Immunotherapy of a plasmacytoma with attenuated salmonella

Recombinant Attenuated Salmonella enterica as a Delivery System of Heterologous Molecules in Cancer Therapy

Simple Summary

Cancer is among the main causes of death of millions of individuals worldwide. Although survival has improved with conventional treatments, the appearance of resistant cancer cells leads to patient relapses. It is, therefore, necessary to find new antitumor therapies that can completely eradicate transformed cells. Bacteria-based tumor therapy represents a promising alternative treatment, particularly the use of live-attenuated Salmonella enterica, with its potential use as a delivery system of antitumor heterologous molecules such as tumor-associated antigens, cytotoxic molecules, immunomodulatory molecules, pro-apoptotic proteins, nucleic acids, and nanoparticles. In this review, we present the state of the art of current preclinical and clinical research on the use of Salmonella enterica as a potential therapeutic ally in the war against cancer.

Abstract

Over a century ago, bacterial extracts were found to be useful in cancer therapy, but this treatment modality was obviated for decades. Currently, in spite of the development and advances in chemotherapies and radiotherapy, failure of these conventional treatments still represents a major issue in the complete eradication of tumor cells and has led to renewed approaches with bacteria-based tumor therapy as an alternative treatment. In this context, live-attenuated bacteria, particularly Salmonella enterica, have demonstrated tumor selectivity, intrinsic oncolytic activity, and the ability to induce innate or specific antitumor immune responses. Moreover, Salmonella enterica also has strong potential as a delivery system of tumor-associated antigens, cytotoxic molecules, immunomodulatory molecules, pro-apoptotic proteins, and nucleic acids into eukaryotic cells, in a process known as bactofection and antitumor nanoparticles. In this review, we present the state of the art of current preclinical and clinical research on the use of Salmonella enterica as a potential therapeutic ally in the war against cancer.

Cancer Immunotherapy: Priming the Host Immune Response with Live Attenuated Salmonella enterica

In recent years, cancer immunotherapy has undergone great advances because of our understanding of the immune response and the mechanisms through which tumor cells evade it. A century after the first immunotherapy attempt based on bacterial products described by William Coley, the use of live attenuated bacterial vectors has become a promising alternative in the fight against cancer. This review describes the role of live attenuated Salmonella enterica as an oncolytic and immunotherapeutic agent, due to its high affinity for tumor tissue and its ability to activate innate and adaptive antitumor immune response. Furthermore, its potential use as delivery system of tumor antigens and immunomodulatory molecules that induce tumor regression is also reviewed.

Comparative Evaluation of the Acute Toxic Effects in Monkeys, Pigs and Mice of a Genetically Engineered Salmonella Strain (VNP20009) Being Developed as an Antitumor Agent

The objective of these studies was to perform a comparative evaluation of the acute toxicity of VNP20009, a genetically engineered Salmonella strain, in monkeys, pigs, and mice. It is hypothesized that mice would be more susceptible than other animal species to the toxic effects of VNP20009, because mice are the most sensitive natural host for the parental wild-type Salmonella typhimurium strain. These studies also compared the virulence of VNP20009 and the parental Salmonella in mice. In Cynomolgus monkeys and Yorkshire pigs ( n = 2/dose), various doses (expressed as colony forming units [cfu] per animal) of VNP20009, or vehicle, were administered as a single IV injection (∼ 1 ml/min). The body weight, body temperature, clinical signs, clinical pathology (serum chemistry and hematology), and ophthalmic examinations (only in monkeys) were evaluated at various times. Necropsy was performed on day 15 in the pigs, and necropsy and histopathology on days 8 or 15 in the monkeys. In C57BL/6 mice ( n = 10/dose), various doses of VNP20009, or the parental Salmonella, were administered as a single IV bolus injection. The mice were observed daily over 3 weeks. The results from monkeys showed that VNP20009-related changes in clinical pathology were primarily confined to fiver enzymes and fiver function tests (i.e., cholesterol, triglyceride, alanine aminotransferase, and aspartate aminotransferase levels). Significant toxicological changes occurred only at the dose of 1 × 1010 cfu/monkey, but not at the doses of 1 × 108 or 3 × 109 cfu/monkey. Gross necropsy and histology findings were primarily confined to the spleen (enlargement, weight increase, and reticuloendothelial hyperplasia), thymus (size and weight reduction and lymphoid depletion), mesenteric lymph node (enlargement), and lung (weight increase). Most of these necropsy and microscopic findings, which occurred mostly in the high-dose group, may be related to the physiological responses to infection, rather than related to the intrinsic toxicity of VNP20009. The results from pigs showed that VNP20009 induced toxicological effects only at the dose of 3 × 109 cfu/pig, but not at the doses of 3 × 108 or 3 × 1010 cfu/pig. Both pigs treated with 3 × 1010 cfu/pig died within the first 2 days post-treatment. Necropsy showed the presence of abdominal transudate fluid, skin blotching, and pulmonary-and gall bladder-associated edema. Therefore, the pig mortality may have been related to the physical damage induced by the sudden systemic presence of large amounts of suspension. The results from mice showed that VNP20009, at doses as high as 1 × 106 cfu/mouse, did not induce any mortality. A 30% mortality rate was induced by 3 × 106 cfu/mouse, and 100% mortality by 1 × 107 cfu/mouse. The parental Salmonella, at a dose of 1 × 102 or 3 × 102 cfu/mouse, induced a 100% mortality. In conclusion, the doses of VNP20009 that induced acute toxicity are very high, suggesting that VNP20009 may be a safe agent. The virulence is 50,000 × less in VNP20009 than the parental Salmonella.

[Salmonella enterica: an ally in the therapy of cancer]

Salmonella enterica, a species of facultative anaerobic bacteria, has demonstrated success as a live-attenuated bacterial vector for vaccination. S. enterica has also demonstrated promise as a therapeutic agent against cancer. Pre-clinical and clinical trials have shown that S. enterica is localized in both solid and semi-solid tumors as well as in metastatic tumors. Moreover, S. enterica reduces resistance to treatment with other agents. In this review we present the novel therapeutic anti-cancer approaches that use S. enterica both for its ability as a delivery system for heterologous moieties against cancer and for its direct anti-cancer properties.

Salmonella-mediated tumor regression involves targeting of tumor myeloid suppressor cells causing a shift to M1-like phenotype and reduction in suppressive capacity

The effectiveness of attenuated Salmonella in inhibiting tumor growth has been demonstrated in many therapeutic models, but the precise mechanisms remain incompletely understood. In this study, we show that the anti-tumor capacity of Salmonella depends on a functional MyD88-TLR pathway and is independent of adaptive immune responses. Since myeloid suppressor cells play a critical role in tumor growth, we investigated the consequences of Salmonella treatment on myeloid cell recruitment, phenotypic characteristics, and functional activation in spleen and tumor tissue of B16.F1 melanoma-bearing mice. Salmonella treatment led to increased accumulation of splenic and intratumoral CD11b(+)Gr-1(+) myeloid cells, exhibiting significantly increased expression of various activation markers such as MHC class II, costimulatory molecules, and Sca-1/Ly6A proteins. Gene expression analysis showed that Salmonella treatment induced expression of iNOS, arginase-1 (ARG1), and IFN-γ in the spleen, but down-regulated IL-4 and TGF-β. Within the tumor, expression of iNOS, IFN-γ, and S100A9 was markedly increased, but ARG1, IL-4, TGF-β, and VEGF were inhibited. Functionally, splenic CD11b(+) cells maintained their suppressive capacity following Salmonella treatment, but intratumoral myeloid cells had significantly reduced suppressive capacity. Our findings demonstrate that administration of attenuated Salmonella leads to phenotypic and functional maturation of intratumoral myeloid cells making them less suppressive and hence enhancing the host's anti-tumor immune response. Modalities that inhibit myeloid suppressor cells may be useful adjuncts in cancer immunotherapy.

Antioxidant oils and Salmonella enterica Typhimurium reduce tumor in an experimental model of hepatic metastasis

Fruit seeds high in antioxidants have been shown to have anticancer properties and enhance host protection against microbial infection. Recently we showed that a single oral dose of Salmonella enterica serovar Typhimurium expressing a truncated human interleukin-2 gene (SalpIL2) is avirulent, immunogenic, and reduces hepatic metastases through increased natural killer cell populations in mice. To determine whether antioxidant compounds enhance the antitumor effect seen in SalpIL2-treated animals, we assayed black cumin (BC), black raspberry (BR), and milk thistle (MT) seed oils for the ability to reduce experimental hepatic metastases in mice. In animals without tumor, BC and BR oil diets altered the kinetics of the splenic lymphocyte response to SalpIL2. Consistent with previous reports, BR and BC seed oils demonstrated independent antitumor properties and moderate adjuvant potential with SalpIL2. MT oil, however, inhibited the efficacy of SalpIL2 in our model. Based on these data, we conclude that a diet high in antioxidant oils promoted a more robust immune response to SalpIL2, thus enhancing its antitumor efficacy.

Development of Salmonella strains as cancer therapy agents and testing in tumor cell lines

Despite significant progress in the development of new drugs and radiation, deaths due to cancer remain high. Many novel therapies are in clinical trials and offer better solutions, but more innovative approaches are needed to eradicate the various subpopulations that exist in solid tumors. Since 1997, the use of bacteria for cancer therapy has gained increased attention. Salmonella Typhimurium strains have been shown to have a remarkably high affinity for tumor cells. The use of bacterial strains to target tumors is a relatively new research method that has not yet reached the point of clinical success. The first step in assessing the effectiveness of bacterial tumor therapy will require strain development and preclinical comparisons of candidate strains, which is the focus of this chapter. Several investigators have developed strains of Salmonella with reduced toxicity and capacity to deliver anti-tumor agents. Although methods for obtaining safe therapeutic strains have been relatively successful, there is still need for further genetic engineering before successful clinical use in human patients. As described by Forbes et al. in 2003, the main stumbling block is that, while bacteria preferentially embed within tumor cells, they fail to spread within the tumor and finish the eradication process. Further engineering might focus on creating Salmonella that remove motility limitations, including increased affinity toward tumor-generated chemotactic attractants and induction of matrix-degrading enzymes.

Implications of Salmonella-induced nitric oxide (NO) for host defense and vaccines: NO, an antimicrobial, antitumor, immunosuppressive and immunoregulatory molecule

Attenuated Salmonella induce immunosuppressive, microbicidal and tumoricidal macrophages in mice. All three effects are mediated by activated macrophages producing nitric oxide (NO). NO is induced by the innate immune response pathway involving IL-12, NK cells and IFN-gamma in response to infection. NO has beneficial and detrimental effects on the host.

Lipid A mutant Salmonella with suppressed virulence and TNFalpha induction retain tumor-targeting in vivo

Systemically administered tumor-targeted Salmonella has been developed as an anticancer agent, although its use could be limited by the potential induction of tumor necrosis factor alpha (TNFalpha)-mediated septic shock stimulated by lipid A. Genetic modifications of tumor-targeting Salmonella that alter lipid A and increase safety must, however, retain the useful properties of this bacteria. We report here that disruption of the Salmonella msbB gene reduces TNFalpha induction and increases the LD50 of this pathogenic bacteria by 10,000-fold. Notwithstanding this enormous difference, Salmonella retains its tumor-targeting properties, exhibiting tumor accumulation ratios in excess of 1000:1 compared with normal tissues. Administration of this bacteria to mice bearing melanoma results in tumors that are less than 6% the size of tumors in untreated controls at day 18. Thus, the antitumor activity previously demonstrated using tumor-targeting Salmonella with normal lipid A is retained. Lipid modification of tumor-specific bacterial vectors provides a means for reducing septic shock and further suggests that the antitumor activity of these bacteria may be independent of TNFalpha.

Interleukin-12 is critical for induction of nitric oxide-mediated immunosuppression following vaccination of mice with attenuated Salmonella typhimurium

Studies from our laboratory have shown that infection of mice with an attenuated strain of Salmonella typhimurium causes a marked suppression in the capacity of splenocytes to generate an in vitro plaque-forming cell (PFC) response to sheep erythrocytes. The suppression has been shown to be mediated by mature, adherent macrophages (Mphis) and nonadherent, precursor Mphis. Nitric oxide has been identified as the suppressor factor. The present study investigated the role of interleukin-12 (IL-12) in the generation of nitric oxide-mediated immunosuppression in this model. Salmonella inoculation resulted in marked suppression of PFC responses and high levels of nitrite production. When mice were treated with anti-IL-12 prior to inoculation, nitrite levels in splenocyte cultures were reduced by 75% and the suppression of PFC responses was prevented. The nonadherent splenocyte fraction from Salmonella-inoculated mice, which contains precursor Mphis and is weakly immunosuppressive, was treated with IL-12 in vitro. IL-12 augmented the capacity of this fraction to suppress PFC responses by normal splenocytes in a coculture system. Additionally, IL-12 induced nitrite and gamma interferon (IFN-gamma) production in a dose-dependent manner. Treatment with anti-IFN-gamma blocked nitrite production and suppression, indicating that IFN-gamma is an important intermediary in the pathway of IL-12-induced immunosuppression. These results indicate that IL-12 is critical for the induction of nitric oxide-mediated immunosuppression following S. typhimurium inoculation and, through its ability to stimulate IFN-gamma production, can induce nitric oxide-producing suppressor Mphis.

Is prevention of cancer possible with tumor-specific vaccines?

Cancer is one of the leading causes of death today, and new therapeutic approaches are continuously being explored. In recent years, cancer vaccines have been tried with the aim of induction of an active specific immune response against the tumor. Although some successful results are obtained, cure rates are still disappointing. The main reason for failure is that vaccines are applied to patients diagnosed with cancer; in these patients, tumor cell burden is so high that it is almost impossible to overcome the disease by increasing the immune response with vaccines at this stage. This hypothesis is based on the following idea: since therapy for cancer is unsuccessful in the majority of patients, methods of prevention should be improved. Immunization against microorganisms has largely prevented morbidity and mortality from infectious disease. In a similar way, if we immunize people before the development of malignant disease with tumor-specific vaccines, then prevention of cancer may be possible.

Macrophage nitric oxide mediates immunosuppression in infectious inflammation

A vaccine strain of live, attenuated Salmonella typhimurium induces profound immunosuppression in inoculated mice 7 days after injection. Immunosuppression to mitogens and inability to mount plaque-forming responses to sheep red blood cells occurs in spite of many parameters of upregulated macrophage function and protection against challenge with virulent Salmonella. Studies show that macrophage nitric oxide mediates the immunosuppression and presumably also the early-onset protective capacity of the vaccine. A model of "bystander lymphocyte autotoxicity" is presented to explain the mechanism of immunosuppression. The model proposes that Salmonella-activated macrophages generate nitric oxide which inactivates lymphocytes in the vicinity, so they become dysfunctional. Inhibition of nitric oxide by NG-monomethyl-L-arginine reverses immunosuppression. Evidence is presented that supports a relationship between the microbial burden in the spleen, the degree of nitric oxide produced, and the extent of immunosuppression. It is proposed that this model of microbial immunosuppression mediated by nitric oxide is generalizable for understanding immunosuppression and loss of delayed-type hypersensitivity induced by other microbes, such as Mycobacteria and measles virus. The model could account for anergy during mycobacterial infections, particularly when the burden of acid-fast bacilli is high, as well as loss of skin test reactivity to tuberculin during measles infection.