Novel molecular mechanisms underlying pathogen killing in macrophages

Antibiotics have been used to treat bacterial infections for the past century and have significantly contributed to the increase in life expectancy over this period. However, the past few decades have seen a dramatic increase in bacterial resistance to antibiotic treatment. The emergence of antibiotic resistance is creating major difficulties in the treatment of many bacterial diseases, and novel approaches to kill infecting bacteria are desperately required.

Most healthy people with an efficient immune system are well equipped to defeat the majority of bacterial infections. Understanding exactly how our immune system recognizes and kills bacteria will therefore help identify new ways to fight bacterial infections efficiently and enhance the potential of our immune system. Macrophages are important cells in our immune system, whose main function is to eat, kill and digest microbes such as bacteria. But how macrophages do this is only partially understood.

Salmonella enterica is a major bacterial pathogen. There are more than two thousand different types (serovars) of Salmonella enterica that cause serious infections in humans, farm animals and pets. These pathogens are becoming increasingly resistant to antibiotics, making them a serious global health threat. We recently identified two molecular components essential for macrophages to kill Salmonella enterica. In this project we will investigate these antimicrobial molecules, taking advantage of state-of-the-art tools and expertise available in our laboratory. Central to this project are advanced technologies that will allow us to identify specific molecules from complex cellular extracts ("omic" technologies) that contribute to pathogen killing. Understanding the role of these novel antimicrobial molecules and associated proteins, which are used by macrophages to kill bacteria, will represent a significant breakthrough in the understanding of the function of the immune system. The insights we obtain will be fundamental to the identification of ways to enhance the molecular antimicrobial activities of macrophages and boost immune responses in both humans and farm animals.

Macrophages are fundamental players in host immune response, and one of their central functions is to kill pathogens. However, the mechanisms used by macrophages to kill bacterial pathogens are only partially understood. We recently showed that mouse macrophages kill the human pathogen S. Typhi through a novel antimicrobial trafficking pathway that is dependent on the Rab32 GTPase and its guanine nucleotide exchange factor BLOC-3. This pathway also appears to be important in controlling the growth of other intracellular pathogens, such as mycobacteria. However, nothing is known about the regulation of this novel pathway in macrophages. In addition, very little is known about the differences between the mouse and the human pathways that enable S. Typhi to survive in human macrophages but not in mouse macrophages. Therefore, this project will elucidate the function and regulation of this novel antimicrobial pathway, and in doing so will define novel strategies used by macrophages to kill bacterial pathogens. First, I will investigate the role of Rab32 interactors, recently identified in my laboratory through a co-affinity purification approach, in S. Typhi killing. I will also use an analogous approach to identify Rab32 and BLOC-3 interacting proteins both in mouse and human macrophages. This will clarify the key mechanistic differences between mouse and human pathways that enable S. Typhi to infect humans. Finally, I will reveal how specific manipulations can modulate the activity of this pathway and affect pathogen killing and clearance. The results will provide invaluable insights into the molecular mechanisms that underlie pathogen killing and bring remarkable opportunities to exploit this antimicrobial pathway to boost the human immune response against bacterial pathogens.

The objective of this proposal is to elucidate the molecular mechanisms underlying the clearance of S. Typhi and other bacterial pathogens. S. Typhi is the cause of typhoid fever, a life-threatening bacterial disease that affects 27 million people and kills 200,000 individuals every year. I identified a macrophage trafficking pathway that mediates killing of S. Typhi. Several observations suggest that this pathway can be a general surveillance pathway in humans, controlling the growth of other intracellular bacterial pathogens, such as Mycobacteria. Through this research I will identify other components of this trafficking pathway and elucidate how to activate this pathway to enhance pathogen clearance.

This project will greatly benefit the society and will, in the long term, have an important impact on biology and medicine by affecting the understanding of mechanisms underlying bacterial diseases. By elucidating new antimicrobial mechanisms, this research can bring fundamental insights into new therapeutic approaches to control infectious diseases and will have a significant impact on the opportunities to treat typhoid fever and Salmonella infections in farm animals. The results have the potential to impact on the treatment of tuberculosis and leprosy.

In academia, this research will benefit the fields of bacterial pathogenesis, host-pathogen interaction, immunology, cell biology and biochemistry. By identifying basic mechanisms underlying macrophage function, this research will fundamentally contribute to the basic knowledge in immunology and host cell biology.

These studies will also have a strong impact on health care and industry. Indeed, a greater understanding of this novel macrophage antimicrobial pathway can lead to the development of novel strategies to boost the host defences against bacterial infections. The identification of novel components of an antimicrobial pathway has the potential to define novel targets for therapeutic interventions.

Finally, this project will also benefit the postdoctoral researcher associate, who will be trained in a laboratory frontrunner in the field of S. Typhi host-pathogen interaction and part of a network of world leaders in the field. This will definitely help her/his future steps in a scientific career and contribute to the economic competitiveness of the United Kingdom in the field of bacterial infections.