Synergistic BM-DC activation and immune induction by the oral vaccine vector Streptococcus gordonii and exogenous tumor necrosis factor

Do Bacteria Provide an Alternative to Cancer Treatment and What Role Does Lactic Acid Bacteria Play?

Cancer is one of the leading causes of mortality and morbidity worldwide. According to 2022 statistics from the World Health Organization (WHO), close to 10 million deaths have been reported in 2020 and it is estimated that the number of cancer cases world-wide could increase to 21.6 million by 2030. Breast, lung, thyroid, pancreatic, liver, prostate, bladder, kidney, pelvis, colon, and rectum cancers are the most prevalent. Each year, approximately 400,000 children develop cancer. Treatment between countries vary, but usually includes either surgery, radiotherapy, or chemotherapy. Modern treatments such as hormone-, immuno- and antibody-based therapies are becoming increasingly popular. Several recent reports have been published on toxins, antibiotics, bacteriocins, non-ribosomal peptides, polyketides, phenylpropanoids, phenylflavonoids, purine nucleosides, short chain fatty acids (SCFAs) and enzymes with anticancer properties. Most of these molecules target cancer cells in a selective manner, either directly or indirectly through specific pathways. This review discusses the role of bacteria, including lactic acid bacteria, and their metabolites in the treatment of cancer.

Identification of a Thiol-Disulfide Oxidoreductase (SdbA) Catalyzing Disulfide Bond Formation in the Superantigen SpeA in Streptococcus pyogenes

Mechanisms of disulfide bond formation in the human pathogen Streptococcus pyogenes are currently unknown. To date, no disulfide bond-forming thiol-disulfide oxidoreductase (TDOR) has been described and at least one disulfide bonded protein is known in S. pyogenes. This protein is the superantigen SpeA, which contains 3 cysteine residues (Cys 87, Cys90, and Cys98) and has a disulfide bond formed between Cys87 and Cys98. In this study, candidate TDORs were identified from the genome sequence of S. pyogenes MGAS8232. Using mutational and biochemical approaches, one of the candidate proteins, SpyM18_2037 (named here SdbA), was shown to be the catalyst that introduces the disulfide bond in SpeA. SpeA in the culture supernatant remained reduced when sdbA was inactivated and restored to the oxidized state when a functional copy of sdbA was returned to the sdbA-knockout mutant. SdbA has a typical C₄₆XXC₄₉ active site motif commonly found in TDORs. Site-directed mutagenesis experiments showed that the cysteines in the CXXC motif were required for the disulfide bond in SpeA to form. Interactions between SdbA and SpeA were examined using cysteine variant proteins. The results showed that SdbA_C49A formed a mixed disulfide with SpeA_C87A, suggesting that the N-terminal Cys46 of SdbA and the C-terminal Cys98 of SpeA participated in the initial reaction. SpeA oxidized by SdbA displayed biological activities suggesting that SpeA was properly folded following oxidation by SdbA. In conclusion, formation of the disulfide bond in SpeA is catalyzed by SdbA and the findings represent the first report of disulfide bond formation in S. pyogenes. IMPORTANCE Here, we reported the first example of disulfide bond formation in Streptococcus pyogenes. The results showed that a thiol-disulfide oxidoreductase, named SdbA, is responsible for introducing the disulfide bond in the superantigen SpeA. The cysteine residues in the CXXC motif of SdbA are needed for catalyzing the disulfide bond in SpeA. The disulfide bond in SpeA and neighboring amino acids form a disulfide loop that is conserved among many superantigens, including those from Staphylococcus aureus. SpeA and staphylococcal enterotoxins lacking the disulfide bond are biologically inactive. Thus, the discovery of the enzyme that catalyzes the disulfide bond in SpeA is important for understanding the biochemistry of SpeA production and presents a target for mitigating the virulence of S. pyogenes.

Inhibition of MDM2 Promotes Antitumor Responses in p53 Wild-Type Cancer Cells through Their Interaction with the Immune and Stromal Microenvironment

p53 is a transcription factor that plays a central role in guarding the genomic stability of cells through cell-cycle arrest or induction of apoptosis. However, the effects of p53 in antitumor immunity are poorly understood. To investigate the role of p53 in controlling tumor-immune cell cross-talk, we studied murine syngeneic models treated with HDM201, a potent and selective second-generation MDM2 inhibitor. In response to HDM201 treatment, the percentage of dendritic cells increased, including the CD103⁺ antigen cross-presenting subset. Furthermore, HDM201 increased the percentage of Tbet⁺Eomes⁺ CD8⁺ T cells and the CD8⁺/Treg ratio within the tumor. These immunophenotypic changes were eliminated with the knockout of p53 in tumor cells. Enhanced expression of CD80 on tumor cells was observed in vitro and in vivo, which coincided with T-cell-mediated tumor cell killing. Combining HDM201 with PD-1 or PD-L1 blockade increased the number of complete tumor regressions. Responding mice developed durable, antigen-specific memory T cells and rejected subsequent tumor implantation. Importantly, antitumor activity of HDM201 in combination with PD-1/PD-L1 blockade was abrogated in p53-mutated and knockout syngeneic tumor models, indicating the effect of HDM201 on the tumor is required for triggering antitumor immunity. Taken together, these results demonstrate that MDM2 inhibition triggers adaptive immunity, which is further enhanced by blockade of PD-1/PD-L1 pathway, thereby providing a rationale for combining MDM2 inhibitors and checkpoint blocking antibodies in patients with wild-type p53 tumors. SIGNIFICANCE: This study provides a mechanistic rationale for combining checkpoint blockade immunotherapy with MDM2 inhibitors in patients with wild-type p53 tumors.

Microbiome, inflammation and colorectal cancer

Chronic inflammation is linked to the development of multiple cancers, including those of the colon. Inflammation in the gut induces carcinogenic mutagenesis and promotes colorectal cancer initiation. Additionally, myeloid and lymphoid cells infiltrate established tumors and propagate so called "tumor-elicited inflammation", which in turn favors cancer development by supporting the survival and proliferation of cancer cells. In addition to the interaction between cancer cells and tumor infiltrating immune cells, the gut also hosts trillions of bacteria and other microbes, whose roles in colorectal inflammation and cancer have only been appreciated in the past decade or so. Commensal and pathobiotic bacteria promote colorectal cancer development by exploiting tumor surface barrier defects following cancer initiation, by invading normal colonic tissue and inducing local inflammation, and by generating genotoxicity against colonic epithelial cells to accelerate their oncogenic transformation. On the other hand, a balanced population of microbiota is important for the prevention of colorectal cancer due to their roles in providing certain bacterial metabolites and inhibiting intestinal inflammation. In this review we summarize our current knowledge regarding the link between microbiota, inflammation, and colorectal cancer, and aim to delineate the mechanisms by which gut microbiome and inflammatory cytokines regulate colorectal tumorigenesis.

Expression, purification, and functional analysis of an antigen-targeting fusion protein composed of CD40 ligand and the C-terminal fragment of ovalbumin

Delivering antigen via molecules specifically targeting receptors on the surface of antigen-presenting cells is a strategy to improve immune responses. In this study, an antigen-targeting fusion protein (OVA-CD40LS) composed of the C-terminal fragment of ovalbumin and the extracellular domain of mouse CD40 ligand was constructed by genetic fusion. The OVA-CD40LS and the control OVA (rOVA) genes were cloned in Escherichia coli and over-expressed as insoluble proteins. The rOVA protein was purified from the insoluble fraction of E. coli cell lysate by nickel affinity chromatography and refolded by step-wise dialysis to give a yield of 11.8 mg/L of culture. The OVA-CD40LS was purified by a 'two-round' nickel affinity and on-column protein-refolding chromatography. The yield was 528 μg/L of culture. The purified OVA-CD40LS, but not the rOVA, was able to simulate the production of pro-inflammatory cytokines and up-regulate cell surface marker proteins in mouse bone marrow-derived dendritic cells. The purified OVA-CD40LS elicited a robust immune response when injected submucosally in the oral cavity of mice. Collectively, the results indicate that the OVA-CD40LS fusion protein was biologically active, functioning as an antigen-targeting protein.

The microbiome and its potential as a cancer preventive intervention

It is becoming increasingly clear that microbiota inhabiting our bodies influence cancer predisposition and etiology. In addition to pathogens with oncogenic properties, commensal and symbiotic microbiota have tumor-suppressive properties. Diet and other environmental factors can modulate the abundance of certain members of microbial communities within the gastrointestinal tract and at other anatomical sites. Furthermore, some dietary factors are metabolized by commensal/symbiotic gut microbiota into bioactive food components believed to prevent cancer. For example, dietary fiber undergoes bacterial fermentation in the colon to yield butyrate, which is a short-chain fatty acid and histone deacetylase (HDAC) inhibitor that suppresses the viability and growth of colorectal cancer cell lines. A recent study using gnotobiotic mouse models demonstrates that fiber can protect against colorectal tumorigenesis in a microbiota- and butyrate-dependent manner that involves the Warburg effect. This and other examples suggest that some of the inter-individual variation observed in epidemiology and intervention studies that have investigated associations between diet and cancer risk might be explained by differences in microbiota among the participants. Data from basic research studies also support the idea that probiotics and prebiotics could be plausible chemoprevention strategies that may be utilized to a greater extent in the future.

In vivo dose response and in vitro mechanistic analysis of enhanced immunoglobulin A production by Lactobacillus plantarum AYA

Secretory immunoglobulin A (IgA) mediates the mucosal immune system, which provides the first line of defense against inhaled and ingested pathogenic bacteria and viruses. Lactobacillus plantarum AYA increases the IgA level of Peyer’s patch (PP) cells, but the recommended amount of consumption and the mechanism of action remains unclear. Better understanding of these is essential to development of L. plantarum AYA for use in functional foods. Therefore, we investigated the dose-response effect (in vivo) and mechanism (in vitro) of IgA enhancement induced by L. plantarum AYA. In the small intestine of the mice fed a diet containing 0.03% or 0.3% of L. plantarum AYA powder for 4 weeks, the IgA levels were significantly increased. Thus, it is suggested that the recommended amount of consumption of L. plantarum AYA is about 0.72 mg per day. In addition, the bacterial cell wall fraction significantly enhanced the IgA production level of murine PP cells in the in vitro assay. The ability of whole cells and the cell wall fraction to enhance IgA levels was significantly inhibited by an anti-Toll-like receptor-2 (TLR-2) antibody, which suggests that the cell wall fraction of L. plantarum AYA increases the IgA level via TLR-2. These findings indicate that L. plantarum AYA is a potential functional food source that maintains mucosal immunity.

Emerging roles of lactic acid bacteria in protection against colorectal cancer

Colorectal cancer (CRC) is the third leading cause of cancer deaths worldwide and the fourth most common cancer diagnosed among men and women in the United States. Considering the risk factors of CRC, dietary therapy has become one of the most effective approaches in reducing CRC morbidity and mortality. The use of probiotics is increasing in popularity for both the prevention and treatment of a variety of diseases. As the most common types of microbes used as probiotics, lactic acid bacteria (LAB) are comprised of an ecologically diverse group of microorganisms united by formation of lactic acid as the primary metabolite of sugar metabolism. LAB have been successfully used in managing diarrhea, food allergies, and inflammatory bowel disease. LAB also demonstrated a host of properties in preventing colorectal cancer development by inhibiting initiation or progression through multiple pathways. In this review, we discuss recent insights into cellular and molecular mechanisms of LAB in CRC prevention including apoptosis, antioxidant DNA damages, immune responses, and epigenetics. The emerging experimental findings from clinical trials as well as the proposed mechanisms of gut microbiota in carcinogenesis will also be briefly discussed.

Development of a Streptococcus gordonii vaccine strain expressing Schistosoma japonicum Sj-F1 and evaluation of using this strain for intranasal immunization in mice

Schistosomiasis is a worldwide parasitic disease. Currently, chemotherapy is the main effective method to treat schistosomiasis; however, it does not prevent reinfection. No effective vaccine is currently available to prevent schistosomiasis. Sj-F1 (GenBank accession number AY261995) is a novel gene that was discovered through screening adult Schistosoma japonicum worm cDNA library with female S. japonicum antigen-immunized sera. Streptococcus gordonii, a normal inhabitant of the human oral cavity, has been a prime candidate in recent investigations toward developing a live oral vaccine vector. One of the approaches for the surface expression of heterologous antigens in S. gordonii is to surface-localize them with the M6 protein from Streptococcus pyogenes. Here, we develop a recombinant S. gordonii strain that expresses the M6-Sj-F1 fusion protein on the bacterial surface. Intranasal immunization in mice with such M6-Sj-F1-expressing S. gordonii bacteria induced strong serum IgG, serum IgA, and saliva IgA against Sj-F1. The results of protective immunity against a challenge with cercariae of S. japonicum showed statistically significant protection following this treatment, with a worm reduction rate of 21.45% and an egg reduction rate of 34.77%. Our data indicate that the described M6-Sj-F1-expressing S. gordonii is highly immunogenic and can partially protect mice from challenge infection with S. japonicum. Intranasal immunization with recombinant S. gordonii may be an alternative to developing a novel S. japonicum vaccine in a safe, effective, and feasible way.

Abating colon cancer polyposis by Lactobacillus acidophilus deficient in lipoteichoic acid

An imbalance of commensal bacteria and their gene products underlies mucosal and, in particular, gastrointestinal inflammation and a predisposition to cancer. Lactobacillus species have received considerable attention as examples of beneficial microbiota. We have reported previously that deletion of the phosphoglycerol transferase gene that is responsible for lipoteichoic acid (LTA) biosynthesis in Lactobacillus acidophilus (NCK2025) rendered this bacterium able to significantly protect mice against induced colitis when delivered orally. Here we report that oral treatment with LTA-deficient NCK2025 normalizes innate and adaptive pathogenic immune responses and causes regression of established colonic polyps. This study reveals the proinflammatory role of LTA and the ability of LTA-deficient L. acidophilus to regulate inflammation and protect against colonic polyposis in a unique mouse model.

Microbes, intestinal inflammation and probiotics

Inflammatory bowel disease (IBD) is known for causing disturbed homeostatic balance among the intestinal immune compartment, epithelium and microbiota. Owing to the emergence of IBD as a major cause of morbidity and mortality, great efforts have been put into understanding the sequence of intestinal inflammatory events. Intestinal macrophages and dendritic cells act in a synergistic fashion with intestinal epithelial cells and microbiota to initiate the triad that governs the intestinal immune responses (whether inflammatory or regulatory). In this review, we will discuss the interplay of intestinal epithelial cells, bacteria and the innate immune component. Moreover, whether or not genetic intervention of probiotic bacteria is a valid approach for attenuating/mitigating exaggerated inflammation and IBD will also be discussed.

Regulation of d-alanylation of lipoteichoic acid in Streptococcus gordonii

d-Alanyl esters on lipoteichoic acid (LTA) are involved in adhesion, biofilm formation, resistance to cationic antimicrobial peptides, and immune stimulation. There is evidence that bacteria can modulate the level of d-alanyl esters on LTA in response to challenge, but the mechanism of regulation appears to be different among bacteria. In this study, expression of the dlt operon responsible for d-alanylation of LTA was examined in the commensal bacterium Streptococcus gordonii. dlt expression was assessed using the dlt promoter–lacZ reporter gene assay, LTA d-alanine content measurements and dlt mRNA quantification. The results showed that dlt expression was growth phase-dependent, with the greatest expression at the mid-exponential phase of growth. In contrast to Staphylococcus aureus, dlt expression in Strep. gordonii was not affected by the exogenous addition of Mg2+ or K+. Interestingly, dlt expression was upregulated under acidic conditions or when cells were stressed with polymyxin B, indicating that cell envelope stress may be a signal for dlt expression. In view of these results, mutants defective in the cell envelope stress LiaSR two-component regulatory system were constructed. The liaS and liaR mutants showed an increase in dlt expression over the parent strain at neutral pH. The mutants failed to respond to low pH and polymyxin B stress; dlt expression remained the same in the presence or absence of these stresses. These results suggest that dlt expression in Strep. gordonii is regulated by the LiaSR regulatory system in response to environmental signals such as pH and polymyxin B. The regulation appears to be complex, involving both repression and activation mechanisms.

Regulation of induced colonic inflammation by Lactobacillus acidophilus deficient in lipoteichoic acid

Imbalance in the regulatory immune mechanisms that control intestinal cellular and bacterial homeostasis may lead to induction of the detrimental inflammatory signals characterized in humans as inflammatory bowel disease. Induction of proinflammatory cytokines (i.e., IL-12) induced by dendritic cells (DCs) expressing pattern recognition receptors may skew naive T cells to T helper 1 polarization, which is strongly implicated in mucosal autoimmunity. Recent studies show the ability of probiotic microbes to treat and prevent numerous intestinal disorders, including Clostridium difficile-induced colitis. To study the molecular mechanisms involved in the induction and repression of intestinal inflammation, the phosphoglycerol transferase gene that plays a key role in lipoteichoic acid (LTA) biosynthesis in Lactobacillus acidophilus NCFM (NCK56) was deleted. The data show that the L. acidophilus LTA-negative in LTA (NCK2025) not only down-regulated IL-12 and TNFα but also significantly enhanced IL-10 in DCs and controlled the regulation of costimulatory DC functions, resulting in their inability to induce CD4(+) T-cell activation. Moreover, treatment of mice with NCK2025 compared with NCK56 significantly mitigated dextran sulfate sodium and CD4(+)CD45RB(high)T cell-induced colitis and effectively ameliorated dextran sulfate sodium-established colitis through a mechanism that involves IL-10 and CD4(+)FoxP3(+) T regulatory cells to dampen exaggerated mucosal inflammation. Directed alteration of cell surface components of L. acidophilus NCFM establishes a potential strategy for the treatment of inflammatory intestinal disorders.