Immune Responses Induced by Recombinant Bacillus Subtilis Expressing the Hemagglutinin Protein of H5N1 in chickens

Important role of Bacillus subtilis as a probiotic and vaccine carrier in animal health maintenance

Bacillus subtilis is a widespread Gram-positive facultative aerobic bacterium that is recognized as generally safe. It has shown significant application value and great development potential in the animal farming industry. As a probiotic, it is frequently used as a feed growth supplement to effectively replace antibiotics due to its favourable effects on regulating the intestinal flora, improving intestinal immunity, inhibiting harmful microorganisms, and secreting bioactive substances. Consequently, the gut health and disease resistance of farmed animals can be improved. Both vegetative and spore forms of B. subtilis have also been utilized as vaccine carriers for delivering the antigens of infectious pathogens for over a decade. Notably, its spore form is regarded as one of the most prospective for displaying heterologous antigens with high activity and stability. Previously published reviews have predominantly focused on the development and applications of B. subtilis spore surface display techniques. However, this review aims to summarize recent studies highlighting the important role of B. subtilis as a probiotic and vaccine carrier in maintaining animal health. Specifically, we focus on the beneficial effects and underlying mechanisms of B. subtilis in enhancing disease resistance among farmed animals as well as its potential application as mucosal vaccine carriers. It is anticipated that B. subtilis will assume an even more prominent role in promoting animal health with in-depth research on its characteristics and genetic manipulation tools.

Surface Display of Duck Hepatitis A Virus Type 1 VP1 Protein on Bacillus subtilis Spores Elicits Specific Systemic and Mucosal Immune Responses on Mice

Duck viral hepatitis, primarily caused by duck hepatitis A virus type 1 (DHAV-1), poses a significant threat to the global duck industry. Bacillus subtilis is commonly utilized as a safe probiotic in the development of mucosal vaccines. In this study, a recombinant strain of B. subtilis, designated as B. subtilis RV, was constructed to display the DHAV-1 capsid protein VP1 on its spore surface using the outer coat protein B as an anchoring agent. The immunogenicity of this recombinant strain was evaluated in a mouse model through mixed feeding immunization. The results indicated that B. subtilis RV could elicit specific systemic and mucosal immune responses in mice, as evidenced by the high levels of serum IgG, intestinal secretory IgA, and potent virus-neutralizing antibodies produced. Furthermore, the recombinant strain significantly upregulated the expression levels of IL-2, IL-6, IL-10, TNF-α, and IFN-γ in the intestinal mucosa. Thus, the recombinant strain maintained the balance of the Th1/Th2 immune response and demonstrated an excellent mucosal immune adjuvant function. In summary, this study suggests that B. subtilis RV can be a novel alternative for effectively controlling DHAV-1 infection as a vaccine-based feed additive.

The use of Bacillus subtilis as a cost-effective expression system for production of Cholera Toxin B fused factor VIII epitope regions applicable for inducing oral immune tolerance

Coagulation factor replacement therapy for the X-linked bleeding disorder Haemophilia, characterized by a deficiency of coagulation protein factor VIII (FVIII), is severely complicated by antibody (inhibitors) formation. The development of FVIII inhibitors drastically alters the quality of life of the patients and is associated with a tremendous increase in morbidity as well as treatment costs. The ultimate goal of inhibitor control is antibody elimination. Immune tolerance induction (ITI) is the only clinically established approach for developing antigen-specific tolerance to FVIII. This work aims to establish a novel cost-effective strategy to produce FVIII molecules in fusion with cholera toxin B (CTB) subunit at the N terminus using the Bacillus subtilis expression system for oral tolerance, as the current clinical immune tolerance protocols are expensive. Regions of B-Domain Deleted (BDD)-FVIII that have potential epitopes were identified by employing Bepipred linear epitope prediction; 2 or more epitopes in each domain were combined and cDNA encoding these regions were fused with CTB and cloned in the Bacillus subtilis expression vector pHT43 and expression analysis was carried out. The expressed CTB-fused FVIII epitope domains showed strong binding affinity towards the CTB-receptor GM1 ganglioside. To conclude, Bacillus subtilis expressing FVIII molecules might be a promising candidate for exploring for the induction of oral immune tolerance.

Construction and Immunogenicity Evaluation of Recombinant Bacillus subtilis Expressing HA1 Protein of H9N2 Avian Influenza Virus

The H9N2 subtype of the avian influenza virus (AIV) is one of the main subtypes of low pathogenic AIV, and it seriously affects the poultry breeding industry. Currently, vaccination is still one of China's main strategies for controlling H9N2 avian influenza. In this study, we selected MW548848.1 on the current popular main branch h9.4.2.5 as the reference strain, and we optimized the amino acid sequence of HA1 to make it suitable for expression in Bacillus subtilis. The B. subtilis expression vector showed good safety and stress resistance; therefore, this study constructed a recombinant B. subtilis expressing H9N2 HA1 protein and evaluated its immunogenicity in mice. The following results were obtained: the sIgA level of HA1 protein in small intestine fluid and the IgG level of PHT43-HA1/B. subtilis in serum were significantly improved (P < 0.01); PHT43-HA1/B. subtilis can cause a special immune response in mice; and cytokine detection interferon-gamma (IFN-γ) (P < 0.05) and Interleukin 2 (IL-2) (P < 0.01) expressions significantly increased. Additionally, the study found that PHT43-HA1/B. subtilis can alleviate the attack of H9N2 AIV in the spleen, lungs, and small intestine of mice. This study was the first to use an oral recombinant B. subtilis-HA1 vaccine candidate, and it provides theoretical data and technical reference for the creation of a new live vector vaccine against H9N2 AIV.

Bacillus subtilis expressing duck Tembusu virus E protein induces immune protection in ducklings

Duck Tembusu virus (DTMUV) is an infectious disease that emerged in China in 2010. It has caused serious economic losses to the poultry industry and may pose a threat to public health. We aimed to develop a new Bacillus subtilis (B. subtilis)-based oral vaccine to control DTMUV transmission among poultry; to this end, we constructed a B. subtilis strain that can secrete DTMUV E protein. Ducklings were orally immunized, and serum antibodies, mucosal antibodies, and splenic cytokines were detected. The results showed that, in addition to high levels of specific IgG, there were also high levels of specific secretory immunoglobulin A (sIgA) in ducklings orally treated with recombinant B. subtilis. In addition, the levels of IFN-γ, IL-2, IL-4, and IL-10 in spleens were significantly boosted by recombinant B. subtilis. Recombinant B. subtilis could effectively enhance ducklings resistance to DTMUV and significantly reduce viral load (p<0.01), along with pathological damage in the brain, heart, and spleen. This is the first study to apply a B. subtilis live-vector vaccine platform for DTMUV disease prevention and control, and our results suggest that B. subtilis expressing DTMUV E protein may be a candidate vaccine against DTMUV.

Strategies for enhancing immunity against avian influenza virus in chickens: A review

Poultry infection with avian influenza viruses (AIV) is a continuous source of concern for poultry production and human health. Uncontrolled infection and transmission of AIV in poultry increases the potential for viral mutation and reassortment, possibly resulting in the emergence of zoonotic viruses. To this end, implementing strategies to disrupt the transmission of AIVs in poultry, including a wide array of traditional and novel methods, is much needed. Vaccination of poultry is a targeted approach to reduce clinical signs and shedding in infected birds. Strategies aimed at enhancing the effectiveness of AIV vaccines are multi-pronged and include methods directed towards eliciting immune responses in poultry. Strategies include producing vaccines of greater immunogenicity via vaccine type and adjuvant application and increasing bird responsiveness to vaccines by modification of the gastrointestinal tract (GIT) microbiome and dietary interventions. This review provides an in-depth discussion of recent findings surrounding novel AIV vaccines for poultry, including reverse genetics vaccines, vectors, protein vaccines and virus like particles, highlighting their experimental efficacy among other factors such as safety and potential for use in the field. In addition to the type of vaccine employed, vaccine adjuvants also provide an effective way to enhance AIV vaccine efficacy, therefore, research on different types of vaccine adjuvants and vaccine adjuvant delivery strategies is discussed. Finally, the poultry gastrointestinal microbiome is emerging as an important factor in the effectiveness of prophylactic treatments. In this regard, current findings on the effects of the chicken GIT microbiome on AIV vaccine efficacy are summarized here.

Bacillus subtilis spores as delivery system for nasal Plasmodium falciparum circumsporozoite surface protein immunization in a murine model

Malaria remains a widespread public health problem in tropical and subtropical regions around the world, and there is still no vaccine available for full protection. In recent years, it has been observed that spores of Bacillus subtillis can act as a vaccine carrier and adjuvant, promoting an elevated humoral response after co-administration with antigens either coupled or integrated to their surface. In our study, B. subtillis spores from the KO7 strain were used to couple the recombinant CSP protein of P. falciparum (rPfCSP), and the nasal humoral-induced immune response in Balb/C mice was evaluated. Our results demonstrate that the spores coupled to rPfCSP increase the immunogenicity of the antigen, which induces high levels of serum IgG, and with balanced Th1/Th2 immune response, being detected antibodies in serum samples for 250 days. Therefore, the use of B. subtilis spores appears to be promising for use as an adjuvant in a vaccine formulation.

Probiotic supplementation containing Bacillus velezensis enhances expression of immune regulatory genes against pigeon circovirus in pigeons (Columba livia)

AIMS

In this study, we aimed to isolate and evaluate the efficacy of Bacillus velezensis as a probiotic and to assess its activity towards pigeons infected with pigeon circovirus (PiCV).

METHODS AND RESULTS

Bacillus velezensis, isolated from pigeon faeces, was orally administered to pigeons for 60 days. After pigeons were challenged with PiCV, the PiCV viral load and expression of indicator genes for innate immunity were detected in spleen tissue and faeces of pigeons. Bacillus velezensis significantly reduced the PiCV viral load in the faeces and spleen of pigeons 5 days post-challenge (dpc). The mRNA expression levels of treated pigeons showed that interferon-gamma (IFN-γ), myxovirus resistance 1 (Mx1), and signal transducers and activators of transcription 1 (STAT1) genes were upregulated, whereas no expression of interleukin-4 (IL-4) was detected. Moreover, toll-like receptor 2 (TLR2) and 4 (TLR4) were significantly upregulated in probiotic-treated pigeons (P < 0·05).

CONCLUSIONS

This is the first report showing that probiotic supplementation can effectively enhance the T-helper type 1 immune response and decrease the PiCV viral loads in pigeons.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study proposes that the administration of a probiotic strain, B. velezensis, to pigeons can protect against PiCV infection.

Application of Bacillus subtilis as a live vaccine vector: A review

Bacillus subtilis is widely used as a probiotic in various fields as it regulates intestinal flora, improves animal growth performance, enhances body immunity, has short fermentation cycle, and is economic. With the rapid development of DNA recombination technology, B. subtilis has been used as a potential vaccine expression vector for the treatment and prevention of various diseases caused by bacteria, viruses, and parasites as it can effectively trigger an immune response in the body. In this review, we refer to previous literature and provide a comprehensive analysis and overview of the feasibility of using B. subtilis as a vaccine expression vector, with an aim to provide a valuable reference for the establishment of efficient vaccines.

Comparison of oral and nasal immunization with inactivated porcine epidemic diarrhea virus on intestinal immunity in piglets

Porcine epidemic diarrhea virus (PEDV) has proven to be a major problem for the porcine industry worldwide. Conventional injectable vaccines induce effective systemic immune responses but are less effective in preventing PEDV at mucosal invasion sites, including the nasal or oral mucosa. Additionally, antigens delivered orally are easily degraded. Nasal immunization induces intestinal mucosal immune responses, which can aid in blocking viral invasion, and requires fewer antigen inoculation doses. Therefore, nasal immunizations are considered to be a potential approach to overcome viral infections. In the present study, nasal immunization of piglets was performed using inactivated PEDV combined with Bacillus subtilis as an immunopotentiator and the efficacy of nasal immunization was assessed. The results demonstrated that compared with oral immunization, piglets from the nasal immunization group exhibited higher levels of neutralizing antibodies (P<0.01) in the intestine, PEDV-specific immunoglobulin (Ig)G (P<0.01) in serum and PEDV-specific secretory IgA (SIgA) in saliva (P<0.01) and nasal secretions (P<0.01). An increased number of intestinal CD3⁺ T cells, IgA-secreting cells and intraepithelial lymphocytes (P<0.05) were also observed. Furthermore, the protein expression levels of interleukin-6 and interferon-γ, relative to the control PEDV infection, were also significantly elevated (P<0.05). The results of the present study indicate that nasal immunization is more effective at inducing the intestinal mucosal immune response, and provide new insights into a novel vaccination strategy that may be used to decrease the incidence of PEDV infections.

Immune response in piglets orally immunized with recombinant Bacillus subtilis expressing the capsid protein of porcine circovirus type 2

BACKGROUND

Porcine circovirus type 2 (PCV2) is the causative agent of postweaning multisystemic wasting syndrome, and is associated with a number of other diseases. PCV2 is widely distributed in most developed swine industries, and is a severe economic burden. With an eye to developing an effective, safe, and convenient vaccine against PCV2-associated diseases, we have constructed a recombinant Bacillus subtilis strain (B. subtilis-Cap) that expresses the PCV2 capsid protein (Cap).

METHODS

Electroporation of a plasmid shuttle vector encoding the PCV2 Cap sequence was use to transform Bacillus subtilis. Flow cytometry was used to evaluate in vitro bone marrow derived dendritic cell (BM-DC) maturation and T cell proliferation induced by B. subtilis-Cap. Orally inoculated piglets were used for in vivo experiments; ELISA and western blotting were used to evaluate B. subtilis-Cap induced PCV2-specific IgA and IgG levels, as well as the secretion of cytokines and the expression of Toll-like receptor 2 (TLR2) and Toll-like receptor 9 (TLR9).

RESULTS

We evaluated the immune response to B. subtilis-Cap in vitro using mouse BM-DCs and in vivo using neonatal piglets orally inoculated with B. subtilis-Cap. Our results showed that the recombinant B. subtilis-Cap activated BM-DCs, significantly increased co-stimulatory molecules (CD40 and CD80) and major histocompatibility complex II, and induced allogenic T cells proliferation. Piglets immunized with B. subtilis-Cap had elevated levels of PCV2-specific IgA in the mucosal tissues of the digestive and respiratory tract, and PCV2-specific IgG in serum (P < 0.05 or P < 0.01). Ileal immunocompetent cells, such as the IgA-secreting cells (P < 0.01), intestinal intraepithelial lymphocytes (IELs) (P < 0.01), CD3+ T lymphocytes (P < 0.01) and CD4+ T lymphocytes (P < 0.01) increased significantly in the B. subtilis-Cap immunized piglets. Additionally, B. subtilis-Cap inoculation resulted in increased the expression of TLR2 and TLR9 (P < 0.01), and induced the secretion of cytokines IL-1β, IL-6, interferon-γ, and β-defensin 2 (P < 0.01).

CONCLUSIONS

We constructed a prototype PCV2 vaccine that can be administered orally and elicits a more robust humoral and cellular immunity than inactivated PCV2. B. subtilis-Cap is a promising vaccine candidate that is safe, convenient, and inexpensive. Further in vivo research is needed to determine its full range of efficacy in pigs.

Nanoparticles in influenza subunit vaccine development: Immunogenicity enhancement

The threat of novel influenza infections has sparked research efforts to develop subunit vaccines that can induce a more broadly protective immunity by targeting selected regions of the virus. In general, subunit vaccines are safer but may be less immunogenic than whole cell inactivated or live attenuated vaccines. Hence, novel adjuvants that boost immunogenicity are increasingly needed as we move toward the era of modern vaccines. In addition, targeting, delivery, and display of the selected antigens on the surface of professional antigen-presenting cells are also important in vaccine design and development. The use of nanosized particles can be one of the strategies to enhance immunogenicity as they can be efficiently recognized by antigen-presenting cells. They can act as both immunopotentiators and delivery system for the selected antigens. This review will discuss on the applications, advantages, limitations, and types of nanoparticles (NPs) used in the preparation of influenza subunit vaccine candidates to enhance humoral and cellular immune responses.

Mucosal immune responses induced by oral administration recombinant Bacillus subtilis expressing the COE antigen of PEDV in newborn piglets

Porcine epidemic diarrhea (PED) is a highly contagious disease in newborn piglets and causes substantial economic losses in the world. PED virus (PEDV) spreads by fecal–oral contact and can be prevented by oral immunization. Therefore, it is necessary to develop an effective oral vaccine against PEDV infection. Currently, Bacillus subtilis as recombinant vaccine carrier has been used for antigen delivery and proved well in immune effect and safety. The present study evaluated the immunogenicity of recombinant Bacillus subtilis (B. subtilis-RC) in piglets via oral administration. After oral immunization in piglets, B. subtilis-RC significantly increased the local mucosal immune responses. Oral administration with B. subtilis-RC significantly improved the level of specific mucosal immunoglobulin A (IgA) antibodies against PEDV infection, through enlarging the area of Peyer’s patches (PPs) and increasing the number of ileum IgA⁺ secreting (SIgA) cells. In the meantime, B. subtilis-RC remarkably increased the number of intraepithelial lymphocytes (IELs). We also observed that oral administration of B. subtilis-RC significantly increased CD3⁺T lymphocytes’ numbers and up-regulated the ratio of CD4⁺/CD8⁺ T cells. Furthermore, high titers of specific serum immunoglobulin G (IgG) revealed satisfactory systemic immune response against PEDV infection. In summary, our study demonstrated that oral administration of B. subtilis-RC could trigger a high level of local and systemic immune responses and would be a promising candidate vaccine against PEDV infection in piglets.

Effects of intranasal administration with Bacillus subtilis on immune cells in the nasal mucosa and tonsils of piglets

The nasal mucosa is the body's first barrier against pathogens entering through the respiratory tract. The respiratory immune system of pigs has more similarities with humans than the mouse respiratory system does, and so was selected as the animal model in the present study. To evaluate the effects of Bacillus subtilis as a potential probiotic to stimulate local immune responses, piglets were intranasally administered with Dylight 488-labeled B. subtilis (WB800-green fluorescent protein). The results revealed that B. subtilis was able to reach the lamina propria of the nasal mucosa, nasopharyngeal tonsils and soft palate tonsils. Piglets were subsequently administered intranasally with B. subtilis (WB800) at 3, 12 and 28 days. The results revealed that, following administration with B. subtilis, the number of dendritic cells, immunoglobulin A⁺ B cells and T cells in the nasal mucosa and tonsils significantly increased (P<0.05). No obvious differences were observed in the morphological structure following B. subtilis administration. There were no statistical differences were observed in the expression of interleukin (IL)-1β, tumor necrosis factor-α and IL-8 mRNA between the B. subtilis treated group and the control group in the nasal mucosa, nasopharyngeal tonsil or soft palate tonsil. Toll-like receptor (TLR)-2 and TLR-9 mRNA expression in the tonsils was significantly increased following B. subtilis administration compared with the control group (P<0.05). The results demonstrate that B. subtilis administration increases the number of immune cells in the nasal mucosa and tonsils of piglets and stimulates nasal mucosal and tonsillar immunity. The present study lays the foundation for further study into the intranasal administration of B. subtilis in humans to enhance the immunity of human nasal mucosa to respiratory diseases.