Evaluation of the Lon-deficient Salmonella strain as an oral vaccine candidate

Oral Immunization with Lactobacillus casei Expressing the Porcine Circovirus Type 2 Cap and LTB Induces Mucosal and Systemic Antibody Responses in Mice

Porcine circovirus type 2 (PCV2) causes many diseases in weaned piglets, leading to serious economic losses to the pig industry. This study investigated the immune response following oral administration of Lactobacillus casei ATCC393 (L. casei 393) expressing PCV2 capsid protein (Cap) fusion with the Escherichia coli heat-labile toxin B subunit (LTB) in mice. Recombinant L. casei strains were constructed using plasmids pPG611.1 and pPG612.1. The expression and localization of proteins from recombinant pPG611.1-Cap-LTB (pPG-1-Cap-LTB)/L. casei 393 and pPG612.1-Cap-LTB (pPG-2-Cap-LTB)/L. casei 393 were detected. All recombinant strains were found to be immunogenic by oral administration in mice and developed mucosal and systemic immune responses against PCV2. The titers of specific antibodies in mice administered pPG-2-Cap-LTB/L. casei 393 were higher than those in mice administered pPG-1-Cap-LTB/L. casei 393 in serum and the mucosal samples. The mucosal immune response was not only limited to the gastrointestinal tract but was also generated in other mucosal parts. Thus, the application of recombinant L. casei could aid in vaccine development for PCV2.

Salmonella SiiE prevents an efficient humoral immune memory by interfering with IgG+ plasma cell persistence in the bone marrow

Serum IgG, which is mainly generated from IgG-secreting plasma cells in the bone marrow (BM), protects our body against various pathogens. We show here that the protein SiiE of Salmonella is both required and sufficient to prevent an efficient humoral immune memory against the pathogen by selectively reducing the number of IgG-secreting plasma cells in the BM. Attenuated SiiE-deficient Salmonella induces high and lasting titers of specific and protective Salmonella-specific IgG and qualifies as an efficient vaccine against Salmonella A SiiE-derived peptide with homology to laminin β1 is sufficient to ablate IgG-secreting plasma cells from the BM, identifying laminin β1 as a component of niches for IgG-secreting plasma cells in the BM, and furthermore, qualifies it as a unique therapeutic option to selectively ablate IgG-secreting plasma cells in autoimmune diseases and multiple myeloma.

Mouse models for assessing the cross-protective efficacy of oral non-typhoidal Salmonella vaccine candidates harbouring in-frame deletions of the ATP-dependent protease lon and other genes

In BALB/c mouse models of Salmonella enterica serovar Typhimurium infection, a single oral immunization with a mutant strain with an insertion of the chloramphenicol resistance gene into the ATP-dependent protease clpP or lon gene decreased the number of salmonellae in each tissue sample 5 days after oral challenge with virulent S. Typhimurium at weeks 26 and 54 post-immunization. These data suggested that an oral immunization with the ClpP- or Lon-disrupted S. Typhimurium strain could provide long-term protection against oral challenge with virulent S. Typhimurium. Accordingly, recombinant oral non-typhoidal Salmonella (NTS) vaccines were constructed by incorporating mutants of both S. Typhimurium and S. enterica serovar Enteritidis harbouring stable in-frame markerless deletions of the clpP-lon-sulA (suppressor of lon), lon-sulA or lon-msbB (acyltransferase) genes. Amongst these orally administered vaccine candidates, those with the lon-sulA gene deletion mutants of S. Typhimurium and S. Enteritidis protected BALB/c and C57BL/6J mice against oral challenge with both virulent S. Typhimurium and virulent S. Enteritidis. Therefore, the in-frame markerless lon-sulA gene deletion mutant of S. Typhimurium or S. Enteritidis could be a promising cross-protective NTS live vaccine candidate for practical use in humans.

Evaluation of the live vaccine efficacy of virulence plasmid-cured, and phoP- or aroA-deficient Salmonella enterica serovar Typhimurium in mice

We evaluated the protective efficacy of 94-kb virulence plasmid-cured, and phoP- or aroA-deficient strains of Salmonella enterica serovar Typhimurium (ΔphoP or ΔaroA S. Typhimurium) as oral vaccine candidates in BALB/c mice. Two weeks after the completion of 3 oral immunizations with 1 × 10⁸ colony-forming units (CFU) of virulence plasmid-cured, and ΔphoP or ΔaroA S. Typhimurium at 10-day intervals, S. Typhimurium lipopolysaccharide (LPS)-specific mucosal secretory immunoglobulin A (s-IgA) antibody titers were detected in the cecal homogenate, bile and lung lavage fluid, but not in the intestinal lavage fluid. In addition, the increases in S. Typhimurium LPS-specific immunoglobulin G (IgG) and IgA antibody titers in the serum were also observed 2 weeks after completing 3 oral immunizations with virulence plasmid-cured, and ΔphoP or ΔaroA S. Typhimurium. The series of 3 oral immunizations protected the mice against an oral challenge with 5 × 10⁸ CFU of the virulent strain of S. Typhimurium, suggesting that both the virulence plasmid-cured, and ΔphoP and ΔaroA S. Typhimurium strains are promising candidates for safe and effective live S. Typhimurium vaccines.

Enhancement of immune responses by an attenuated Salmonella enterica serovar Typhimurium strain secreting an Escherichia coli heat-labile enterotoxin B subunit protein as an adjuvant for a live Salmonella vaccine candidate

A plasmid harboring eltB, the gene encoding heat-labile enterotoxin (LTB), was constructed by insertion of eltB into an Asd(+) β-lactamase signal plasmid (pMMP65). This was introduced into the Δlon ΔcpxR Δasd Salmonella enterica serovar Typhimurium strain and designated the LTB adjuvant strain. LTB protein production and secretion from the strain were demonstrated with an immunoblot assay and enzyme-linked immunosorbent assay. The LTB strain was evaluated for enhancement of immunity and protection efficacy induced by a previously constructed live Salmonella vaccine candidate. In addition, immunization strategies using the LTB strain were optimized for effective salmonellosis protection. Seventy female BALB/c mice were divided into seven groups (A to G; n = 10 mice per group). Mice were primed at 6 weeks of age and boosted at 9 weeks of age. All mice were orally challenged with a virulent wild-type strain at week 3 postbooster. Serum IgG and IgA titers from mice immunized with the LTB strain alone or with a mixture of the LTB strain and the vaccine candidate were significantly increased. The secretory IgA titers from mice immunized with the LTB strain alone or with the mixture were at least 2.2 times greater than those of control mice. In addition, all group E mice (primed with the vaccine-LTB mixture and boosted with the vaccine candidate) were free of clinical signs of salmonellosis and survived a virulent challenge. In contrast, death due to the challenge was 100% in control mice, 80% in group A mice (single immunization with the vaccine candidate), 60% in group B mice (primed and boosted with the vaccine candidate), 40% in group C mice (single immunization with the LTB strain), 30% in group D mice (primed and boosted with the LTB strain), and 30% in group F mice (primed and boosted with the vaccine-LTB mixture). These results suggest that vaccination with the LTB strain, especially when added at the prime stage only, effectively enhances immune responses and protection against salmonellosis.

Dermal mast cells reduce progressive tissue necrosis caused by subcutaneous infection with Streptococcus pyogenes in mice

A single subcutaneous (s.c.) infection with 1×10(7) c.f.u. GAS472, a group A streptococcus (GAS) serotype M1 strain isolated from the blood of a patient suffering from streptococcal toxic shock syndrome, led to severe damage of striated muscle layers in the feet of mast cell (MC)-deficient WBB6F(1)-Kit(W)/Kit(W-v) (W/W(v)) mice 72 h after infection. In contrast, no damage was recognized in striated muscle layers in the feet of the control WBB6F(1)-Kit(+/+) (+/+) mice 72 h after infection. In addition, adoptively transferred MCs reduced progressive tissue necrosis of the feet of W/W(v) mice after infection. However, there was no significant difference in the mortality rates between the W/W(v) and +/+ mice, or between the human CD46-expressing transgenic (Tg) mouse bone marrow-derived cultured MC-reconstituted W/W(v) and non-Tg mouse bone marrow-derived cultured MC-reconstituted W/W(v) mice after infection. Consequently, although MCs can help to reduce the severity of necrosis of the feet caused by s.c. infection with GAS472, such reduction of tissue necrosis scarcely improves the mortality rates of these mice. Moreover, human CD46 does not play a crucial role in the MC-mediated innate immune defence against GAS infection.

Recombinant porcine rotavirus VP4 and VP4-LTB expressed in Lactobacillus casei induced mucosal and systemic antibody responses in mice

Background

Porcine rotavirus infection is a significant cause of morbidity and mortality in the swine industry necessitating the development of effective vaccines for the prevention of infection. Immune responses associated with protection are primarily mucosal in nature and induction of mucosal immunity is important for preventing porcine rotavirus infection.

Results

Lactobacillus casei expressing the major protective antigen VP4 of porcine rotavirus (pPG612.1-VP4) or VP4-LTB (heat-labile toxin B subunit from Echerichia coli) (pPG612.1-VP4-LTB) fusion protein was used to immunize mice orally. The expression of recombinant pPG612.1-VP4 and pPG612.1-VP4-LTB was confirmed by SDS-PAGE and Western blot analysis and surface-displayed expression on L. casei was verified by immunofluorescence. Mice orally immunized with recombinant protein-expressing L. casei produced high levels of serum immunoglobulin G (IgG) and mucosal IgA. The IgA titters from mice immunized with pPG612.1-VP4-LTB were higher than titters from pPG612.1-VP4-immunized mice. The induced antibodies demonstrated neutralizing effects on RV infection.

Conclusion

These results demonstrated that VP4 administered in the context of an L. casei expression system is an effective method for stimulating mucosal immunity and that LTB served to further stimulate mucosal immunity suggesting that this strategy can be adapted for use in pigs.

CD46 transgenic mouse model of necrotizing fasciitis caused by Streptococcus pyogenes infection

We developed a human CD46-expressing transgenic (Tg) mouse model of subcutaneous (s.c.) infection into both hind footpads with clinically isolated 11 group A streptococcus (GAS) serotype M1 strains. When the severity levels of foot lesions at 72 h and the mortality rates by 336 h were compared after s.c. infection with 1x10(7) CFU of each GAS strain, the GAS472 strain, isolated from the blood of a patient suffering from streptococcal toxic shock syndrome (STSS), induced the highest severity levels and mortality rates. GAS472 led to a 100% mortality rate in CD46 Tg mice after only 168 h postinfection through the supervention of severe necrotizing fasciitis (NF) of the feet. In contrast, GAS472 led to a 10% mortality rate in non-Tg mice through the supervention of partial necrotizing cutaneous lesions of the feet. The footpad skin sections of CD46 Tg mice showed hemorrhaging and necrotic striated muscle layers in the dermis, along with the exfoliation of epidermis with intracellular edema until 48 h after s.c. infection with GAS472. Thereafter, the bacteria proliferated, reaching a 90-fold or 7-fold increase in the livers of CD46 Tg mice or non-Tg mice, respectively, for 24 h between 48 and 72 h after s.c. infection with GAS472. As a result, the infected CD46 Tg mice appeared to suffer severe liver injuries. These findings suggest that human CD46 enhanced the progression of NF in the feet and the exponential growth of bacteria in deep tissues, leading to death.

Comparative efficacies of different antibiotic treatments to eradicate nontypeable Haemophilus influenzae infection

Background

Nonencapsulated and nontypeable Haemophilus influenzae (NTHi) is a major cause of human respiratory tract infections. Some strains of NTHi can cause invasive diseases such as septicemia and meningitis, even if H. influenzae is not generally considered to be an intracellular pathogen. There have been very few reports about the therapeutic efficacy of antibiotics against respiratory tract infection caused by NTHi in mice because it is difficult for H. influenzae to infect mice. Therefore, we evaluated the efficacy of antibiotics against NTHi in both a cell culture model and a mouse model of infection.

Methods

We used six strains of NTHi isolated from adult patients with chronic otitis media, namely three β-lactamase-negative ampicillin (AMP)-resistant (BLNAR) strains and three β-lactamase-negative AMP-susceptible (BLNAS) strains, to evaluate the efficacy of AMP, cefcapene (CFPN), levofloxacin (LVX), clarithromycin (CLR), and azithromycin (AZM) in both a cell culture infection model and a mouse infection model. In the cell culture infection model, strains that invade A549 human alveolar epithelial cells were treated with each antibiotic (1 μg/ml). In the mouse infection model, female C57BL/6 mice were intraperitoneally injected with cyclophosphamide (200 mg/kg) three days before intranasal infection with 1 × 10⁹ colony-forming units (CFU) of NTHi and on the day of infection. After infection, the mice were orally administered each antibiotic three times daily for three days, except for AZM, which was administered once daily for three days, at a dose of 100 mg/kg/day.

Results

In the cell culture infection model, it was found that two BLNAR strains were able to enter the cell monolayers by the process of macropinocytosis, and treatment with LVX yielded good bactericidal activity against both strains inside the cells. In the mouse infection model, no bacteria were detected by means of plating the lung homogenates of LVX-treated mice at day 4 after infection, while more than 10⁵ CFU of bacteria per tissue sample were detected in nontreated mice.

Conclusion

Our findings show the outcome and rich benefits of fluoroquinolone treatment of respiratory infections caused by either invasive or noninvasive BLNAR strains of NTHi.

Oral vaccination: where we are?

As early as 900 years ago, the Bedouins of the Negev desert were reported to kill a rabid dog, roast its liver and feed it to a dog-bitten person for three to five days according to the size and number of bites [1] . In sixteenth century China, physicians routinely prescribed pills made from the fleas collected from sick cows, which purportedly prevented smallpox. One may dismiss the wisdom of the Bedouins or Chinese but the Nobel laureate, Charles Richet, demonstrated in 1900 that feeding raw meat can cure tuberculous dogs - an approach he termed zomotherapy. Despite historical clues indicating the feasibility of oral vaccination, this particular field is notoriously infamous for the abundance of dead-end leads. Today, most commercial vaccines are delivered by injection, which has the principal limitation that recipients do not like needles. In the last few years, there has been a sharp increase in interest in needle-free vaccine delivery; new data emerges almost daily in the literature. So far, there are very few licensed oral vaccines, but many more vaccine candidates are in development. Vaccines delivered orally have the potential to take immunization to a fundamentally new level. In this review, the authors summarize the recent progress in the area of oral vaccines.

A new concept to stimulate mucosal as well as systemic immunity by parenteral vaccination as applied to the development of a live attenuated Salmonella enterica serovar Dublin vaccine

A new concept of slow "drip feeding" that enables activation of mucosal as well as systemic immunity following parenteral vaccination was demonstrated using Salmonella Dublin in a mouse model. The live vaccine candidate, N-RM25, generated from a wild S. Dublin strain utilising metabolic-drift (spontaneous chromosomal) mutations had a unique sensitivity to bile and restricted growth in the presence of a very low concentration of bile salts No. 3 (0.075% (w/v)) but also had the ability to survive in a high concentration (19.2%) of the substance. Following intraperitoneal administration with 10(7) cfu, N-RM25 colonised and survived (10(1)-10(3) cfu/g) in the liver and spleen of mice for over 24 days without causing disease. A small number of the mutant organisms also penetrated the gall bladder and gut, most likely via the enterohepatic circulation. N-RM25 induced significant levels of serum IgG, IgA and intestinal secretory IgA. A second metabolic-drift mutant (R-NM29) that was rapidly eliminated from the liver and spleen and highly unlikely to penetrate the gall bladder and gut, stimulated some systemic immunity, but induced no mucosal immunity because it did not reach the immune stimulation sites within the gut. In vaccine trials, N-RM25 was significantly more effective in eliminating the homologous challenge bacteria (S. Dublin wild strain FD436) from the internal organs and intestinal lumen when compared to R-NM29 and the negative control. N-RM25 prevented the development of systemic infection and produced 100% protection.

ExpressedSalmonellaantigens within macrophages enhance the proliferation of CD4+and CD8+T lymphocytes by means of bystander dendritic cells

ATP-dependent Lon protease-deficient Salmonella enterica serovar Typhimurium (strain CS2022) appeared to invade successfully the mesenteric lymph nodes (MLN) and Peyer's patches (PP) of BALB/c mice and appeared to be easily eradicated by the host after oral immunization. As detected by flow cytometry, the population of major histocompatibility complex class I (MHC-I)-expressing macrophages and dendritic cells (DCs) was increased in the PP of mice immunized with CS2022 on day 6 after immunization. Thereafter, the population of splenic surface CD69(+) T lymphocytes prepared from mice immunized with CS2022 6 weeks prior to measurement increased as a result of the administration of the extracellular vesicles of RAW264.7 macrophage-like cells derived by Salmonella challenge. In addition, the proliferation of CD8(+) and even of CD4(+)T cells isolated from mouse spleens immunized with CS2022 was enhanced after cocultivation with naive DCs in the presence of the extracellular vesicles. These findings indicate that the extracellular vesicles prepared from the Salmonella-challenged macrophages carried salmonellae antigens to bystander DCs, thereby stimulating T-cell responses. Therefore, as antigen presentation after phagocytosis should be a central process in the T-cell activation that occurs in response to Salmonella infection, an oral immunization with CS2022 sufficiently induces T cell-mediated immunity in mice.

An oralSalmonellavaccine promotes the down-regulation of cell surface Toll-like receptor 4 (TLR4) and TLR2 expression in mice

A single oral immunization with the Lon-protease-deficient Salmonella enterica serovar Typhimurium (strain CS2022) induced protective immunity in mice against a subcutaneous challenge with virulent Listeria monocytogenes as well as virulent Salmonella serovar Typhimurium. The populations of cell surface Toll-like receptor 4 (TLR4) and TLR2 on peritoneal macrophages decreased at week 6 after immunization. This population decrease was not reversed after a challenge with either Salmonella or Listeria. These results suggest that oral immunization with CS2022 induced immune tolerance correlated with the down-regulation of cell surface TLR expression. This down-regulation may in part account for the development of cross-protection against a Listeria challenge by immunization with CS2022.