basis of a homopolymer of 2-acetamido-2-deoxyglucose phosphate [20] the high specificity of this MAb is particularly useful when ELISA assays are used to quantitate multivalent meningococcal-CP based vaccines that contain the above mentioned serogroups. Previous serological studies on MenX CP have shown that its antiserum is highly specific for the position of linkage of the phosphodiester bonds [20]. Based on these results, the specificity of the 10B5F10 MAb could be related with the a1-4 phosphodiester bond present in MenX CP which differs of a1-6 phosphodiester bond present in MenA CP. On the other hand, the use of a high affinity MAb is regarded as a decisive factor for improvement of the sensitivity of an immunoassay such as ELISA [21]. 3.2. MenX CP sandwich ELISA MAb 10B5F10 was used either as the capture and peroxidaseconjugated MAb in a sandwich ELISA assay. Calibration curve is shown in Fig. 2 and a four-parameter fit model was applied to obtain the function describing a sigmoid model with a coefficient of determination (R2) 0.99. The LOD and LOQ were found to be 0.12 ng/mL and 0.2 ng/mL respectively and could be related with the Ka value calculated for the MAb in the previous experiment. Assessment of the content of polysaccharide is a requirement for meningococcal polysaccharide vaccines because it is directly related to the product dose. Several methods are available and suitable for use in determining the polysaccharide concentration. These include colorimetric methods and monosaccharide analysis following hydrolysis by HPAEC-PAD, CZE, gas chromatography or HPLC with fluorescence detection. Immunological methods that may be suitable include ELISA or rate nephelometry. The suitability of these methods depends on the availability of appropriate reagents, especially when multivalent vaccines must be analyzed. For example, colorimetric assays can't be used in multivalent meningococcal vaccines (trivalent, tetravalent, pentavalent) because phosphorus is present in MenA and MenX CPs, while N-acetylneuraminic acid is common to MenC, MenW and MenY CPs. In this regard, specific MAbs or polyclonal preparations used in immunological methods are crucial for the identification of each polysaccharide [22]. Typically, plain meningococcal polysaccharide vaccines contain 50 mg of each serogroup in a single human dose (0.5 mL). The Fig. 2. Calibration curve obtained from sandwich ELISA using the 4 PL equation. MenX CP was used as a standard. Error bars represent standard deviations of quadruplicate samples. available European Pharmacopoeia guidelines are ±30% for acceptable range of target dosage amount for plain meningococcal CP-based vaccines [23]. The MenX CP content using the proposed sandwich ELISA is shown in Table 1. All of the three lots of the experimental vaccine were found into the accepted range (±30%) of CP content and coefficients of variation (CVs) were <10% compared to the acceptance criterion of <15% CV for immunoassays [24]. A recent paper described the use of HPAEC-PAD for MenX polysaccharide quantitation [12], however, this method require prior conversion to monosaccharides by acid digestion and the use of strong acids, often at elevated temperature, represents a potential safety concern for laboratory staff and has a cost associated with material disposal [14]. To our knowledge, this is the first report of development and evaluation of a MAb against MenX CP. On the other hand, the performance of this MAb in a sandwich ELISA to properly quantitate the CP in three lots of an experimental meningococcal multivalent CP-based vaccine is clearly a novel result of our work. Previously, our group obtained a set of MAbs suitable for the specific identification of serogroups A, C, Y and W CPs [16]. In addition, these MAbs were used to establish four sandwich ELISAs for the quantitation of each CP in registered multivalent CP-based vaccines with excellent results [25]. Therefore, the obtention of anti-MenX CP MAb would complete the collection of MAbs against the main serogroups of N. meningitidis. Using appropriate immunological methods, the MAb can be used for monitoring polysaccharide identity, serogroup identification or rapid diagnostic of meningococcal disease due to MenX. Also, the proposed sandwich ELISA could be useful for the quantitation of bound and unbound (free) CP in conjugated vaccines prior separation of the unbound CP from conjugated-CP in such vaccines. Table 1 Serogroup polysaccharide content of meningococcal CP-based vaccines. Experimental vaccines Fig. 1. Cross-reactivity of MAb 10B5F10 by indirect ELISA. MenX, MenA, MenC, MenW, MenY CPs: Neisseria meningitidis serogroups X, A, C, W and Y respectively; TT: tetanus toxoid. Error bars represent standard deviations of three independent experiments. ACYWX 101 ACYWX 102 ACYWX 103 a MenX CP (mg/dose) Averagea CV (%) 59.5 58.3 50.6 9 6.8 9.1 Mean of triplicate determinations from three independent experiments. F. Reyes et al. / Biologicals 42 (2014) 312e315 Acknowledgments lez and Felix Cardoso (BioThe authors thank Majela Gonza molecular Chemistry Center, Cuba) for generously providing MenXTT. References [1] Micoli F, Romano MR, Tontini M, Cappelletti E, Gavini M, Proietti D, et al. Development of a glycoconjugate vaccine to prevent meningitis in Africa caused by meningococcal serogroup X. Proc Natl Acad Sci U S A 2013;110(47): 19077e82. [2] Harrison LH, Trotter CL, Ramsay ME. Global epidemiology of meningococcal disease. Vaccine 2009;27(Suppl. 2):B51e63. [3] Mutonga D, Pimentel G, Muindi J, Nzioka C, Mutiso J, Klena J, et al. 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