Abstract
Most of the current knowledge on Burkholderia pseudomallei-induced inflammasome activation and cell death in macrophages is derived from murine systems. Little is known about the involved bacterial structures and mechanisms in primary human macrophages. This is of particular relevance since murine and human macrophages as well as primary cells and cell lines differ in many aspects of inflammasome activation, including the proteins involved in the recognition of bacterial patterns. In this study, we therefore aimed (i) to establish an in vitro B. pseudomallei infection model with human monocyte-derived primary macrophages from single donors as these cells more closely resemble macrophages in the human host and (ii) to analyze B. pseudomallei-triggered cell death and bacterial elimination in those cells. Our results show that B. pseudomallei-infected primary human macrophages not only release the inflammasome-independent pro-inflammatory cytokines IL-8 and TNF-α, but are also engaged in canonical inflammasome activation as evidenced by caspase-1 and gasdermin D processing. Absence of the B. pseudomallei T3SS-3 needle protein BsaL, a potent activator of the canonical inflammasome, abolished lytic cell death, reduced IL-1β release, and caspase-1 and gasdermin D processing. IFN-γ, known to promote non-canonical inflammasome activation, did not influence pyroptosis induction or IL-1β release from infected primary human macrophages. Nevertheless, it reduced intracellular B. pseudomallei loads, an effect which was partially antagonist by the inhibition of NADPH oxidase. Overall, our data implicate T3SS-3 dependent inflammasome activation and IFN-γ induced immune mechanisms as critical defense mechanisms of human macrophages against B. pseudomallei. In addition, our infection model provides a versatile tool to study human host-pathogen interactions and has the potential to elucidate the role of human individual genetic variations in B. pseudomallei infections.
Highlights
A huge population is constantly exposed to the melioidosis causing environmental pathogen Burkholderia pseudomallei [1,2,3,4,5]
Studies based on primary murine cells and cell lines of murine and human origin led to advances in the understanding of immune defense mechanisms against bacterial infections including B. pseudomallei
We established and validated a macrophage-based model system derived from human peripheral blood monocytes, which yields high amounts of genetically identical cells more closely resembling cells found in the human host
Summary
A huge population is constantly exposed to the melioidosis causing environmental pathogen Burkholderia pseudomallei [1,2,3,4,5] This results in a high proportion of asymptomatic infections as evidenced by serological studies or a broad clinical spectrum involving virtually any site [3, 6]. Most of our knowledge about the interaction of B. pseudomallei with macrophages results from studies using cell lines or primary murine macrophages, both being highly informative for a wide range of research questions Both cell sources have their limitations and some findings based on these models might not be transferable to primary human cells and human infections. These models do not provide a tool to investigate genetic diversity in humans, which is suggested to affect innate immunity [8]
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