Monoclonal antibodies to nucleoprotein and glycoprotein of the virus of infectious haematopoietic necrosis of salmonids (IHNV) and their use in immunoperoxidase test

Monoclonal antibodies to the nucleoprotein and glycoprotein of infectious haematopoietic necrosis virus (IHNV) were prepared. Hybridomas were selected by ELISA according to the production of murine immunoglobulins. The specificity of the monoclonal antibodies was tested by Western blotting after SDS-PAGE. The monoclonal antibodies reacted only with a panel of nine IHNV isolates differing in electropherotype and geographical origin. No cross-reactivity with strains of the related viral haemorrhagic septicaemia virus was observed. Larger amounts of the monoclonal antibodies were raised in vivo and used for the preparation of direct peroxidase-labelled conjugates for the demonstration of IHNV in infected cell cultures. Introduction Infectious haematopoietic necrosis (IHN) is an acute viral disease affecting young salmonid fish. The high mortality, reaching up to 100%, is due to fatal damage to haematopoietic tissues of the cranial part of kidney and spleen (Wolf, 1988). The disease was first identified at North American fish farms at the Pacific coast (Rucker et al., 1953, McAllister, 1979; Pilcher & Fryer, 1980, Groberg & Fryer, 1983, Wolf, 1988). Later the infection spread to Asia and was detected in Taiwan (Luqi & Zhizhuang, 1988) and Japan (Sano et al., 1977). The first outbreaks in several European countries were reported in the late eighties (Bovo et al., 1987; Laurencin, 1987; Arkush et al., 1989; Hattenberger-Baudouy et al., 1989). Infectious haematopoietic necrosis is caused by a rhabdovirus in the genus Novirhabdovirus, which also includes also the causal agents of viral haemorrhagic septicaemia of trout (VHSV) and hirame rhabdovirus (HIRRV). The virion has a characteristic bullet-like shape and consists of five structural proteins including RNA polymerase (L), envelope glycoprotein (G), nucleoprotein (N), phosphoprotein (P) and matrix protein (M). The relative molecular weights of the proteins have been determined by SDS-PAGE (Hill et al., 1975; Lenoir & de Kinkelin, 1975; Leong et al., 1981; Hsu et al., 1984; Schutze et al., 1995). Moreover, the viral RNA codes for the production of a unique non-structural protein synthesized in infected cells, but not present in complete virions (Kurath & Leong, 1985). Bull. Eur. Ass. Fish Pathol., 24(5) 2004, 219 The IHN virus (IHNV) is pathogenic for rainbow trout (Oncorhynchus mykiss) and other species of this genus (O. nerca, O. tschawytscha, O. keta, O. masou, and O. kisutch). Atlantic salmon (Salmo salar) and other salmonids (Wolf, 1988) are also susceptible, and susceptibility of pike (Esox Iucius) has been mentioned (OIE Diagnostic Manual, 1995). The virus attacks haematopoietic tissues and causes petechial haemorrhages in the cranial part of the kidney and spleen. The infected fish are anaemic due to the damage to the haematopoietic tissues (Wolf, 1988). Similar clinical picture and lesions can develop in fish affected by other diseases and are not pathognomic for IHN. Therefore, the demonstration of the causal agent is preferred as a reliable diagnostic method. Diagnostic methods for IHN are based on direct detection of IHNV without or after propagation in cell culture. Virus isolation in a suitable cell line, such as FHM, RTG-2, CHSE, BF-2, or EPC, is used most frequently. The latter line is preferred owing to its high sensitivity to IHNV. The replicated virus is then detected by neutralization or immunofluorescence test, or ELISA using specific antibodies (OIE Diagnostic Manual, 1995). Traditional diagnostic identification methods are based on specific antibodies. Generally, the replacement in serological tests of polyclonal sera with monoclonal antibodies (MAbs) has contributed to the elimination of non-specific reactions and standardization of the procedures. Therefore, MAbs to IHNV proteins were prepared in several laboratories and used in immunofluorescence tests (Ristow & Arnzen, 1989; Danton et al., 1994), Western blotting (Schultz et al., 1989; Ristow et al., 1991), or ELlSA (Huang et al., 1994). MAbs which were produced mainly against American IHNV isolates and were considered to be universal reagents to IHNV did not recognise some of European isolates (Danton et al., 1994). The present work was aimed to production of MAbs suitable for a sensitive and specific identification of IHNV. Materials and methods Cell cultures The cell line EPC (Epithelioma Papulosum Cyprini; Fijan et al., 1983) was used for the replication of IHNV strains. The cells were cultured in minimal essential medium with Earle’s salts (MEM, Sigma Biosciences, USA) supplemented with 10% bovine foetal serum (Veterinary and Pharmaceutical University, Brno, Czech Republic), 2 mM glutamine (Sigma, USA), and the antibiotics penicillin 100 IU/ml (Biotika, Slovakia), streptomycin 100 μg/ml (Galenika, Yugoslavia), and gentamycin 100 μg/ml (LPCC, Slovenia). pH of the medium was adjusted to 7.6. Virus strains and culture conditions Mice were immunized with the American isolate IHNV US-OSV (Winton et al., 1988), and the first Italian isolate IHNV I-4008 (Bovo et al., 1987) before fusion. The panels of IHNV (Table 1) and VHSV (Table2) strains were used in tests of specificity of MAbs. All the strains were propagated in the cell line EPC. The virus was inoculated onto a 24-h-old monolayer after the removal of the medium and allowed to adsorb for 1 h at 15 °C. Then the medium was added and the infected cultures were inBull. Eur. Ass. Fish Pathol., 24(5) 2004, 220 cubated at 15 °C not longer than 7 days. The incubation was interrupted if the whole monolayer was destroyed by cytopathic effect (CPE) before the end of this period. Flasks containing the replicated virus were frozen. Purification Infected cell cultures with a fully developed CPE were frozen-thawn and sonicated (Artek Sonic Dismembrator, USA). Afterwards cell debris was removed by centrifugation (3 000 x g, 4 °C, 15 min; Beckman J-2 21 M) and the virus was pelleted by ultracentrifugation (60 000 x g, 4 °C, 2.5 h; Beckman L8 80M). The resulting pellet was resuspended in 0.1 M TrisHCI buffer, pH 7.0, to obtain a 100x concentrate of the original volume. The concentrated virus was used as antigen for immunization, in ELISA and in SDS-PAGE. A non-infected # Electropherotype according to the method of Hsu et al., 1986. * Serotype according to Olesen et al., 1986; NP-not published (serotyped by N.J.Olesen according to Olesen et al., 1993). Table 2. Panel of VHSV isolates. e t a l o s I r a e Y . p s t s o H e c n e r e f e R 1 F K D V S H V 2 6 9 1 s s i k y m . O ) * I ( 5 6 9 1 , n e s n e J 5 7 / 3 2 R F V S H V 5 7 9 1 a t t u r t . S ) * I I ( 7 7 9 1 , e r r e B e L & n i l e k n i K e d 1 3 1 5 K D V S H V 8 8 9 1 s s i k y m . O ) * I I ( 3 9 9 1 , . l a t e n e s e l O o v o B / a d r a G o i p r a c a t t u r t . S 5 9 9 1 , . l a t e o v o B 1 5 1 5 K D V S H V 8 8 9 1 s s i k y m . O ) * I I I ( 3 9 9 1 , . l a t e n e s e l O 1 6 2 5 K D V S H V 8 8 9 1 s s i k y m . O ) * I I ( P N 6 7 2 5 K D V S H V 8 8 9 1 s s i k y m . O ) * I I ( P N 1 3 2 5 K D V S H V 8 8 9 1 s s i k y m . O ) * I I ( P N 5 5 5 3 K D V S H V 5 8 9 1 s s i k y m . O ) * I ( 1 9 9 1 , . l a t e n e s n e g r o J e t a l o s I r a e Y . p s t s o H e c n e r e f e R 0 1 1 3 9 1 S U V N H I 4 8 9 1 s s i k y m . O ) # 2 ( 8 8 9 1 , . l a t e n o t n i W V S O S U V N H I 8 5 9 1 a c r e n . O ) # 1 ( 8 8 9 1 , . l a t e n o t n i W R E S U V N H I 6 8 9 1 a h c s t y w a h s t . O ) # 1 ( 3 9 9 1 , . l a t e a r t a P a L 7 8 / 2 3 R F V N H I 7 8 9 1 s s i k y m . O 9 8 9 1 , . l a t e r e g r e b n e t t a H 8 0 0 4 T I V N H I 7 8 9 1 s s i k y m . O 7 8 9 1 , . l a t e o v o B n a m e l o C S U V N H I 7 6 9 1 a h c s t y w a h s t . O f l o W n e K . r D y b d e d i v o r P R T S U V N H I 6 8 9 1 a h c s t y w a h s t . O ) # 3 ( 3 9 9 1 , . l a t e a r t a P a L W D S U V N H I 6 8 9 1 s s i k y m . O ) # 3 ( 3 9 9 1 , . l a t e a r t a P a L G A H S U N H I 3 8 9 1 s s i k y m . O ) # 2 ( 3 9 9 1 , . l a t e a r t a P a L Table 1. Panel of IHNV isolates. Bull. Eur. Ass. Fish Pathol., 24(5) 2004, 221 EPC culture was processed in the same way to obtain a negative control antigen for ELISA. Immunization of mice Mice of the inbred line BALB/c were immunized with the American strain IHNV USOSV and the Italian strain IHNV I-4008, by subcutaneously repeated injections with the virus emulgated in AI-Span-Oil adjuvant (Sevac, Prague). The dynamics of antibody responses were monitored during the immunization period by titration of antibodies in blood serum samples collected from the tail vein. The serum titre was defined as the highest dilution at which OD > 0.1. Three to four days before fusion, the mice were injected intraperitoneally with the virus suspended in PBS, pH 7.2. The mice were sacrificed on the day of fusion, spleens were collected and homogenized under sterile conditions and the density of the suspension (cell count per 1 ml) was determined. Hybridomas Spleen lymphocytes of the immunized mice were fused with cells of the mouse myeloma line Sp 2/0 using 50% PEG 1500 (Sigma, USA) as the fusigen. Hybridoma cells were cultured in the selective medium HAT RPMI 1640 (Sigma, USA) supplemented with 10% bovine foetal serum. After one week, the medium was replaced with HT-RPMI 1640 and later with the complete medium RPMI 1640. Detailed description of the hybridoma technique is published by Galfré & Milstein (1981). ELISA for the titration of murine antibodies to IHNV Wells of odd and even columns of polystyrene microtitre plates (Gama, Ceske Budìjovice, Czech Republic) were filled with 50 μl IHNV and 50 μl negative control (EPC) antigens, respectively, diluted 1:500 with carbonate-bicarbonate buffer, pH 9.6. The plates were incubated in a humid chamber at 4 °C overnight. On the following day, the wells were washed three times with PBS containing 0.1 % Tween 20 (PBST), 50 μl of the tested serum diluted 1: 200 was added to the upper pair of wells and serial twofold dilutions up to 1 : 102 400 were prepared using PBST + 1 % lactalbumin hydrolysate (PBST -LAH) (Difco, USA). The contents of the wells were replaced with peroxidase conjugate of swine serum to murine immunoglobulins (HRP-SwAMolgG; Veterinary Research Institute, Brno, Czech Republic) after 1 h of incubation at 37 °C and triple washing with PBST. Then the wells were washed three times again with PBST and filled with 100 μl substrate solution (H20 2 + 3.3',5,5' tetramethylbenzidine in 0.1 M acetate buffer, pH 5.8). The reaction was stopped after 10 min by adding 100 μl 0.1 M sulphuric acid and optical density was measured at 450 nm (SLT Spectra, Austria). The antibody titre was defined as the highest dilution at which the optical density (OD) > 0.1. ELISA for the selection of hybridomas ELISA was used for the selection of hybridomas producing antibodies to IHNV antigens. The procedure differed from that described for the titration of murine antibodies to IHNV in the following points: 1. Separate microtitre plates were used for the IHNV and the negative control antigens for the sake of a better arrangement of individual clones; 2. Hybridoma supernatants were diluted 1:1 with PBST -LAH; 3. Serum of immunized mice and negative murine serum, both diluted 1:200, were included in each plate as internal Bull. Eur. Ass. Fish Pathol., 24(5) 2004, 222 controls; Clones yielding optical density (OD) values > 1.0 on the plate containing the virus antigen and OD < 0.1 with the control antigen were selected. SDS-PAGE Samples for SDS-PAGE were pre-treated by boiling for 3 min in the same volume of a reducing solution composed of Tris-HCI buffer, pH 6.8, 2% SDS, 10% glycerol, bromphenol blue (as a marker of separation speed), and 5% mercaptoethanol (Sigma). The LMW calibration kit (Pharmacia, Sweden) with the range of 14.4 94 kDa was used as the internal standard of relative molecular weights (Mw). The samples were fractionated in 12.5% polyacrylamide gel in the presence of sodium dodecylsulphate (SDS) (Laemmli, 1970) in a constant potential gradient of 10 V per 1 cm gel in the Mini Protean II (Bio Rad, USA) apparatus. After the separation, the proteins were stained with silver (Moeremans et al., 1985) or with Coomassie brilliant blue R 250 (Serva, Heidelberg, Germany). Finally, the gels were equilibrated in 25% methanol containing 3% glycerol and dried between cellophane foils to keep them transparent for photographic documentation. Western blotting The electrophoretically separated proteins were transferred onto a nitrocellulose membrane (pore size of 0.45 μm; Bio Rad, USA). The blotting was done at a constant potential of 5V per 1 cm in 0.25M Tris and 1.92M glycine buffer, pH 8.3. The nitrocellulose replica was incubated overnight in PBST-LAH, pH 7.2. to block unoccupied sites. Thereafter, the membrane was incubated at 20°C for 1 h with MAbs to IHNV diluted with PBST-LAH. A thorough washing with PBST (3x5 min) was followed by 1-h incubation with HRPSwAMoIgG, another triple washing with PBST (3x5 min) and addition of the substrate solution (diaminobenzidine in 0.1 M Tris-HCI buffer, pH 7.6, and 0.01 % hydrogen peroxide). The reaction was stopped with 0.5% solution of sodium azide. In the case of selection of individual clones, the purified IHN virus strain OSV was applied onto the whole width of the gel. After electrophoretic separation, blotting and blocking of unoccupied sites, the nitrocellulose replica was cut into strips that were subsequently stained with colloid gold (Moeremans et al., 1985), or incubated with individual MAbs diluted 1:1 in PBST-LAH. The procedure continued as described above. In vivo preparation of MAbs and purification of Ig fraction The selected hybridomas after cloning were propagated in vivo after intraperitoneal inoculation of 0.2 ml of the respective hybridoma suspension to BALB/c mice that had been irritated with paraffin oil (0.1 ml per animal) three weeks earlier. The mice were sacrificed approx. 10 days after the inoculation and ascitic fluid to be used for the purification of immunoglobulins were collected. The immunoglobulin fraction was precipitated in 50%saturated solution of ammonium sulphate. The precipitate was pelleted by centrifugation at 5 000 r.p.m. for 15 min, the supernatant was decanted and the pellet was resuspended in the original volume of distilled water. The precipitation and resuspension procedure was repeated twice more to remove all ballast protein and the resulting euglobulin fraction was dialyzed against water for 48 h to Bull. Eur. Ass. Fish Pathol., 24(5) 2004, 223 remove all residues of sulphate ions. Then the fraction was transferred in 0.02M phosphate buffer, pH 8.0, and applied onto a DEAE Sephadex A 5 column (Pharmacia, Uppsala). Ion-exchange chromatography was run using the same buffer (isocratic ion-exchange chromatography). Under the above conditions, the IgG fraction was eluted, while all the remaining proteins were bound to the ion exchanger. The IgG fraction was precipitated from the eluate with 50% ammonium sulphate, pelleted by centrifugation at 5 000 rpm for 15 min and disolved in PBS, pH 7.2. The concentration of Ig was calculated from light absorption at 280 nm (photometer Perkin-Elmer Model 550 S) and the IgG fraction was stored at -80°C. Immunoperoxidase staining Monolayers of the EPC cell line grown on slides were infected with IHNV and fixed with 80% acetone 24 h later. Endogenous peroxidase was inactivated by 30-min incubation at room temperature in PBS containing 0.1% sodium azide and 0.3% hydrogen peroxide. Then the cultures were incubated at room temperature for 30 min with purified MAbs and, after thorough washing with PBST, with HRP-SwAMoIgG diluted 1:1 000 with PBSTLAH. The reaction was visualized by the addition of 3,3' diaminobenzidine in 0.1 M TrisHCI buffer, pH 7.6, The staining was stopped with 0.5% sodium azide and the cultures were embedded into glycerol. Conjugate for direct immunoperoxidase test The purified monoclonal antibodies were conjugated with horseradish peroxidase of RZ 3 purity (Boehringer, Mannheim, Germany) using the periodate technique (Boorsma & Streefkerk, 1979). Results Immunization of mice for fusion The titre of specific antibodies reacting with antigenic determinants of IHNV prior to fusion was 1:51 200. Titres of non-specific antibodies reacting with antigen determinants of the cell line EPC (used for virus propagation)did not exceed 1:400. A comparison of antibody titres after the first and the third (last) immunization showed an increase in the titres of specific antibodies by three dilution steps from 1:6 400 to 1:51 200, while the titre of non-specific antibodies rose only from 1:200 to 1:400. The result of the comparison is suggestive of an effective purification of the antigen used for immunization and of a low proportion of cellular antigens inducing the formation of non-specific antibodies. Selection of hybridoma fluids after fusion The selection of hybridomas was done on the basis of production of murine immunoglobulins reacting in ELISA. Hybridomas producing culture supernatants yielding OD > 1.0 in wells containing the positive virus antigen and OD < 0.1 in wells containing the negative control EPC antigen were selected for further experiments. The selection resulted in 32 clones yielding OD 1.0 to 1.256 with the viral antigen and OD < 0.065 with the cellular antigen. Sera of non-immunized and immunized mice were used as internal controls in ELISA. Selection of hybridomas with respect to antigenic specificity The selected hybridomas were tested for antigenic specificity by immunoblotting. Seventeen of the strains reacted with the nucleoprotein (N antigen) of IHNV whereas twelve of the selected hybridomas recognised the viral glycoprotein (G antigen). The Bull. Eur. Ass. Fish Pathol., 24(5) 2004, 224 hybridomas 2F4 and 2E9, producing antibodies to the N antigen and G antigen respectively, were selected for continued cloning using the end-point titration technique and checks by ELISA and immunoblotting. The clones 2F4/F3, specific for the N antigen, and 2E9/F6, specific for the G antigen (Figure 1), were the final products of this selection. Figure 1. Western blot analysis of antigenic specificity of selected MAbs to IHNV. Lane 1: MAb 2E9/F6. Lane 2: MAb 2F4/F3. Lane 3: MAb C9/C9. Lane 4: MAb HYB 136-3 (Denmark). Lane 5: colloidal gold staining. Larger amounts of MAbs were then prepared by in vivo culture of the clones. SDS-PAGE analysis of concentrated virus samples Samples of the panels of IHNV and VHSV strains propagated in the EPC cell line and concentrated by ultracentrifugation were analysed under reducing conditions in SDSPAGE. The gels containing the IHNV and VHSV fractions were stained with silver and Coomassie blue, respectively. The reason of the difference in staining was that the concentration of VHSV suspension applied onto the gel was ten times higher to enable the detection of possible non-specific (cross) reactivity of MAbs to VHSV in the subsequent immunoblotting. The staining of gels revealed the presence of main viral antigens as well as residual antigens of the cell line used for virus replication (Figures 2 and 4). Relative molecular weights of the structural IHNV proteins were derived from the calibration curve for molecular weight standards. Figure 2. Electrophoretic analysis of a panel of IHNV isolates (silver staining). Lane 1: IHNV US193-110. Lane 2: IHNV US-OSV. Lane 3: IHNV USER. Lane 4: IHNV FR-32/87. Lane 5: IHNV IT-4008. Lane 6: IHNV US-Coleman. Lane 7: IHNV US-TR. Lane 8: IHNV US-DW. Lane 9: IHN US-HAG. S: LMW standards. Bull. Eur. Ass. Fish Pathol., 24(5) 2004, 225 Specificity tests of MAbs The specificity and reactivity of MAb 2F4/F3 to the nucleoprotein of IHNV and MAb 2E9/ F6 to the viral glycoprotein was tested using immunological staining of fractionated structural proteins of IHNV and VHSV. In this test, MAb 2F4/F3 reacted with all the strains of IHNV under study (Figure 3). No cross reactivity with antigenic determinants of VHSV was observed although approx. ten times higher concentration of the fractionated and blotted samples were used. This result has also been confirmed by a positive result of a test with the IHNV strain 193-110 used as an internal standard (Figure 5). The same results were obtained with MAb 2E9/F6. Immunoperoxidase test Selected 2F4/F3 and E9/F6 hybridomas were used for the preparation of ascitic fluids and isolation of monoclonal antibodies therefrom. The hybridomas were separated from the fluids by centrifugation and MAbs were isolated by triple precipitation in 50%-saturated ammonium sulphate. After dialysis against PBS, the concentration of immunoglobulins was determined photometrically and adjusted to 10 mg per 1 ml. The MAbs were used for the demonstration of IHNV in infected EPC Figure 3. Western blotting of IHNV strains and immunologocal staining with the MAb 2F4/F3. Lane 1: IHNV US-193-110. Lane 2: IHNV US-OSV. Lane 3: IHNV US-ER. Lane 4: IHNV FR-32/87. Lane 5: IHNV IT-4008. Lane 6: IHNV US-Coleman. Lane 7: IHNV US-TR. Lane 8: IHNV US-DW. Lane 9: IHN US-HAG. Figure 4. Electrophoretic analysis of a panel of VHSV isolates (Coomasie blue staining). Lane 1: IHNV US-193-110 (inner standard). Lane 2: VHSV DKF1. Lane 3: VHSV FR-23/75. Lane 4: VHSV DK5131. Lane 5: Garda/Bovo (Carpione rhabdovirus). Lane 6: VHSV DK-5151. Lane 7: VHSV DK-5261. Lane 8: VHSV DK-5276. Lane 9: VHSV DK-5231. Lane 10: VHSV DK-3555. Figure 5. Western blotting of VHSV strains and immunologocal staining with the MAb 2F4/F3. Lane 1: IHNV US-193-110 (internal standard). Lane 2: VHSV DKF1. Lane 3: VHSV FR-23/75. Lane 4: VHSV DK-5131. Lane 5: Garda/Bovo (Carpione rhabdovirus). Lane 6: VHSV DK-5151. Lane 7: VHSV DK-5261. Lane 8: VHSV DK-5276. Lane 9: VHSV DK-5231. Lane 10: VHSV DK-3555. Bull. Eur. Ass. Fish Pathol., 24(5) 2004, 226 cell cultures by direct immunoperoxidase test. Foci of IHNV-infected EPC cells were stained and clearly visible. The results of the test are shown in Figures 6 and 7. Discussion Although the purity of the antigen used for the immunization of mice is not of crucial importance in the preparation of monoclonal antibodies, the chances of a successful fusion and preparation of MAbs with a satisfactory specificity increase if a quality antigen with a sufficient immunogenicity is used. Therefore IHNV free of cell debris and partially purified by ultracentrifugation was used for the immunization of mice in our experiments. Further purification procedures were omitted in order to retain as much viral antigen as possible. At the assessment criterion chosen for our experiment, the antibody titres in the immunized mice reached 1:51 200, but the actual titre was almost by one dilution step higher, i.e. 1:100 000. These values are similar to those obtained in mice immunized with the objective to prepare monoclonal antibodies to Figure 6. Demonstration of IHNV in a cell culture using the direct peroxidase conjugate HRP-MAb 2F4/F3 (anti N). VHSV (Veselý et al., 1995). A comparison of antibody titers reached in mice immunized in our laboratory with other viral antigens (data not shown here) supports the view of a rather weak antigenicity of rhabdoviruses. Our selection of hybridomas was based on the demonstration by ELISA of the production of specific antibodies and confirmation of their antigenic specificity by immunological staining after Western blotting. The use of HRPSwAMoIgG in ELISA eliminated a part of hybridomas producing IgM which is often less suitable in diagnostic tests. Checks of antigenic specificity after fusion revealed a prevalence of hybridomas producing MAbs specific to epitopes of the viral nucleoprotein. This finding was confirmed also by results of another two fusions that resulted in 90% hybridomas specific to the N antigen (data not presented here). One explanation could be that the N-protein was the most prevalent viral protein in the ELISA assay used for the initial selection of hybridomas. The smaller number of clones producing MAbs to the viral glycoprotein (G) may also have been due to the separation of Figure 7. Demonstration of IHNV in a cell culture using the direct peroxidase conjugate HRP-MAb 2E9/F6 (anti G). Bull. Eur. Ass. Fish Pathol., 24(5) 2004, 227 structural proteins of IHNV in SDS-PAGE under reducing conditions. Some of anti-G MAbs are associated with conformation specificity to the viral glycoprotein and only some of them react with linear epitopes that are accesible in immunoblotting also under reducing conditions (Huang et al. 1994; Huang et al., 1996). The clone 2F4/F3, producing the antibody to the N protein of IHNV and showing the highest absorbance values in ELISA, was selected for further IHNV identification studies. Favourable results of the use of MAbs to viral nucleoprotein were reported by Chou et al. (1993) and Ristow & Arnzen (1991). Lorenzen et al. (1988) used the antibody to the N protein for the identification of the related VHSV. The specificity of the antibody produced by 2F4/F3 was tested using a panel of IHNV and VHSV strains. The strains were propagated and their antigens were separated by SDSPAGE under reducing conditions. After the separation, the gel was stained and used for the determination of molecular weight of individual fractions. Our investigations showed lower values for the antigens L, P and M than were those reported earlier by Leong et al. (1981) and Hsu et al. (1984), and corresponded rather to the results published by Schutze (1995). i.e. 225 kDa, 57 kDa, 42 kDa, 26 kDa, and 22 kDa for the antigens L, G, N, P, and M, respectively. The separated structural proteins of IHNV and VHSV were transferred onto a nitrocellulose membrane by Western blotting and incubated with MAb 2F4/F3. The antibody proved to be specific, reacted with all the 9 strains of IHNV, but with none of the strains of VHSV. Ristow and Arnzen (1991) reported production 7 of 17 MAbs to the nucleoprotein, which detected all examined IHNV isolates. But following experiments revealed that isolates from different European localities are not detected (Danton et al., 1994). That is why our further studies of the specificity of MAb 2F4/F3 using a larger set of IHNV strains might give an answer to the question, whether the epitope reacting with the antibody is located in a more preserved part of the virus and whether this antibody is suitable for the identification of strains isolated in various parts of the world as presumed by Chou et al. (1993). The differences in immunoblot staining intensity appear to be due to different concentrations of the separated protein fractions, this assumption being supported by the results of silver staining. After the preparation in vivo, ballast proteins were removed from MAbs 2F4/F3 (anti-N) and 2E9/F6 (anti-G) by precipitation with neutral salts and the MAbs were conjugated with horse radish peroxidase to be used in the direct immunoperoxidase test (for the detection of IHNV in infected cell cultures) allowing the identification of the virus after a single incubation with the conjugate and staining with the substrate and chromogen. Moreover, only a light microscope is necessary for the reading of results. Our continuing studies will concentrate on the preparation of antibodies to be used in ELISA. IHNV of salmonid fish is included in list B of the OIE International Code for the Health of Aquatic Animals and has been reported from several European countries. Therefore, a rapid Bull. Eur. Ass. Fish Pathol., 24(5) 2004, 228 and reliable method is urgently needed for health checks of farmed fish to confirm or reject the diagnosis in cases of suspected IHNV infection. 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