Bacillus anthracis Targets Involved in Chemokine-Mediated Antimicrobial Activity Chemokines are chemotactic cytokines that function in host defense by orchestrating leukocyte migration to sites of infection. However, a number of chemokines have also been found to directly kill a range of pathogenic microorganisms through an as yet undefined mechanism. We previously reported that the interferon-inducible CXC chemokines, CXCL9, CXCL10, and CXCL11, block Bacillus anthracis spore germination, reduce spore viability, and kill vegetative cells, with CXCL10 being the most effective. We also reported that C57BL/6 mice, which are resistant to pulmonary B. anthracis Sterne strain infection, produced significant levels of CXCL9, CXCL10, and CXCL11 in their lungs following spore challenge; whereas, highly susceptible A/J mice did not generate significant levels of these chemokines during pulmonary infection. In vivo neutralization of CXCL9, CXCL9/CXCL10, or CXCL9/CXCL10/CXCL11 rendered C57BL/6 mice susceptible to pulmonary anthrax whereas neutralization of their shared receptor CXCR3, which is the receptor expressed on leukocytes recruited to the site of infection by CXCL9, CXCL10, CXCL11, had no impact on survival. These findings support that CXCL9, CXCL10, and CXCL11 have direct antimicrobial effects against B. anthracis both in vitro and in vivo. To identify the vegetative cell target(s) of CXCL10, we screened a B. anthracis transposon mutant library and found that disruption of ftsX, which encodes the transmembrane protein of a widely conserved prokaryotic ABC transporter, resulted in a CXCL10-resistant phenotype. Deletion of the ftsX gene in B. anthracis (i.e., DftsX) resulted in resistance of vegetative cells to CXCL10, and complementation of ftsX restored CXCL10 susceptibility. In contrast, DftsX spores remained susceptible to CXCL10, suggesting that spores have a different CXCL10 target. To further investigate the role of FtsX, as well as other B. anthracis targets of CXCL10, we propose three Specific Aims: 1) Determine the role of FtsX in susceptibility of vegetative cells to CXCL10; 2) Identify spore target(s) of CXCL10; and 3) Determine the role of spore and vegetative bacterial targets of CXCL10 during in vivo infection. These studies will provide a key foundation for the development of innovative therapeutic strategies for treating infections caused by not only B. anthracis but also a range of pathogenic, potentially multi-drug resistant microorganisms.
We are studying the ability of immune response proteins to fight bacteria that invade the lungs of humans and cause severe infections. These proteins are known as chemokines, and we discovered that a select group of them exerts direct antimicrobial or killing effects against Bacillus anthracis, a bacterium that causes anthrax. We recently identified a bacterial protein that is required for the antimicrobial activity of the chemokine, CXCL10. This project focuses on studying the interaction of CXCL10 with this bacterial protein to determine the mechanism by which CXCL10 kills bacteria and acts as a new type of antibiotic. This project should provide critically important insight into how CXCL10 fights infection caused by a pulmonary pathogen and should lead to future development of new and innovative therapeutic agents for use against not only B. anthracis but also a broad array of infectious organisms such as multi-drug resistant bacterial pathogens.
