Early immunity to Mycobacterium tuberculosis: new insights into macrophage heterogeneity and vaccine-mediated responses
Tuberculosis (TB), caused by the bacteria Mycobacterium tuberculosis (Mtb), kills more individuals than any other single infectious agent. The only approved vaccine, BCG, poorly prevents the spread of disease, highlighting the fact that new vaccines are needed to eradicate TB. These efforts would benefit from a mechanistic understanding of how immunity to TB can be achieved as well as insight into the barriers that restrict immunity. However, a number of gaps in our knowledge of TB immunity remain. In particular, the interaction between Mtb and its target cell, the macrophage, during the early stages of infection remains incompletely understood. Here, we characterized the distinct response programs which are initiated in lung-resident alveolar macrophages (AM) and lung-infiltrating monocyte-derived macrophages (MDM). We find that infected AM harbor many more Mtb bacilli than infected MDM during both the innate and adaptive phases of the immune response. Using RNA-seq, we identified multiple differentially expressed pathways between the two cell types. While infected MDM upregulate canonical proinflammatory signaling pathways associated with Mtb control, infected AM are enriched for pathways relating to proliferation and fatty acid metabolism. Additionally, a number of genes relating to phagosome maturation and nitric oxide production were also differentially expressed. However, validation studies using gene knockout animals did not support a cell-intrinsic, protective role afforded by these genes. In addition to these host studies, we also investigated the transcriptional response of the Mtb itself. Here we found that Mtb in MDM have a transcriptional signature associated with late hypoxia, a known Mtb stress condition. We next used a mouse model of prior BCG immunization in order to understand how this vaccine alters early Mtb immunity. Here we found that BCG promotes a dramatic shift in the cell types targeted for Mtb infection. While AM are the major infected cell for the first two weeks of Mtb infection in unimmunized animals, BCG promotes the accelerated transfer of bacteria into neutrophils and MDM. To determine the requirements for this transfer, we characterized the dynamics of the T cell response using tetramers. BCG-specific CD4 and CD8 T cells were present in the lungs of immunized animals prior to infection, and as early as D10 post-infection, there were 5-fold more of these cells compared to controls. Using T cell-depleting antibodies, we found that the transfer of infection out of AM was dependent on CD4, but not CD8, T cells yet did not require infected AM-intrinsic antigen recognition. Finally, using confocal microscopy, we saw that Mtb-specific T cells were first activated in lung regions devoid of infected cells. Importantly, these events preceded BCG-induced control of the bacterial burden, which occurred only after the co-localization of T cells and infected cells. Together, these studies further our understanding of the early immune response to Mtb. Specifically, they show that the cell types targeted for infection differ profoundly in their ability to control Mtb. In addition, vaccination begins to shape immunity much earlier than previously appreciated by accelerating the infection of recruited macrophages, however, the delayed colocalization of T cells with infected cells in the lung represents a barrier to vaccine-induced immunity. These results could help to inform more rationale vaccine design for TB.
Networked Digital Library of Theses and Dissertations