Characterization of the Eimeria tenella rhoptry protein with a nuclear localization sequence (EtROP30)

Evaluation of the immunoprotective effect of the recombinant Eimeria intestinalis rhoptry protein 25 and rhoptry protein 30 on New Zealand rabbits

BACKGROUND

Rabbit coccidiosis is a parasitism caused by either one or multiple co-infections of Eimeria species. Among them, Eimeria intestinalis is the primary pathogen responsible for diarrhea, growth retardation, and potential mortality in rabbits. Concerns regarding drug resistance and drug residues have led to the development of recombinant subunit vaccines targeting Eimeria species as a promising preventive measure. The aim of this study was to assess the immunoprotective efficacy of recombinant subunit vaccines comprising EiROP25 and EiROP30 (rhoptry proteins (ROPs)) against E. intestinalis infection in rabbits.

METHODS

Cloning, prokaryotic expression, and protein purification were performed to obtain EiROP25 and EiROP30. Five groups of fifty 35-day-old Eimeria-free rabbits were created (unchallenged control group, challenged control group, vector protein control group, rEiROP25 group, and rEiROP30 group), with 10 rabbits in each group. Rabbits in the rEiROP25 and rEiROP30 groups were immunized with the recombinant proteins (100 μg per rabbit) for primary and booster immunization (100 μg per rabbit) at a two-week intervals, and challenged with 7 × 104 oocysts per rabbit after an additional two-week interval. Two weeks after the challenge, the rabbits were euthanized for analysis. Weekly collections of rabbit sera were made to measure changes in specific IgG and cytokine level. Clinical symptoms and pathological changes after challenge were observed and recorded. At the conclusion of the animal experiment, lesion scores, the relative weight increase ratio, the oocyst reduction rate, and the anticoccidial index were computed.

RESULTS

Rabbits immunized with rEiROP25 and rEiROP30 exhibited relative weight gain ratios of 56.57% and 72.36%, respectively. Oocysts decreased by 78.14% and 84.06% for the rEiROP25 and rEiROP30 groups, respectively. The anticoccidial indexes were 140 and 155. Furthermore, there was a noticeable drop in intestinal lesions. After the primary immunization with rEiROP25 and rEiROP30, a week later, there was a notable rise in specific IgG levels, which remained elevated for two weeks following challenge (P < 0.05). Interleukin (IL)-2 levels increased markedly in the rEiROP25 group, whereas IL-2, interferon gamma (IFN-γ), and IL-4 levels increased substantially in the rEiROP30 group (P < 0.05).

CONCLUSION

Immunization of rabbits indicated that both rEiROP25 and rEiROP30 are capable of inducing an increase in specific antibody levels. rEiROP25 triggered a Th1-type immune protection response, while rEiROP30 elicited a Th1/Th2 mixed response. EiROP25 and EiROP30 can generate a moderate level of immune protection, with better efficacy observed for EiROP30. This study provides valuable insights for the promotion of recombinant subunit vaccines targeting rabbit E. intestinalis infection.

Host protein EPCAM interacting with EtMIC8-EGF is essential for attachment and invasion of Eimeria tenella in chickens

The five epidermal growth factor-like domains (EGF) of Eimeria tenella microneme protein 8 (EtMIC8) (EtMIC8-EGF) plays a vital role in host cell attachment and invasion. These processes require interactions between parasite proteins and receptors on the surface of host cells. In this study, five chicken membrane proteins potentially interacting with EtMIC8-EGF were identified using the GST pull-down assay and mass spectrometry analysis, and only chicken (Gallus gallus) epithelial cell adhesion molecule (EPCAM) could bind to EtMIC8-EGF. EPCAM-specific antibody and recombinant EPCAM protein (rEPCAM) inhibited the EtMIC8-EGF binding to host cells in a concentration-dependent manner. Furthermore, the rEPCAM protein showed a binding activity to sporozoites in vitro, and a significant reduction of E. tenella invasion in DF-1 cells was further observed after pre-incubation of sporozoites with rEPCAM. The specific anti-EPCAM antibody further significantly decreased weight loss, lesion score and oocyst output during E. tenella infection, displaying partial inhibition of E. tenella infection. These results indicate that chicken EPCAM is an important EtMIC8-interacting host protein involved in E. tenella-host cell adhesion and invasion. The findings will contribute to a better understanding of the role of adhesion-associated microneme proteins in E. tenella.

Molecular characterization and immune protective efficacy of 3 Eimeria tenella antigens

Avian coccidiosis caused by Eimeria is a serious parasitic disease that poses a threat to the poultry industry. Currently, prevention and treatment mainly rely on the administration of anticoccidials and live oocyst vaccines. However, the prevalence of drug resistance and the inherent limitations of live vaccines have driven the development of novel vaccines. In this study, the surface protein (Et-SAG14), a previously annotated rhoptry protein (Eten5-B), and a gametocyte phosphoglucomutase (Et-PGM1) were characterized and the vaccine potential of the recombinant proteins were evaluated. Et-SAG14 was dispersed in the form of particles in the sporozoite and merozoite stages, whereas Et-PGM1 was distributed in the apical part of the sporozoite and merozoite stages. The previously annotated rhoptry Eten5-B was found not to be located in the rhoptry but distributed in the cytoplasm of sporozoites and merozoites. Immunization with rEten5-B significantly elevated host interferon gamma (IFN-γ) and interleukin 10 (IL-10) transcript levels and exhibited moderate anticoccidial effects with an anticoccidial index (ACI) of 161. Unexpectedly, both recombinant Et-SAG14 and Et-PGM1 immunization significantly reduced host IFN-γ and IL-10 transcription levels, and did not show protection against E. tenella challenge (ACI < 80). These results suggest that the rEten5-B protein can trigger immune protection against E. tenella and may be a potential and effective subunit vaccine for the control of coccidiosis in poultry.

A candidate virulence factor of Eimeria tenella (EtROP30) predicted by virulence enhancement of transgenic Toxoplasma gondii

Difficulties of in vitro culture and genetic manipulation of Eimeria tenella have hindered the screening of virulence factors in this parasite. In this study, the E. tenella rhoptry protein 30 (EtROP30) was expressed in Toxoplasma gondii (RH∆Ku80-EtROP30), and its effect on the proliferation and virulence of parasites was investigated. The results revealed that the expression of EtROP30 had no impact on the invasion and egress processes. However, the RH∆Ku80-EtROP30 strain formed larger plaques compared to the RH∆Ku80, indicating that the EtROP30 expression promotes T. gondii proliferation. Furthermore, the RH∆Ku80-EtROP30 strain exhibited greater pathogenicity, resulting in earlier mortality and shorter overall survival time compared to RH∆Ku80. These results imply that EtROP30 expression facilitates parasite intracellular proliferation and virulence in mice, suggesting that EtROP30 might be a candidate virulence factor of E. tenella.

A rhoptry protein, localizing in the bulb region of rhoptries, could induce protective immunity against Eimeria tenella infection

Background

Rhoptry organelle proteins (ROPs) secreted by apicomplexan parasites play important roles during parasites invasion and survival in host cells, and are potential vaccine candidates against apicomplexan diseases. Eimeria tenella (E. tenella) is one of the most noteworthy apicomplexan species, which causes hemorrhagic pathologies. Although dozens of putative E. tenella ROP sequences are annotated, most ROP proteins are not well studied.

Methods

In this study, an E. tenella ROP21 gene was identified and the recombinant EtROP21 protein (rEtROP21) was expressed in Escherichia coli. The developmental expression levels, localization, and protective efficacy against E. tenella infection in chickens were studied.

Results

An EtROP21 gene fragment with an open reading frame (ORF) of 981 bp was obtained from the Beijing strain of E. tenella. The rEtROP21 has a molecular weight of approximately 50 kDa and was recognized by rEtROP21-immunized mouse serum. Two specific protein bands, about 43 KDa and 95 KDa in size, were detected in the whole sporozoite proteins using the rEtROP21-immunized chicken serum. RT-qPCR analysis of the E. tenella ROP21 gene (EtROP21) revealed that its mRNA levels were higher in merozoites and sporozoites than in sporulated and unsporulated oocysts. Immunofluorescence and immunoelectron analyses showed that the EtROP21 protein predominantly localizes in the bulb region of rhoptries distributed at anterior, posterior, and perinuclear regions of E. tenella sporozoites. Immunization and challenge experiments revealed that immunizing chickens with rEtROP21 significantly increased their average body weight gain while decreasing mean lesion score and oocyst output (P <0.05). When compared with the challenged control group, the rEtROP21-immunized group was associated with a significantly higher relative weight gain (90.2%) and a greater reduction in oocyst output (67%) (P <0.05). The anticoccidial index of the rEtROP21-immunized group was 163.2. Chicken serum ELISA revealed that the levels of the specific anti- rEtROP21 antibody, IFN-γ, and IL-4 were significantly higher in the rEtROP21-immunized group than in the challenged control group (P <0.05).

Conclusion

These results indicate that rEtROP21 can induce a high level of specific immune response and it is a potential candidate for the development of vaccines against E. tenella infection in chickens.

The Immunoprotective Effect of ROP27 Protein of Eimeria tenella

Eimeria tenella rhoptry protein has the properties of a protective antigen. EtROP27 is a pathogenic gene that is detected via a transcriptome, but its expression pattern, immunogenicity, and potency are unknown. Therefore, a gene segment of EtROP27 was amplified and transplanted into the pET28a prokaryotic vector for the expression of the recombinant protein, and it subsequently purified for the generation of a polyclonal antibody. Then, RT-PCR and Western blotting were performed to understand the expression pattern of EtROP27. Subsequently, animal experiments were conducted to evaluate the immunoprotective effect of the recombinant protein with different immunizing doses (50, 100, and 150 μg). The results showed that the expression of EtROP27 gradually increased with the prolongation of infection time, reaching the highest level at 96 h and then decreasing. Additionally, EtROP27 is a natural antigen of coccidia that can stimulate the body to produce high levels of IgY. As with recombinant protein vaccines, the results of immune protection evaluation tests showed that the average weight gain rates of the immune challenge groups were significantly higher than that of the challenged control group, and their average lesion scores were significantly lower than that of the challenged control group. Furthermore, the oocyst excretion decreased by 81.25%, 86.21%, and 80.01%, and the anticoccidial index was 159.45, 171.47, and 166.75, respectively, for these groups. EtROP27 is a promising antigen gene candidate for the development of a coccidiosis vaccine.

Characterization of the novel glucose-methanol-choline (GMC) oxidoreductase EnOXIO1 in Eimeria necatrix

Eimeria species are intracellular obligate parasites, among the most common pathogens affecting the intensive poultry industry. Oxidoreductases are members of a class of proteins with redox activity and are widely found in apicomplexan protozoans. However, there have been few reports related to Eimeria species. In this study, total RNA was extracted from the gametocytes of E. necatrix Yangzhou strain to amplify the EnOXIO1 gene using reverse-transcription polymerase chain reaction. After cloning and sequence analysis, the prokaryotic expression vector pET-28a(+)-EnOXIO1 was constructed and transformed into Escherichia coli BL21(DE3), and the recombinant protein rEnOXIO1 was expressed by induction with isopropyl ß-D-1-thiogalactopyranoside. The full length EnOXIO1 gene was 2535 bp encoding 844 amino acids, and the EnOXIO1 protein had a molecular weight of about 100 kDa and was mainly expressed in inclusion bodies. Western blot analysis indicated that the rEnOXIO1 protein had good antigenicity and cross-reactivity and was specifically recognized by a 6 ×HIS labeled monoclonal antibody, mouse anti-recombinant protein polyclonal antibody, and recovery serum from chickens infected with E. necatrix, E. acervulina, and E. tenella sporulated oocysts. The results of laser confocal immunofluorescence localization showed that the EnOXIO1 protein was mainly located on the wall-forming bodies in gametocytes and played an important role in the formation of the oocyst wall. Quantitative PCR analysis revealed that transcript levels of EnOXIO1 were highest in the gametocyte stage. Protein expression levels of EnOXIO1 were higher in the gametocyte stage than in other developmental stages according to western blot analysis. Vaccination of chickens against E. necatrix was achieved with recombinant protein rEnOXIO1, which triggered humoral immunity and antibody production, increased average body weight gain, reduced oocyst output and alleviated lesions after E. necatrix infection. The highest ACI value (172.36) was observed in chickens that received 200 μg rEnOXIO1 compared with other immunization groups.

Overexpression of Eimeria tenella Rhoptry Kinase 2 Induces Early Production of Schizonts

Eimeria tenella is an obligate intracellular parasite responsible for avian coccidiosis. Like other apicomplexan parasites, such as Toxoplasma gondii, cell invasion and intracellular development rely on apical organelle content discharge, named micronemes and rhoptries. Some rhoptry (ROP) kinases (ROPK) are key virulence factors in T. gondii. To date, among the 28 ropk genes carried by E. tenella, only two to four were confirmed by proteomic analysis or immunostaining to be expressed at the sporozoite stage. We have previously shown that EtROP1 is implicated in the inhibition of host cell apoptosis by interacting with the cellular p53. This work functionally described the second ROP kinase expressed at the sporozoite stage in E. tenella. EtROP2 is an active kinase that phosphorylates cell substrates of approximately 50 kDa. Its overexpression leads to the shortening of the prepatent period and to the early development of first-generation schizonts. Conduction of RNA sequencing analysis and reverse transcriptase quantitative PCR (RT-qPCR) on the host cell allowed us to identify the mitogen-activated protein kinase (MAPK) pathway and the transcription factor cFos to be upregulated by EtROP2. We also showed by immunofluorescence assay that the active kinase EtROP2 is implicated in the p38 MAPK pathway activation. We established here that EtROP2 activates the p38 MAPK pathway through a direct or indirect phosphorylation, leading to the overexpression of the master transcription factor cFos known to be implicated in E. tenella development. IMPORTANCE Rhoptries are specialized secretory organelles found in zoite stages of apicomplexan parasites. In addition to well-conserved rhoptry neck proteins, their protein consists mostly of kinase proteins, highly divergent from eukaryotic kinases. Some of those kinases are described as major virulence factors in Toxoplasma gondii, secreted into the host cell to hijack signaling pathways. Most of those kinases remain to be characterized in Eimeria tenella. Deciphering their cellular function is a prerequisite to supporting their relevance as a druggable target in development of new means of Eimeria tenella control. Secreted divergent kinases that interact with host cell partners to modulate pathways are good candidates, as they coevolve with their host targets to ensure their function within the host and are less prone to mutations that would lead to drug resistance. The absence of any orthologous kinase in host cells makes these parasite kinases a promising drug target candidate.

Molecular characterization and protective efficacy of vacuolar protein sorting 29 from Eimeria tenella

Introduction

Vacuolar protein sorting 29 (VPS29) is a core component of the retromer-retriever complex and is essential for recycling numerous cell-surface cargoes from endosomes. However, there are no reports yet on VPS29 of Eimeria spp.

Methods

Here, we cloned and prokaryotically expressed a partial sequence of Eimeria tenella VPS29 (EtVPS29) with RT-PCR and engineered strain of Escherichia coli respectively. The localization of the VPS29 protein in E. tenella sporozoites was investigated with immunofluorescence (IFA) and overexpression assays. And its protective efficacy against E. tenella infection was investigated in chickens with the animal protection test.

Results

An EtVPS29 gene fragment with an ORF reading frame of 549 bp was cloned. The band size of the expressed recombinant protein, rEtVPS29, was approximately 39 kDa and was recognized by the chicken anti-E. tenella positive serum. EtVPS29 protein was observed widely distributing in the cytoplasm of E. tenella sporozoites in the IFA and overexpression assays. rEtVPS29 significantly increased average body weight gain and decreased mean lesion score and oocyst output in chickens. The relative weight gain rate in the rEtVPS29-immunized group was 62.9%, which was significantly higher than that in the unimmunized and challenged group (P < 0.05). The percentage of reduced oocyst output in the rEtVPS29 immunized group was 32.2%. The anticoccidial index of the rEtVPS29-immunized group was 144.2. Serum ELISA also showed that rEtVPS29 immunization induced high levels of specific antibodies in chickens.

Discussion

These results suggest that rEtVPS29 can induce a specific immune response and is a potential candidate for the development of novel vaccines against E. tenella infections in chickens.

Identification and Characterization of Eimeria tenella Rhoptry Protein 35 (EtROP35)

Simple Summary

Rhoptry proteins (ROPs) of phylum Apicomplexa parasites are important secretory virulence factors as well as candidate vaccines. However, studies on ROPs of Eimeria tenella are limited. In this study, the coding sequence of E. tenella rhoptry protein 35 (EtROP35) was cloned, and then its localization, expression in parasite, potential role within invasion, and protective efficacy were investigated. Sequence analysis and subcellular localization revealed that EtROP35 is a rhoptry protein of E. tenella. Sporozoite invasion-blocking assay and protective efficacy indicated that EtROP35 might be involved in the parasite invasion process and may be a potential vaccine candidate against E. tenella.

Abstract

Rhoptry proteins (ROPs) of Apicomplexa are crucial secreted virulence factors and sources of vaccine candidates. To date, Eimeria tenella ROPs are not well studied. This study identified and characterized a novel E. tenella ROP (EtROP35), which showed the highest levels among 28 putative ROPs in previous sporozoite and merozoite transcriptomes. Sequence analysis showed that EtROP35 contains an N-terminal secretory signal and a protein kinase domain including eight conserved ROP35-subfamily motifs. Subsequent experiments confirmed that it is a secretory protein. Subcellular localization revealed it localized at the apical end of the sporozoites and merozoites, which was consistent with the ROPs of other Apicomplexan parasites. To further understand the biological meaning of EtROP35, expression levels in different developmental stages and sporozoite invasion-blocking assay were investigated. EtROP35 showed significantly higher levels in sporozoites (6.23-fold) and merozoites (7.00-fold) than sporulated oocysts. Sporozoite invasion-blocking assay revealed that anti-EtROP35 polyclonal antibody significantly reduced the sporozoite invasion rate, suggesting it might participate in host cell invasion and be a viable choice as a vaccine candidate. The immunological protective assays showed that EtROP35 could induce a high level of serum IgY and higher mean body weight gain, and lower cecum lesion score and oocysts excretion than the challenged control group. These data indicated that EtROP35 had good immunogenicity and may be a promising vaccine candidate against E. tenella.

Transcriptional changes in LMH cells induced by Eimeria tenella rhoptry kinase family protein 17

Though a number of Eimeria tenella rhoptry kinase family proteins have been identified, little is known about their molecular functions. In the present study, the gene fragment encoding the matured peptide of E. tenella rhoptry kinase family protein 17 (EtROP17) was used to construct a recombinant vector, followed by transfection into leghorn male hepatoma (LMH) cells. Then, the transcriptional changes in the transfected cells were determined by RNA-seq. The expression of EtROP17 in LMH cells was validated by both Western blot and indirect immunofluorescence analysis. Our analysis showed that EtROP17 altered the expression of 309 genes (114 downregulated genes and 195 upregulated genes) in LMH cells. The quantitative real-time polymerase chain reaction (qRT-PCR) results of the selected differentially expressed genes (DEGs) were consistent with the RNA-seq data. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that DEGs were significantly enriched in nine pathways, such as toll-like receptor signaling pathway, ECM-receptor interaction, intestinal immune network for IgA production and focal adhesion. These findings reveal several potential roles of EtROP17, which contribute to understanding the molecular mechanisms underlying the host-parasite interplay.