Oxidized Mitochondrial DNA Engages TLR9 to Activate the NLRP3 Inflammasome in Myelodysplastic Syndromes

Myelodysplastic Syndromes (MDSs) are bone marrow (BM) failure malignancies characterized by constitutive innate immune activation, including NLRP3 inflammasome driven pyroptotic cell death. We recently reported that the danger-associated molecular pattern (DAMP) oxidized mitochondrial DNA (ox-mtDNA) is diagnostically increased in MDS plasma although the functional consequences remain poorly defined. We hypothesized that ox-mtDNA is released into the cytosol, upon NLRP3 inflammasome pyroptotic lysis, where it propagates and further enhances the inflammatory cell death feed-forward loop onto healthy tissues. This activation can be mediated via ox-mtDNA engagement of Toll-like receptor 9 (TLR9), an endosomal DNA sensing pattern recognition receptor known to prime and activate the inflammasome propagating the IFN-induced inflammatory response in neighboring healthy hematopoietic stem and progenitor cells (HSPCs), which presents a potentially targetable axis for the reduction in inflammasome activation in MDS. We found that extracellular ox-mtDNA activates the TLR9-MyD88-inflammasome pathway, demonstrated by increased lysosome formation, IRF7 translocation, and interferon-stimulated gene (ISG) production. Extracellular ox-mtDNA also induces TLR9 redistribution in MDS HSPCs to the cell surface. The effects on NLRP3 inflammasome activation were validated by blocking TLR9 activation via chemical inhibition and CRISPR knockout, demonstrating that TLR9 was necessary for ox-mtDNA-mediated inflammasome activation. Conversely, lentiviral overexpression of TLR9 sensitized cells to ox-mtDNA. Lastly, inhibiting TLR9 restored hematopoietic colony formation in MDS BM. We conclude that MDS HSPCs are primed for inflammasome activation via ox-mtDNA released by pyroptotic cells. Blocking the TLR9/ox-mtDNA axis may prove to be a novel therapeutic strategy for MDS.

Somatic gene mutations expose cytoplasmic DNA to co-opt the cGAS-STING-NLRP3 axis in Myelodysplastic syndromes

NLRP3 inflammasome and IFN-stimulated gene (ISG) induction are key biological drivers of ineffective hematopoiesis and inflammation in myelodysplastic syndromes (MDSs). Gene mutations involving mRNA splicing and epigenetic regulatory pathways induce inflammasome activation and myeloid lineage skewing in MDSs through undefined mechanisms. Using immortalized murine hematopoietic stem and progenitor cells harboring these somatic gene mutations and primary MDS BM specimens, we showed accumulation of unresolved R-loops and micronuclei with concurrent activation of the cytosolic sensor cyclic GMP-AMP synthase. Cyclic GMP-AMP synthase/stimulator of IFN genes (cGAS/STING) signaling caused ISG induction, NLRP3 inflammasome activation, and maturation of the effector protease caspase-1. Deregulation of RNA polymerase III drove cytosolic R-loop generation, which upon inhibition, extinguished ISG and inflammasome response. Mechanistically, caspase-1 degraded the master erythroid transcription factor, GATA binding protein 1, provoking anemia and myeloid lineage bias that was reversed by cGAS inhibition in vitro and in Tet2^–/– hematopoietic stem and progenitor cell–transplanted mice. Together, these data identified a mechanism by which functionally distinct mutations converged upon the cGAS/STING/NLRP3 axis in MDS, directing ISG induction, pyroptosis, and myeloid lineage skewing.

Lactate production by Staphylococcus aureus biofilm inhibits HDAC11 to reprogram the host immune response during persistent infection

Staphylococcus aureus (S. aureus) is a leading cause of prosthetic joint infection (PJI) typified by biofilm formation. Our laboratory has identified preferential myeloid-derived suppressor cell (MDSC) recruitment as a critical mechanism for biofilm persistence, as MDSCs are a main source of IL-10. We screened the Nebraska Transposon Mutant Library to identify S. aureus mutants impaired in their ability to trigger MDSC IL-10 production. Significant hits in lactate biosynthesis were identified. A S. aureus Δddh/ldh1/ldh2 mutant, defective in both D- and L-lactate production, decreased leukocyte IL-10 expression and biofilm-associated monocytes were reprogrammed to a pro-inflammatory state, resulting in reduced biofilm burden. Histone acetylation plays a central role in regulating gene expression and histone deacetylase (HDAC) activity was significantly increased in leukocytes recovered from mice infected with Δddh/ldh1/ldh2 compared to WT, resulting in decreased global histone 3 (H3) acetylation at the IL-10 promoter in MDSCs from Δddh/ldh1/ldh2-infected mice. Lactate inhibited HDAC11, resulting in unchecked activity of HDAC6, a positive regulator of Il-10 transcription, and increased IL-10 production. MDSCs and macrophages produced significantly less IL-10 when treated with HDAC6 inhibitor tubastatin A during co-culture with WT but not Δddh/ldh1/ldh2 biofilm. D-lactate was elevated in synovial fluid of patients with PJI compared to aseptic controls and lactate promoted IL-10 production by human monocyte-derived macrophages. Collectively, S. aureus lactate-mediated inhibition of HDAC11 plays a critical role during biofilm infection, leading to epigenetic changes that reprogram the host immune response.

Transcriptional Diversity and Niche-Specific Distribution of Leukocyte Populations during Staphylococcus aureus Craniotomy-Associated Biofilm Infection

Neurosurgery for brain tumor resection or epilepsy treatment requires a craniotomy to gain access to the brain. Despite prophylactic measures, infectious complications occur at a frequency of 1-3%, with approximately half caused by Staphylococcus aureus (S. aureus) that forms a biofilm on the bone flap and is recalcitrant to antibiotics. Using single-cell RNA sequencing in a mouse model of S. aureus craniotomy infection, this study revealed the complex transcriptional heterogeneity of resident microglia and infiltrating monocytes in the brain, in addition to transcriptionally diverse granulocyte subsets in the s.c. galea and bone flap. In the brain, trajectory analysis identified the transition of microglia from a homeostatic/anti-inflammatory to proinflammatory and proliferative populations, whereas granulocytes in the brain demonstrated a trajectory from a granulocyte myeloid-derived suppressor cell (MDSC)-like phenotype to a small population of mature polymorphonuclear neutrophils (PMNs). In the galea, trajectory analysis identified the progression from two distinct granulocyte-MDSC-like populations to PMN clusters enriched for IFN signaling and cell cycle genes. Based on their abundance in the galea and bone flap, PMNs and MDSCs were depleted using anti-Ly6G, which resulted in increased bacterial burden. This revealed a critical role for PMNs in S. aureus containment because MDSCs were found to attenuate PMN antibacterial activity, which may explain, in part, why craniotomy infection persists in the presence of PMN infiltrates. These results demonstrate the existence of a transcriptionally diverse leukocyte response that likely influences the chronicity of S. aureus craniotomy infection.

TP53 mutations in myelodysplastic syndromes and secondary AML confer an immunosuppressive phenotype

Somatic gene mutations are key determinants of outcome in patients with myelodysplastic syndromes (MDS) and secondary AML (sAML). In particular, patients with TP53 mutations represent a distinct molecular cohort with uniformly poor prognosis. The precise pathogenetic mechanisms underlying these inferior outcomes have not been delineated. In this study, we characterized the immunological features of the malignant clone and alterations in the immune microenvironment in patients with TP53-mutant and wild-type MDS or sAML. Notably, PDL1 expression is significantly increased in hematopoietic stem cells of patients with TP53 mutations, which is associated with MYC upregulation and marked downregulation of MYC's negative regulator miR-34a, a p53 transcription target. Notably, patients with TP53 mutations display significantly reduced numbers of bone marrow-infiltrating OX40+ cytotoxic T cells and helper T cells, as well as decreased ICOS+ and 4-1BB+ natural killer cells. Further, highly immunosuppressive regulatory T cells (Tregs) (ie, ICOShigh/PD-1-) and myeloid-derived suppressor cells (PD-1low) are expanded in cases with TP53 mutations. Finally, a higher proportion of bone marrow-infiltrating ICOShigh/PD-1- Treg cells is a highly significant independent predictor of overall survival. We conclude that the microenvironment of TP53 mutant MDS and sAML has an immune-privileged, evasive phenotype that may be a primary driver of poor outcomes and submit that immunomodulatory therapeutic strategies may offer a benefit for this molecularly defined subpopulation.

Synthesis and SAR Studies of 1H-Pyrrolo[2,3-b]pyridine-2-carboxamides as Phosphodiesterase 4B (PDE4B) Inhibitors

Herein we report the synthesis, SAR, and biological evaluation of a series of 1H-pyrrolo[2,3-b]pyridine-2-carboxamide derivatives as selective and potent PDE4B inhibitors. Compound 11h is a PDE4B preferring inhibitor and exhibited acceptable in vitro ADME and significantly inhibited TNF-α release from macrophages exposed to pro-inflammatory stimuli (i.e., lipopolysaccharide and the synthetic bacterial lipopeptide Pam3Cys). In addition, 11h was selective against a panel of CNS receptors and represents an excellent lead for further optimization and preclinical testing in the setting of CNS diseases.

Staphylococcus aureus biofilm-derived lactate inhibits HDAC11 to augment leukocyte IL-10 production and promote infection persistence

Staphylococcus aureus (S. aureus) is a leading cause of biofilm-associated prosthetic joint infections (PJIs), where leukocyte IL-10 production is critical for biofilm persistence. We screened a S. aureus mutant library that identified the importance of lactate biosynthetic pathways to elicit IL-10 production. A S. aureus Δddh/ldh1/ldh2 mutant, defective in both D- and L-lactate production, led to significantly decreased leukocyte IL-10 expression and re-programmed biofilm-associated monocytes to a pro-inflammatory state, resulting in reduced biofilm burden. Histone acetylation plays a central role in regulating gene expression, and we found that histone deacetylase (HDAC) activity was increased in Δddh/ldh1/ldh2-infected mice compared to WT concomitant with significant decreases in global histone 3 (H3) acetylation and H3 acetylation at the IL-10 promoter, demonstrating that S. aureus-derived lactate is a HDAC inhibitor. Lactate was found to inhibit HDAC11, which resulted in unregulated HDAC6 activity and increased IL-10 production. MDSCs and macrophages produced significantly less IL-10 in the presence of the HDAC6 inhibitor tubastatin A during co-culture with WT but not Δddh/ldh1/ldh2 biofilms, while HDAC11 KO MDSCs and macrophages co-cultured with Δddh/ldh1/ldh2 biofilms produced significantly more IL-10 compared to WT cells due to unchecked HDAC6 action. D-lactate was elevated in synovial fluid of patients with PJI compared to aseptic controls and lactate also promoted IL-10 production by human monocyte-derived macrophages. Collectively, S. aureus lactate-mediated inhibition of HDAC11 plays a critical role during biofilm infection, leading to epigenetic changes that reprogram the host immune response.

TLR2 and caspase-1 signaling are critical for bacterial containment but not clearance during craniotomy-associated biofilm infection

BACKGROUND

A craniotomy is required to access the brain for tumor resection or epilepsy treatment, and despite precautionary measures, infectious complications occur at a frequency of 1-3%. Approximately half of craniotomy infections are caused by Staphylococcus aureus (S. aureus) that forms a biofilm on the bone flap, which is recalcitrant to antibiotics. Our prior work in a mouse model of S. aureus craniotomy infection revealed a critical role for myeloid differentiation factor 88 (MyD88) in bacterial containment and pro-inflammatory mediator production. Since numerous receptors utilize MyD88 as a signaling adaptor, the current study examined the importance of Toll-like receptor 2 (TLR2) and TLR9 based on their ability sense S. aureus ligands, namely lipoproteins and CpG DNA motifs, respectively. We also examined the role of caspase-1 based on its known association with TLR signaling to promote IL-1β release.

METHODS

A mouse model of craniotomy-associated biofilm infection was used to investigate the role of TLR2, TLR9, and caspase-1 in disease progression. Wild type (WT), TLR2 knockout (KO), TLR9 KO, and caspase-1 KO mice were examined at various intervals post-infection to quantify bacterial burden, leukocyte recruitment, and inflammatory mediator production in the galea, brain, and bone flap. In addition, the role of TLR2-dependent signaling during microglial/macrophage crosstalk with myeloid-derived suppressor cells (MDSCs) was examined.

RESULTS

TLR2, but not TLR9, was important for preventing S. aureus outgrowth during craniotomy infection, as revealed by the elevated bacterial burden in the brain, galea, and bone flap of TLR2 KO mice concomitant with global reductions in pro-inflammatory mediator production compared to WT animals. Co-culture of MDSCs with microglia or macrophages, to model interactions in the brain vs. galea, respectively, also revealed a critical role for TLR2 in triggering pro-inflammatory mediator production. Similar to TLR2, caspase-1 KO animals also displayed increased S. aureus titers coincident with reduced pro-inflammatory mediator release, suggestive of pathway cooperativity. Treatment of caspase-1 KO mice with IL-1β microparticles significantly reduced S. aureus burden in the brain and galea compared to empty microparticles, confirming the critical role of IL-1β in limiting S. aureus outgrowth during craniotomy infection.

CONCLUSIONS

These results demonstrate the existence of an initial anti-bacterial response that depends on both TLR2 and caspase-1 in controlling S. aureus growth; however, neither pathway is effective at clearing infection in the WT setting, since craniotomy infection persists when both molecules are present.

Large-Scale and Rapid Preparation of Nanofibrous Meshes and Their Application for Drug-Loaded Multilayer Mucoadhesive Patch Fabrication for Mouth Ulcer Treatment

Electrospinning provides a simple and convenient method to fabricate nanofibrous meshes. However, the nanofiber productivity is often limited to the laboratory scale, which cannot satisfy the requirements of practical application. In this study, we developed a novel needleless electrospinning spinneret based on a double-ring slit to fabricate drug-loaded nanofibrous meshes. In contrast to the conventional single-needle electrospinning spinneret, our needless spinneret can significantly improve nanofiber productivity due to the simultaneous formation of multiple jets during electrospinning. Curcumin-loaded poly(l-lactic acid) (PLLA) nanofiber meshes with various concentrations and on the large scale were manufactured by employing our developed needleless spinneret-based electrospinning device. We systematically investigated the drug release behaviors, antioxidant properties, anti-inflammatory attributes, and cytotoxicity of the curcumin-loaded PLLA nanofibrous meshes. Furthermore, a bilayer nanofibrous composite mesh was successfully generated by electrospinning curcumin-loaded PLLA solution and diclofenac sodium loaded poly(ethylene oxide) solution in a predetermined time sequence, which revealed potent antibacterial properties. Subsequently, novel mucoadhesive patches were assembled by combining the bilayer composite nanofibrous meshes with (hydroxypropyl)methyl cellulose based mucoadhesive film. The multilayered mucoadhesive patch has excellent adhesion properties on the porcine buccal mucosa. Overall, our double-ring slit spinneret can provide a novel method to rapidly produce large-scale drug-loaded nanofibrous meshes to fabricate mucoadhesive patches. The multiple-layered mucoadhesive patches enable the incorporation of multiple drugs with different targets of action, such as analgesic, anti-inflammatory, and antimicrobial compounds, for mouth ulcer or other oral disease treatments.

Platelet-Rich Plasma for the Treatment of Tissue Infection: Preparation and Clinical Evaluation

IMPACT STATEMENT

The clinical application of platelet-rich plasma (PRP) has been widely studied for its effects on trauma or injury repair/regeneration, however the antibacterial property of PRP has been overlooked. Increasing evidence suggests PRP as a good antibacterial agent and that it could help prevent/treat tissue infection. This review emphasizes the importance of PRP's antibacterial property and summarizes the preclinical and clinical findings regarding the application of PRP in the prevention and treatment of wound and bone infection. The use of biocompatible PRP may be advantageous for tissue infection treatment due to its inherent antibacterial and healing promoting properties.

TLR2 deficiency leads to increased Th17 infiltrates in experimental brain abscesses

TLR2 plays a pivotal role in recognizing Staphylococcus aureus, a common etiologic agent of CNS parenchymal infections, such as brain abscess. We previously reported that brain abscesses of TLR2 knockout (KO) mice exhibited elevated IL-17 levels, suggesting the presence of an alternative pathway available to respond to S. aureus infection that may involve Th17 cells. Both CD4(+) and CD8(+) T cell infiltrates were elevated in brain abscesses of TLR2 KO mice at days 3, 7, and 14 postinfection compared with wild-type animals. Intracellular cytokine staining revealed a significant increase in the frequency of IL-17-producing Th17 cells in TLR2 KO mice with relatively few IFN-gamma-positive cells. gammadelta T cells were also a source of IL-17 in brain abscesses. Microglia, astrocytes, and macrophages were shown to express both IL-17RA and IL-17RC. Despite receptor expression, IL-17 was relatively ineffective at eliciting glial activation, whereas the cytokine augmented the ability of TNF-alpha to induce CXCL2 and CCL2 expression by macrophages. Based on the ability of IL-17 to elicit the release of chemokines and other proinflammatory mediators, we propose that the exaggerated IL-17 response that occurs in TLR2 KO mice functions in a compensatory manner to control brain abscess pathogenesis, with cells other than glia as targets for IL-17 action. This is supported by our findings in which innate immune infiltrates were not significantly different between TLR2 KO and wild-type mice in conjunction with the lack of prolonged alterations in the synthesis of other proinflammatory molecules during the course of infection.