Cellular GWAS of the host-pathogen interaction in melioidosis

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Melioidosis is a tropical infection caused by inoculation, inhalation, or ingestion of the Gram-negative soil saprophyte and Tier 1 select agent Burkholderia pseudomallei. Disease is often severe and treatment is hampered by difficulty eradicating the pathogen due to its extensive antimicrobial resistance. Pneumonia and bacteremia are common presentations and often result in sepsis and multi-organ failure. The overall melioidosis mortality rate is 40% in southeast Asia and 20% in northern Australia. No vaccine is available and efforts to reduce environmental exposure to the bacterium have been challenging. Although the distribution of B. pseudomallei in the environment is not fully determined, recent modeling work suggests that it may be widespread and that an extraordinary 165,000 cases of human melioidosis and 89,000 deaths occur annually worldwide. Studies show that there is marked inter-individual variation in the whole blood innate immune response to bacterial ligands in vitro. There is also significant inter-individual variation in the ability of B. pseudomallei to enter and replicate in primary human cells such as alveolar macrophages and monocytes. Remarkably, it has been reported that the heritability – or proportion of the observed phenotype explained by genetic factors – of Salmonella invasion of cells exceeds 50%. Combined, this suggests that naturally occurring phenotypic variation in vitro can be exploited to identify variants in genes that regulate host defense against invading bacteria such as B. pseudomallei. EBV-transformed lymphoblastoid cell lines (LCLs) derived from extensively genotyped and sequenced individuals in the HapMap/100 Genomes Projects are a useful tool for in vitro screening of genetic regulators of disease-related traits. In vitro phenotypes can be merged with readily available and unparalleled genomic structure data to perform cellular genome wide association studies (GWAS) efficiently and cheaply. LCLs infected with B. pseudomallei demonstrate differential invasion and replication dynamics. This likely reflects significant heritability in the host response to infection, and indicates that LCL-based cellular GWAS can be effectively used to study modulators of host defense in melioidosis. Understanding the host determinants of successful B. pseudomallei invasion and replication within cells is crucial to developing strategies to prevent infection with this resourceful pathogen. The hypothesis of this project is that novel host genetic variation modulates the dynamics of host cell invasion in melioidosis and that these genetic modulators can be identified by performing cellular GWAS of quantitative LCL phenotypes. The aims of this project are to 1) identify genetic determinants of B. pseudomallei invasion and survival in lymphoblastoid cell lines and 2) validate observed genetic associations in primary human lymphocytes and monocytes. This exploratory project offers exciting potential to open up new avenues of knowledge about host defense in melioidosis. Such new knowledge is essential to tackle this increasingly global public health threat.