Caspase-3-independent apoptotic pathways contribute to interleukin-32gamma-mediated control of Mycobacterium tuberculosis infection in THP-1 cells
SourceBMC Microbiology, 15, (2015), pp. 39
Article / Letter to editor
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SubjectRadboudumc 4: lnfectious Diseases and Global Health RIMLS: Radboud Institute for Molecular Life Sciences
BACKGROUND: Macrophages are the primary effector cells responsible for killing Mycobacterium tuberculosis (MTB) through various mechanisms, including apoptosis. However, MTB can evade host immunity to create a favorable environment for intracellular replication. MTB-infected human macrophages produce interleukin-32 (IL-32). IL-32 is a pro-inflammatory cytokine and has several isoforms. We previously found that IL-32gamma reduced the burden of MTB in human macrophages, in part, through the induction of caspase-3-dependent apoptosis. However, based on our previous studies, we hypothesized that caspase-3-independent death pathways may also mediate IL-32 control of MTB infection. Herein, we assessed the potential roles of cathepsin-mediated apoptosis, caspase-1-mediated pyroptosis, and apoptosis-inducing factor (AIF) in mediating IL-32gamma control of MTB infection in THP-1 cells. RESULTS: Differentiated human THP-1 macrophages were infected with MTB H37Rv alone or in the presence of specific inhibitors to caspase-1, cathepsin B/D, or cathepsin L for up to four days, after which TUNEL-positive cells were quantified; in addition, MTB was quantified by culture as well as by the percentage of THP-1 cells that were infected with green fluorescent protein (GFP)-labeled MTB as determined by microscopy. AIF expression was inhibited using siRNA technology. Inhibition of cathepsin B/D, cathepsin L, or caspase-1 activity significantly abrogated the IL-32gamma-mediated reduction in the number of intracellular MTB and of the percentage of GFP-MTB-infected macrophages. Furthermore, inhibition of caspase-1, cathepsin B/D, or cathepsin L in the absence of exogenous IL-32gamma resulted in a trend toward an increased proportion of MTB-infected THP-1 cells. Inhibition of AIF activity in the absence of exogenous IL-32gamma also increased intracellular burden of MTB. However, since IL-32gamma did not induce AIF and because the relative increases in MTB with inhibition of AIF were similar in the presence or absence of IL-32gamma, our results indicate that AIF does not mediate the host-protective effect of IL-32gamma against MTB. CONCLUSIONS: The anti-MTB effects of IL-32gamma are mediated through classical caspase-3-dependent apoptosis as well as caspase-3-independent apoptosis.
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