Approximately eight million people develop active tuberculosis (TB) each year with two million dying from the disease. In addition, it is estimated that one third of the world's population is chronically infected with Mycobacterium tuberculosis (Mtb). Most individuals respond to infection with Mtb by mounting a strong cellular immune response that prevents active disease but does not sterilize the infection. Mtb has developed strategies to persist within macrophages, its major host cells, even in the face of fully developed T cell immunity. Thus, there is a fine balance between the host immune response that controls infection and the pathogen's ability to evade and manipulate this response.

Our work focuses on the interactions of Mtb with its host and addresses both, the role of macrophage in the immune response to and control of Mtb and the molecular mechanisms used by the pathogen to establish persistent infections.

The initial interaction of Mtb with its primary host cell, the macrophage, can determine the outcome of a Mtb infection. We investigate how host cells recognize Mtb, which signal transduction pathways are required for the induction of an efficient immune response, and which regulatory immune mechanisms are engaged by Mtb to counter-regulate and modify the immune response. We began addressing these questions with genomic approaches that identified molecules critical for the host cell response to Mtb. Our current work focuses on macrophage receptors that participate in pathogen recognition and intracellular signal transduction within infected cells. These studies will shed light on the biology of the host-pathogen interface and may lead to insights that are relevant to other pathogens and diseases.

Mtb is able to persist indefinitely in the lungs of an infected host and must have evolved effective mechanisms of defense against the attack of the immune response. To better understand the molecular basis for its ability to resist host defense we characterize Mtb mutants that are susceptible to stresses likely encountered by the bacterium during persistence within the host. Mtb primarily resides within the phagosomes of macrophages and is able to arrest phagosome maturation. However, after cytokine activation of the macrophage, the phagosome matures along the endosomal-lysosomal pathway. Mtb persists within mature phagosomes, indicating that the bacterium possesses resistance mechanisms against defenses of activated macrophages, such as low pH, nitric oxide and others. Using genetic approaches we have identified Mtb mutants that are hyper susceptible to such stress conditions. Many of these mutants are also attenuated in the mouse model of TB. Identification and characterization of the mechanisms underlying loss of stress resistance and loss of virulence of these mutants will help better understand the intracellular environment encountered by Mtb and reveal how the pathogen resists host defense mechanisms.

In collaboration with Dr. Schnappinger's laboratory (Department of Microbiology, Weill Medical College of Cornell University) we recently developed TetR-controlled expression systems that allow silencing of mycobacterial genes in vitro and in vivo. We are applying these systems to create conditional knock-downs of mycobacterial genes that are important for growth and persistence within the host. These conditional knock-downs allow us to investigate if a mycobacterial gene is required at all or only at specific stages of an infection. They also are also used for drug target evaluation and studies of essential mycobacterial genes.

Keywords:

infectious disease, macrophage, host response, microbial pathogen, microarray

Email address:sae2004@med.cornell.edu