The emerging role of exosomal miRNAs as a diagnostic and therapeutic biomarker in Mycobacterium tuberculosis infection

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), has been the world's driving fatal bacterial contagious disease globally. It continues a public health emergency, and around one-third of the global community has been affected by latent TB infection (LTBI). This is mostly due to the difficulty in diagnosing and treating patients with TB and LTBI. Exosomes are nanovesicles (40-100 nm) released from different cell types, containing proteins, lipids, mRNA, and miRNA, and they allow the transfer of one's cargo to other cells. The functional and diagnostic potential of exosomal miRNAs has been demonstrated in bacterial infections, including TB. Besides, it has been recognized that cells infected by intracellular pathogens such as Mtb can be secreting an exosome, which is implicated in the infection's fate. Exosomes, therefore, open a unique viewpoint on the investigative process of TB pathogenicity. This study explores the possible function of exosomal miRNAs as a diagnostic biomarker. Moreover, we include the latest data on the pathogenic and therapeutic role of exosomal miRNAs in TB.

Intracellular localisation of Mycobacterium marinum in mast cells

AIM: To study the bacteriocidal or bacteriostatic role of mast cells during infection with Mycobacterium.

METHODS: Mycobacterium marinum (M. marinum) (BAA-535/M strain) was investigated for its ability to grow at a temperature relevant to the mammalian host. Primary mast cells were differentiated from bone marrows of mice, a human mast cell line (HMC-1) and a human monocytic cell line (MonoMac6) were maintained in culture. Mice were stimulated by intraperitoneal injection of heat-killed M. marinum to study cytochemically the degranulation of peritoneal mast cells. HMC-1 cells were stimulated with M. marinum to analyse mRNA expression for inflammatory reactant genes, while HMC-1 and primary mouse mast cells were infected with M. marinum to establish in parallel cell viability (lactate dehydrogenase release and cell counts) and viable mycobacterial counts. Flow cytometry was used to assess intracellular presence of fluorescein isothiocyanate labelled M. marinum after trypan blue quenching and to measure the extent of infection-induced apoptosis or necrosis in HMC-1. A GFP expressing recombinant M. marinum strain was used to assess intracellular location by fluorescence microscopy. Light microscopy of osmium tetroxide and Gram Twort stained sections of 0.5 μm and transmission electron microscopy were undertaken as sensitive methods.

RESULTS: Since its isolation, M. marinum has adapted to grow at 37 °C. This study found that M. marinum infects HMC-1 cells and primary murine mast cells, where they survive, replicate, and cause dose dependent cell damage over the analysis period of up to 120 h. Amikacin was an effective aminoglycoside antibiotic to eliminate extracellular or membrane attached M. marinum in order to adequately quantify the intracellular bacterial loads. In vivo, intraperitoneal injection of heat-killed M. marinum led to the release of mast cell granules in mice. HMC-1 cells stimulated with M. marinum showed a biphasic pattern of increased mRNA expression for LL-37 and COX-2/TNF-α during 24 h of stimulation. In HMC-1, M. marinum localised to the cytoplasm whereas in primary mast cells, M. marinum were found in vacuoles.

CONCLUSION: The effector role of mast cells in infection with M. marinum can be studied in vitro and in vivo.