Activation of apoptosis by Salmonella pathogenicity island-1 effectors through both intrinsic and extrinsic pathways in Salmonella-infected macrophages

Background: Salmonella enterica serovar Typhimurium, a non-typhoidal food borne pathogen, causes acute enterocolitis, bacteremia, extraintestinal focal infections in humans. Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2) contribute to invading into host cellular cytosol, residing in Salmonella-containing vacuoles for intracellular survival, and inducing cellular apoptosis. This study aimed to better understand the mechanism underlying apoptosis in Salmonella-infected macrophages. Methods: S. Typhimurium SL1344 was used to evaluate extrinsic and intrinsic apoptosis pathways in THP-1 monocyte-derived macrophages in response to Salmonella infection. Results: Activated caspase-3-induced apoptosis pathways, including extrinsic (caspase-8mediated) and intrinsic (caspase-9-mediated) pathways, in Salmonella-infected macrophages were verified. THP-1 cells with dysfunction of TLR-4 and TLR-5 and Salmonella SPI-1 and SPI-2 mutants were constructed to identify the roles of the genes associated with programmed cell death in the macrophages. Caspase-3 activation in THP-1 macrophages was induced by Salmonella through TLR-4 and TLR-5 signaling pathways. We also identified that SPI-1 structure protein PrgH and effectors SipB and SipD, but not SPI-2 structure protein SsaV, could induce apoptosis via caspase-3 activation and reduce the secretion of inflammation marker TNF-a in the Salmonella-infected cells. The two effectors also reduced the translocation of the p65 subunit of NF-kB into the nucleus and the expression of TNF-a, and then inflammation was diminished. Conclusion: Non-typhoid Salmonella induced apoptosis of macrophages and thereby reduced inflammatory cytokine production through the expression of SPI-1. This mechanism in host-pathogen interaction may explain why Salmonella usually manifests as occult bacteremia with less systemic inflammatory response syndrome in the bloodstream infection of children. Copyright 2020, Taiwan Society of Microbiology. Published by Elsevier Taiwan LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).

Automated summary

The study revealed that Salmonella induces apoptosis in macrophages through caspase-3 activation, especially via the expression of SPI-1 structure proteins and effectors. This mechanism may help explain why Salmonella infections in children often present as mild bacteremia with less systemic inflammatory response.