The main concern of gene therapy is to target the gene of interest to intended cell tissues for optimizing treatment efficiency. Genetically engineered bacteria have been developed as shuttle vectors for localized delivery of therapeutics. Their success depends upon their tropism to target cells and the efficiency of the engaged delivery system. Bodies of evidence clearly indicate the great potential of recombinant bacteria in gene therapy, although most of the studies were just looking for proof-of-concept rather than a ready-to-use final product. This part will provide an overview of our current understanding of bacteria-based delivery of therapeutic genes and heterologous antigens for prophylactic strategies.
- Recombinant bacteria
- Gene delivery
- Gene therapy
1.1. Concept of genetically engineered microorganisms as delivery vectors
Although significant progress has been made in physical and chemical methods for gene delivery, these nonmicrobial strategies still present some drawbacks related to specificity and efficiency of gene transfer [1–6]. For example, new formulations of lipid nanoparticles have led to great improvement in gene stability and transfer, yet there remains a lack of a targeting system that would favor the gene transfer to particular cell tissues . Live avirulent microbial vectors such as viruses and bacteria are a promising approach for gene delivery that may serve to fill in those blanks [8–14]. As such, microbial vectors are able to not only serve as cell factories for the production of the transgene but also as vehicles that deliver the transgene to specific cells for which they have a naturally high tropism. Gene transduction with recombinant viruses is generally based on the use of an expression cassette encompassing a transgene [8–11], while in bacteria, the classic approach of gene transfer is based on plasmid-encoded genes [12–14]. The gene of interest must be delivered to the cell’s nucleus to allow an efficient manufacturing of the corresponding protein. DNA escape from intracellular bacteria to host cell cytosol may occur following their phagocytosis and lysosomal degradation within the cell. This is, however, not the case for intracellular bacteria that resist or subvert the phagolysosomal processing such as
2. Bacteria as delivery vectors in gene therapy
Recombinant bacteria are being considered as an in vivo cell factory that could be used for the delivery of therapeutic genes to target cells. In this process known as “bactofection,” a number of bacterial species have been developed as delivery vectors for their application in different therapeutic approaches.
2.1. Attenuated mutant bacteria
The most known bacteria for such purposes are
2.2. Naturally occurring nonpathogenic bacteria
Gene therapy in cancer has been also investigated using a food-grade microorganism
3. Type III delivery system: A promising strategy for targeting intended cell tissues
A broad spectrum of pathogenic bacteria (
3.1. Application in immunoprophylaxy
The first attempt in using the TTSS for the delivery of heterologous antigens for vaccine purposes was performed with attenuated
The experimental approach of the bacterial TTSS in vaccination studies has been investigated in cancer models as well. Studies in mice indicated that oral administration of recombinant
Besides their role in the delivery of heterologous antigens, bacterial vectors present major advantages over nonmicrobial adjuvant vaccines in that they are endowed with the ability to induce innate immunity through pathogen-associated molecular patterns (PAMPs). These specific microbial motifs include lipoproteins, lipopolysaccharides, single-strand RNA, and nonmethylated DNA sequences that can trigger the maturation process of antigen-presenting cells through binding to their specific Toll-like receptors and consequently induce the production of inflammatory cytokines . This is particularly interesting for vaccination strategies aiming to optimize the protection efficacy .
3.2. Application in therapeutic development
Optimal efficiency of any microbial vector in gene therapy relies particularly on its ability to deliver a sufficient amount of the drug to targeted cell tissues while preserving healthy tissue. The fact that
4. Issues to overcome for better translating the generated proofs-of-concept to effective treatments in human
Bodies of evidence clearly indicate that bacterial vectors are a promising strategy for gene delivery. Many experimental investigations have shown proof-of-concept examples of the feasibility of such an approach, yet steps forward are still needed not only to translate these concepts into effective treatments for humans but also to find the perfect delivery system for each disease situation.
For safety reasons, nonpathogenic food-grade bacteria remain more attractive as live vectors for vaccine and therapeutic strategies. Some concerns exist, however, about targeting issues which is crucial for optimal efficiency. The best example is the potential use of
Recombinant bacteria have shown great potential in the preclinical trials. Their clinical potential relies on their safety and biological containment. Most of the studies were just looking for proof-of-concept rather than a final product that could be put directly to use. Given the global needs, future research challenges should focus on the balance between the optimization of gene therapy through effectiveness of gene delivery to target cells and the biological control of recombinant bacteria to ensure not only an appropriate shutoff mechanism but also to minimize the risks of insertional mutagenesis and aberrant genomic location of delivered genes.
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