Optimized Doxycycline-Inducible Gene Expression System for Genetic Programming of Tumor-Targeting Bacteria

Tumor Pre-Targeting System Using Streptavidin-Expressing Bacteria

PURPOSE

A major obstacle to targeted cancer therapy is identifying suitable targets that are specifically and abundantly expressed by solid tumors. Certain bacterial strains selectively colonize solid tumors and can deliver genetically encoded cargo molecules to the tumor cells. Here, we engineered bacteria to express monomeric streptavidin (mSA) in tumors, and developed a novel tumor pre-targeting system by visualizing the presence of tumor-associated mSA using a biotinylated imaging probe.

PROCEDURES

We constructed a plasmid expressing mSA fused to maltose-binding protein and optimized the ribosome binding site sequence to increase solubility and expression levels. E. coli MG1655 was transformed with the recombinant plasmid, expression of which is driven by the pBAD promotor. Expression of mSA was induced by L-arabinose 4 days after injection of bacteria into mice bearing CT26 mouse colon carcinoma cells. Selective accumulation of mSA in tumor tissues was visualized by optical imaging after administration of a biotinylated fluorescent dye. Counting of viable bacterial cells was also performed.

RESULTS

Compared with a conventional system, the novel expression system resulted in significantly higher expression of mSA and sustained binding to biotin. Imaging signals in tumor tissues were significantly stronger in the mSA-expressing group than in non-expressing group (P = 0.0005). Furthermore, the fluorescent signal in tumor tissues became detectable again after multiple inductions with L-arabinose. The bacterial counts in tumor tissues showed no significant differences between conditions with and without L-arabinose (P = 0.45). Western blot analysis of tumor tissues confirmed expression and binding of mSA to biotin.

CONCLUSIONS

We successfully engineered tumor-targeting bacteria carrying a recombinant plasmid expressing mSA, which was targeted to, and expressed in, tumor tissues. These data demonstrate the potential of this novel tumor pre-targeting system when combined with biotinylated imaging probes or therapeutic agents.

Reprogramming a Doxycycline-Inducible Gene Switch System for Bacteria-Mediated Cancer Therapy

PURPOSE

Attenuated Salmonella typhimurium is a potential biotherapeutic antitumor agent because it can colonize tumors and inhibit their growth. The present study aimed to develop a doxycycline (Doxy)-inducible gene switch system in attenuated S. typhimurium and assess its therapeutic efficacy in various tumor-bearing mice models.

PROCEDURES

A Doxy-inducible gene switch system comprising two plasmids was engineered to trigger the expression of cargo genes (Rluc8 and clyA). Attenuated S. typhimurium carrying Rluc8 were injected intravenously into BALB/c mice bearing CT26 tumors, and bioluminescence images were captured at specified intervals post-administration of doxycycline. The tumor-suppressive effects of bacteria carrying clyA were evaluated in BALB/c mice bearing CT26 tumors and in C57BL/6 mice bearing MC38 tumors.

RESULTS

Expression of the fimE gene, induced only in the presence of Doxy, triggered a unidirectional switch of the POXB20 promoter to induce expression of the cargo genes. The switch event was maintained over a long period of bacterial culture. After intravenous injection of transformed Salmonella into mice bearing CT26 tumors, the bacteria transformed with the Doxy-inducible gene switch system for Rluc8 targeted only tumor tissues and expressed the payloads 2 days after Doxy treatment. Notably, bacteria carrying the Doxy-inducible gene switch system for clyA effectively suppressed tumor growth and prolonged survival, even after just one Doxy induction.

CONCLUSIONS

These results suggest that attenuated S. typhimurium carrying this novel gene switch system elicited significant therapeutic effects through a single induction triggering and were a potential biotherapeutic agent for tumor therapy.

Synergistic cancer immunotherapy utilizing programmed Salmonella typhimurium secreting heterologous flagellin B conjugated to interleukin-15 proteins

The use of appropriately designed immunotherapeutic bacteria is an appealing approach to tumor therapy because the bacteria specifically target tumor tissue and deliver therapeutic payloads. The present study describes the engineering of an attenuated strain of Salmonella typhimurium deficient in ppGpp biosynthesis (SAM) that could secrete Vibrio vulnificus flagellin B (FlaB) conjugated to human (hIL15/FlaB) and mouse (mIL15/FlaB) interleukin-15 proteins in the presence of L-arabinose (L-ara). These strains, named SAMphIF and SAMpmIF, respectively, secreted fusion proteins that retained bioactivity of both FlaB and IL15. SAMphIF and SAMpmIF inhibited the growth of MC38 and CT26 subcutaneous (sc) tumors in mice and increased mouse survival rate more efficiently than SAM expressing FlaB alone (SAMpFlaB) or IL15 alone (SAMpmIL15 and SAMphIL15), although SAMpmIF had slightly greater antitumor activity than SAMphIF. The mice treated with these bacteria showed enhanced macrophage phenotype shift, from M2-like to M1-like, as well as greater proliferation and activation of CD4⁺ T, CD8⁺ T, NK, and NKT cells in tumor tissues. After tumor eradication by these bacteria, ≥50% of the mice show no evidence of tumor recurrence upon rechallenge with the same tumor cells, indicating that they had acquired long-term immune memory. Treatment of mice of 4T1 and B16F10 highly malignant sc tumors with a combination of these bacteria and an immune checkpoint inhibitor, anti-PD-L1 antibody, significantly suppressed tumor metastasis and increased mouse survival rate. Taken together, these findings suggest that SAM secreting IL15/FlaB is a novel therapeutic candidate for bacterial-mediated cancer immunotherapy and that its antitumor activity is enhanced by combination with anti-PD-L1 antibody.

Small molecule-inducible gene regulatory systems in mammalian cells: progress and design principles

Small molecule-inducible gene circuits are some of the most important tools in biology because they provide a convenient way to exert precise regulation of biological systems. These systems typically are designed to govern gene activation, repression, or disruption at multiple levels, such as through genome modification, transcription, translation, or post-translational regulation of protein activity. Due to their importance, many new systems have been created in the past few years to address different needs or afford orthogonality. They can be broadly characterized based on the inducer used, the mode of regulation, and the effector protein enabling the regulation. Furthermore, each synthetic circuit has varying performance metrics and design considerations. Here, we provide a concise comparison of recently developed tools and recommend standardized metrics for evaluating their performance and potential as biological interrogators or therapeutics.

Progress of engineered bacteria for tumor therapy

Recently, with the rapid development of bioengineering technology and nanotechnology, natural bacteria were modified to change their physiological activities and therapeutic functions for improved therapeutic efficiency of diseases. These engineered bacteria were equipped to achieve directed genetic reprogramming, selective functional reorganization and precise spatio-temporal control. In this review, research progress in the basic modification methodologies of engineered bacteria were summarized, and representative researches about their therapeutic performances for tumor treatment were illustrated. Moreover, the strategies for the construction of engineered colonies based on engineering of individual bacteria were summarized, providing innovative ideas for complex functions and efficient anti-tumor treatment. Finally, current limitation and challenges of tumor therapy utilizing engineered bacteria were discussed.