Team:CPU CHINA/Background
Cellular Immunotherapy
Cellular immunotherapy treatments harness the patient's immune system to eliminate cancer cells or invaded pathogens and provide durable remissions. In 2017, U.S. Food and Drug Administration (FDA) approved the first cellular immunotherapy to treat B-cell acute lymphoblastic leukemia. In August 2018, two cell-based therapies, Kymriah and Yescarta were approved by European Medicines Agency (EMA) on the same day. The approval of these products indicates that cellular immunotherapy comes of age.
Although most of the current researches focus on the cell-based cancer immunotherapy, closed-loop gene networks programming mammalian cells are also designed for treating other serious illnesses. The "post-antibiotic era" poses a serious threat to the health of human beings, and conventional methods are getting increasingly difficult to deal with plenty of infectious diseases. Therefore, we put forward the idea to design " immune-like cells" to fight against various infectious diseases.
Infectious diseases
Infection is the invasion of disease-causing agents into an organism, which leads to infectious diseases, also known as communicable disease. Infectious disease is the chief reason for mortality and morbidity worldwide, claiming millions of lives [1]. Among the top 10 causes of deaths in 2016, three items are related to infection, including lower respiratory infections, diarrhoeal diseases and tuberculosis [2]. Furthermore, owing to continuing emergence of new pathogens and drug-resistant microorganisms, the protracted war between infectious diseases and human beings will be extremely tough.
Basic facts about tuberculosis
Tuberculosis (TB) is the leading cause of death from an infectious agent, accounting for 1.6 million deaths and a global economic burden of $12 billion in 2017 [3]. Although drug-susceptible TB is curable under 6-12 months treatment with 4 antimicrobial drugs, we should not ignore that non-compliance to anti-TB treatment is also a serious problem in TB control [4]. Besides, the World Health Organization (WHO) estimates that about 490,000 new cases are multidrug-resistant tuberculosis (MDR-TB) with resistance to at least 2 powerful first-line anti-TB drugs (Isoniazid and Rifampicin), of which 8.5% had extensively drug-resistant TB (XDR-TB) in 2017. Nearly half of the global MDR-TB cases occur in 3 countries - India, China and the Russian Federation [5]. However, only 50% of patients with MDR-TB and 30% of patients with XDR-TB could be cured with current treatment regimens [6]. Thus, it is extremely urgent to develop candidate strategies for MDR-TB treatment.
TB is an airborne infectious disease caused by Mycobacterium tuberculosis (Mtb) that most often affects lungs [7]. When patients with pulmonary TB cough, sneeze or spit, they propel the germs into surrounding environment. A person will become infected as long as he or she inhale a few of these germs. In fact, more than a quarter of the world's population are infected with Mtb (latent TB) and there is an approximately 5-10% risk of falling ill with TB in their lifetime (active TB)[8].
Mtb navigates multiple intrinsic barriers of the upper respiratory tract, and then encounters resident alveolar macrophages, which is the first line of defense against Mtb invasion. Typically, macrophages recognize and ingest the invaded pathogen into phagosome via cell surface receptors, which refer to complement receptors, C-type lectin receptors and Fcγ receptors in the case of Mtb. Then, phagosome matures and fuses with the lysosome, resulting in generation of antimicrobial products and degradation of the phagosomal contents [9] . However, after cellular uptake, Mtb manipulates normal cell biological process using plenty of strategies to survive and replicate in infected macrophages [10] . Furthermore, the germs in blood circulation also threaten the patients' health and increase the risk of extrapulmonary tuberculosis. Consequently, we aim to develop and integrate two separated strategies to fight against extracellular and intracellular Mtb simultaneously.
How to fight against Mycobacterium tuberculosis?
(1) Target extracellular Mtb: Granulysin
It is generally recognized that cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells play vital roles in host defense against pathogenic bacteria, which could exhibit cytolytic activity against microbes via producing various antimicrobial substances, such as perforin, granzyme and granulysin [12] . Granulysin is a cationic protein possessing broad antimicrobial activity, especially fatal to Mtb. It directly kills extracellular Mtb by damaging membrane integrity and disrupting lipid metabolism of the bacillus [13 ,14]. Moreover, granulysin significantly decreased the number of bacteria in Mtb H37Rv-infected mice model and there was no obvious cytotoxicity of granulysin [15]. Therefore, we choose granulysin as a weapon to directly attack extracellular Mtb in blood circulation.
(2) Target intracellular Mtb: hsa-let-7f
MicroRNAs (miRNAs) are a group of non-coding RNA molecules of 19-25 nt in length that regulates post-transcriptional gene expression [16]. Mature miRNA could block gene expression via forming RNA-induced silencing complex (RISC) with the 3' UTR region of a targeted mRNA and then facilitates mRNA degradation [17]. Due to enormous therapeutic benefits of RNA interference (RNAi), RNAi-based gene therapy for treating infectious diseases has become an extremely prevalent topic [18].
MicroRNA hsa-let-7f is reported to regulate human immune responses to Mtb via control of A20, an inhibitor of the NF-κB pathway [19]. The Nuclear factor-kappa B (NF-κB) family of transcription factor plays an essential role in regulating the expression of pro-inflammatory cytokine and chemokine genes, which is regarded as the central mediator of inflammatory process [20].
Studies have shown that Mtb attenuates the level of hsa-let-7f in macrophages, which causes the increase of A20 and suppression of NF-κB signaling pathway, leading to much more bacterial survival in macrophage. Meanwhile, Kumar's study demonstrated that macrophage expression of hsa-let-7f reduces Mtb survival. This indicates hsa-let-7f to be a promising agent for killing intracellular Mtb, thus we put forward a novel strategy of RNAi-based host-directed therapeutics utilizing hsa-let-7f against TB.
How to deliver miRNA into macrophages?
Despite of its great therapeutic potential, clinical application of RNAi-based therapy is limited by lack of safe and effective drug delivery vectors. In recent years, exosomes have attracted tremendous attention due to their intrinsic role as natural transporters. Exosomes are nanoscale (40-120) extracellular vesicles that bear mRNA, non-coding RNAs, proteins and lipids, mediating intercellular communication between different cells [21].
As a promising drug carrier, exosome possesses a great many advantages. Primarily, exosomes are biocompatible vehicles with low immunogenicity, which means they are safe enough to be used as therapeutic agents. In addition, exosomes provide a stable environment for miRNAs to avoid potential degradation by various endonucleases in blood circulation and extracellular matrix [22]. Moreover, exosomes targeting Mtb-infected macrophages can be obtained by displaying a variety of targeting molecules on the outer surface of exosomes [23]. Given their superior properties, we decided to adopt a targeting exosome system to deliver hsa-let-7f into Mtb-infected macrophages.
References
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