Development of Vaccines to Prevent Tuberculosis

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications & trials

Abstract

Mycobacterium tuberculosis (Mtb) is a respiratory pathogen and the leading cause of death from infection worldwide. Intradermal (ID) vaccination with BCG has variable efficacy against pulmonary tuberculosis, the major cause of mortality and disease transmission. We showed that the route and dose of BCG vaccination influence circulating and lung resident T cells and subsequent protection against Mtb infection and TB disease in a highly-susceptible nonhuman primate (NHP) model. NHP immunized with BCG by the intravenous (IV) route induced substantially higher antigen-specific CD4 (Th1 or Th17) and CD8 responses in blood, spleen, bronchoalveolar lavage (BAL, airway), and lung lymph nodes compared to the same BCG dose administered by ID or aerosol (AE) routes. Moreover, IV immunization was the only route that elicited a high frequency of antigen-specific tissue resident T cells in lung tissue (site of TB disease). Six months after BCG vaccination, NHP were challenged in the lung with virulent Mtb. Strikingly, 9 of 10 NHP that received BCG IV were highly protected, with 6 NHP showing no detectable infection (sterile protection) as determined by PET CT imaging, mycobacterial culture, pathology, granuloma formation, or de novo immune responses to Mtb-specific antigens. Follow up efficacy experiments in NHP have repeated these initial findings and shown that IV BCG immunization provides robust protection against Mtb challenge even at 10-100-fold lower doses. Interestingly, a recent study conducted in our lab using alternative strains of BCG that are either replication-limited (auxotrophic) or non-live (irradiated) demonstrated that IV non-live BCG was not protective against TB compared to its live, non-replicating counterpart. The finding that live BCG IV mediates an unprecedented level of protection in NHP provides a model for determining immune correlates and mechanisms of protection against Mtb and has important implications for TB vaccine development. We interrogated correlates of protection (immune responses that can be used clinically to predict protection) in an IV BCG dose-ranging study in macaques. This analysis revealed a highly integrated and coordinated immune response in the airway, some features of which could be predicted by early (day 2) innate signatures in whole blood (transcriptional profiling). Antigen-specific cytokine-producing CD4 T cells (Th1/Th17) and natural killer (NK) cell numbers in the airway were among features most strongly correlated with protection. A parallel systems serology analysis of the study identified humoral signatures associated with protection. Together, these data suggested that IV BCG elicits a multi-faceted immune response that mediates high-level protection against infection and disease. Our group continues to interrogate protective immune correlates in blood and tissues using increasingly advanced technologies. Our current work is focusing on a large analysis of single cell transcriptional data generated from blood, BAL and lung tissue of NHPs before and after BCG vaccination and Mtb challenge. These studies will provide further clarity on the functional profiles of immune cells that participate in protection, and how these cells communicate with one another. They may also identify blood transcriptional signatures of protection that could facilitate clinical vaccine development. Additionally, we have developed a pipeline for high dimensional fluorescent microscopy from NHP tissues following vaccination with BCG and infection with Mtb. This analysis aims to characterize the spatial landscape in the lungs of animals that are either protected or not protected against TB. To understand whether antigen-specific T cell responses were indeed required for protection against Mtb challenge, we performed a series of studies in which individual T cell subsets were depleted from IV BCG-immunized NHP just prior to Mtb challenge. Compared to undepleted vaccinated animals (that were highly protected), protection was completed abrogated in vaccinated animals depleted of CD4 T cells. These data demonstrate that CD4 T cells are required for IV BCG-mediated protection. Of note, depletion of CD8 alpha, but not CD8 beta expressing cells also diminished protection implicating non-conventional (innate) CD8 T cells (or NK cells) in protection after IV BCG. An additional aspect of this work was the ability to characterize Mtb establishment and dissemination in the context of T cell depletion. This analysis was made possible by infecting with a genetically barcoded library of Mtb such that animals were infected with (~20) individual bacteria, each labelled with a unique barcode. Typically, the number of unique barcoded Mtb detected in unvaccinated animals is nearly equivalent to the infectious dose (ie, each infecting bacteria established infection), whereas IV BCG-immunized animals significantly reduce the number of bacteria that establish (in the lung) and disseminate (to lung lymph nodes). Unexpectedly, despite the high bacterial burden in CD4- and CD8alpha-depleted BCG-immunized animals, the number of unique barcodes that established infection were restricted—similar to undepleted/vaccinated animals. These data indicate that there is an initial bottleneck (reduction) following Mtb infection that occurs independent of T cells. A possible explanation for this initial bottleneck is a contribution of Trained Immunity (TI) to TB protection. TI is innate immune memory—the concept that refers to long-term functional reprogramming of innate cells that results in an enhanced response to a secondary encounter with a different pathogen-derived stimulus. BCG has been at the forefront of the TI field—it has long been recognized that neonatal BCG immunization provides off-target protection against unrelated pathogens. In mice, TI has been shown to contribute to protection against TB after IV BCG immunization through epigenetic reprogramming of myeloid precursor cells in the bone marrow. Thus, a continued focus in the lab is to investigate whether IV BCG induces trained immunity in NHP, whether it contributes to protection against infection, and whether TI can be harnessed to improve the efficacy of other vaccine modalities.