Mycobacterium tuberculosis is the causative agent of tuberculosis, a primarily pulmonary disease that is responsible for approximately one million deaths annually. M. tuberculosis is exquisitely adapted to survival within the eukaryotic host and is capable of persisting in a latent state for decades after infection. The ability of M. tuberculosis to evade and manipulate the host immune response is believed to contribute to its survival for such long periods of time after initial infection. My thesis work has focused on the bacterial mechanisms, particularly an alternate protein secretion system, underlying the modulation of macrophage cytokine production by M. tuberculosis.
In this work, I first describe the results of a genetic screen to identify bacterial genes required for the suppression of macrophage cytokine production. This work revealed that two major virulence determinants in M. tuberculosis, the ESX-1 secretion system and the pathogenic lipid phthiocerol dimycocerosate, are critical to modulation of macrophage innate immune responses. I further characterize the macrophage signaling pathways that are targeted for manipulation by the ESX-1 system and identify a small subset of macrophage genes regulated by bacterial infection that are specifically modulated by the ESX-1 pathway. Many of the genes identified in this work do not have previously described functions in the immune system, suggesting novel avenues for study of bacterial pathogenesis and the resistance to M. tuberculosis infection.
In the second part of this work, I describe the characterization of a novel member of the ESX-1 protein secretion pathway, Rv3849. Through a number of complementary approaches, I demonstrate that Rv3849 is a transcriptional activator of an operon required for function of the ESX-1 pathway. Furthermore, I show that Rv3849 is itself a secreted substrate of the ESX-1 system. I propose and provide data supporting a model in which Rv3849 transcriptionally activates ESX-1 secretion in response to intracellular cues and then is exported through the ESX-1 pathway as a form of negative feedback. The identification of a regulator of ESX-1 secretion and characterization of its activity is an important step forward in understanding a critical element in the interaction between M. tuberculosis and the host.