Tumor Vaccination by Salmonella typhimurium After Transformation with a Eukaryotic Expression Vector in Mice

Therapeutic benefit of Salmonella attributed to LPS and TNF-α is exhaustible and dictated by tumor susceptibility

The potential of bacteria-mediated tumor therapy (BMTT) is highlighted by more than a century of investigation. Attenuated Salmonella has prevailed as promising therapeutic agents. For BMTT - categorized as an immune therapy - the exact contribution of particular immune reactions to the therapeutic effect remains ambiguous. In addition, one could argue for or against the requirement of bacterial viability and tumor targeting. Herein we evaluate the isolated therapeutic efficacy of purified LPS and TNF-α, which together account for a dominant immunogenic pathway of gram negative bacteria like Salmonella. We show that therapeutic efficacy against CT26 tumors does not require bacterial viability. Analogous to viable Salmonella SL7207, tumor regression by a specific CD8⁺ T cell response can be induced by purified LPS or recombinant TNF-α (rTNF-α). Conversely, therapeutic effects against RenCa tumors were abrogated upon bacterial avitalization and limited using isolated adjuvants. This argues for an alternative mechanistic explanation for SL7207 against RenCa that depends on viability and persistence. Unable to boost bacterial therapies by co-injection of rTNF-α suggested therapeutic effects along this axis are exhausted by the intrinsic adjuvanticity of bacteria alone. However, the importance of TNF-α for BMTT was highlighted by its support of tumor invasion and colonization in concert with lower infective doses of Salmonella. In consideration, bacterial therapeutic effectiveness along the axis of LPS and TNF-α appears limited, and does not offer the necessary plasticity for different tumors. This emphasizes a need for recombinant strengthening and vehicular exploitation to accommodate potency, plasticity and distinctiveness in BMTT.

Live-attenuated bacteria as a cancer vaccine vector

In the emerging field of active and specific cancer immunotherapy, strategies using live-attenuated bacterial vectors have matured in terms of academic and industrial development. Different bacterial species can be genetically engineered to deliver antigen to APCs with strong adjuvant effects due to their microbial origin. Proteic or DNA-encoding antigen delivery routes and natural bacterial tropisms might differ among species, permitting different applications. After many academic efforts to resolve safety and efficacy issues, some firms have recently engaged clinical trials with live Listeria or Salmonella spp. We describe here the main technological advances that allowed bacteria to become one of the most promising vectors in cancer immunotherapy.

Microcin C: biosynthesis and mechanisms of bacterial resistance

Nonhydrolyzable aminoacyl-adenylates that inhibit protein synthesis provide a promising route towards the development of novel antibiotics whose mechanism of action limits the appearance of bacterial drug resistance. The 'Trojan horse' antibiotic microcin C (McC) consists of a nonhydrolyzable aspartyl-adenylate that is efficiently imported into bacterial cells owing to a covalently attached peptide carrier. Once inside the cell, the carrier is removed by proteolytic processing to release a potent aspartyl tRNA synthetase inhibitor. The focus of this article is on the mechanism of biosynthesis of McC. We also examine the strategies utilized by McC-producing strains to overcome toxicity due to unwanted, premature processing of the drug. This article will discuss how McC biosynthesis can be systematically manipulated for the development of derivatives that will target the entire battery of aminoacyl tRNA synthetases in various bacteria.

Induction of effective antitumor response after mucosal bacterial vector mediated DNA vaccination with endogenous prostate cancer specific antigen

PURPOSE

The induction of systemic immune responses against antigenic targets that are over expressed by cancer cells represents a powerful therapeutic strategy to target metastatic cancer. We generated specific antitumor immune responses in a murine model of prostate cancer by oral administration of an attenuated strain of Salmonella typhimurium containing a plasmid coding for murine prostate stem cell antigen.

MATERIALS AND METHODS

Trafficking of S. typhimurium SL7207 in the initial 10 hours after gavage feeding was determined using a bacterial lux expressing strain and live bioluminescence imaging. For vaccination trials male C57 BL/6 mice were gavage fed SL7207/murine prostate stem cell antigen expressing plasmid or controls twice at 2-week intervals. One week after the last feeding the mice were challenged subcutaneously with TRAMPC1 murine prostate carcinoma cells. Tumor dynamics and animal survival were recorded.

RESULTS

Clearance of bacterial vector from animals was complete 9 hours after feeding. Delivery of vector transformed with a firefly luciferase reporter plasmid resulted in maximal eukaryotic reporter gene expression in splenocytes 48 hours after feeding. Induction of tumor protective immunity was achieved by feeding the mice murine prostate stem cell antigen expressing plasmid bearing bacteria and greater than 50% of immunized mice remained tumor free. No significant toxicity was observed. Induction of T-helper type 1 immune responses was determined by measuring interferon-γ produced by splenocytes from vaccinated mice. When adoptively transferred to naive animals, splenocytes from vaccinated mice prevented tumor growth in 66% of challenged animals.

CONCLUSIONS

Endogenous prostate cancer antigen gene delivery using a bacterial vector resulted in breaking immune tolerance to murine prostate stem cell antigen and significant retardation of tumor growth.

Concomitant tumor and autoantigen vaccination supports renal cell carcinoma rejection

Efficient tumor vaccination frequently requires adjuvant. Concomitant induction of an autoimmune response is discussed as a means to strengthen a weak tumor Ag-specific response. We asked whether the efficacy of dendritic cell (DC) vaccination with the renal cell carcinoma Ags MAGE-A9 (MAGE9) and G250 could be strengthened by covaccination with the renal cell carcinoma autoantigen GOLGA4. BALB/c mice were vaccinated with DC loaded with MHC class I-binding peptides of MAGE9 or G250 or tumor lysate, which sufficed for rejection of low-dose RENCA-MAGE9 and RENCA-G250 tumor grafts, but only retarded tumor growth at 200 times the tumor dose at which 100% of animals will develop a tumor. Instead, 75-100% of mice prevaccinated concomitantly with Salmonella typhimurium transformed with GOLGA4 cDNA in a eukaryotic expression vector rejected 200 times the tumor dose at which 100% of animals will develop tumor. In a therapeutic setting, the survival rate increased from 20-40% by covaccination with S. typhimurium-GOLGA4. Autoantigen covaccination significantly strengthened tumor Ag-specific CD4(+) and CD8(+) T cell expansion, particularly in peptide-loaded DC-vaccinated mice. Covaccination was accompanied by an increase in inflammatory cytokines, boosted IL-12 and IFN-gamma expression, and promoted a high tumor Ag-specific CTL response. Concomitant autoantigen vaccination also supported CCR6, CXCR3, and CXCR4 upregulation and T cell recruitment into the tumor. It did not affect regulatory T cells, but slightly increased myeloid-derived suppressor cells. Thus, tumor cell eradication was efficiently strengthened by concomitant induction of an immune response against a tumor Ag and an autoantigen expressed by the tumor cell. Activation of autoantigen-specific Th cells strongly supports tumor-specific Th cells and thereby CTL activation.

Novel cancer vaccine based on genes of Salmonella pathogenicity island 2

Although tumors express potentially immunogenic tumor-associated antigens (TAAs), cancer vaccines often fail because of inadequate antigen delivery and/or insufficient activation of innate immunity. Engineering nonpathogenic bacterial vectors to deliver TAAs of choice may provide an efficient way of presenting TAAs in an immunogenic form. In this study, we used genes of Salmonella pathogenicity island 2 (SPI2) to construct a novel cancer vaccine in which a TAA, survivin, was fused to SseF effector protein and placed under control of SsrB, the central regulator of SPI2 gene expression. This construct uses the type III secretion system (T3SS) of Salmonella and allows preferential delivery of tumor antigen into the cytosol of antigen-presenting cells for optimal immunogenicity. In a screen of a panel of attenuated strains of Salmonella, we found that a double attenuated strain of Salmonella typhimurium, MvP728 (purD/htrA), was not toxic to mice and effectively expressed and translocated survivin protein inside the cytosol of murine macrophages. We also found that a ligand for CD1d-reactive natural killer T (NKT) cells, alpha-glucuronosylceramide (GSL1), enhanced MvP728-induced interleukin-12 production in human dendritic cells and that in vivo coadministration of a NKT ligand with MvP728-Llo or MvP728-survivin enhanced effector-memory cytotoxic T lymphocyte (CTL) responses. Furthermore, combined use of MvP728-survivin with GSL1 produced antitumor activity in mouse models of CT26 colon carcinoma and orthotopic DBT glioblastoma. Therefore, the use of TAA delivery via SPI-2-regulated T3SS of Salmonella and NKT ligands as adjuvants may provide a foundation for new cancer vaccines.

Thymus repopulation after allogeneic reconstitution in hematological malignancies

OBJECTIVE

Active vaccination in the allogeneically reconstituted tumor-bearing host essentially requires donor T-cell tolerance. To create a basis for vaccination in the allogeneically reconstituted, lymphoma-bearing host, we elaborate a reconstitution protocol that supports thymus repopulation and tolerance induction.

METHODS

Myeloreductively conditioned, lymphoma-bearing mice were vaccinated after reconstitution with hematopoietic progenitor cells. Readout systems included recovery of donor-derived T cells, graft vs host disease (GVHD), anti-host and anti-lymphoma cytotoxicity, as well as tumor growth rate and tumor rejection.

RESULTS

In tumor-free mice, myeloreductive conditioning, together with natural killer cell depletion of the host and transfer of T cell-depleted bone marrow cells, allows reconstitution without severe GVHD. However, in hematological malignancies, donor-derived T-progenitor cells hardly immigrated into the thymus. As a consequence, the frequency of severe GVHD was significantly increased, which prohibited active vaccination. Thymus repopulation became improved by strengthening myeloreductive conditioning; by supporting thymocyte expansion via interleukin-7; and, most strongly, by a small dose of donor-derived CD4(+)CD8(+) thymocytes, which preferentially homed into the thymus. Active vaccination, in combination with this reconstitution protocol, did not strengthen GVHD, but significantly improved survival time and survival rate of lymphoma-bearing mice.

CONCLUSION

The negative impact of hematological malignancies on thymus repopulation and central tolerance induction can, at least in part, be corrected by application of a small number of donor-derived T-progenitor cells.

Salmonella typhimurium stimulation combined with tumour-derived heat shock proteins induces potent dendritic cell anti-tumour responses in a murine model

Appropriate activation of the immune system and effective targeting of tumour cells are the primary hurdles to be overcome for cancer immunotherapy to be successful and applicable to a wide range of tumour types. Our studies have examined the ability of bacterial-stimulated dendritic cells (DCs), loaded with tumour-associated antigens, to inhibit tumour growth in a murine model. Immature murine bone marrow-derived DCs were stimulated in vitro with the cytoplasmic fraction (CM) of Salmonella typhimurium in combination with heat shock proteins (hsps) from 4T1 tumours, isolated using heparin affinity chromatography. Activated DCs were administered subcutaneously. Tumours were generated by orthotopic inoculation of 4T1 cells in Balb/c mice. Primary tumour growth was measured using Vernier calipers, while lung metastases were measured using the clonogenic assay. S. typhimurium CM induced potent tumour necrosis factor (TNF)-alpha responses from DCs accompanied by significant up-regulation of CD80 and CD86 expression. When injected into mice, bacterial-stimulated DCs loaded with 4T1 hsps inhibited the formation of new 4T1 tumours and reduced the growth rate of established tumours. In addition, the number of lung metastatic nodules was reduced significantly in the DC-treated mice (1.6 +/- 0.6 versus 245.9 +/- 55.6, P = 0.0015). DCs stimulated with CM alone, exposed to tumour hsps alone or exposed to tumour hsps from an unrelated tumour cell line did not induce a protective immune response. Dendritic cells primed with a proinflammatory bacterial stimulus and tumour-associated antigens induce a protective anti-tumour immune response in this murine model.

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.