Immunomodulatory Properties of Bacterium-Like Particles Obtained From Immunobiotic Lactobacilli: Prospects for Their Use as Mucosal Adjuvants

A novel bacterium-like particles platform displaying antigens by new anchoring proteins induces efficacious immune responses

Bacterium-like particles (BLP) are the peptidoglycan skeleton particles of lactic acid bacteria, which have high safety, mucosal delivery efficiency, and adjuvant effect. It has been widely used in recent years in the development of vaccines. Existing anchoring proteins for BLP surfaces are few in number, so screening and characterization of new anchoring proteins are necessary. In this research, we created the OACD (C-terminal domain of Escherichia coli outer membrane protein A) to serve as an anchoring protein on the surface of BLP produced by the immunomodulatory bacteria Levilactobacillus brevis 23017. We used red fluorescent protein (RFP) to demonstrate the novel surface display system's effectiveness, stability, and ability to be adapted to a wide range of lactic acid bacteria. Furthermore, this study employed this surface display method to develop a novel vaccine (called COB17) by using the multi-epitope antigen of Clostridium perfringens as the model antigen. The vaccine can induce more than 50% protection rate against C. perfringens type A challenge in mice immunized with a single dose and has been tested through three routes. The vaccine yields protection rates of 75% for subcutaneous, 50% for intranasal, and 75% for oral immunization. Additionally, it elicits a strong mucosal immune response, markedly increasing levels of specific IgG, high-affinity IgG, specific IgA, and SIgA antibodies. Additionally, we used protein anchors (PA) and OACD simultaneous to show several antigens on the BLP surface. The discovery of novel BLP anchoring proteins may expand the possibilities for creating mucosal immunity subunit vaccines. Additionally, it may work in concert with PA to provide concepts for the creation of multivalent or multiple vaccines that may be used in clinical practice to treat complex illnesses.

Lactobacilli-derived adjuvants combined with immunoinformatics-driven multi-epitope antigens based approach protects against Clostridium perfringens in a mouse model

Clostridium perfringens is ubiquitously distributed and capable of secreting toxins, posing a significant threat to animal health. Infections caused by Clostridium perfringens, such as Necrotic Enteritis (NE), result in substantial economic losses to the livestock industry annually. However, there is no effective commercial vaccine available. Hence, we set out to propose an effective approach for multi-epitope subunit vaccine construction utilizing biomolecules. We utilized immunoinformatics to design a novel multi-epitope antigen against C. perfringens (CPMEA). Furthermore, we innovated novel bacterium-like particles (BLPs) through thermal acid treatment of various Lactobacillus strains and selected BLP23017 among them. Then, we detailed the structure of CPMEA and BLPs and utilized them to prepare a multi-epitope vaccine. Here, we showed that our vaccine provided full protection against C. perfringens infection after a single dose in a mouse model. Additionally, BLP23017 notably augmented the secretion of secretory immunoglobulin A (sIgA) and enhanced antibody production. We conclude that our vaccine possess safety and high efficacy, making it an excellent candidate for preventing C. perfringens infection. Moreover, we demonstrate our approach to vaccine construction and the preparation of BLP23017 with distinct advantages may contribute to the prevention of a wider array of diseases and the novel vaccine development.

Proof of concept in utilizing the peptidoglycan skeleton of pathogenic bacteria as antigen delivery platform for enhanced immune response

Subunit vaccines are becoming increasingly important because of their safety and effectiveness. However, subunit vaccines often exhibit limited immunogenicity, necessitating the use of suitable adjuvants to elicit robust immune responses. In this study, we demonstrated for the first time that pathogenic bacteria can be prepared into a purified peptidoglycan skeleton without nucleic acids and proteins, presenting bacterium-like particles (pBLP). Our results showed that the peptidoglycan skeletons screened from four pathogens could activate Toll-like receptor1/2 receptors better than bacterium-like particles from Lactococcus lactis in macrophages. We observed that pBLP was safe in mouse models of multiple ages. Furthermore, pBLP improved the performance of two commercial vaccines in vivo. We confirmed that pBLP successfully loaded antigens onto the surface and proved to be an effective antigen delivery platform with enhanced antibody titers, antibody avidity, balanced subclass distribution, and mucosal immunity. These results indicate that the peptidoglycan skeleton of pathogenic bacteria represents a new strategy for developing subunit vaccine delivery systems.

Microbiota-Derived Extracellular Vesicles as a Postbiotic Strategy to Alleviate Diarrhea and Enhance Immunity in Rotavirus-Infected Neonatal Rats

Rotavirus (RV) infection is a major cause of acute gastroenteritis in children under 5 years old, resulting in elevated mortality rates in low-income countries. The efficacy of anti-RV vaccines is limited in underdeveloped countries, emphasizing the need for novel strategies to boost immunity and alleviate RV-induced diarrhea. This study explores the effectiveness of interventions involving extracellular vesicles (EVs) from probiotic and commensal E. coli in mitigating diarrhea and enhancing immunity in a preclinical model of RV infection in suckling rats. On days 8 and 16 of life, variables related to humoral and cellular immunity and intestinal function/architecture were assessed. Both interventions enhanced humoral (serum immunoglobulins) and cellular (splenic natural killer (NK), cytotoxic T (Tc) and positive T-cell receptor γδ (TCRγδ) cells) immunity against viral infections and downregulated the intestinal serotonin receptor-3 (HTR3). However, certain effects were strain-specific. EcoR12 EVs activated intestinal CD68, TLR2 and IL-12 expression, whereas EcN EVs improved intestinal maturation, barrier properties (goblet cell numbers/mucin 2 expression) and absorptive function (villus length). In conclusion, interventions involving probiotic/microbiota EVs may serve as a safe postbiotic strategy to improve clinical symptoms and immune responses during RV infection in the neonatal period. Furthermore, they could be used as adjuvants to enhance the immunogenicity and efficacy of anti-RV vaccines.

Bacterium-like particles derived from probiotics: progress, challenges and prospects

Bacterium-like particles (BLPs) are hollow peptidoglycan particles obtained from food-grade Lactococcus lactis inactivated by hot acid. With the advantage of easy preparation, high safety, great stability, high loading capacity, and high mucosal delivery efficiency, BLPs can load and display proteins on the surface with the help of protein anchor (PA), making BLPs a proper delivery system. Owning to these features, BLPs are widely used in the development of adjuvants, vaccine carriers, virus/antigens purification, and enzyme immobilization. This review has attempted to gather a full understanding of the technical composition, characteristics, applications. The mechanism by which BLPs induces superior adaptive immune responses is also discussed. Besides, this review tracked the latest developments in the field of BLPs, including Lactobacillus-derived BLPs and novel anchors. Finally, the main limitations and proposed breakthrough points to further enhance the immunogenicity of BLPs vaccines were discussed, providing directions for future research. We hope that further developments in the field of antigen delivery of subunit vaccines or others will benefit from BLPs.

Lactobacillus co-fermentation of Cerasus humilis juice alters chemical properties, enhances antioxidant activity, and improves gut microbiota

Fermentation with Lactobacillus has been shown to improve the nutritional value of juice. In this study, Cerasus humilis juice was fermented using two commercial probiotics, namely, Lactobacillus acidophilus and Lactobacillus plantarum. The total antioxidant capacity (TAOC), viable count, chemical properties, antioxidant activity after in vitro digestion, and alterations in the gut microbiota composition of the fermented juice were investigated. After fermentation, the TAOC increased from 107.66 U mL-1 to 126.72 U mL-1; viable count increased from 5.85 lg (CFU mL-1) to 8.17 lg (CFU mL-1); and the contents of total phenols, total flavonoids, proanthocyanins, four organic acids, and 29 amino acids had changed. Overall, 47 compounds were identified in the juice, 20 of which were enriched after fermentation. Furthermore, Lactobacillus co-fermentation improved the antioxidant properties of the juice after in vitro digestion and increased the abundance of probiotics to regulate the gut microbiota. These findings illustrate the potential use of Lactobacillus acidophilus and Lactobacillus plantarum in the co-fermentation of C. humilis juice to enhance its nutritional and functional properties.

Intranasally inoculated bacterium-like particles displaying porcine epidemic diarrhea virus S1 protein induced intestinal mucosal immune response in mice

Porcine epidemic diarrhea virus (PEDV) causes acute watery diarrhea and high mortality in newborn piglets. Activation of intestinal mucosal immunity is crucial to anti-PEDV infection. To develop a vaccine capable of stimulating intestinal mucosal immunity, we prepared a bacterium (Lactococcus lactis)-like particle (BLP) vaccine (S1-BLPs) displaying the S1 protein, a domain of PEDV spike protein (S), based on gram-positive enhancer matrix (GEM) particle display technology. We further compared the effects of different vaccination routes on mucosal immune responses in mice induced by S1-BLPs. The specific IgG titer in serum of intramuscularly immunized mice with S1-BLPs was significantly higher than that of the intranasally administered. The specific IgA antibody was found in the serum and intestinal lavage fluid of mice vaccinated intranasally, but not intramuscularly. Moreover, the intranasally inoculated S1-BLPs induced higher levels of IFN-γ and IL-4 in serum than the intramuscularly inoculated. In addition, the ratio of serum IgG2a/IgG1 of mice inoculated intramuscularly was significantly higher with S1-BLPs compared to that of with S1 protein, suggesting that the immune responses induced by S1-BLPs was characterized by helper T (Th) cell type 1 immunity. The results indicated that S1-BLPs induced systemic and local immunity, and the immunization routes significantly affected the specific antibody classes and Th immune response types. The intranasally administered S1-BLPs could effectively stimulate intestinal mucosal specific secretory IgA response. S1-BLPs have the potential to be developed as PEDV mucosal vaccine.

The Respiratory Commensal Bacterium Corynebacterium pseudodiphtheriticum as a Mucosal Adjuvant for Nasal Vaccines

Previously, we demonstrated that nasally administered Corynebacterium pseudodiphteriticum 090104 (Cp) or its bacterium-like particles (BLPs) increase the resistance of mice against bacterial and viral respiratory pathogens by modulating the innate immunity. In this work, we evaluated the ability of Cp and BLPs to stimulate alveolar macrophages, and to enhance the humoral immune response induced by a commercial vaccine against Streptococcus pneumoniae. In the first set of experiments, Cp or the BLPs were incubated with primary cultures of murine alveolar macrophages and the phagocytic activity, and the production of cytokines was evaluated. The results revealed that Cp and BLPs were efficiently phagocyted by respiratory macrophages and that both treatments triggered the production of TNF-α, IFN-γ, IL-6, and IL-1β. In the second set of experiments, 3-week-old Swiss mice were intranasally immunized at days 0, 14, and 28 with the pneumococcal vaccine Prevenar®13 (PCV), Cp + PCV, or BLPs + PCV. On day 33, samples of bronco-alveolar lavages (BAL) and serum were collected for the study of specific antibodies. In addition, immunized mice were challenged with S. pneumoniae serotypes 6B or 19F on day 33 and sacrificed on day 35 (day 2 post-infection) to evaluate the resistance to the infection. Both Cp + PCV and BLPs + PCV groups had higher specific serum IgG and BAL IgA antibodies than the PCV control mice. In addition, the mice that were immunized with Cp + PCV or BLPs + PCV had lower lung and blood pneumococcal cell counts as well as lower levels of BAL albumin and LDH, indicating a reduced lung damage compared to the control mice. Improved levels of anti-pneumococcal antibodies were also detected in the serum and BAL samples after the challenges with the pathogens. The results demonstrated that C. pseudodiphteriticum 090104 and its bacterium-like particles are capable of stimulating the respiratory innate immune system serving as adjuvants to potentiate the adaptive humoral immune response. Our study is a step forward in the positioning of this respiratory commensal bacterium as a promising mucosal adjuvant for vaccine formulations aimed at combating respiratory infectious diseases.

Nano selenium-enriched probiotic Lactobacillus enhances alum adjuvanticity and promotes antigen-specific systemic and mucosal immunity

Nano selenium-enriched probiotics have been identified to improve immune responses, such as alleviating inflammation, antioxidant function, treatment of tumors, anticancer activity, and regulating intestinal flora. However, so far, there is little information on improving the immune effect of the vaccine. Here, we prepared nano selenium-enriched Levilactobacillus brevis 23017 (SeL) and heat-inactivated nano selenium-enriched L. brevis 23017 (HiSeL) and evaluated their immune enhancing functions on the alum-adjuvanted, inactivated Clostridium perfringens type A vaccine in mouse and rabbit models, respectively. We found that SeL enhanced immune responses of the vaccine by inducing a more rapid antibody production, eliciting higher immunoglobulin G (IgG) antibody titers, improving secretory immunoglobulin A (SIgA) antibody level and cellular immune response, and regulating Th1/Th2 immune response, thus helping to induce better protective efficacy after challenge. Moreover, we confirmed that the immunoenhancement effects are related to regulating oxidative stress, cytokine secretion, and selenoprotein expression. Meanwhile, similar effects were observed in HiSeL. In addition, they show enhanced humoral immune response at 1/2 and 1/4 standard vaccine doses, which confirms their prominent immune enhancement effect. Finally, the effect of improving vaccine immune responses was further confirmed in rabbits, which shows that SeL stimulates the production of IgG antibodies, generates α toxin-neutralizing antibodies rapidly, and reduces the pathological damage to intestine tissue. Our study demonstrates that nano selenium-enriched probiotics improve the immune effect of the alum adjuvants vaccine and highlight its potential usage in remedying the disadvantages of alum adjuvants.

The adjuvant effect of bacterium-like particles depends on the route of administration

Direct administration of vaccines to mucosal surfaces, such as via oral or nasal vaccination, represents an attractive alternative, or complement, to current parenteral vaccination because it has a potential to induce antigen-specific immunity both at mucosal and systemic tissues. Although bacterium-like particles (BLPs), peptidoglycan structures derived from lactic acid bacteria, have been investigated as a novel adjuvant for oral or nasal vaccines, it remains unclear whether the administration routes differ the adjuvant effect of BLPs. Here, we showed that the adjuvant effect of BLPs from Lactococcus lactis NZ9000 is greater with the nasal administration than with the oral administration. We conjugated BLPs with Tir, a virulence factor of Citrobacter rodentium, as a model adjuvant-antigen complex, and found that nasal, but not oral, immunization of mice with BLP-Tir induced robust antigen-specific IgA responses at the respiratory and intestinal mucosa, IgG2b-skewed systemic responses, and Th17 cellular responses. As one of the underlying mechanisms, we demonstrated that the nasal administration has a greater delivery efficiency (~1,000-fold) of the BLPs-conjugated antigens to mucosal-associated lymphoid tissues than the oral administration. Furthermore, the nasal, but not oral, administration of BLP-Tir elicited robust innate immune responses that were characterized by the expression of various pro-inflammatory cytokines and chemokines in the mucosal-associated lymphoid tissues. Considering these findings together, we anticipate that BLPs can be an attractive novel adjuvant for nasal vaccines targeting not only respiratory but also gastrointestinal infectious diseases.

The Mucus Binding Factor Is Not Necessary for Lacticaseibacillus rhamnosus CRL1505 to Exert Its Immunomodulatory Activities in Local and Distal Mucosal Sites

Both viable and non-viable orally administered Lacticaseibacillus rhamnosus CRL1505 modulate immunity in local (intestine) and distal (respiratory) mucosal sites. So, intestinal adhesion and colonization are not necessary for this probiotic strain to exert its immunomodulatory effects. In this work, a mucus-binding factor knockout CRL1505 strain (ΔmbfCRL1505) was obtained and the lack of binding ability to both intestinal epithelial cells and mucin was demonstrated in vitro. In addition, two sets of in vivo experiments in 6-week-old Balb/c mice were performed to evaluate ΔmbfCRL1505 immunomodulatory activities. (A) Orally administered ΔmbfCRL1505 prior to intraperitoneal injection of the Toll-like receptor 3 (TLR3) agonist poly(I:C) significantly reduced intraepithelial lymphocytes (CD3⁺NK1.1⁺CD8αα⁺) and pro-inflammatory mediators (TNF-α, IL-6 and IL-15) in the intestinal mucosa. (B) Orally administered ΔmbfCRL1505 prior to nasal stimulation with poly(I:C) significantly decreased the levels of the biochemical markers of lung tissue damage. In addition, reduced recruitment of neutrophils and levels of pro-inflammatory mediators (TNF-α, IL-6 and IL-8) as well as increased IFN-β and IFN-γ in the respiratory mucosa were observed in ΔmbfCRL1505-treated mice when compared to untreated control mice. The immunological changes induced by the ΔmbfCRL1505 strain were not different from those observed for the wild-type CRL1505 strain. Although it is generally accepted that the expression of adhesion factors is necessary for immunobiotics to induce their beneficial effects, it was demonstrated here that the mbf protein is not required for L. rhamnosus CRL1505 to exert its immunomodulatory activities in local and distal mucosal sites. These results are a step forward towards understanding the mechanisms involved in the immunomodulatory capabilities of L. rhamnosus CRL1505.

The Ability of Postimmunobiotics from L. rhamnosus CRL1505 to Protect against Respiratory Syncytial Virus and Pneumococcal Super-Infection Is a Strain-Dependent Characteristic

Previously, we demonstrated that the non-viable strain Lacticaseibacillus rhamnosus CRL1505 (NV1505) or its purified peptidoglycan (PG1505) differentially modulated the respiratory innate antiviral immune response triggered by Toll-like receptor (TLR)-3 activation in infant mice, improving the resistance to primary respiratory syncytial virus (RSV) infection and secondary pneumococcal pneumonia. In this work, we evaluated the effect of other non-viable L. rhamnosus strains and their peptidoglycans on the respiratory immune response and their impact on primary and secondary respiratory infections. In addition, the duration of the protective effect induced by NV1505 and PG1505 as well as their ability to protect against different Streptococcus pneumoniae serotypes were evaluated. Our results showed that among the five selected L. rhamnosus strains (CRL1505, CRL498, CRL576, UCO25A and IBL027), NV1505 and NVIBL027 improved the protection against viral and pneumococcal infections by modulating the respiratory immune response. Of note, only the PG1505 presented immunomodulatory activities when compared with the other purified peptidoglycans. Studies on alveolar macrophages showed that NV1505 and PG1505 differentially modulated the expression of IL-6, IFN-γ, IFN-β, TNF-α, OAS1, RNAseL and IL-27 genes in response to RSV infection, and IL-6, IFN-γ, IL-1β, TNF-α, CCL2, CXCL2, CXCL10 and IL-27 in response to pneumococcal challenge. Furthermore, we demonstrated that NV1505 and PG1505 treatments protected mice against secondary pneumococcal pneumonia produced by different serotypes of S. pneumoniae until 30 days after stimulation with poly(I:C). This work advances the characterization of the protective effect of NV1505 and PG1505 by demonstrating that they increase resistance against the pneumococcal serotypes 3, 6B, 14 and 19F, with an effect that lasts up to 30 days after the primary viral inflammation. The results also confirm that the immunomodulatory properties of NV1505 and PG1505 are unique and are not shared by other members of this species, and suggest the existence of a capacity to stimulate trained immunity in alveolar macrophages.

NOD2 signaling in CD11c + cells is critical for humoral immune responses during oral vaccination and maintaining the gut microbiome

Nucleotide-binding oligomerization domain containing 2 (NOD2) is a critical regulator of immune responses within the gastrointestinal tract. This innate immune receptor is expressed by several cell types, including both hematopoietic and nonhematopoietic cells within the gastrointestinal tract. Vaccination targeting the gastrointestinal mucosal immune system is especially difficult due to both physical and mechanistic barriers to reaching inductive sites. The use of lactic acid bacteria is appealing due to their ability to persist within harsh conditions, expression of selected adjuvants, and manufacturing advantages. Recombinant Lactobacillus acidophilus (rLA) has shown great promise in activating the mucosal immune response with minimal impacts on the resident microbiome. To better classify the kinetics of mucosal vaccination with rLA, we utilized mice harboring knockouts of NOD2 expression specifically within CD11c + cells. The results presented here show that NOD2 signaling in CD11c + cells is necessary for mounting a humoral immune response against exogenous antigens expressed by rLA. Additionally, disruption of NOD2 signaling in these cells results in an altered bacterial microbiome profile in both control mice and mice receiving L. acidophilus strain NCK1895 and vaccine strain LaOVA.

Dietary emulsifier glycerol monodecanoate affects gut microbiota contributing to regulating lipid metabolism, insulin sensitivity and inflammation

Glycerol monodecanoate (GMD) is a medium-chain monoacylglycerol that possesses emulsifying and antibacterial properties. Common emulsifiers carboxymethylcellulose and polysorbate-80 have been reported to cause intestinal microbiota dysbiosis and metabolic disturbances. While...

Novel LysM motifs for antigen display on lactobacilli for mucosal immunization

We characterized two LysM domains of Limosilactobacillus fermentum, belonging to proteins Acglu (GenBank: KPH22907.1) and Pgb (GenBank: KPH22047.1) and bacterium like particles (BLP) derived from the immunomodulatory strain Lacticaseibacillus rhamnosus IBL027 (BLPs027) as an antigen display platform. The fluorescence protein Venus fused to the novel LysM domains could bind to the peptidoglycan shell of lactobacilli and resisted harsh conditions such as high NaCl and urea concentrations. Acglu with five LysM domains was a better anchor than Pgb baring only one domain. Six-week-old BALB/c mice were nasally immunized with the complex Venus-Acglu-BLPs027 at days 0, 14 and 28. The levels of specific serum IgG, IgG1 and IgG2a and the levels of total immunoglobulins (IgT) and IgA in broncho-alveolar lavage (BAL) were evaluated ten days after the last boosting. Venus-Acglu-BLPs027, nasally administered, significantly increased specific BAL IgT and IgA, and serum IgG levels. In addition, spleen cells of mice immunized with Venus-Acglu-BLPs027 secreted TNF-α, IFN-γ and IL-4 when stimulated ex vivo in a dose-dependent manner. We constructed a Gateway compatible destination vector to easily fuse the selected LysM domain to proteins of interest for antigen display to develop mucosal subunit vaccines.

Alterations in Faecal Microbiota and Elevated Levels of Intestinal IgA Following Oral Administration of Lacticaseibacillus casei in mice

The intestinal microbiota has been identified as a crucial regulator of the overall health, with studies describing its influence in a variety of disorders and developmental processes throughout the body. A widely accepted approach of influencing the microbiota and regulating its functionality in health or disease is the consumption of probiotics. In this study, we aimed to identify the impact of probiotic Lacticaseibacillus casei ATCC393 on the intestinal microbiota of mice and circulating soluble products of microbial origin or the immune system. Investigation of the gut microflora using next-generation sequencing analysis revealed alterations in the microbial populations following consumption of the probiotic. Abundance of taxa classified as Muribaculaceae was increased in lactobacilli-fed animals, while abundance of taxa classified as Lachnospiraceae and Oscillospiraceae was decreased. In addition, the composition of the intestinal microbiota was modified by the administration of L. casei, as evident by the clustering of test subjects when inspecting beta diversity, without however any significant effect on the alpha diversity of the animals. Finally, production of IgA in the intestinal lumen of mice that had received the microorganism was significantly increased, as was the concentration of lactic acid, while levels of acetic acid were noticeably lower in the L. casei group. The findings suggest that L. casei can be considered a potential candidate strain for the modulation of intestinal homeostasis and a component of dietary interventions aiming to improve overall health.

Immunobiotic Lactobacilli Improve Resistance of Respiratory Epithelial Cells to SARS-CoV-2 Infection

Previously, we reported that immunomodulatory lactobacilli, nasally administered, beneficially regulated the lung antiviral innate immune response induced by Toll-like receptor 3 (TLR3) activation and improved protection against the respiratory pathogens, influenza virus and respiratory syncytial virus in mice. Here, we assessed the immunomodulatory effects of viable and non-viable Lactiplantibacillus plantarum strains in human respiratory epithelial cells (Calu-3 cells) and the capacity of these immunobiotic lactobacilli to reduce their susceptibility to the acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Immunobiotic L. plantarum MPL16 and CRL1506 differentially modulated IFN-β, IL-6, CXCL8, CCL5 and CXCL10 production and IFNAR2, DDX58, Mx1 and OAS1 expression in Calu-3 cells stimulated with the TLR3 agonist poly(I:C). Furthermore, the MPL16 and CRL1506 strains increased the resistance of Calu-3 cells to the challenge with SARS-CoV-2. L. plantarum MPL16 induced these beneficial effects more efficiently than the CRL1506 strain. Of note, neither non-viable MPL16 and CRL1506 strains nor the non-immunomodulatory strains L. plantarum CRL1905 and MPL18 could modify the resistance of Calu-3 cells to SARS-CoV-2 infection or the immune response to poly(I:C) challenge. To date, the potential beneficial effects of immunomodulatory probiotics on SARS-CoV-2 infection and COVID-19 outcome have been extrapolated from studies carried out in the context of other viral pathogens. To the best of our knowledge, this is the first demonstration of the ability of immunomodulatory lactobacilli to positively influence the replication of the new coronavirus. Further mechanistic studies and in vivo experiments in animal models of SARS-CoV-2 infection are necessary to identify specific strains of beneficial immunobiotic lactobacilli like L. plantarum MPL16 or CRL1506 for the prevention or treatment of the COVID-19.

The Respiratory Commensal Bacterium Dolosigranulum pigrum 040417 Improves the Innate Immune Response to Streptococcus pneumoniae

Previously, we demonstrated that the nasal administration of Dolosigranulum pigrum 040417 differentially modulated the respiratory innate immune response triggered by the activation of Toll-like receptor 2 in infant mice. In this work, we aimed to evaluate the beneficial effects of D. pigrum 040417 in the context of Streptococcus pneumoniae infection and characterize the role of alveolar macrophages (AMs) in the immunomodulatory properties of this respiratory commensal bacterium. The nasal administration of D. pigrum 040417 to infant mice significantly increased their resistance to pneumococcal infection, differentially modulated respiratory cytokines production, and reduced lung injuries. These effects were associated to the ability of the 040417 strain to modulate AMs function. Depletion of AMs significantly reduced the capacity of the 040417 strain to improve both the reduction of pathogen loads and the protection against lung tissue damage. We also demonstrated that the immunomodulatory properties of D. pigrum are strain-specific, as D. pigrum 030918 was not able to modulate respiratory immunity or to increase the resistance of mice to an S. pneumoniae infection. These findings enhanced our knowledge regarding the immunological mechanisms involved in modulation of respiratory immunity induced by beneficial respiratory commensal bacteria and suggested that particular strains could be used as next-generation probiotics.

Ligilactobacillus salivarius Strains Isolated From the Porcine Gut Modulate Innate Immune Responses in Epithelial Cells and Improve Protection Against Intestinal Viral-Bacterial Superinfection

Previously, we constructed a library of Ligilactobacillus salivarius strains from the intestine of wakame-fed pigs and reported a strain-dependent capacity to modulate IFN-β expression in porcine intestinal epithelial (PIE) cells. In this work, we further characterized the immunomodulatory activities of L. salivarius strains from wakame-fed pigs by evaluating their ability to modulate TLR3- and TLR4-mediated innate immune responses in PIE cells. Two strains with a remarkable immunomodulatory potential were selected: L. salivarius FFIG35 and FFIG58. Both strains improved IFN-β, IFN-λ and antiviral factors expression in PIE cells after TLR3 activation, which correlated with an enhanced resistance to rotavirus infection. Moreover, a model of enterotoxigenic E. coli (ETEC)/rotavirus superinfection in PIE cells was developed. Cells were more susceptible to rotavirus infection when the challenge occurred in conjunction with ETEC compared to the virus alone. However, L. salivarius FFIG35 and FFIG58 maintained their ability to enhance IFN-β, IFN-λ and antiviral factors expression in PIE cells, and to reduce rotavirus replication in the context of superinfection. We also demonstrated that FFIG35 and FFIG58 strains regulated the immune response of PIE cells to rotavirus challenge or ETEC/rotavirus superinfection through the modulation of negative regulators of the TLR signaling pathway. In vivo studies performed in mice models confirmed the ability of L. salivarius FFIG58 to beneficially modulate the innate immune response and protect against ETEC infection. The results of this work contribute to the understanding of beneficial lactobacilli interactions with epithelial cells and allow us to hypothesize that the FFIG35 or FFIG58 strains could be used for the development of highly efficient functional feed to improve immune health status and reduce the severity of intestinal infections and superinfections in weaned piglets.

Lactiplantibacillus plantarum as a Potential Adjuvant and Delivery System for the Development of SARS-CoV-2 Oral Vaccines

The most important characteristics regarding the mucosal infection and immune responses against the Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2) as well as the current vaccines against coronavirus disease 2019 (COVID-19) in development or use are revised to emphasize the opportunity for lactic acid bacteria (LAB)-based vaccines to offer a valid alternative in the fight against this disease. In addition, this article revises the knowledge on: (a) the cellular and molecular mechanisms involved in the improvement of mucosal antiviral defenses by beneficial Lactiplantibacillus plantarum strains, (b) the systems for the expression of heterologous proteins in L. plantarum and (c) the successful expressions of viral antigens in L. plantarum that were capable of inducing protective immune responses in the gut and the respiratory tract after their oral administration. The ability of L. plantarum to express viral antigens, including the spike protein of SARS-CoV-2 and its capacity to differentially modulate the innate and adaptive immune responses in both the intestinal and respiratory mucosa after its oral administration, indicates the potential of this LAB to be used in the development of a mucosal COVID-19 vaccine.

The Ability of Respiratory Commensal Bacteria to Beneficially Modulate the Lung Innate Immune Response Is a Strain Dependent Characteristic

We investigated whether the ability of commensal respiratory bacteria to modulate the innate immune response against bacterial and viral pathogens was a shared or strain-specific characteristic. Bacterial strains belonging to the Corynebacterium pseudodiphtheriticum and Dolosigranulum pigrum species were compared by studying their influence in the Toll-like receptor (TLR)-2- and TLR3-triggered immune responses in the respiratory tract, as well as in the resistance to Respiratory Syncytial Virus (RSV) and Streptococcus pneumoniae infections. We demonstrated that nasally administered C. pseudodiphteriticum 090104 or D. pigrum 040417 were able to modulate respiratory immunity and increase the resistance against pathogens, while other strains of the same species did not influence the respiratory immune responses, demonstrating a clear strain-dependent immunomodulatory effect of respiratory commensal bacteria. We also reported here that bacterium-like particles (BLP) and cell walls derived from immunomodulatory respiratory commensal bacteria are an interesting alternative for the modulation of the respiratory immune system. Our study is a step forward in the positioning of certain strains of respiratory commensal bacteria as next-generation probiotics for the respiratory tract.