Characterisation of Mycobacterium Marinum EsxG.EsxH Complex Expression in Mycobacterial-Infected Cells: Towards an Understanding of Tuberculosis Infection

Tuberculosis is a disease caused by Mycobacterium tuberculosis, transmitted from one person to another through inhalation of contaminated aerosols and remains a major threat despite the availability of treatments and prevention globally. Recently, a novel secretion system in M. tuberculosis has been identified known as the Type VII secretion system (T7SS) that allows ESX proteins to be secreted by M. tuberculosis and other mycobacterial species. In this study, the purified ESX protein complexes: EsxA.EsxB, EsxO.EsxP and EsxG.EsxH were analyzed to observe their effects on two different cell lines; J774.1 macrophages and 16-human bronchial epithelial (16HBE). It was observed that only EsxA.EsxB and EsxO.EsxP bound to the surfaces of J774.1 macrophages and 16HBE, while EsxG.EsxH did not. This observation suggests the possible sharing of receptors that are present on both cell lines. The localization of EsxG.EsxH was investigated by immunofluorescence microscopy of Mycobacterium marinum-infected macrophages. EsxG-EsxH was found to be present close to the surface of M. marinum, predominantly outside phagosome in infected macrophages. The production of EsxG.EsxH was estimated at 25% and 29% of the total infected macrophages counted at 24- and 48-hours-post-infection (hpi) respectively suggesting the production of EsxG.EsxH was independent to time-length of infection. Not all mycobacteria in infected macrophages expressed the protein and it was rarely detected in phagosomal bacteria. This rare occurrence did not support the claim of synergism between EsxG.EsxH and Hrs in interrupting the phagolysosomal maturation. Possible co-localisation of EsxG.EsxH with another secreted protein Mpm70 was investigated but these proteins did not co-localise.