Role of Canonical and Non-canonical Inflammasomes During Burkholderia Infection

Detecting DNA: An Overview of DNA Recognition by Inflammasomes and Protection against Bacterial Respiratory Infections

The innate immune system plays a key role in modulating host immune defense during bacterial disease. Upon sensing pathogen-associated molecular patterns (PAMPs), the multi-protein complex known as the inflammasome serves a protective role against bacteria burden through facilitating pathogen clearance and bacteria lysis. This can occur through two mechanisms: (1) the cleavage of pro-inflammatory cytokines IL-1β/IL-18 and (2) the initiation of inflammatory cell death termed pyroptosis. In recent literature, AIM2-like Receptor (ALR) and Nod-like Receptor (NLR) inflammasome activation has been implicated in host protection following recognition of bacterial DNA. Here, we review current literature synthesizing mechanisms of DNA recognition by inflammasomes during bacterial respiratory disease. This process can occur through direct sensing of DNA or indirectly by sensing pathogen-associated intracellular changes. Additionally, DNA recognition may be assisted through inflammasome-inflammasome interactions, specifically non-canonical inflammasome activation of NLRP3, and crosstalk with the interferon-inducible DNA sensors Stimulator of Interferon Genes (STING) and Z-DNA Binding Protein-1 (ZBP1). Ultimately, bacterial DNA sensing by inflammasomes is highly protective during respiratory disease, emphasizing the importance of inflammasome involvement in the respiratory tract.

Porphyromonas gingivalis lipopolysaccharide affects oral epithelial connections via pyroptosis

Background/purpose

Pyroptosis is a form of programmed cell death dependent on the activation of caspase-1. Porphyromonas gingivalis (P. gingivalis) is a major pathogenic bacterium in periodontitis and its lipopolysaccharide (LPS) can trigger inflammation. However, whether P. gingivalis-LPS affects epithelial connections or triggers pyroptosis in the gingival epithelium is unknown.

Materials and methods

Gingival samples from human donors were collected and the expression levels of E-cadherin, nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3), caspase-1/4/5, interleukin (IL)-18, and IL-1β were examined. P. gingivalis-LPS was injected into rat gingival sulcus to establish gingivitis models, and the expression levels of E-cadherin, NLRP3, caspase-1/11, IL-18, and IL-1β were compared via immunohistochemistry. The mRNA levels of E-cadherin, caspase-1, IL-18, and IL-1β were evaluated in oral mucosa epithelial cells (OMECs) and rat gingival tissues.

Results

In the present study, NLRP3 (p < 0.01), caspase-1 (p < 0.01), caspase-4 (p = 0.044), and IL-18 (p = 0.036) expression was greater in the human inflammatory gingival samples, whereas E-cadherin (p = 0.045) had the opposite presentation. Gingivitis models were successfully established in rats with the injection of P. gingivalis-LPS. NLRP3 (p = 0.015), caspase-1 (p < 0.01), caspase-11 (p < 0.01), and IL-18 (p = 0.041) were upregulated, whereas E-cadherin (p = 0.038) expression was decreased. Furthermore, E-cadherin mRNA was decreased while caspase-1, IL-18, and IL-1β mRNA levels were increased. The addition of a caspase-1 inhibitor reversed the expression changes.

Conclusion

P. gingivalis-LPS may effectively destroy the epithelial connection by triggering pyroptosis.

Hierarchical Cell Death Program Disrupts the Intracellular Niche Required for Burkholderia thailandensis Pathogenesis

Burkholderia infections can result in serious diseases with high mortality, such as melioidosis, and they are difficult to treat with antibiotics. Innate immunity is critical for cell-autonomous clearance of intracellular pathogens like Burkholderia by regulating programmed cell death. Inflammasome-dependent inflammatory cytokine release and cell death contribute to host protection against Burkholderia pseudomallei and Burkholderia thailandensis; however, the contribution of apoptosis and necroptosis to protection is not known. Here, we found that bone marrow-derived macrophages (BMDMs) lacking key components of pyroptosis died via apoptosis during infection. BMDMs lacking molecules required for pyroptosis, apoptosis, and necroptosis (PANoptosis), however, were significantly resistant to B. thailandensis-induced cell death until later stages of infection. Consequently, PANoptosis-deficient BMDMs failed to limit B. thailandensis-induced cell-cell fusion, which permits increased intercellular spread and replication compared to wild-type or pyroptosis-deficient BMDMs. Respiratory B. thailandensis infection resulted in higher mortality in PANoptosis-deficient mice than in pyroptosis-deficient mice, indicating that, in the absence of pyroptosis, apoptosis is essential for efficient control of infection in vivo. Together, these findings suggest both pyroptosis and apoptosis are necessary for host-mediated control of Burkholderia infection. IMPORTANCE Burkholderia infections result in a high degree of mortality when left untreated; therefore, understanding the host immune response required to control infection is critical. In this study, we found a hierarchical cell death program utilized by infected cells to disrupt the intracellular niche of Burkholderia thailandensis, which limits bacterial intercellular spread, host cell-cell fusion, and bacterial replication. In macrophages, combined loss of key PANoptosis components results in extensive B. thailandensis infection-induced cell-cell fusion, bacterial replication, and increased cell death at later stages of infection compared with both wild-type (WT) and pyroptosis-deficient cells. During respiratory infection, mortality was increased in PANoptosis-deficient mice compared to pyroptosis-deficient mice, identifying an essential role for multiple cell death pathways in controlling B. thailandensis infection. These findings advance our understanding of the physiological role of programmed cell death in controlling Burkholderia infection.

Gasdermin D Protects from Melioidosis through Pyroptosis and Direct Killing of Bacteria

Gasdermin D (GSDMD) cleavage by caspase-1 or caspase-11 inflammasomes triggers pyroptosis, a lytic form of cell death protective against intracellular bacteria. In this study, we examine the role of GSDMD in a mouse model of melioidosis. Gsdmd^-/- mice were more susceptible than wild-type mice to intranasal infection with Burkholderia thailandensis Production of IL-18, but not IL-1β, was decreased in Gsdmd^-/- infected mice. Despite lower IL-18, IFN-γ was produced in similar amounts in wild-type and Gsdmd^-/- mice. In vitro, secretion of both IL-1β and IL-18 by macrophages or dendritic cells infected with B. thailandensis was dependent on GSDMD. Surprisingly, wild-type or GSDMD-deficient neutrophils secreted similar amounts of IL-1β, suggesting these cells may be the source of the GSDMD-independent IL-1β detected in vivo. Recombinant GSDMD was able to directly kill B. thailandensis in vitro upon processing by active caspase-1. Moreover, bacteria harvested from wild-type, but not Gsdmd^-/- , macrophages were more susceptible to the microbicidal effect of hydrogen peroxide or human β-defensin-3. Finally, we provide evidence that pyroptosis of in vitro infected macrophages is directly microbicidal. Taken together, these results indicate that the protective action of GSDMD in melioidosis is primarily due to induction of pyroptosis and direct killing of bacteria rather than production of cytokines.