Bacteria can be engineered to effectively deliver therapeutics in specific tissues, including tumours. Two articles have now described different approaches to improve the design and efficacy of engineered therapeutic bacteria in cancer. Tal Danino and colleagues have designed a tunable system to reduce the toxicity of immunogenic bacteria, whereas Claudia Gravekamp and colleagues have designed bacteria that can deliver immunogenic antigens to the tumour, making it more susceptible to host immune attack. Both approaches were efficient in inhibiting tumour growth in mouse models of cancer.
Credit: N.Smith/Springer Nature Limited
Which bacterial strain is selected to develop a therapeutic platform depends, among other factors, on the safety profile. “To reduce toxicity due to immunogenicity, many groups have genetically attenuated bacteria or coated them with surface molecules”, explains Danino, “but these approaches can reduce the efficacy and functions of the bacteria.”
In their paper, Danino and colleagues describe an approach to balance immunogenicity and efficacy. They have designed a synthetic gene circuit that expresses a polysaccharide coating (CAP, a surface polymer that protects bacteria from opsonization and phagocytosis) to encapsulate Escherichia coli Nissle 1917 (EcN) — a strain with a safe clinical profile — for initial delivery. Over time, expression of the coating can be switched off, reducing immune escape and thereby retaining safety by increasing bacteria clearance.
EcN has a complex CAP with polymers encoded by different genes. First, the authors generated a library of knocked-down strains to identify key CAP genes capable of altering response to antibacterial factors encountered during therapeutic delivery. They identified kfiC as the optimal candidate in regulating bacterial protection. They then engineered a tunable CAP (iCAP) by cloning kfiC under the control of the lac promoter, which can be induced by the galactopyranoside, IPTG. In vitro, addition of IPTG increased expression of CAP and bacterial survival by reducing phagocytosis.
Next, to enhance efficacy, the authors engineered EcN iCAP to express theta toxin, which has shown antitumour effects in previous studies. In syngeneic colorectal and spontaneous breast cancer models, a single administration of EcN iCAP induced with IPTG resulted in increased tumour growth inhibition compared with a single injection of EcN.
The authors also tested the ability of EcN iCAP to reach distant lesions in three models of colorectal and breast cancer. EcN iCAP increased the frequency of distal site colonization when the mice were fed IPTG. “Activating such a behaviour in situ would be difficult to do with other attenuation or coating strategies,” says Tetsuhiro Harimoto, first author of the study. Once EcN iCAP reached distant lesions, induction of the theta toxin resulted in tumour inhibition.
Danino is hopeful the results will translate to patients: “Humans are known to be ~250-fold more sensitive to bacterial endotoxins and thus encapsulation may have a higher impact in clinical settings than in our studies”. He is also optimistic about the possibilities of the system, as there are many types of capsular polysaccharides, which provides opportunities for a similar approach in other bacteria types.
In their study, Gravekamp and collaborators have used attenuated bacterium Listeria monocytogenes to deliver tetanus toxoid (TT) into tumour cells in mouse models of pancreatic ductal adenocarcinoma (PDAC), a notoriously poorly immunogenic and difficult to treat cancer. Listeria attract and infect myeloid-derived suppressor cells (MDSCs), which then migrate into the tumour site, where the infection can spread from the MDSCs into tumour cells. Because most people are vaccinated against tetanus, expression of TT on tumour cells can reactivate pre-existing TT-specific memory T cells that migrate to the tumour site and eliminate infected cells.
First, the authors vaccinated KPC mice — a mouse model of PDAC — with the human tetanus vaccine before tumour development. Once the tumour was established, they treated the vaccinated mice with Listeria-TT in combination with gemcitabine, a standard treatment for PDAC. Tumours from mice treated with the combination were much more immunogenic than tumours treated with Listeria-TT or gemcitabine alone, with a higher presence of cytotoxic T cells, activation of interferon pathways and upregulation of MHC and chemokines. The treatment reduced tumour size by 80%, the number of metastases by 87%, and increased survival by 40%.
Gravekamp and colleagues are already preparing the first clinical trial in patients with PDAC and are planning on exploring the approach with other cancer types. She is also looking forward to exploring how this approach can avoid the problems of immunosenescence: “Age-related reduction in naïve T cells is an underrecognized field, particularly in cancer immunotherapy,” she explains. “We believe that our approach of reactivating memory T cells generated during childhood overcomes the need for naïve T cells at older age.”
