Poster Presentation Lorne Infection and Immunity 2014

Outzincing host defence: Salmonella employs multiple mechanisms to subvert zinc-mediated antimicrobial responses in macrophages (#166)

Ronan Kapetanovic 1 , Nilesh J. Bokil 1 , Maud E. S. Achard 2 , Kate Peters 2 , Minh-Duy Phan 2 , Kate Schroder 1 , Katharine M. Irvine 3 , Bernadette M. Saunders 4 , Katryn J. Stacey 2 , Alastair G. McEwan 2 , Mark A. Schembri 2 , Matthew J. Sweet 1
  1. Institute for Molecular Bioscience, Centre for Inflammation and Disease Research and Australian Infectious Diseases Research Centre, The University of Queensland , St Lucia, QLD, Australia
  2. School of Chemistry and Molecular Biosciences and Australian Infectious Diseases Research Centre, The University of Queensland, St Lucia, QLD, Australia
  3. School of Medicine, The University of Queensland, Wooloongabba, QLD, Australia
  4. Mycobacterial Research Program, Centenary Institute, Locked Bag No. 6, Sydney, NSW, Australia

Zinc deficiency predisposes to infectious diseases and zinc supplementation is associated with improved outcomes in severe diarrheal disease.Here we investigated potential antimicrobial effects of zinc in human macrophages. We show that Toll-like Receptor (TLR) 4 signalling promotes the delayed accumulation of vesicular zinc within both primary human monocyte-derived macrophages (HMDM) and PMA-differentiated THP-1 cells. This vesicular zinc is delivered to non-pathogenic Escherichia coli (E. coli) strain MG1655 for microbial clearance. In contrast, Salmonella enterica serovar Typhimurium (S. Typhimurium) evades this response via a mechanism dependent upon Salmonella pathogenicity island-1 (SPI-1); a ΔSPI-1, but not wild type S. Typhimurium, co-localized with zinc-containing vesicles. Treatment of human macrophages with exogenous zinc increased the intracellular zinc pool and was able to overcome SPI-1-dependent zinc evasion. However, the antimicrobial effect of exogenous zinc was more pronounced with E. coli than with S. Typhimurium, suggesting that S. Typhimurium may employ additional mechanisms to evade zinc-mediated antimicrobial responses. Indeed, intramacrophage S. Typhimurium upregulated expression of the zinc exporter zntA, which effluxes zinc when concentrations reach cytotoxic levels. The ΔSPI1 mutant displayed prolonged up-regulation of zntA within macrophages, consistent with its sustained exposure to zinc in this environment. Finally, we show that lipopolysaccharide and S. Typhimurium specifically upregulate mRNA expression of the human zinc transporter SLC39A8 in macrophages. Gene knock-down studies demonstrated that SLC39A8 is required for effective clearance of E. coli from HMDM, whereas this transporter is actually required for intracellular survival of S. Typhimurium. Our data thus suggest that S. Typhimurium employs multiple mechanisms of zinc evasion including SPI-1-dependent subversion of zinc delivery, upregulation of zntA and possible hijacking of SLC39A8 function. In total, our findings identify roles for zinc and zinc trafficking in human macrophage antimicrobial pathways against Gram-negative bacteria, and provide insights into host subversion.