Prokaryotic Expression of Eimeria magna SAG10 and SAG11 Genes and the Preliminary Evaluation of the Effect of the Recombinant Protein on Immune Protection in Rabbits

Eimeria magna is a common coccidia in the intestines of rabbits, causing anorexia, weight loss, diarrhea, and bloody stools. This study cloned and determined the expression levels of four Eimeria surface antigens (EmSAGs) at different developmental stages and showed that EmSAG10 and EmSAG11 are highly expressed at the merozoite stage. Rabbits were immunized with rEmSAG10 and rEmSAG11, and then challenged with E. magna after 2 weeks. Serum-specific antibodies and cytokine levels were detected using ELISA. Immune protection was evaluated based on the rate of the oocysts decrease, the output of oocysts (p < 0.05), the average weight gain, and the feed: meat ratio. Our results showed that rabbits immunized with rEmSAG10 and rEmSAG11 had a higher average weight gain (62.7%, 61.1%), feed; meat ratio (3.8:1, 4.5:1), and the oocysts decrease rate (70.8%, 81.2%) than those in the control group, and also significantly reduced intestinal lesions. The specific IgG level increased one week after the first rEmSAG10 and rEmSAG11 immunization and was maintained until two weeks after the challenge (p < 0.05). The TGF-β, IL-4, and IL-10 levels in the serum increased significantly after the secondary immunization with rEmSAG10 and rEmSAG11, while the IL-2 levels increased significantly after the secondary immunization with rEmSAG11 (both p < 0.05), suggesting that rEmSAG10 can induce a humoral and cellular immunity, while rEmSAG11 can only induce a humoral immunity. Therefore, rEmSAG10 is a candidate antigen for E. magna recombinant subunit vaccines.


Introduction
Eimeria magna is a common coccidia in rabbit intestines. It mainly parasitizes the jejunum and ileum of rabbits. It causes anorexia, weight loss, and diarrhea in rabbits. Infection leads to bloody stools and seriously affects rabbit reproduction [1][2][3][4][5][6]. Currently, the control of rabbit coccidiosis relies mainly on anticoccidial drugs. Although it is inexpensive to add anticoccidial drugs to feed, drug resistance and drug residues remain major problems. Live, attenuated vaccines have good anticoccidial effects, but in some cases, there is a risk of restoring virulence [7,8]. The recombinant subunit vaccine is safer, stable, easy to make in large quantities, and costs less than the live attenuated vaccine [9].
Coccidia have a complex life history, and their antigen composition and distribution are inconsistent across different developmental stages [10]. The immunogenicity of coccidia in the early stages of endogenesis is stronger than in the later stages of sexual reproduction [10]. Located on the surface of coccidia during invasion, the Eimeria surface antigens (SAGs) family is large, and their structural characteristics are obvious [11][12][13]. Recombinant chicken coccidia SAGs can effectively reduce intestinal lesions and weight loss, which have certain There was no signal peptide region, transmembrane helix region and GPI anchor site in EmSAG2 (Table 1). The multiple sequence alignment is invalid because its amino acid sequence is too short.
There was no signal peptide region in EmSAG10, and its transmembrane region was a 195-214 amino acid sequence. The GPI anchor site is amino acid 193 (Table 1). EmSAG10 encoding a protein with a predicted MW of 41 kDa. The multiple sequence comparison of EmSAG10 and other Eimeria coccidia showed that the similarities ranged from 23.4%-34.3% ( Figure 1).
The amino acid sequence of the EmSAG11 signal peptide was a 1-23 amino acid sequence, and its transmembrane domain was a 224-246 amino acid sequence. The GPI anchor site is amino acid 225 (Table 1). EmSAG11 encoding a protein with a predicted MW of 42 kDa. The multiple sequence comparison of EmSAG11 and other Eimeria coccidia showed that the similarity ranged from 26.47%-36.73% (Figure 1). There was no signal peptide region, transmembrane helix region and GPI anchor site in EmSAG2 (Table 1). The multiple sequence alignment is invalid because its amino acid sequence is too short.
There was no signal peptide region in EmSAG10, and its transmembrane region was a 195-214 amino acid sequence. The GPI anchor site is amino acid 193 (Table 1). EmSAG10 encoding a protein with a predicted MW of 41 kDa. The multiple sequence comparison of

Analysis of the Transcription Level Differences in Different Development Stages of EmSAGs
The expression of EmSAG1 (p < 0.001) and EmSAG2 (p < 0.0001) in the sporidized stage of E. magna was higher than that in other stages (Figure 2), and the expression of EmSAG10 (p < 0.0001) and EmSAG11 (p < 0.0001) in the merozoite stage was higher than that in the other stages (Figure 2), the difference was extremely significant (p < 0.0001).

Analysis of the Transcription Level Differences in Different Development Stages of EmSAG
The expression of EmSAG1 (p < 0.001) and EmSAG2 (p < 0.0001) in the sporidiz stage of E. magna was higher than that in other stages (Figure 2), and the expression EmSAG10 (p < 0.0001) and EmSAG11 (p < 0.0001) in the merozoite stage was higher th that in the other stages (Figure 2), the difference was extremely significant (p < 0.0001).

Expression of rEmSAG10 and rEmSAG11
The successfully constructed prokaryotic expression recombinant plasmids of t two genes were transformed into E. coli BL21 (DE3) competent cells for the induction the expression. pET-32a is a tagged fusion protein and we saw that a band of about 40 kD for rEmSAG10 and rEmSAG11, which contained the signature fusion protein was about kDa, and the predicted sizes were 21 kDa and 22 kDa, respectively, so the predicted combinant proteins were 41 kDa and 42 kDa, and our results were consistent with t predicted size. Both pET-32a (+) -rEmSAG10 and pET-32a (+) -rEmSAG11 were express in the inclusion bodies. pET-32a (+) -rEmSAG10 ( Figure 3a) was best expressed at 37 ° and pET-32a (+) -rEmSAG11 ( Figure 3b) was induced at different temperatures and w best expressed at 18 °C. The induction results at different time periods showed that t induction effect improved with an increase in time in the range of 10 h.

Expression of rEmSAG10 and rEmSAG11
The successfully constructed prokaryotic expression recombinant plasmids of the two genes were transformed into E. coli BL21 (DE3) competent cells for the induction of the expression. pET-32a is a tagged fusion protein and we saw that a band of about 40 kDa for rEmSAG10 and rEmSAG11, which contained the signature fusion protein was about 20 kDa, and the predicted sizes were 21 kDa and 22 kDa, respectively, so the predicted recombinant proteins were 41 kDa and 42 kDa, and our results were consistent with the predicted size. Both pET-32a (+) -rEmSAG10 and pET-32a (+) -rEmSAG11 were expressed in the inclusion bodies. pET-32a (+) -rEmSAG10 ( Figure 3a) was best expressed at 37 • C, and pET-32a (+) -rEmSAG11 ( Figure 3b) was induced at different temperatures and was best expressed at 18 • C. The induction results at different time periods showed that the induction effect improved with an increase in time in the range of 10 h.

Western Blotting of rEmSAG10 and rEmSAG11
A western blot analysis showed that the rabbit serum infected with E. magna could recognize rEmSAG10 and rEmSAG11 ( Figure 4, lanes 1 and 2), and the pET-32a tag protein could not be recognized by the rabbit serum infected with E. magna ( Figure 4, lane 3), whereas the rabbit-negative serum did not recognize rEmSAG10 or rEmSAG11 (Figure 4, lanes 4 and 5).

Protective Efficacy of rEmSAG10 and rEmSAG11
Following the first and second immunizations, there were no adverse reactions in the experimental rabbits in each group. According to the average weight gain after the immunization before the challenge, there was no significant difference between the groups (p > 0.05), indicating that rEmSAG10 and rEmSAG11 were in our experiment. It has a good safety profile at the doses used (Table 2).

Western Blotting of rEmSAG10 and rEmSAG11
A western blot analysis showed that the rabbit serum infected with E. magna could recognize rEmSAG10 and rEmSAG11 (  . Western blotting analysis of rEmSAG10 and rEmSAG11. M: Bio-red protein molecular weight standard; lanes 1 and 2 were the identified as rEmSAG10 and rEmSAG11 in the rabbit serum infected with E. magna; Lane 3 was the identified as the pET-32a tag protein protein in the rabbit serum infected with E. magna; lanes 4 and 5 were identified as rEmSAG10 and rEmSAG11 in the rabbit-negative serum for E. magna without infection.

Western Blotting of rEmSAG10 and rEmSAG11
A western blot analysis showed that the rabbit serum infected with E. magna could recognize rEmSAG10 and rEmSAG11 (  . Western blotting analysis of rEmSAG10 and rEmSAG11. M: Bio-red protein molecular weight standard; lanes 1 and 2 were the identified as rEmSAG10 and rEmSAG11 in the rabbit serum infected with E. magna; Lane 3 was the identified as the pET-32a tag protein protein in the rabbit serum infected with E. magna; lanes 4 and 5 were identified as rEmSAG10 and rEmSAG11 in the rabbit-negative serum for E. magna without infection. . Western blotting analysis of rEmSAG10 and rEmSAG11. M: Bio-red protein molecular weight standard; lanes 1 and 2 were the identified as rEmSAG10 and rEmSAG11 in the rabbit serum infected with E. magna; Lane 3 was the identified as the pET-32a tag protein protein in the rabbit serum infected with E. magna; lanes 4 and 5 were identified as rEmSAG10 and rEmSAG11 in the rabbit-negative serum for E. magna without infection.

Protective Efficacy of rEmSAG10 and rEmSAG11
None of the rabbits in the control group died after infection, with symptoms of mental depression, anorexia or obvious diarrhea. The typical pathological changes were observed in the middle and lower segments of the small intestine. The intestine was filled with mucus, blood vessels in the intestinal wall, congestion, and bleeding in the intestinal mucosa (Figure 5a-c); whereas in the unchallenged control group, there were no lesions in the middle and lower segments of the small intestine ( Figure 5d). In the immune groups of rEmSAG10 and rEmSAG11, no rabbit died after infection, the rabbits were active, had a normal appetite, and did not have diarrhea symptoms. An autopsy showed a small number of hemorrhagic filaments in the rEmSAG10 and rEmSAG11 groups (Figure 5e), and there were a few bleeding spots in the crypt of the intestinal tract in the immune group of rEmSAG11 (Figure 5f). The data are presented as the mean ± standard deviation (SD). In each column, there is a significant difference between the data marked with different letters (p < 0.05), and there is no significant difference between the data marked with the same letter (p > 0.05), All values were estimated by ANOVA (Duncan's post hoc) at p ≤ 0.05.
Following the first and second immunizations, there were no adverse reactions in the experimental rabbits in each group. According to the average weight gain after the immunization before the challenge, there was no significant difference between the groups (p > 0.05), indicating that rEmSAG10 and rEmSAG11 were in our experiment. It has a good safety profile at the doses used ( Table 2).
None of the rabbits in the control group died after infection, with symptoms of mental depression, anorexia or obvious diarrhea. The typical pathological changes were observed in the middle and lower segments of the small intestine. The intestine was filled with mucus, blood vessels in the intestinal wall, congestion, and bleeding in the intestinal mucosa (Figure 5a-c); whereas in the unchallenged control group, there were no lesions in the middle and lower segments of the small intestine (Figure 5d). In the immune groups of rEmSAG10 and rEmSAG11, no rabbit died after infection, the rabbits were active, had a normal appetite, and did not have diarrhea symptoms. An autopsy showed a small number of hemorrhagic filaments in the rEmSAG10 and rEmSAG11 groups (Figure 5e), and there were a few bleeding spots in the crypt of the intestinal tract in the immune group of rEmSAG11 (Figure 5f).
Compared with the control group (challenged control, Quil-A saponin, Recombinant pET-32a tag protein), the average specific IgG levels in the serum of the rEmSAG10 ( Figure  6a) and rEmSAG11 (both p < 0.05) (Figure 6b) immunized groups reached a higher level one week after the first immunization, and the specific IgG level after the second immunization tended to be stable, which could be maintained at a higher level until two weeks after the challenge.

Discussion
In this study, SAG10 and SAG11, which are highly expressed in the merozoite stage of E. magna, were selected, and then the rEmSAG10 and rEmSAG11 proteins were obtained as subunit vaccines through the prokaryotic expression. The animal experiments showed that rEmSAG10 and rEmSAG11 immunization led to average weight gain and decreased oocyst output compared to the control group (PBS-infected, Quil-A saponin, Recombinant

Detection of Specific Antibody Levels
Compared with the control group (challenged control, Quil-A saponin, Recombinant pET-32a tag protein), the average specific IgG levels in the serum of the rEmSAG10 (Figure 6a) and rEmSAG11 (both p < 0.05) (Figure 6b) immunized groups reached a higher level one week after the first immunization, and the specific IgG level after the second immunization tended to be stable, which could be maintained at a higher level until two weeks after the challenge.

Discussion
In this study, SAG10 and SAG11, which are highly expressed in the merozoite stage of E. magna, were selected, and then the rEmSAG10 and rEmSAG11 proteins were obtained as subunit vaccines through the prokaryotic expression. The animal experiments showed that rEmSAG10 and rEmSAG11 immunization led to average weight gain and decreased oocyst output compared to the control group (PBS-infected, Quil-A saponin, Recombinant pET-32a tag protein). The relative weight gain rates of rEmSAG10 and rEmSAG11 were 62.7% and 61.1%, respectively, and their oocyst decrease rate were 70.8% and 81.2%, respectively. The results showed that they had a protective effect.
Eimeria magna is an intestinal coccidia found in rabbits. It mainly invades intestinal epithelial cells, which causes intestinal bleeding and huge economic losses to the rabbit industry. Compared with drug prevention and live vaccines, subunit vaccines are a promising alternative strategy for control [22][23][24]. In recent years, subunit vaccines of chicken coccidia, such as MIC2, GAM56, and SAG4, have shown better protective effects [25][26][27].
A surface antigen is one of the candidate antigens for the development of a new generation of vaccines [15,16,28]. Its core function may be to attach itself to host cells before the parasite invasion [29], and the early infection of parasites on host cells can stimulate the host to produce a good immune response [30,31]. Researchers inoculated chickens with rEtSAG to induce humoral and cellular immunity and to protect against E. tenella [32]. Subsequent studies have also confirmed that rEtSAG, as a recombinant vaccine, can induce high levels of antibodies in the host and has a certain effect on chicken coccidiosis infection [33]. The invasion of E. tenella SAG mainly manifests in the sporozoite and merozoite stages [34].
IFN-γ and IL-2 are Th1-type cytokines and play an important role in the resistance to Eimeria infection [35]. IL-2 promotes the growth and differentiation of many immune cells [36,37]. In the present study, compared with the control group, the secretion level of IL-2 in the experimental rabbits inoculated with rEmSAG10 was significantly increased (p < 0.05), indicating that the anticoccidial immunity was activated in the experimental rabbits inoculated with rEmSAG10. However, the secretion level of IFN-γ is not significant. However, the secretion level of IFN-γ did not change significantly in this study. The reason for this phenomenon may be that the increase of IL-10 (Th2 cytokine) promotes the occurrence of the Th2 response, thereby inhibiting the secretion of IFN-γ [38][39][40].
The Th2-type cytokines, mainly IL-4, can regulate humoral immunity, activate helper B cells [41,42], and as other studies have shown, elevated IL-4 levels after inoculation with various recombinant proteins [43,44]. In this study, compared with the control group, we detected IL-4 levels in the serum after two immunizations with rEmSAG10 and rEmSAG11, and observed that IL-4 in the serum was significantly increased, and the antibody response in this study showed that after inoculation with rEmSAG10 and rEmSAG11, the level of IgG in the serum was significantly increased, and both IL-4 and IgG increased, indicating that rEmSAG10 and rEmSAG11 can induce humoral immunity, thereby playing a certain role in the resistance to Eimeria infection.
TGF-β is produced by regulatory T cells and it is an important inflammatory regulator that exhibits pro-inflammatory properties at low concentrations and anti-inflammatory effects at high concentrations [45,46], and stimulates the repair of the damaged mucosal epithelial integrity following injury [47]. IL-10 can inhibit host injury and reduce the production of pro-inflammatory cytokines [48][49][50][51]. In this study, the expression of IL-10 and TGF-β in rEmSAG10 and rEmSAG11 after the second immunization was compared, indicating that rEmSAG10 and rEmSAG11 can inhibit the occurrence of inflammation, reduce host intestinal damage, and thus suggests that TGF-β and IL-10 play a role not only in E. magna infection, but also in the immune response to E. magna antigens.
In summary, EmSAG10, and EmSAG11 are highly expressed during the merozoite stage. The specific IgG antibody levels in the serum of rabbits immunized with rEmSAG10 and rEmSAG11 were still high two weeks after the challenge, and TGF-β, IL-2, IL-4, and IL-10 levels were significantly increased in the serum after the second immunization with rEmSAG10, and TGF-β, IL-4, and IL-10 levels were significantly increased in the serum after the second immunization with rEmSAG11, suggesting that rEmSAG10 can induce humoral and cellular immunity. The results of animal experiments also showed that rEmSAG10 could increase the average body weight and decrease the output of oocysts, indicating that rEmSAG10 is more suitable as a candidate antigen for recombinant subunit vaccines of E. magna.

Animals, Parasites, and Serum
The Beijing strain of E. magna was provided by Professor Xianyong Liu of China Agricultural University. It was subcultured and preserved at the Department of Parasitology of Sichuan Agricultural University. Four stages of E. magna, including unsporulated oocysts, sporulated oocysts, merozoites, and gametophytes, were isolated from a rabbit artificially infected with E. magna.
Forty-eight 35-day-old coccidian-free New Zealand rabbits (850 ± 97.7 g, 24 males and 24 females) were provided by the Department of Parasitology, College of Veterinary Medicine, Sichuan Agricultural University and kept strictly in a coccidian-free environment according to the method described by Shi et al. [52]. The young rabbits were weaned at the age of 18 days, and fed high-temperature sterilized feeding pellets (in-house prepared at this laboratory) in combination with human infant formula until 30 days of age, during which boiled drinking water and feed were provided ad libitum. Anticoccidial drugs were used in cross-rotation in water, and rabbit cages were regularly flame sterilized to prevent contamination with other Eimeria species.
Negative and positive serum samples of E. magna were provided by our laboratory.

Total RNA Extraction and Reverse Transcription
The total RNAs from the four stages of E. magna were extracted using the RNA Extraction Kit (TaKaRa, Dalian, China) and reverse transcribed into cDNA using a reverse transcription system kit (PrimeScript™ RT reagent kit with gDNA Eraser, TaKaRa, Dalian, China), and stored at −80 • C until use.

Transcription Level of the SAGs Genes
The cDNA was prepared by the above-mentioned method to obtain the unsporidized, sporidized, merozoites, and gametophytes stages of rabbit E. magna as a template, and EmSAG1, EmSAG2, EmSAG10, and EmSAG11 (Table 3) genes were amplified by real-time quantitative PCR (Roche, Switzerland), and β-Tubulin and GAPDH (Table 3) were the two housekeeping reference genes for comparison. A 20-µL reaction system consisted of 10 µL TB Green Premix Ex TaqII (Tli RNaseH Plus) (2X) (TaKaRa, Dalian, China), 0.8 µL Forward primer (Sangong, Shanghai, China), 0.8 µL Reverse primer (Sangong, Shanghai, China), 2 µL cDNA template, and 6.4 µL ddH2O. The procedure was as follows: one-step 95 • C for 30 s; two-step 95 • C for 5 s, 60 • C for 30 s; 40 cycles, 95 • C for 15 s, 60 • C for 60 s, and 95 • C for 15 s. Three parallel repeats were performed for each of the four genes. We screened two stable reference genes, Em-GAPDH and Em-β-tubulin, and Em-β-tubulin was selected as the reference gene due to its stability among the different developmental stages; we took the unsporulated stage as the calibrator, and by subtracting the expression level of Em-β-tubulin, we were able to compare the expression level of other developmental stages. The relative transcription levels of the four genes at each developmental stage of E. magna were analyzed using the Ct (Qr = 2 -Ct ) method [53], and subsequently analyzed by one-way ANOVA with a Tukey's post-hoc test using GraphPad Prism version 8.4.

Prokaryotic Expression and Purification
The cDNA of E. magna prepared using the above method was used as a template, and specific primers were designed with reference to the full-length coding sequences of the EmSAG10 and EmSAG11 genes (Table 4). The extracted E. manga cDNA was used as a template for the amplification. The reaction conditions were as follows: 95 • C for 3 min; 35 cycles of 95 • C for 45 s, 60 • C for 45 s, 72 • C for 30 s; 72 • C for 10 min; and 4 • C for 5 min. The PCR products were ligated into the pMD-19 T vector (TaKaRa, Dalian, China) and transformed into DH5α competent cells (Tiangen, Beijing, China). Once a single colony was identified as positive by PCR, the plasmids were extracted and sent to Shanghai Bioengineering Co., Ltd. for sequencing. Once sequencing was correct, the above four were subjected to a fluorescence quantitative PCR to select EmSAG10 and EmSAG11, which were highly expressed in the large merozoite stage, and then the successful construction of pMD19-T-Em-SAG10 and pMD19-T-Em-SAG11 plasmid was subcloned into pET-32a(+) vector (Tiangen, Beijing, China) after the double-enzyme digestion. The constructed pet-32a (+) -Em-SAG10 and pet-32a (+) -Em-SAG11 were transformed into E. coli BL21 (DE3) (TianGen, Beijing, China) and induced with IPTG (Sigma, Cream Ridge, NJ, USA) at 37 • C for 12 h. The cells were collected and sonicated, and SDS-PAGE analyzed the supernatant and precipitate to verify the expression of the recombinant protein. The recombinant proteins were purified using an Ni2-affinity chromatography column (Qiagen, Dusseldorf, Germany), and the purity was detected by 12% SDS-PAGE (Solebao, Beijing, China).

Western Blot Analysis of rEmSAG10 and rEmSAG11
The purified rEmSAG10 and rEmSAG11 were subjected to SDS-PAGE and transferred to a nitrocellulose membrane (Kemaijie, Chengdu, China) using a semi-dry membrane transfer tank (Bio-Rad Laboratories, Hercules, CA, USA). Once cleaning the membrane in TBST three times (5 min each time), the membrane was sealed in TBST with 5% skimmed milk powder (Sangong, Shanghai, China) at 4 • C for 2 h. Once the sealing was completed, the positive and negative serums of E. magna (dilution 1:100) were added, and incubated at 4 • C overnight. Following the washing of the membrane with TBST three times, it was incubated with horseradish peroxidase-labeled goat anti-rabbit IgG (HRP-IgG) (Boster, Wuhan, China) at a dilution of 1:1000 at room temperature for 2 h. Following the washing of the membrane with TBST five times, the reaction was stopped with a diaminobenzidine (DAB) substrate solution (Tiangen, Beijing, China).

Immunization Procedure and Experimental Grouping
A subcutaneous injection into the neck was used for immunization. The immunization and vaccination statuses of each group are shown in Table 5. The rabbits were immunized for the first time at 42 days of age and then immunized with the same dose 14 days later. The blood samples were collected and stored at −20 • C, The body weight was recorded every 7 days. Except for the unchallenged control group, each rabbit in each group was orally inoculated with 1 × 10 5 newly collected sporulated oocysts. Each experimental rabbit was infected with 100 g of anticoccidial-free feed every day.

Evaluation of the Protective Efficacy of rEmSAG10 and rEmSAG11
Safety observation: The body weight was measured at the time of the first immunization, second immunization, and the challenge, and the health status of all experimental rabbits was observed after immunization.
Following infection, the mental state, appetite for food, and diarrhea were observed, and all experimental rabbits were sacrificed 14 days after infection. The lesions of the jejunum and ileum were observed, photographed and recorded, and the relative weight gain rate, feed to meat ratio, and the oocyst decrease rate were measured. According to the national standard for the diagnosis of coccidiosis in animals (GB/T18647-2020), the appropriate amount of rectal fecal samples were collected for the quantitative examination of oocyst output. The average oocysts per gram (OPG) of each group was counted, and the reduction rate of the oocysts was calculated.
The average weight gain before the challenge was equal to the body weight before the first immunization. The average weight gain after the challenge was equal to the body weight before the sacrifice and before the challenge. The relative weight gain rate = (average weight gain of each experimental group/average weight gain of unchallenged group) × 100%; The rate of feed to meat = (total feed before feeding − total amount of residual feed after sacrificed)/total weight gain after infection; Output of oocysts = experimental group OPG/number of animal; Oocyst decrease rate (%) = (challenged control group OPG -immune group OPG)/(challenged control group OPG) × 100%.

Detection of the Serum Antibody IgG
The serum was collected by weekly blood sampling before the first vaccination until the sacrifice period, and the levels of the specific IgG of rEmSAG10 and rEmSAG11 were determined by indirect ELISA, and the serum samples (1:100 dilution) of goat anti-rabbit IgG (Doctor De, Wuhan, China) were detected by the HRP labeled secondary antibody.

Cytokine Detection
An ELISA kit (CUSABIO, Wuhan, China) was used to determine the concentrations of IL-2, IL-4, IL-10, IL-17, TGF-β, and IFN-γ in the serum of the experimental rabbits after the second immunization (before the challenge) according to the instructions.

Data Analysis
All data were analyzed using SPSS software (IBM SPSS version 20.0; SPSS Inc., Chicago, IL, USA) using one-way analysis of variance and the Duncan post-hoc multiple range test. There was no significant difference between the different experimental groups when p > 0.05, but there was a significant difference when p < 0.05. The difference was extremely significant (p < 0.01). All data are expressed as mean ± standard deviation (SD). Graphs were generated using GraphPad Prism 8.4 software.

Conclusions
Our results indicate that rEmSAG10 can induce a humoral and cellular immunity and have certain immune protective effects, and rEmSAG10 can be used as a candidate antigen for recombinant subunit vaccines of Eimeria magna in rabbits. All animal procedures used in this study were performed in accordance with the Guide for the Care and Use of Laboratory Animals (National Research Council, Bethesda, MD, USA) and the recommendations of the Animal Research: Reporting of In Vivo Experiments (ARRIVE) guidelines (http://www.nc3rs.org.uk/arrive-guidelines) (accessed on 28 April 2022). All applicable institutional and national guidelines for the care and use of animals were followed.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The datasets supporting the conclusions of this study are included in this article.