Host-directed therapy of tuberculosis: what is in it for microRNA?

Abstract

Tuberculosis (TB) is a major health threat and current intervention measures are far from satisfactory. MicroRNAs (miRs) have become major targets of investigations for different diseases due to their propensity to regulate gene expression in various biological processes. More recently, miRs have been found to play key roles in the control of infectious diseases. Consequently, the potential of miRs for diagnosis and therapy of TB is being considered. In this editorial, we discuss most recent lines of evidence for regulation of the immune response in TB by miRs that could form the basis for diagnosis and host-directed therapy in adjunct to canonical intervention measures in TB.

Keywords::

• host-directed therapy
• lung
• microRNA
• tuberculosis

1. Introduction

MicroRNAs (miRs) are small noncoding RNAs that regulate diverse biological processes, such as cell growth and differentiation, cellular metabolism and immune response. They control gene expression by targeting RNA transcripts and determining degradation and/or repression of translation [1]. The multifactorial features of miRs have fostered investigations on their roles in complex host responses, such as immunity to chronic bacterial infections [2]. Tuberculosis (TB) is primarily a lung disease caused by the intracellular bacterium Mycobacterium tuberculosis (Mtb), which caused 1.3 million deaths in 2012 [3]. After 40 years of quiescence, very few drugs have been licensed, and increasing incidences of multidrug-resistant (MDR) and extensively drug-resistant (XDR) TB emphasize the necessity to develop novel intervention strategies [4]. Although caused by a bacterium, not only protection against, but also pathology of TB, is driven to a large extent by the host immune response. Therefore, in addition to standard anti-TB chemotherapy, host-directed therapy (HDT), which improves protection and/or ameliorates pathology, are urgently needed [5]. MiRs have been harnessed for treatment of several disorders, and here, we discuss opportunities and challenges for miR-based HDT of TB.

2. MiRs and lung disease

Increasing evidence suggests that miRs regulate diverse cellular processes in pulmonary pathologies. Sato et al. showed that miR-146a [6] is downregulated when fibroblasts, isolated from chronic obstructive pulmonary disease, are incubated in vitro with proinflammatory cytokines and point to prostaglandin E2 as target of miR-146a [6]. In a similar vein, Wu et al. [7] identified specific miR expression in response to H1N1 pandemic influenza virus revealing new potential strategies toward HTD against influenza [7]. In a mouse model, Moschos et al. [8] showed that several miRs are upregulated in response to aerosolized lipopolysaccharide exposition suggesting that miRs play a key role in the host response to bacterial components in the lung [8]. Furthermore, miRs have been implicated in pathogenesis accompanying lung neoplasia. For example, downregulation of let-7 is associated with shortened postoperative survival and cancer progression [9]. Altogether these data suggest a key role of miRs in controlling diverse disease processes affecting the lung.

3. MiRs and TB

Modulation of host miR expression during bacterial infections remains incompletely understood. In TB, miRs were exploited as potential biomarkers to discriminate between healthy individuals and TB patients [10]. Various biological samples have been analyzed, including peripheral blood, serum/plasma [11], saliva [12], and different cell types. Using a tailored signature of differentially expressed miRs, Miotto et al. [13] discriminated healthy controls from TB patients. This study included differential genetic background to increase sensitivity and specificity within groups [13]. Using peripheral blood, Maertzdorf et al. [14] identified a cluster of four miRs that were differentially regulated between the two granulomatous lung diseases of similar pathology, active TB and sarcoidosis. Interestingly, applying a clustering approach, strong correlations between miRs and gene expression were revealed, suggesting interactions between different biological pathways [14]. The biomarker findings are complemented by studies investigating biology of miRs in immunity to TB. The cytokines IFN-γ and TNF-α are key mediators of protective immunity in TB, and chemokines modulate recruitment of inflammatory leukocytes to the lungs [15]. Rajaram et al. [16] showed that bacterial cell-wall components from virulent or avirulent mycobacterial species induce differential miR expression in infected macrophages. Lipomannan from pathogenic Mtb or from the nonpathogenic M. smegmatis stimulated expression of miR-125b or miR-155, respectively. These two miRs differentially induced TNF-α. Yet miR-125b directly targeted TNF-α, while miR-155 modulated the function of SHIP1, a negative regulator of the PI3K/Akt pathway [16]. Thus, components from related bacterial species, but different virulence, modulate host miR expression, and consequently, immunity. Modulation of IFN-γ by miR has been recently analyzed. Ma et al. [17] created a transgenic mouse in which a sponge blocks the endogenous miR-29. These mice showed increased resistance against Mtb challenge, mostly due to reduced inflammation and lower bacterial burden. In a similar vein, the impact of miR-223 on susceptibility to TB was dissected in gene knockout mutant mice. Dorhoi et al. revealed that miR-223 deficiency caused susceptibility to Mtb infection [18]. Absence of miR-223 resulted in aberrant neutrophil migration to the site of infection, followed by tissue destruction and higher bacterial load. CXCL2, CCL3, and IL-6 were identified as direct targets of miR-223. Using neutralizing antibodies against these mediators, the resistant phenotype could be reestablished [18]. As a corollary, distinct miRs are currently being identified as potential targets for TB therapy.

4. MiR delivery techniques

Novel intervention strategies exploit miR-mimics to restore optimal miR abundance or anti-miRs to block aberrantly produced miRs [19]. For efficient delivery of these molecules, attempts have been made to improve endonuclease resistance and to optimize binding affinity to the homologous target, including chemical modifications, for example, in the sugar backbone or the nucleotide structure. Technologies to deliver miRs in vivo include lentiviral vectors, lipid conjugates, or small exosome-like vesicles. Using a vesicle-based system, primary B cells, transfected with a specific anti-miR, have been shown to deliver this molecule to antigen-activated T cells [20]. Moreover, antigen-specific exosome-like nanovesicles isolated by affinity chromatography have been transfected with mimics or anti-miR to restore miR expression in target cells [21]. These techniques hold promise for treatment of TB, where granulomas, representing the hallmark tissue response, are not easily accessible to drugs. In this context, antigen-specific exosome-like vesicles loaded with specific miRs could modulate cellular composition within granulomas and fine-tune immune responses in situ. However, these delivery methods for miR-based HDTs need to be first carefully assessed in animal models mimicking human TB pathology.

5. Anti-infective therapy with miRs

Attempts to stabilize anti-miR have been advanced for hepatitis C virus (HCV) treatment. High expression of miR-122, which is associated with HCV infection, protects the viral genome by endonuclease degradation. Consequently, an anti-miR drug composed of locked nucleic acid sequence complementary to mature miR-122, has been developed [22]. In HCV patients treated with this compound (Miravirsen), a dose-dependent viral RNA reduction was observed in a Phase II clinical trial [22]. The miR-mimics are also being developed as therapeutics. In a mouse model, administration of exogenous miR-590-3p and miR-199a-3p into the heart was shown to enhance cardiomiocyte growth after induced heart damage [23]. Intranasal administration of miR let-7 for lung cancer HDT maintained tumor suppressor activity [24] leading to a reduction of tumor size. Thus, anti-miR and miR-mimics promise to become powerful measures for readjustment of aberrant miR expression in infection, inflammation, and malignant disorder.

6. Expert opinion

MiRs represent promising tools as biomarker and as accessible targets to modulate host immune responses. Early diagnosis is essential for effective control of TB and analysis of miR expression supports their applicability as reliable biomarkers (alone or combined with other types of markers) for TB diagnosis and prognosis. In addition, miRs could be targeted for HDT approaches, to reduce pathology and strengthen protection. Administration of miR-223 mimics could restrict migration of inflammatory cells to the site of infection and thereby diminish inflammation. Anti-miR constructs targeting miR-29 and miR-125a could be harnessed to augment concentrations of IFN-γ and TNF-α, respectively. Administration of miR-based HDTs to modulate inflammation in adjunct to canonical drug therapy could shorten the duration and increase efficacy of treatment of MDR and XDR TB. Although significant advances have been accomplished concerning delivery of miRs into the lung, numerous questions still await clarification. Further studies will define the most appropriate route of delivery, kinetics of miR-mimics/anti-miR HDT as well as pharmacovigilance issues. The encouraging results reported for intranasal administration of miRs in lung cancer and those describing modulation of TGF-β1 expression via pulmonary delivery of siRNA in the TB mouse model [25], provide a valid basis for future miR-based HDTs. Detailed mechanistic insights into the biological role of miRs remain scarce, and specifics of mode-of-action of anti-miR/miR-mimics await further clarification. Yet, although miR HDT options are still in their infancy, exciting opportunities for the betterment of TB control appear on the horizon.

Declaration of interest

The authors state no conflict of interest and have received no payment in preparation of this manuscript.

Acknowledgements

The authors thank Mary Louise Grossman for help preparing the manuscript.