Defining the molecular basis of host-pathogen interaction in bovine TB

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The sustainable control of Bovine tuberculosis (bTB) is a major challenge for UK agriculture. bTB is caused by a bacterium, Mycobacterium bovis, that can infect a range of wild and domesticated animals, as well as man. Control of bTB is therefore important for public health as well as animal health and welfare. However, current bTB control measures have been ineffective in the UK. The number of herds newly infected with bTB has doubled every nine years; in 2013 over 6.2 million bTB tests were performed in England leading to the slaughter of over 26,000 cattle at a cost to the taxpayer of almost £100Million. New approaches to halt the spread of infection are desperately needed. Mycobacterium bovis (Mbv) is a close relative of Mycobacterium tuberculosis (Mtb), the causative agent of TB in humans. While some of the mechanisms used by Mtb to cause disease in humans are well characterised, not much is known about the strategies used by Mbv to cause disease in cattle. DNA sequencing of the genomes of Mtb and Mbv have revealed that they share more than 99.9% genetic identity. While both Mbv and Mtb can infect humans, Mbv rarely transmits between humans and is instead spread by contaminated dairy products. On the other hand, Mtb can only cause a mild form of the disease in cattle. The distinct host preference of Mbv and Mtb indicates that the two mycobacterial species have evolved distinct strategies to infect and cause disease in their respective hosts. Through previous studies and our own work, a range of discrete molecular variations have been identified between Mtb and Mbv; these include differences in the expression of proteins, termed MPB70 and MPB83, and lipids, such as 'sulfolipid', between these bacteria. The role of these proteins and lipids in the virulence of Mbv for cattle has however not been defined; in this study we will explore the hypothesis that these molecular differences are among the key reasons why Mbv causes disease in cattle. We will test our hypothesis by first generating genetically-engineered Mbv strains that are 'Mtb-like' with regards to the expression of these lipids and proteins. These strains will then be tested along with their parental Mtb and Mbv strains in cellular infection models. These models will focus on laboratory grown cell cultures of bovine macrophages, cells of the bovine immune system that are the first to encounter and engulf invading Mbv cells. We have initial data to show that these cells respond differently to infection with Mbv and Mtb, and will use the genetically engineered strains of 'Mtb-like' Mbv strains to elucidate the mechanisms used by Mbv to establish infection in cattle. To achieve this we will develop an array of tools and new methodologies, such as the use of fluorescently-labelled Mbv and genetic-knockdown techniques, to study the infected macrophages. These tools will help us monitor critical intracellular components of the macrophage machinery, as well as providing tools for the wider bovine immunology research community. Our findings will shed new light on the molecular components that Mbv uses to cause disease in cattle, information that will help in the long term development of vaccines and diagnostics against this devastating disease for UK agriculture.