Qualification of Assays to Measure Human T-cell Responses Against Mycobacterial Lipid Antigens

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PROJECT SUMMARY (ABSTRACT)   Mycobacterium tuberculosis (M.tb) was responsible for more than 1.5 million deaths in 2013, making it a leading infectious cause of death worldwide. Currently, Mycobacterium bovis bacille Calmette-Guérin (BCG) is the only licensed vaccine for tuberculosis. BCG provides protection against disseminated forms of the disease in children but is inconsistent in preventing the development of pulmonary tuberculosis in adults. Since adults with pulmonary tuberculosis are highly infectious, control of the epidemic will not be achieved with BCG, and new vaccines are urgently needed. There are a number of vaccines under development, including recombinant BCG and attenuated M.tb strains. A successful immune response to M.tb depends critically on T- cells, which are typically activated by bacterial peptide antigens bound to major histocompatibility complex (MHC) molecules. Like BCG, candidate whole cell vaccines are poly-antigenic and contain both peptide and non-peptide antigens that are recognized by human T cells. Mycobacterial cell wall lipids have conclusively been shown to activate human T cells when bound to CD1 proteins on antigen-presenting cells. The CD1 system is conserved among mammals and mostly absent from other vertebrates, suggesting it evolved to perform an important function that is non-redundant with the MHC system. However, there are no validated assays to measure human T-cell responses against non-peptide antigens, such as lipids. We have established partnerships with experts in lipid antigen discovery, flow cytometry, vaccines, and computational biology to tackle this technical challenge over the last five years. We recently developed soluble CD1 tetramers and an activation-based T-cell profiling assay as tools to facilitate population-based studies of T-cell responses to mycobacterial lipids. We have also developed unbiased computational approaches to the analysis of high-dimensional flow cytometry data. In Aim 1, we will optimize and qualify an assay using lipid- loaded CD1 tetramers to study the memory phenotype and activation status of lipid-specific T cells at rest. In Aim 2, we will optimize and qualify a complementary activation-based assay to study the effector functions of T cells activated by lipid antigens. Unlike MHC proteins, CD1 proteins are virtually non-polymorphic so these assays can be applied independent of genetic background. In Aim 3, we will use both assays to study the effect of mycobacterial vaccination on lipid-specific T-cell responses in humans using BCG as a model system. Major collaborators on this proposal are the HIV Vaccine Trials Network (HVTN) and the South African Tuberculosis Vaccine Initiative (SATVI), which have extensive experience in developing immune-based assays for candidate HIV and tuberculosis vaccine trials, respectively. By the end of the funding period, we will have qualified a suite of assays that will supplement existing approaches to identifying correlates of protective immunity in efficacy studies of whole cell mycobacterial vaccines.