Person:

Booty, Matthew Gregory

IL-21 is produced predominantly by activated CD4+ T cells and has pleiotropic effects on immunity via the IL-21 receptor (IL-21R), a member of the common gamma chain (γc) cytokine receptor family. We show that IL-21 signaling plays a crucial role in T cell responses during Mycobacterium tuberculosis infection by augmenting CD8+ T cell priming, promoting T cell accumulation in the lungs, and enhancing T cell cytokine production. In the absence of IL-21 signaling, more CD4+ and CD8+ T cells in chronically infected mice express the T cell inhibitory molecules PD-1 and TIM-3. We correlate these immune alterations with increased susceptibility of IL-21R−/− mice, which have increased lung bacterial burden and earlier mortality compared to WT mice. Finally, to causally link the immune defects with host susceptibility, we use an adoptive transfer model to show that IL-21R−/− T cells transfer less protection than WT T cells. These results prove that IL-21 signaling has an intrinsic role in promoting the protective capacity of T cells. Thus, the net effect of IL-21 signaling is to enhance host resistance to M. tuberculosis. These data position IL-21 as a candidate biomarker of resistance to tuberculosis.

Granulomas are the pathological hallmark of tuberculosis (TB). However, their function and mechanisms of formation remain poorly understood. To understand the role of granulomas in TB, we analyzed the proteomes of granulomas from subjects with tuberculosis in an unbiased fashion. Using laser capture microdissection, mass spectrometry and confocal microscopy, we generated detailed molecular maps of human granulomas. We found that the centers of granulomas possess a pro-inflammatory environment characterized by anti-microbial peptides, ROS and pro-inflammatory eicosanoids. Conversely, the tissue surrounding the caseum possesses a comparatively anti-inflammatory signature. These findings are consistent across a set of six subjects and in rabbits. While the balance between systemic pro- and anti-inflammatory signals is crucial to TB disease outcome, here we find that these signals are physically segregated within each granuloma. The protein and lipid snapshots of human and rabbit lesions analysed here suggest that the pathologic response to TB is shaped by the precise anatomical localization of these inflammatory pathways during the development of the granuloma.

T cell vaccines against Mycobacterium tuberculosis (Mtb) and other pathogens are based on the principle that memory T cells rapidly generate effector responses upon challenge, leading to pathogen clearance. Despite eliciting a robust memory CD8+ T cell response to the immunodominant Mtb antigen TB10.4 (EsxH), we find the increased frequency of TB10.4-specific CD8+ T cells conferred by vaccination to be short-lived after Mtb challenge. To compare memory and naïve CD8+ T cell function during their response to Mtb, we track their expansions using TB10.4-specific retrogenic CD8+ T cells. We find that the primary (naïve) response outnumbers the secondary (memory) response during Mtb challenge, an effect moderated by increased TCR affinity. To determine whether the expansion of polyclonal memory T cells is restrained following Mtb challenge, we used TCRβ deep sequencing to track TB10.4-specific CD8+ T cells after vaccination and subsequent challenge in intact mice. Successful memory T cells, defined by their clonal expansion after Mtb challenge, express similar CDR3β sequences suggesting TCR selection by antigen. Thus, both TCR-dependent and -independent factors affect the fitness of memory CD8+ responses. The impaired expansion of the majority of memory T cell clonotypes may explain why some TB vaccines have not provided better protection.

Approximately one-third of the world’s population is currently infected with Mycobacterium tuberculosis (Mtb), the bacillus that causes tuberculosis. Globally, it is the second leading cause of death by a single infectious agent. An effective vaccine is needed to stop this ongoing pandemic, but efforts to design one are hampered by our limited understanding of host immunity to this pathogen.

CD8+ T cells are elicited during tuberculosis and are required for optimum host resistance. They produce cytokines such as IFN-γ and can directly lyse infected cells. During infection, the expansion and differentiation of effector CD8+ T cells is a dynamically regulated process that is influenced by the inflammatory milieu of the infected host. Currently, the signals governing CD8+ T cell responses during tuberculosis are not well characterized.

Utilizing a mouse model of disease, we address the effects of key cytokines on CD8+ T cells, beginning with IL-12, type 1 interferons (IFN), and IL-27. All three of these cytokines are produced by innate immune cells during tuberculosis and have profound effects on host resistance. IL-12 proves most essential for robust CD8+ T cell expansion and IFN-γ production and also drives the terminal differentiation of short-lived effector cells. However, IL-12 is not acting alone, and type 1 IFN and IL-27 each have non-redundant roles supporting expansion in infected lungs. Thus, CD8+ T cells reflect the inflammatory environment of the host, responding in different degrees to each cytokine present.

We next examine the role of IL-21, a cytokine produced by activated CD4+ T cells. In the absence of IL-21 signaling, CD8+ T cell expansion and effector functions are severely compromised. IL-21 is also essential to prevent CD8+ T cell exhaustion at later time points during disease. These observations are the first to describe an essential role for IL-21 in the host immune response to Mtb.

Together, these studies establish IL-12 and IL-21 as essential regulators of CD8+ T cells during tuberculosis, and indicate type 1 IFN and IL-27 support expansion in the lungs. We believe these observations have implications for future immunotherapies and rational vaccine design.