Megakaryocytes possess a STING pathway that is transferred to platelets to potentiate activation

Platelets display unexpected roles in immune and coagulation responses. Emerging evidence suggests that STING is implicated in hypercoagulation. STING is an adaptor protein downstream of the DNA sensor cyclic GMP-AMP synthase (cGAS) that is activated by cytosolic microbial and self-DNA during infections, and in the context of loss of cellular integrity, to instigate the production of type-I IFN and pro-inflammatory cytokines. To date, whether the cGAS-STING pathway is present in platelets and contributes to platelet functions is not defined. Using a combination of pharmacological and genetic approaches, we demonstrate here that megakaryocytes and platelets possess a functional cGAS-STING pathway. Our results suggest that in megakaryocytes, STING stimulation activates a type-I IFN response, and during thrombopoiesis, cGAS and STING are transferred to proplatelets. Finally, we show that both murine and human platelets contain cGAS and STING proteins, and the cGAS-STING pathway contributes to potentiation of platelet activation and aggregation. Taken together, these observations establish for the first time a novel role of the cGAS-STING DNA sensing axis in the megakaryocyte and platelet lineage.

Activation of stimulator of interferon genes in dendritic cells induces interferon-lambda and subset- and species-specific dendritic cell death

Stimulator of Interferon (IFN) Genes (STING), a cytosolic DNA sensor that recognises cyclic dinucleotides (CDNs), has been an adjuvant target in many cancer immunotherapies. Activation of STING results in the production of Type I IFN through the phosphorylation of TANK-binding kinase 1 (TBK1) and IFN regulatory factor 3 (IRF3) and it is thought this enhances cross-presentation of tumour antigens by dendritic cells (DCs). However, DCs interrogated in many of these studies use in vitro-generated DCs instead of putative ex vivo DCs, and the direct effects of STING activation on different DC subsets are not completely understood. Here, we report that mouse and humanised mouse splenic DC subsets as well as human blood DCs are activated by CDN stimulation and produce Type III IFN (IFN-lambda) but only type 2 conventional DCs (cDC2s) and plasmacytoid DCs (pDCs) produce Type I IFN in response to CDN stimulation. However, only mouse pDCs aberrantly express extremely high levels of CD86 and CD80 and are ablated rapidly after STING activation. Some DC death was also observed in mouse and human cDC2s, but not in human pDCs. This ablation is STING-dependent and occurs via a cell-intrinsic mechanism involving intrinsic apoptosis. These observations demonstrate the differential effects STING activation has on DC subsets as well as highlight a discordance amongst mouse and human DCs during activation, which serve as an important consideration in the translation of animal cancer models and the use of STING ligands as adjuvants in cancer immunotherapy.

Human erythroleukemia genetics and transcriptomes identify master transcription factors as functional disease drivers

Acute erythroleukemia (AEL or acute myeloid leukemia [AML]-M6) is a rare but aggressive hematologic malignancy. Previous studies showed that AEL leukemic cells often carry complex karyotypes and mutations in known AML-associated oncogenes. To better define the underlying molecular mechanisms driving the erythroid phenotype, we studied a series of 33 AEL samples representing 3 genetic AEL subgroups including TP53-mutated, epigenetic regulator-mutated (eg, DNMT3A, TET2, or IDH2), and undefined cases with low mutational burden. We established an erythroid vs myeloid transcriptome-based space in which, independently of the molecular subgroup, the majority of the AEL samples exhibited a unique mapping different from both non-M6 AML and myelodysplastic syndrome samples. Notably, >25% of AEL patients, including in the genetically undefined subgroup, showed aberrant expression of key transcriptional regulators, including SKI, ERG, and ETO2. Ectopic expression of these factors in murine erythroid progenitors blocked in vitro erythroid differentiation and led to immortalization associated with decreased chromatin accessibility at GATA1-binding sites and functional interference with GATA1 activity. In vivo models showed development of lethal erythroid, mixed erythroid/myeloid, or other malignancies depending on the cell population in which AEL-associated alterations were expressed. Collectively, our data indicate that AEL is a molecularly heterogeneous disease with an erythroid identity that results in part from the aberrant activity of key erythroid transcription factors in hematopoietic stem or progenitor cells.

Abstract 176: Platelet Cyclophilin D Mediates Neutrophil Recruitment and Ischemic Stroke Outcomes in Mice

Rationale: Besides their role in thrombosis, platelets also mediate inflammation through platelet-neutrophil aggregates (PNA). Recently, cyclophilin D (CypD)-mediated platelet necrosis emerged as a potential mediator of detrimental PNA during thrombosis. However, the role of platelet CypD in ischemic stroke has never been examined.

Objective: Here, we investigate the contribution of platelet CypD following ischemic stroke.

Methods: We generated mice lacking CypD specifically in platelets (KO). Both wild-type (WT) and KO mice were subjected to a 1h transient middle cerebral artery occlusion (tMCAO) stroke model. Twenty-four hours after occlusion, neurological and motor function, and stroke infarct size were determined. We also examined if the CypD pathway was altered in human platelets after ischemic stroke.

Results: Loss of platelet CypD significantly improved neurological (p<0.001) and motor (p<0.005) functions compared to WT mice after tMCAO. Furthermore, platelet CypD deficient significantly reduced ischemic stroke infarct volume (39.1±15.7mm 3 vs. 78.6±27.7mm 3 , n=15; p<0.0001). Smaller infarcts in KO mice was not due to difference in blood flow during the ischemia stage. Twenty-four hours after stroke, a greater than 2-fold reduction in neutrophils was observed in the brains from KO mice (p<0.0001). In addition, we observed significantly fewer circulating and cerebral PNA (p<0.01). Depletion of neutrophils significantly (p<0.05) reduced infarct size and neurological damage following ischemic stroke in WT mice, however, no additional protective effect was observed in KO mice, suggesting necrotic PNAs are critical during ischemic stroke. RNA-sequencing on platelets isolated from ischemic stroke patients (n=8) and healthy aged, gender matched controls (n=7) revealed significant increases in several targets involved in CypD-mediated necrosis, including MCUR1, TMEM16F and calpain2 (p<0.005).

Conclusion: Our results demonstrate necrotic platelets interact with neutrophils to exacerbate brain injury following ischemic stroke. As inhibiting platelet necrosis does not compromise hemostasis, targeting platelet CypD may be a potential therapeutic strategy to limit brain damage after ischemic stroke.

Increased expression of the dopamine transporter leads to loss of dopamine neurons, oxidative stress and l-DOPA reversible motor deficits

The dopamine transporter is a key protein responsible for regulating dopamine homeostasis. Its function is to transport dopamine from the extracellular space into the presynaptic neuron. Studies have suggested that accumulation of dopamine in the cytosol can trigger oxidative stress and neurotoxicity. Previously, ectopic expression of the dopamine transporter was shown to cause damage in non-dopaminergic neurons due to their inability to handle cytosolic dopamine. However, it is unknown whether increasing dopamine transporter activity will be detrimental to dopamine neurons that are inherently capable of storing and degrading dopamine. To address this issue, we characterized transgenic mice that over-express the dopamine transporter selectively in dopamine neurons. We report that dopamine transporter over-expressing (DAT-tg) mice display spontaneous loss of midbrain dopamine neurons that is accompanied by increases in oxidative stress markers, 5-S-cysteinyl-dopamine and 5-S-cysteinyl-DOPAC. In addition, metabolite-to-dopamine ratios are increased and VMAT2 protein expression is decreased in the striatum of these animals. Furthermore, DAT-tg mice also show fine motor deficits on challenging beam traversal that are reversed with l-DOPA treatment. Collectively, our findings demonstrate that even in neurons that routinely handle dopamine, increased uptake of this neurotransmitter through the dopamine transporter results in oxidative damage, neuronal loss and l-DOPA reversible motor deficits. In addition, DAT over-expressing animals are highly sensitive to MPTP-induced neurotoxicity. The effects of increased dopamine uptake in these transgenic mice could shed light on the unique vulnerability of dopamine neurons in Parkinson's disease.