Identification of a novel linear B-cell epitope within the collagenase equivalent domain of porcine epidemic diarrhea virus spike glycoprotein

A Comprehensive View on the Protein Functions of Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea (PED) virus (PEDV) is one of the main pathogens causing diarrhea in piglets and fattening pigs. The clinical signs of PED are vomiting, acute diarrhea, dehydration, and mortality resulting in significant economic losses and becoming a major challenge in the pig industry. PEDV possesses various crucial structural and functional proteins, which play important roles in viral structure, infection, replication, assembly, and release, as well as in escaping host innate immunity. Over the past few years, there has been progress in the study of PEDV pathogenesis, revealing the crucial role of the interaction between PEDV viral proteins and host cytokines in PEDV infection. At present, the main control measure against PEDV is vaccine immunization of sows, but the protective effect for emerging virus strains is still insufficient, and there is no ideal safe and efficient vaccine. Although scientists have persistently delved their research into the intricate structure and functionalities of the PEDV genome and viral proteins for years, the pathogenic mechanism of PEDV remains incompletely elucidated. Here, we focus on reviewing the research progress of PEDV structural and nonstructural proteins to facilitate the understanding of biological processes such as PEDV infection and pathogenesis.

Porcine epidemic diarrhea virus strain CH/HLJ/18 isolated in China: characterization and phylogenetic analysis

BACKGROUND

Porcine epidemic diarrhea (PED) is an infectious disease of the digestive tract caused by the porcine epidemic diarrhea virus (PEDV), characterized by vomiting, severe diarrhea, and high mortality rates in piglets. In recent years, the distribution of this disease in China has remarkably increased, and its pathogenicity has also increased. PEDV has been identified as the main cause of viral diarrhea in piglets. This study aimed to understand the genetic evolution and diversity of PEDV to provide a theoretical basis for the development of new vaccines and the prevention and treatment of PED.

METHODS

A PEDV strain was isolated from the small intestine of a diarrheal piglet using Vero cells. The virus was identified using reverse transcription-polymerase chain reaction (RT-PCR), indirect immunofluorescence assay (IFA), and transmission electron microscopy. The whole genome sequence was sequenced, phylogenetic analysis was conducted using MEGA (version 7.0), and recombination analysis was performed using RDP4 and SimPlot. The S protein amino acid sequence was aligned using Cluster X (version 2.0), and the S protein was modeled using SWISS-MODEL to compare differences in structure and antigenicity. Finally, the piglets were inoculated with PEDV to evaluate its pathogenicity in newborn piglets.

RESULT

PEDV strain CH/HLJ/18 was isolated. CH/HLJ/18 shared 89.4-99.2% homology with 52 reference strains of PEDV belonging to the GII-a subgroup. It was a recombinant strain of PEDV BJ-2011-1 and PEDV CH_hubei_2016 with a breakpoint located in ORF1b. Unique amino acid deletions and mutations were observed in the CH/HLJ/18 S protein. The piglets then developed severe watery diarrhea and died within 7 d of inoculation with CH/HLJ/18, suggesting that CH/HLJ/18 was highly pathogenic to newborn piglets.

CONCLUSION

A highly pathogenic recombinant PEDV GII-a strain, CH/HLJ/18, was identified in China, with unique deletion and mutation of amino acids in the S protein that may lead to changes in protein structure and antigenicity. These results will be crucial for understanding the prevalence and variation of PEDV and for preventing and controlling PED.

Elucidating the biological characteristics and pathogenicity of the highly virulent G2a porcine epidemic diarrhea virus

Since the large-scale outbreak of porcine epidemic diarrhoea (PED) in 2010, caused by the genotype 2 (G2) variant of the porcine epidemic diarrhoea virus (PEDV), pig farms in China, even those vaccinated with the G2b vaccine, have experienced infections from the G2a variant, leading to significant economic losses. This study successfully isolated the G2a strain DY2020 from positive small intestine contents (SICs) by blind passage on Vero cells for four generations. The SICs were taken from Daye, Hubei Province, China. The biological characteristics were identified by indirect immunofluorescence assay (IFA) and transmission electron microscopy (TEM). The growth kinetics of the strain on Vero cells were detected by TCID50, and the virus titre could reach 107.35 TCID50 ml-1 (SD: 5.07×106). The pathogenicity towards colostrum-deprived piglets was conducted by assessing faecal viral shedding, morphometric analysis of intestinal lesions, and immunohistochemical staining. The results showed that DY2020 was highly virulent to colostrum-deprived piglets, with severe watery diarrhoea and other clinical symptoms appeared at 6 h post-infection (h p.i.), and all died within 30 h. Pathological tissue examination results showed that the lesions mainly occurred in the intestines of piglets, causing pathological changes such as shortening of intestinal villi. In summary, the discovery of the G2a strain DY2020 in this study is of great significance for understanding Hubei PEDV and provides an important theoretical basis for the development of new efficient PEDV vaccines.

Accurate location of two conserved linear epitopes of PEDV utilizing monoclonal antibodies induced by S1 protein nanoparticles

Porcine Epidemic diarrhea virus (PEDV), which can result in severe vomiting, diarrhea, dehydration and death in newborn piglets, poses a great threat to the pig industry around the world. The S1 subunit of S protein is crucial for triggering neutralizing antibodies binding to the receptor. Based on the advantages of high immunogenicity and precise assembly of nanoparticles, the mi3 nanoparticles and truncated S1 protein were assembled by the SpyTag/SpyCatcher system and then expressed in HEK293F cells, whereafter high-efficiency monoclonal antibodies (mAbs) were produced and identified. The obtained five mAbs can bind to various genotypes of PEDV, including a mAb (12G) which can neutralize G1 and G2 genotypes of PEDV in vitro. By further identification of monoclonal antibody target sequences, 507FNDHSF512 and 553LFYNVTNSYG562 were first identified as B-cell linear epitopes, in which 553LFYNVTNSYG562 was a neutralizing epitope. Alanine scans identified the key amino acid sites of two epitopes. Moreover, the results of multiple sequence alignment analysis showed that these two epitopes were highly conserved in various subtype variants. In brief, these findings can serve as a basis for additional research of PEDV and prospective resources for the creation of later detection and diagnostic techniques.

Antigen epitopes of animal coronaviruses: a mini-review

Coronaviruses are widespread in nature and can infect mammals and poultry, making them a public health concern. Globally, prevention and control of emerging and re-emerging animal coronaviruses is a great challenge. The mechanisms of virus-mediated immune responses have important implications for research on virus prevention and control. The antigenic epitope is a chemical group capable of stimulating the production of antibodies or sensitized lymphocytes, playing an important role in antiviral immune responses. Thus, it can shed light on the development of diagnostic methods and novel vaccines. Here, we have reviewed advances in animal coronavirus antigenic epitope research, aiming to provide a reference for the prevention and control of animal and human coronaviruses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s44149-023-00080-0.

Development of an Indirect Enzyme-Linked Immunosorbent Assay Based on the Yeast-Expressed CO-26K-Equivalent Epitope-Containing Antigen for Detection of Serum Antibodies against Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea (PED) is a severe contagious intestinal disease caused by the porcine epidemic diarrhea virus (PEDV), which leads to high mortality in piglets. In this study, by analyzing a total of 53 full-length spike genes and COE domain regions of PEDVs, the conserved COE fragment of the spike protein from the dominant strain SC1402 was chosen as the target protein and expressed successfully in Pichia pastoris (P. pastoris). Furthermore, an indirect enzyme-linked immunosorbent assay (iELISA) based on the recombinant COE protein was developed for the detection of anti-PEDV antibodies in pig sera. The results showed that under the optimized conditions, the cut-off value of COE-based indirect ELISA (COE-iELISA) was determined to be 0.12. Taking the serum neutralization test as standard, the relative sensitivity of the COE-iELISA was 94.4% and specificity 92.6%. Meanwhile, no cross-reactivity to other porcine pathogens was noted with this assay. The intra-assay and inter-assay coefficients of variation were less than 7%. Moreover, 164 vaccinated serum samples test showed that overall agreement between COE-iELISA and the actual diagnosis result was up to 99.4%. More importantly, the developed iELISA exhibited a 95.08% agreement rate with the commercial ELISA kit (Kappa value = 0.88), which suggested that the expressed COE protein was an effective antigen in serologic tests and the established COE-iELISA is reliable for monitoring PEDV infection in pigs or vaccine effectiveness.

Development of an enzyme-linked immunosorbent assay based on viral antigen capture by anti-spike glycoprotein monoclonal antibody for detecting immunoglobulin A antibodies against porcine epidemic diarrhea virus in milk

BACKGROUND

Porcine epidemic diarrhea (PED), caused by PED virus (PEDV), is a severe enteric disease burdening the global swine industry in recent years. Especially, the mortality of PED in neonatal piglets approaches 100%. Maternal antibodies in milk, particularly immunoglobulin A (IgA) antibodies, are of great importance for protection neonatal suckling piglets against PEDV infection as passive lactogenic immunity. Therefore, appropriate detection methods are required for detecting PEDV IgA antibodies in milk. In the current study, we prepared monoclonal antibodies (mAbs) against PEDV spike (S) glycoprotein. An enzyme-linked immunosorbent assay (ELISA) was subsequently developed based on PEDV antigen capture by a specific anti-S mAb.

RESULTS

The developed ELISA showed high sensitivity (the maximum dilution of milk samples up to 1:1280) and repeatability (coefficient of variation values < 10%) in detecting PEDV IgA antibody positive and negative milk samples. More importantly, the developed ELISA showed a high coincidence rate with a commercial ELISA kit for PEDV IgA antibody detection in clinical milk samples.

CONCLUSIONS

The developed ELISA in the current study is applicable for PEDV IgA antibody detection in milk samples, which is beneficial for evaluating vaccination efficacies and neonate immune status against the virus.

Antigenicity Alternations of Variant PEDV S Protein Disclosed by Linear B Cell Epitope Mapping

The spike protein (S) plays a crucial role in porcine epidemic diarrhea virus (PEDV) infection and induces neutralizing antibodies. Mutations of the S protein are supposed to provide the main antigenic shift leading to the antigenic escape of PEDVs. It is therefore a significant question how much accumulation of antigenic shift could lead to the antigenic escape of the variant PEDV. To provide an answer in the study, B cell epitopes (BCEs) on the S protein of the PEDV vaccine strain CV777 (S^CV777) and variant strain SD2014 (S^SD2014) were mapped using biosynthetic peptides and rabbit anti-PEDV S serum. Seventy-nine and 68 linear BCEs were identified from S^CV777 and S^SD2014, respectively. While 66.2% of the BCEs of S^SD2014 could be recognized by anti-S^CV777 serum and 67.1% of S^CV777 BCEs could be recognized by anti-S^SD2014 serum, more than 40% of the BCEs identified using anti-S^CV777 serum on S^CV777 could not be recognized by anti-S^SD2014 serum and vice versa. The completely shared BCEs took low percentages of 29.4% and 25.3% for S^SD2014 and S^CV777, respectively. These results indicate a low conservation of antigenicity of the S protein compared to a relatively high amino acid sequence similarity of 92.2% between the two strains. The study provided a BCE shift reference of PEDV antigenic escape and surveillance control.

Immunodominant and Neutralizing Linear B-Cell Epitopes Spanning the Spike and Membrane Proteins of Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea virus (PEDV) is the causative agent of PED, an enteric disease that causes high mortality rates in piglets. PEDV is an alphacoronavirus that has high genetic diversity. Insights into neutralizing B-cell epitopes of all genetically diverse PEDV strains are of importance, particularly for designing a vaccine that can provide broad protection against PEDV. In this work, we aimed to explore the landscape of linear B-cell epitopes on the spike (S) and membrane (M) proteins of global PEDV strains. All amino acid sequences of the PEDV S and M proteins were retrieved from the NCBI database and grouped. Immunoinformatics-based methods were next developed and used to identify putative linear B-cell epitopes from 14 and 5 consensus sequences generated from distinct groups of the S and M proteins, respectively. ELISA testing predicted peptides with PEDV-positive sera revealed nine novel immunodominant epitopes on the S protein. Importantly, seven of these novel immunodominant epitopes and other subdominant epitopes were demonstrated to be neutralizing epitopes by neutralization–inhibition assay. Our findings unveil important roles of the PEDV S2 subunit in both immune stimulation and virus neutralization. Additionally, our study shows the first time that the M protein is also the target of PEDV neutralization with seven neutralizing epitopes identified. Conservancy profiles of the epitopes are also provided. In this study, we offer immunoinformatics-based methods for linear B-cell epitope identification and a more complete profile of linear B-cell epitopes across the PEDV S and M proteins, which may contribute to the development of a greater next-generation PEDV vaccine as well as peptide-based immunoassays.

Novel Neutralizing Epitope of PEDV S1 Protein Identified by IgM Monoclonal Antibody

Porcine epidemic diarrhea virus (PEDV) causes devastating enteric disease that inflicts huge economic damage on the swine industry worldwide. A safe and highly effective PEDV vaccine that contains only the virus-neutralizing epitopes (not enhancing epitope), as well as a ready-to-use PEDV neutralizing antibody for the passive immunization of PEDV vulnerable piglets (during the first week of life) are needed, particularly for PEDV-endemic farms. In this study, we generated monoclonal antibodies (mAbs) to the recombinant S1 domain of PEDV spike (S) protein and tested their PEDV neutralizing activity by CPE-reduction assay. The mAb secreted by one hybrodoma clone (A3), that also bound to the native S1 counterpart from PEDV-infected cells (tested by combined co-immunoprecipitation and Western blotting), neutralized PEDV infectivity. Epitope of the neutralizing mAb (mAbA3) locates in the S1A subdomain of the spike protein, as identified by phage mimotope search and multiple sequence alignment, and peptide binding-ELISA. The newly identified epitope is shared by PEDV G1 and G2 strains and other alphacoronaviruses. In summary, mAbA3 may be useful as a ready-to-use antibody for passive immunization of PEDV-susceptible piglets, while the novel neutralizing epitope, together with other, previously known protective epitopes, have potential as an immunogenic cocktail for a safe, next-generation PEDV vaccine.

Expression of the recombinant C-terminal of the S1 domain and N-terminal of the S2 domain of the spike protein of porcine epidemic diarrhea virus

Background and Aim: Porcine epidemic diarrhea virus (PEDV) causes severe diarrhea in suckling piglets, leading to severe economic losses in the swine industry. Commercial vaccines have limited effectiveness against different genogroups of PEDV and the shedding of virus. The C-terminal of the S1 domain and the N-terminal of the S2 domain (S1-2) protein of the spike (S) protein have four neutralizing epitopes. However, research on the expression of the S1-2 segment of the S gene has been limited. In this study, we expressed a recombinant S1-2 protein of the S protein of the PEDV Thai isolate and characterized the immunological properties of the recombinant S1-2 protein. Materials and Methods: The S1-2 segment of the S gene of the PEDV Thai isolate (G2b) was amplified, cloned into the pBAD202/D-TOPO® vector (Invitrogen, Carlsbad, CA, USA), and expressed in Escherichia coli. The optimum concentration of arabinose and the optimum induction time for the expression of the recombinant S1-2 protein were determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The immunogenic reactivity of the recombinant S1-2 protein was determined using Western blot analysis with rabbit polyclonal antibodies against the SM98 strain of PEDV (G1a). Results: The recombinant S1-2 segment of the S gene of the PEDV Thai isolate protein was cloned and the recombinant S1-2 protein was successfully expressed. The optimum concentration of arabinose and the optimum induction time for the induction of the recombinant S1-2 protein were 0.2% and 8 h, respectively. The recombinant S1-2 protein reacted specifically with both rabbit anti-histidine polyclonal antibodies and rabbit anti-PEDV polyclonal antibodies. Conclusion: The recombinant S1-2 protein reacted with rabbit anti-PEDV polyclonal antibodies induced by the different PEDV genogroup. Therefore, the recombinant S1-2 protein may be a useful tool for the development of a diagnostic test for PEDV or for a vaccine against PEDV.

A monoclonal antibody neutralizes pesudorabies virus by blocking gD binding to the receptor nectin-1

Pseudorabies virus (PRV) was isolated from some human cases recently and the infected patients manifested respiratory dysfunction and acute neurological symptoms. However, no effective drug or vaccine, preventing the progression of PRV infection, is available. Nectin-1 was the only reported receptor for PRV cell entry both swine and human origin, representing an excellent target to block PRV infection, and especially its transmission from pigs to humans. A PRV-gD specific mAbs (10B6) was isolated from hybridomas and its neutralizing activities in vitro and in vivo were determined. 10B6 exhibited effective neutralizing activities in vitro with IC₅₀ = 2.514 μg/ml and 4.297 μg/ml in the presence and absence of complement. And in vivo, 10B6 provided 100% protection against PRV lethal challenge with a dose of 15 mg/kg. Further, 10B6 could bind to a conserved epitope, ³¹⁶QPAEPFP³²², locating in gD pro-fusion domain, and finally blocks the binding of PRV-gD to nectin-1. Moreover, 10B6 showed an effective inhibition on PRV cell-attachment in a cell type-independent manner and could also block the virus spreading among cells. 10B6 exhibited effectively neutralizing activities to Chinese PRV variant strain in vitro and in vivo by blocking gD binding to nectin-1, implied both prophylactic and therapeutic interventions against PRV infections.

Molecular Detection and Characterization of Porcine Epidemic Diarrhea Virus and Porcine Aichivirus C Coinfection in México

Swine enteric viral infections are responsible for substantial economic losses in the pork industry worldwide. Porcine epidemic diarrhea (PEDV) is one of the main causative agents of diarrhea in lactating pigs, and reports of PEDV coinfection with other enteric viruses highlight the importance of viral interactions for disease presentation and outcomes. Using next-generation sequencing (NGS) and sequence analyses from samples taken from piglets with acute diarrhea, we explored the possible interactions between PEDV and other less reported pathogens. PEDV coinfection with porcine kobuvirus (PKV) was detected in 36.4% (27/74) of samples. Full genomes from porcine coronavirus and kobuvirus were obtained, as was a partial porcine sapovirus genome (PSaV). The phylogenetic results show the clustering of these strains corresponding to the geographical relationship. To our knowledge, this is the first full genome and isolation report for porcine kobuvirus in México, as well as the first phylogenetic analysis for porcine sapovirus in the country. The NGS approach provides a better perspective of circulating viruses and other pathogens in affected production units.

Development and characterization of monoclonal antibodies against the N-terminal domain of African swine fever virus structural protein, p54

African swine fever virus (ASFV), a re-emerging DNA virus, causes a highly contagious disease for domestic pigs. It is running rife worldwide and threatening the global swine industry. Protein p54 is an attractive candidate for ASFV diagnostic and vaccine design. In this work, we designed a peptide to mimic the N-terminal domain (NTD) of ASFV p54 and pretested it with sera from ASFV-infected pigs. The peptide could be well recognized by the sera, implying that the NTD of p54 contained some potential linear B cell epitopes. Then, the conjugates of the peptide with bovine serum albumin were used as the immunogen to generate monoclonal antibodies (mAbs). A total of six mAbs specific to the NTD of ASFV p54 protein were developed. Five of them well reacted with ASFV HLJ/18 strain and recognized a same linear B cell epitope ⁵FFQPV⁹. Furthermore, epitope ⁵FFQPV⁹ could be well recognized by ASFV-positive sera from natural infected pigs, suggesting that it was a natural linear B-cell epitope. Conservation analysis indicated that epitope ⁵FFQPV⁹ were highly conserved among ASFV epidemic isolates belonging to genotype I and II. Alanine-scanning mutagenesis further revealed that the residues (6F to 9V) of epitope ⁵FFQPV⁹ were the core binding sites for antibody recognition. This is the first research to characterize specific mAbs against NTD of p54 protein. These findings may help further understand the function of p54 protein and the improvement of ASFV diagnosis.

Fine mapping of linear B cell epitopes on capsid protein of porcine circovirus 3

Porcine circovirus type 3 (PCV3) is an emerging swine pathogen associated with acute porcine dermatitis and nephropathy syndrome (PDNS)-like clinical signs, reproductive failure, and multisystemic inflammation. Current evidence shows that PCV3 is spread worldwide, and its high incidence may pose a threat to the global pig industry. Capsid (Cap) protein is the sole structural protein which plays an important role in inducing protective immunity against PCV3 infection. In this study, monoclonal antibodies (mAbs) against Cap protein of PCV3 were produced by the hybridoma technique. Subsequently, 12 serial overlapping peptides (P1 to P12) spanning the entire region of Cap were synthesized to determine the B cell epitope regions using the mAbs. Results from dot-blot and peptide ELISA identified that P3, P9, and P10 were the major B cell antigenic regions. Fine mapping by shorter N- and C-terminal truncated peptides confirmed that the motifs ⁵⁷NKPWH⁶¹, ¹⁴⁰KHSRYFT¹⁴⁶, and ¹⁶¹QSLFFF¹⁶⁶ were linear B cell epitopes, which were highly conserved among different PCV3 strains. Interestingly, we found that the motif ¹⁴⁰KHSRYFT¹⁴⁶ was highly conserved in all reported types of PCVs (i.e., PCV1, PCV2, PCV3, and PCV4), except for the substitution (Y → K → R) of the first residue. This is the first research to identify B cell epitopes of PCV3 Cap, and these findings may lead to a better understanding of the antibody-antigen interaction and provide some guidance for PCV3 vaccine design.Key points• The recombinant Cap protein of PCV3 was expressed and purified in soluble form. • PCV3 Cap-specific mAbs prepared in this study had no cross-reactivity with PCV1/PCV2 Cap. • This is the first report of three conserved linear B cell epitopes on PCV3 Cap. • The minimal residues of the epitopes were 57-61 aa, 140-146 aa, and 161-166 aa.

Identification of a novel B-cell epitope in the spike protein of porcine epidemic diarrhea virus

Background

Porcine epidemic diarrhea virus (PEDV) infection causes an acute enteric tract infectious disease characterized by vomiting, anorexia, dehydration, weight loss and high mortality in neonatal piglets. During PEDV infection, the spike protein (S) is a major virion structural protein interacting with receptors and inducing neutralizing antibodies. However, the neutralizing B-cell epitopes within PEDV S protein have not been well studied.

Methods

To accurately identify the important immunodominant region of S1, the purified truncated S1 proteins (SA, SB, SC, SD and SE) were used to immunize BALB/c mice to prepare polyclonal antibodies. The antisera titers were determined by indirect ELISA, western blot and IFA after four immunizations to find the important immunodominant region of S1, and then purified the immunodominant region of S1 protein and immunized mice to generate the special antibodies, and then used recombinant peptides to determine the B-cell epitopes of monoclonal antibodies.

Results

Five antisera of recombinant proteins of the spike protein region of PEDV were generated and we found that only the polyclonal antibody against part of the S1 region (signed as SE protein, residues 666–789) could recognize the native PEDV. Purified SE protein was used to immunize BALB/c mice and generate mAb 2E10. Pepscan of the SE protein demonstrated that SE16 (⁷²²SSTFNSTREL⁷³¹) is the minimal linear epitope required for reactivity with the mAb 2E10. Further investigation indicated that the epitope SE16 was localized on the surface of PEDV S protein in the 3D structure.

Conclusions

A mAb 2E10 that is specifically bound to PEDV was generated and identified a specific linear B-cell epitope (SE16, ⁷²²SSTFNSTREL⁷³¹) of the mAb. The epitope region of PEDV S1 localized in the different regions in comparison with the earlier identified epitopes. These findings enhance the understanding of the PEDV spike protein structure for vaccine design and provide a potential use for developing diagnostic methods to detect PEDV.

Monoclonal Antibody Preparation and Epitope Identification for Brucella melitensis Elongation Factor Tu

Elongation factor thermo-unstable (EF-Tu), an abundant multifunctional protein, is pivotal during protein synthesis and is an important antigen. Few studies have addressed the role of this protein in Brucella species, and the epitopes of this protein have not been reported. Here, we describe a monoclonal antibody (McAb), BD₆, for EF-Tu in Brucella melitensis. Using western blotting involving a series of partially overlapping recombinant EF-Tu truncation peptides, a novel linear B-cell epitope, ¹¹⁰QTREHIL¹¹⁶ (EF), was identified. Alanine-scanning mutagenesis revealed that residues Q¹¹⁰, T¹¹¹, R¹¹², I¹¹⁵, and L¹¹⁶ were core residues involved in recognition. Sequence alignment suggested that the epitope peptide was conserved among bacterial species but differed by one amino acid residue (I¹¹⁵) from the host sequence. The epitope peptide was recognized by sera from B. melitensis-infected mice, and while recombinant epitope peptide induced a strong humoral immune response, the corresponding mouse peptide, QTREHLL, did not. These results suggested that I¹¹⁵ may be the key residue for the host immune system to distinguish between bacterial and self epitope EF sequences. Indirect immunofluorescence and western blotting assays showed that epitope peptide could be used in Saccharomyces cerevisiae, human embryonic kidney cell (HEK-293), and chicken fibroblast cell (DF1) expression systems and immunoprecipitation assay. Together, our results suggested that the McAb BD₆ is a useful tool for further investigation of the potential functions of the EF-Tu protein in pathogen-host interactions, and that the epitope tag may be useful for application as a novel affinity tag to identify other bacterial pathogens, especially convenient for the identification of intracellular bacteria.