Elucidating the role of mycobacterial secreted phosphatases in host lipid dynamics and pathogen survival

Tuberculosis (TB) is one of the oldest and most persistent bacterial infectious diseases that affects humans. Despite significant advances to control the spread of TB with the introduction of vaccines and antibiotics, one third of the human population is still infected, and 1.6 million die every year. The rise in drug resistance and the limited arsenal of effective antibiotics is threatening to further expand the disease burden. New therapeutic approaches are urgently needed to fight the disease. Mycobacterium tuberculosis (Mtb), the causing agent of TB, is able to challenge our immune system and to survive in the lungs by evading our natural defences. Generally, our white blood cells will engulf and digest microbes to stop infections, in a process known as phagocytosis. However, TB produces a number of substances, "survival factors", that stop this process allowing the bacteria to grow and multiply inside the white blood cells and then spread to nearby cells. The precise mechanisms that allow the bacteria to survive are still poorly understood. We now want to understand how these survival factors work and what happens when we block their action. A better understanding of these processes will help us to develop new strategies to treat TB and other infectious diseases, leading to the development of new types of medicines to eliminate bacteria. Blocking the action of these essential survival factors, in combination with current antibiotics, has the potential to shorten the time of treatment, increase efficacy against susceptible and drug resistant strains of Mtb, thereby increasing cure rates, and preventing relapse in patients with an impaired inmune system.

TB remains a leading cause of death and poverty in the world. Critical to the persistence of TB infection is the ability of Mtb to subvert the innate immune response and disrupt the antimicrobial activity of host macrophages. It does that by using secreted factors that manipulate the host cells signalling pathways to enhance mycobacterial invasion and survival. Two such factors are the secreted phosphatases MptpB and SapM, essential for persistence of Mtb in macrophages and animal models of TB. These phosphatases show activity towards membrane phosphoinositides involved in phagocytosis and phagosome maturation. We have demonstrated that their inhibition impairs Mtb survival in macrophages. Our hypothesis is that the ability of MptpB and SapM to dephosphorylate phagosomal membrane PIs contributes to maturation arrest and mycobacterial survival in macrophages. To date, the mechanism of action of MptpB and SapM in host cells remains largely unknown. Our aim is to elucidate the molecular and cellular mechanisms of action of MptpB and SapM to understand their role in pathogenesis. Specifically we want to address the following questions: i) how do MptpB and SapM affect PI dynamics during infection?; ii) what is the role of MptpB and SapM in phagosomal maturation?; iii) what is the mechanism by which MptpB and SapM promote Mtb survival?; iv) how does MptpB and SapM inhibition lead to impaired survival?. We propose a multidisciplinary study that combines molecular and cellular biology with live cell imaging to elucidate the spatio-temporal involvement of MptpB and SapM on PI dynamics and phagosomal maturation during Mtb infection to establish their contribution to phagosomal maturation arrest. The results from these studies will lead to a fundamental understanding of the role of survival factors in pathogenesis and persistence of TB and other microbial pathogens (such as Listeria, Salmonella, Shigella) that rely on subversion of PI metabolism to establish infection.

England has the second highest rate of TB in Western Europe. Hence, the economic burden of TB to the NHS is significant, the estimated cost per patient rises from £5K for antibiotic sensitive TB to £100K for MDR/XDR-TB. In addition, non-tuberculous mycobacteria (NTM) present a growing health threat and their infections are particularly difficult to treat since they are mostly drug resistant. There are more than 150 species of NTMs present in the environment, soil and water sources, including domestic water systems. A large population is at risk of infection by NTMs, such as people with asthma, cystic fibrosis and chronic obstructive pulmonary disease (COPD) as well as people with rheumatoid arthritis taking TNF inhibitors therapy. The economic burden is also substantial (£40K/patient).Current treatments for TB and NTMs function by preventing bacterial growth and fail to completely clear the infection, resulting in relapse and selection of drug-resistant strains. Therefore, there is an urgent need for better drugs that reduce treatment duration and increase efficacy against MDR/XDR-TB and NTMs. An alternative approach to antibiotics is to block the action of bacterial survival factors, thereby compromising establishment of the infection and persistence of the pathogen. MptpB and SapM are critical for survival of Mtb in the host, thus blocking their action offers an attractive co-adjuvant strategy for therapy. As a proof of the potential for this approach, we have demonstrated that inhibition of MptpB is effective in reducing bacterial burden of drug sensitive and MDR-TB strains in macrophages and animal models of TB and increases efficacy of first-line antibiotics (Vickers et al 2018). Therefore a better understanding on how we can use this strategy to develop new medicines will have clear translational implications.
Our project will address fundamental questions on how bacterial pathogens manipulate host defence mechanisms to avoid degradation. These studies are applicable to many microbial pathogens and therefore our results could guide the development of new approaches to tackle drug resistance in a wide spectrum of infectious diseases. Therefore the outcome of our studies will impact on the academic and scientific communities as well as on the private sector for example through pharmaceutical companies currently interested in wide spectrum antimicrobials (LifeArc, GSK, Evotec, F2G). We believe that new treatments based on this type of approach could have a critical impact on various patient populations, including patients with active TB, either drug-susceptible or drug-resistant. Other implications would be prevention of disease establishment and reactivation in immunosupressed hosts (diabetics, patients under cancer treatment or immunosupressants, elderly, HIV patients). These patients are at higher risk of developing infectious diseases because of defective adaptive immune response or impaired macrophage activation.
We are committed to the translation of this research as proven by our current collaboration with LifeArc (see letter of support), and we have made very significant progress towards generating a new drug candidate for future clinical evaluation. We anticipate that a new approach to TB treatment will have a significant impact on the clinical and health sciences as well as regulatory agencies and world health organisations (WHO, Stop-TB, GATB, Gates Foundation). In summary, our research is likely to impact on the academic and scientific community (see academic beneficiaries), on the public, on the private sector, to improve health and quality of life on the patient population and their families (societal impact), to impact on the health system and the clinic, and to inform regulatory agencies to elaborate new regimen guidelines. The potential to increase efficacy of treatments, in particular for MDR/XDR-TB, will also significantly reduce the hospitalisation time and overall cost per patient (economic impact)