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Identifying therapeutics to delay, and potentially reverse, age-related cognitive decline is critical in light of the increased incidence of dementia-related disorders forecasted in the growing older population 1 . Here we show that platelet factors transfer the benefits of young blood to the ageing brain. Systemic exposure of aged male mice to a fraction of blood plasma from young mice containing platelets decreased neuroinflammation in the hippocampus at the transcriptional and cellular level and ameliorated hippocampal-dependent cognitive impairments. Circulating levels of the platelet-derived chemokine platelet factor 4 (PF4) (also known as CXCL4) were elevated in blood plasma preparations of young mice and humans relative to older individuals. Systemic administration of exogenous PF4 attenuated age-related hippocampal neuroinflammation, elicited synaptic-plasticity-related molecular changes and improved cognition in aged mice. We implicate decreased levels of circulating pro-ageing immune factors and restoration of the ageing peripheral immune system in the beneficial effects of systemic PF4 on the aged brain. Mechanistically, we identified CXCR3 as a chemokine receptor that, in part, mediates the cellular, molecular and cognitive benefits of systemic PF4 on the aged brain. Together, our data identify platelet-derived factors as potential therapeutic targets to abate inflammation and rescue cognition in old age. Platelet factors transfer the benefits of young blood to the ageing brain in mice through CXCR3, which mediates the cellular, molecular and cognitive benefits of systemic PF4 on the aged brain.

Heparin-induced thrombocytopenia/thrombosis (HIT) is a serious immune reaction to heparins, characterized by thrombocytopenia and often severe thrombosis with high morbidity and mortality. HIT is mediated by IgG antibodies against heparin/platelet factor 4 antigenic complexes. These complexes are thought to activate platelets leading to thrombocytopenia and thrombosis. Here we show that HIT immune complexes induce NETosis via interaction with FcγRIIa on neutrophils and through neutrophil-platelet association. HIT immune complexes induce formation of thrombi containing neutrophils, extracellular DNA, citrullinated histone H3 and platelets in a microfluidics system and in vivo, while neutrophil depletion abolishes thrombus formation. Absence of PAD4 or PAD4 inhibition with GSK484 abrogates thrombus formation but not thrombocytopenia, suggesting they are induced by separate mechanisms. NETs markers and neutrophils undergoing NETosis are present in HIT patients. Our findings demonstrating the involvement of NETosis in thrombosis will modify the current concept of HIT pathogenesis and may lead to new therapeutic strategies. The pathogenesis of heparin-induced thrombocytopenia and thrombosis (HIT) is mediated by heparin-reactive autoantibodies binding to platelets (thrombocytes). Here the authors show neutrophil activation and NETosis are elevated in patients with HIT, and are essential for thrombosis in HIT mouse models.

The blood-retina barrier (BRB), which is disrupted in diabetic retinopathy (DR) and uveitis, is an important anatomical characteristic of the retina, regulating nutrient, waste, water, protein, and immune cell flux. The BRB is composed of endothelial cell tight junctions, pericytes, astrocyte end feet, a collagen basement membrane, and perivascular macrophages. Despite the importance of the BRB, retinal perivascular macrophage function remains unknown. We found that retinal perivascular macrophages resided on postcapillary venules in the superficial vascular plexus and expressed MHC class II. Using single-cell RNA-Seq, we found that perivascular macrophages expressed a prochemotactic transcriptome and identified platelet factor 4 (Pf4, also known as CXCL4) as a perivascular macrophage marker. We used Pf4Cre mice to specifically deplete perivascular macrophages. To model retinal inflammation, we performed intraocular CCL2 injections. Ly6C+ monocytes crossed the BRB proximal to perivascular macrophages. Depletion of perivascular macrophages severely hampered Ly6C+ monocyte infiltration. These data suggest that retinal perivascular macrophages orchestrate immune cell migration across the BRB, with implications for inflammatory ocular diseases including DR and uveitis.

Introduction of vaccines against COVID-19 has provided the most promising chance to control the world-wide COVID-19 pandemic. However, the adenovirus-vector based Oxford/AstraZeneca [ChAdOx1] (AZ) and Johnson & Johnson [Ad26.CoV2.S] COVID-19 vaccines have been linked with serious thromboembolic events combined with thrombocytopenia, denominated Vaccine-induced Immune Thrombocytopenia and Thrombosis (VITT). The pathogenesis of COVID-19 VITT remain incompletely understood; especially the initial events that trigger platelet activation, platelet factor (PF)4 release, complex formation and PF4 antibody production are puzzling. This is a prospective study investigating the impact of different COVID-19 vaccines on inflammation (CRP, TNF-α, IL-1β, IL-6, IL-8, IL-10), vascular endothelial activation (syndecan-1, thrombomodulin, E-selectin, ICAM-1, ICAM-3, VCAM-1), platelet activation (P-selectin, TGF-β, sCD40L) and aggregation (Multiplate ® impedance aggregometry), whole blood coagulation (ROTEM ® ), thrombin generation and PF4 antibodies to reveal potential differences between AZ and mRNA vaccines in individuals without VITT. The study included 80 (55 AZ and 55 mRNA) vaccinated individuals and 55 non-vaccinated age- and gender matched healthy controls. The main findings where that both vaccines enhanced inflammation and platelet activation, though AZ vaccination induced a more pronounced increase in several inflammatory and platelet activation markers compared to mRNA vaccination and that post-vaccination thrombin generation was higher following AZ vaccination compared to mRNA vaccination. No difference in neither the PF4 antibody level nor the proportion of individuals with positive PF4 antibodies were observed between the vaccine groups. This is the first study to report enhanced inflammation, platelet activation and thrombin generation following AZ vaccination compared to mRNA vaccination in a head-to-head comparison. We speculate that specific components of the AZ adenovirus vector may serve as initial trigger(s) of (hyper)inflammation, platelet activation and thrombin generation, potentially lowering the threshold for a cascade of events that both trigger complications related to excessive inflammation, platelet and coagulation activation as observed in epidemiological studies and promote development of VITT when combined with high-titer functionally active PF4 antibodies.

Background Increased circulatory levels of the chemokine CXCL4 have been associated with the presence of interstitial lung disease (ILD) in an observational study of patients with systemic sclerosis (SSc). The purpose of the present study was to evaluate the relationship between baseline CXCL4 level and extent of ILD in the context of a randomized controlled trial and to determine whether changes in CXCL4 levels in response to immunosuppression are associated with future progression of SSc-ILD. Methods A total of 142 SSc-ILD patients from Scleroderma Lung Study (SLS) II were randomized in a double-blind, parallel-arm trial, to receive mycophenolate (MMF) for 2 years or oral cyclophosphamide (CYC) for 1 year followed by 1 year of placebo. Plasma CXCL4 levels were measured at baseline, 12 months, and 24 months in SLS II participants ( N  = 136) and at a single time point in healthy controls ( N  = 67). A mixed-effects model evaluated the relationship between change in CXCL4 levels and SSc-ILD progression. The primary outcome was the course of the forced vital capacity. Results Baseline CXCL4 levels were significantly higher in SSc-ILD patients compared with healthy controls (2699 ± 1489 ng/ml vs 2233 ± 1351 ng/ml (mean ± SD); P  = 0.019). However, no significant correlations were identified between CXCL4 levels and extent of ILD at baseline, as measured by the forced vital capacity, diffusing capacity of carbon monoxide, or radiographic extent of ILD. Plasma CXCL4 decreased significantly from baseline to 12 months in all patients (CYC: P  < 0.001; MMF: P  = 0.006) with no between-treatment differences (CYC vs MMF). Patients with the largest decline in CXCL4 levels during the first 12 months had an improved course of forced vital capacity %-predicted from 12 to 24 months ( P  = 0.040), even after adjusting for baseline disease severity and treatment arm assignment. Conclusions Levels of CXCL4 were higher in patients with SSc-ILD compared with controls and decreased in all patients treated with immunosuppressive therapy. While CXCL4 levels were not correlated with extent of ILD at baseline, changes in CXCL4 at 12 months predicted future progression of SSc-ILD from 12 to 24 months. These findings suggest that intermediate-term changes in CXCL4 may have predictive significance for long-term progression of SSc-ILD in patients receiving immunosuppressive therapy. Trial registration ClinicalTrials.gov NCT00883129 . Registered 16 April 2009.

Previous studies suggest that multiple inflammatory chemokines (e.g., CCL5, CXCL12) bind to the allosteric site of integrins (site 2) and induce allosteric integrin activation and inflammatory signals. PF4 is abundantly present in platelet granules, but PF4 levels are very low in plasma. PF4 is released from damaged platelets and is markedly increased in plasma (>1000×) in pathological conditions. PF4 (tetramer) is an inhibitory chemokine, and the specifics of PF4 signaling are unclear. Docking simulation predicted that PF4 monomer binds to site 2, but PF4 by itself did not induce allosteric integrin activation. Anti-PF4 mAbs KKO and RTO generate complexes with PF4 tetramer and monomer, respectively. We discovered that the PF4/RTO complex induced potent integrin activation, but the PF4/KKO complex did not. We hypothesize that inactive PF4 tetramer is converted by RTO to active monomer. A PF4 mutant (4E), in which four basic amino acid residues in the predicted site 2 binding site were mutated to Glu, did not induce integrin activation and acted as a dominant-negative antagonist, suggesting that the RTO/PF4 complex is required to bind to site 2 for integrin activation. Notably, RTO-like autoantibody was detected in plasma of healthy people. We propose that autoanti-PF4 in healthy controls may not be a problem since plasma PF4 levels are very low. When plasma PF4 tetramer is increased, active PF4 monomer is generated by autoanti-PF4 and plays a role in disease pathogenesis. Notably, anti-inflammatory cytokine neuregulin-1 and anti-inflammatory ivermectin bind to site 2 and suppress integrin activation induced by RTO/PF4 complex, suggesting that neuregulin-1 and ivermectin are potentially useful to suppress PF4/anti-PF4-mediated inflammatory signals.

The malarial parasite Plasmodium can acquire resistance to most mainstay antimalarial drugs, necessitating the development of new antiplasmodial agents with different modes of action. The innate defense protein, human platelet factor 4 (PF4), has a unique antiplasmodial action that involves selective entry into Plasmodium -infected red blood cells (RBC) and subsequent destruction of the parasite’s digestive vacuole (DV). This activity is recapitulated in PF4-derived internalization peptides (PDIPs). Here, we characterized the actions of PDIP analogs and PF4 in live P. falciparum -infected human RBC to understand their kinetics, effects on cell and parasite viability, and molecular requirements for antiplasmodial activity. The entry and accumulation of PDIP, and peptide-induced DV destruction, were distinguishable as ordered and rapidly occurring events that were equivalent to PF4. Both host cell and parasite plasma membranes remained intact and undamaged following destruction of the DV, although modest changes in phosphatidylserine (PS) exposure on the surface of the host cells (indicative of changes to its phospholipid organization) and swelling (but not lysis) of the intracellular parasite were observed. PDIP retained its macrocyclic structure, and its activity depended on elevated levels of PS on the surface of infected versus uninfected cells. Neither the intramolecular disulfide bond of PDIP, nor the parasite’s nutrient and ion transporter functions were required. These actions on the parasite DV were not detected for other antiplasmodial drugs and compounds. In conclusion, this study reveals the unique, rapid, and distinct antiplasmodial actions of PDIP, highlighting its potential for future antimalarial development.

In patients with heparin-induced thrombocytopenia, pathogenic antibodies were found to be monoclonal, which challenges previous views of HIT as a polyclonal disorder and suggests new avenues for diagnosis and treatment.

Background The mechanism by which immune cells regulate metastasis is unclear. Understanding the role of immune cells in metastasis will guide the development of treatments improving patient survival. Methods We used syngeneic orthotopic mouse tumour models (wild-type, NOD/scid and Nude), employed knockout ( CD8 and CD4 ) models and administered CXCL4. Tumours and lungs were analysed for cancer cells by bioluminescence, and circulating tumour cells were isolated from blood. Immunohistochemistry on the mouse tumours was performed to confirm cell type, and on a tissue microarray with 180 TNBCs for human relevance. TCGA data from over 10,000 patients were analysed as well. Results We reveal that intratumoral immune infiltration differs between metastatic and non-metastatic tumours. The non-metastatic tumours harbour high levels of CD8 + T cells and low levels of platelets, which is reverse in metastatic tumours. During tumour progression, platelets and CXCL4 induce differentiation of monocytes into myeloid-derived suppressor cells (MDSCs), which inhibit CD8 + T-cell function. TCGA pan-cancer data confirmed that CD8 low Platelet high patients have a significantly lower survival probability compared to CD8 high Platelet low . Conclusions CD8 + T cells inhibit metastasis. When the balance between CD8 + T cells and platelets is disrupted, platelets produce CXCL4, which induces MDSCs thereby inhibiting the CD8 + T-cell function.

Despite increased understanding of the genomic landscape of Myeloproliferative Neoplasms (MPNs), the pathological mechanisms underlying abnormal megakaryocyte (Mk)-stromal crosstalk and fibrotic progression in MPNs remain unclear. We conducted mass spectrometry-based proteomics on mice with Romiplostim-dependent myelofibrosis to reveal alterations in signaling pathways and protein changes in Mks, platelets, and bone marrow (BM) cells. The chemokine Platelet Factor 4 (PF4)/Cxcl4 was up-regulated in all proteomes and increased in plasma and BM fluids of fibrotic mice. High TPO concentrations sustained in vitro PF4 synthesis and secretion in cultured Mks, while Ruxolitinib restrains the abnormal PF4 expression in vivo. We discovered that PF4 is rapidly internalized by stromal cells through surface glycosaminoglycans (GAGs) to promote myofibroblast differentiation. Cxcl4 gene silencing in Mks mitigated the profibrotic phenotype of stromal cells in TPO-saturated co-culture conditions. Consistently, extensive stromal PF4 uptake and altered GAGs deposition were detected in Romiplostim-treated, JAK2 mice and BM biopsies of MPN patients. BM PF4 levels and Mk/platelet CXCL4 expression were elevated in patients, exclusively in overt fibrosis. Finally, pharmacological inhibition of GAGs ameliorated in vivo fibrosis in Romiplostim-treated mice. Thus, our findings highlight the critical role of PF4 in the fibrosis progression of MPNs and substantiate the potential therapeutic strategy of neutralizing PF4-GAGs interaction.