Macrophage Gene Therapy: opening novel therapeutic avenues for immune disorders

Introduction

Macrophages are known to be involved in the development of various diseases, not only because of their key role in host immunity, but also because of their ability to act as a host and reservoir for certain pathogens^1,2. Their involvement in rheumatoid arthritis, tumorigenesis, AIDS, atherosclerosis, diabetes, and lupus erythematosus has already been well documented (3–6; Figure 1). Since, like other primary cells, primary macrophages also resist transfection by contemporary methods, efficient transfection methods are required⁷. A recent breakthrough in macrophage gene therapy is the successful use of macrophage-specific synthetic promoters. These promoters were made by random combination of macrophage and myeloid cis elements⁸. This promoter was found to have up to 100-fold higher activity than that of a native macrophage promoter with minimal activity in nonmyeloid cells. When this promoter was used with GFP, blood leukocytes showed stable and high levels of CD11b+ specific GFP expression for more than a year. Similarly, hypoxia-response elements (HREs) are being utilized for delivery of therapeutic genes specifically to hypoxic tumour areas⁹. Self-inactivating retroviral vectors that had regulatory cis elements from the macrophage-specific human CD68 gene, successfully directed cell-specific expression of reporter genes in immune cells in vivo¹⁰. Furthermore, a number of gene therapy trials, where macrophage granulocyte-macrophage colony-stimulating factor (GM-CSF) expression was altered, showed promising results¹¹.

Figure 1. Prospects of macrophage gene therapy.

Viral gene therapy

Viruses including adenovirus, lentivirus, and adeno-associated virus are commonly used to transfect macrophages¹². However, a number of non-viral methods have also been used¹³. The biggest disadvantage of non-viral methods is that exogenous DNA entering the macrophage cells by endocytosis is often degraded by nucleases. On the other hand, viral methods generally give higher transfection efficiency and sustained transgene expression¹⁴. Retroviruses (other than the lentivirus sub-family) are generally ineffective in the transfection of primary macrophages as these cells are of non-dividing nature¹⁵. Therefore, macrophages are often generated from retrovirus-infected macrophage precursors that divide more frequently¹⁶. Convenient transfection of macrophage precursors offers a greater advantage of re-introduction of transfected cells into a suitable host. An encouraging macrophage-infection rate of 90% was seen, when a nuclear localization signal (NLS) sequence was introduced into the matrix protein of the C type retrovirus¹⁷. A common problem with retroviruses is that transgene expression from integrated retroviruses is lost after a short duration, possibly due to transcriptional silencing.

Lentiviruses can normally infect non-dividing cells including monocytes. Viral matrix proteins, Vpr, integrase, and pol genes have been shown to be important for the transduction of non-dividing cells in this case¹⁸. Lentiviral vectors might also be less susceptible to transcriptional silencing. Adenoviruses can infect non-dividing cells with high efficiency but with moderate stability. 10–80% transfection efficiencies of human macrophages have been reported using adenoviruses¹⁹. Interestingly, adenoviruses are less capable of infecting monocytes. However, more than 90% infection rate was reported, where primary human monocytes were pre-treated with macrophage-colony stimulating factor⁸. Despite a number of advantages, the major drawback of lentiviral vectors (particularly of human lentiviruses) is the extreme safety concerns over its use⁸. Adeno-associated viruses (AAV) offer some advantages similar to retroviruses; they can insert themself into the host cell chromosome for stable expression of transgenes, and, they can also infect both proliferating and non-proliferating cells²⁰. One of the common drawbacks of using AAV or retroviral vectors for gene therapy is that they offer low size limits (4–8 kb). On the other hand, poxvirus and herpes simplex virus, that have also been used to infect macrophages, have much higher capacity. A major hurdle in viral gene therapy is immune responses generated by viral vectors especially in the case of adenoviruses and AAV. The common solution for these problems is to use a viral vector having a truncated genome and therefore produce fewer viral proteins that can be immunogenic^21,22. Importantly, most of the viruses used so far infect a broad range of cell types, and are not highly specific for macrophages. Altering the cell tropism of viruses to enhance specificity has been an important area of research recently²³. Some of the interesting reports, where an artificial virus cell-binding receptor was engineered, have shown promising results²⁴. Also, genetic manipulation of the genes coding for the adenoviral fiber protein was helpful to gain higher specificity.

Non-viral gene therapy

Both chemical and physical methods including liposomes, lipoplexes, DNA carriers, diethylaminoethyl (DEAE)-dextran, microinjection, and electroporation have been used with variable success rates²⁵. Some of the drawbacks of non-viral methods are low transfection efficiency and transient transgene expression. The other common problems are: limited systemic application, rapid clearing from the circulation, and transfection of non-target tissues^26–28. This can be a problem during gene therapy. Fortunately, this problem can easily be avoided by treating the transfection-ready DNA with a methylases. Although electroporation has been shown to achieve moderate levels of transfection efficiency for human monocytic cell lines and monocyte-derived macrophages, the main disadvantage in this case is increased cell death²⁹. Improved transfection reagents offer benefits like low toxicity, large DNA delivery, and lack of immunogenicity³⁰. However, poor transfection efficiencies were seen in the case of primary macrophages¹⁹. LipofectAMINE along with protamine sulfate was found to be the most effective in macrophage cell line RAW 264.7, followed by Lipofectin, DOTAP, and DEAE-dextran³¹. Cationic liposome/DNA complexes have also been used for transfecting monocytes/macrophages in vivo in the blood, liver, and spleen³². However, a number of non-target cells were transfected in these cases. Several transfection methods have been developed to target specific macrophage cell surface receptors³³. Ligands such as mannose and transferrin have been incorporated into gene transfer vehicles to increase the efficacy of transfection for macrophages. Some of the intracellular microbes that infect macrophage cells were also used as a method of transferring DNA constructs. An attenuated derivative of Salmonella typhimurium containing interferon-expressing plasmid has been used for macrophage-specific interferon expression³⁴. Similarly, attenuated mutants of leishmania have been developed that can safely be used for macrophage gene therapy^35,36.

Conclusion

Macrophages are of central importance in immune system, and also have important physiological roles in major organs including brain and liver. Their role in variety of immune, physiological, and pathogen-centric disorders have been repetitively validated. More and more diseases are showing associations with compromised or exaggerated macrophage functions. With the recent developments in the gene therapy, macropahges have become an attractive target to prevent or cure major diseases including cancer, range of blood-vascular disorders, diabetes, and various immune disorders.