Dendritic cells (DCs) as professional antigen-presenting cells play an important role in the initiation and modulation of the adaptive immune response. However, their role in the innate immune response against bacterial infections is not completely defined. Here we have analyzed the role of DCs and their impact on the innate anti-bacterial host defense in an experimental infection model of Yersinia enterocolitica (Ye). We used CD11c-diphtheria toxin (DT) mice to deplete DCs prior to severe infection with Ye. DC depletion significantly increased animal survival after Ye infection. The bacterial load in the spleen of DC-depleted mice was significantly lower than that of control mice throughout the infection. DC depletion was accompanied by an increase in the serum levels of CXCL1, G-CSF, IL-1α, and CCL2 and an increase in the numbers of splenic phagocytes. Functionally, splenocytes from DC-depleted mice exhibited an increased bacterial killing capacity compared to splenocytes from control mice. Cellular studies further showed that this was due to an increased production of reactive oxygen species (ROS) by neutrophils. Adoptive transfer of neutrophils from DC-depleted mice into control mice prior to Ye infection reduced the bacterial load to the level of Ye-infected DC-depleted mice, suggesting that the increased number of phagocytes with additional ROS production account for the decreased bacterial load. Furthermore, after incubation with serum from DC-depleted mice splenocytes from control mice increased their bacterial killing capacity, most likely due to enhanced ROS production by neutrophils, indicating that serum factors from DC-depleted mice account for this effect. In summary, we could show that DC depletion triggers phagocyte accumulation in the spleen and enhances their anti-bacterial killing capacity upon bacterial infection.

Platelet activation plays a critical role in thrombosis. Inhibition of platelet activation is a cornerstone in treatment of acute organ ischemia. Platelet ACKR3 surface expression is independently associated with all-cause mortality in CAD patients. In a novel genetic mouse strain, we show that megakaryocyte/platelet-specific deletion of ACKR3 results in enhanced platelet activation and thrombosis in vitro and in vivo. Further, we performed ischemia/reperfusion experiments (transient LAD-ligation and tMCAO) in mice to assess the impact of genetic ACKR3 deficiency in platelets on tissue injury in ischemic myocardium and brain. Loss of platelet ACKR3 enhances tissue injury in ischemic myocardium and brain and aggravates tissue inflammation. Activation of platelet-ACKR3 via specific ACKR3 agonists inhibits platelet activation and thrombus formation and attenuates tissue injury in ischemic myocardium and brain. Here we demonstrate that ACKR3 is a critical regulator of platelet activation, thrombus formation and organ injury following ischemia/reperfusion.

The challenging human pathogen Staphylococcus aureus has highly efficient immune evasion strategies for causing a wide range of diseases, from skin and soft tissue to life-threatening infections. Phenol-soluble modulin (PSM) peptides are major pathogenicity factors of community-associated methicillin-resistant S. aureus strains. In previous work, we demonstrated that PSMs in combination with TLR2 ligand from S. aureus induce tolerogenic dendritic cells (DCs) characterized by the production of high amounts of IL-10, but no proinflammatory cytokines. This in turn promotes the activation of regulatory T cells while impairing Th1 response; however, the signaling pathways modulated by PSMs remain elusive. In this study, we analyzed the effects of PSMs on signaling pathway modulation downstream of TLR2. TLR2 stimulation in combination with PSMα3 led to increased and prolonged phosphorylation of NF-κB, ERK, p38, and CREB in mouse bone marrow–derived DCs compared with single TLR2 activation. Furthermore, inhibition of p38 and downstream MSK1 prevented IL-10 production, which in turn reduced the capacity of DCs to activate regulatory T cells. Interestingly, the modulation of the signaling pathways by PSMs was independent of the known receptor for PSMs, as shown by experiments with DCs lacking the formyl peptide receptor 2. Instead, PSMs penetrate the cell membrane most likely by transient pore formation. Moreover, colocalization of PSMs and p38 was observed near the plasma membrane in the cytosol, indicating a direct interaction. Thus, PSMs from S. aureus directly modulate the signaling pathway p38–CREB in DCs, thereby impairing cytokine production and in consequence T cell priming to increase the tolerance toward the pathogen.

DCs are professional APCs playing a crucial role in the initiation of T-cell responses to combat infection. However, systemic bacterial infection with various pathogens leads to DC-depletion in humans and mice. The mechanisms of pathogen-induced DC-depletion remain poorly understood. Previously, we showed that mice infected with Yersinia enterocolitica (Ye) had impaired de novo DC-development, one reason for DC-depletion. Here, we extend these studies to gain insight into the molecular mechanisms of DC-depletion and the impact of different bacteria on DC-development. We show that the number of bone marrow (BM) hematopoietic progenitors committed to the DC lineage is reduced following systemic infection with different Gram-positive and Gram-negative bacteria. This is associated with a TLR4-and IFN-γ−signaling dependent increase of committed monocyte progenitors in the BM and mature monocytes in the spleen upon Ye-infection. Adoptive

Objective: Patients with coronary artery disease (CAD) are at increased risk for cardiac death and respiratory failure following severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Platelets are crucially involved in pathogenesis of CAD and might also contribute to pathophysiology of SARS-CoV-2 infection. Approach and Results: We enrolled a cohort of 122 participants from February 2020 to July 2020 including 55 patients with preexisting CAD and acute SARS-CoV-2 infection (CAD-SARS-CoV-2 positive ), 28 patients with CAD and without SARS-CoV-2 (CAD-SARS-CoV-2 negative ), and 39 healthy controls. Clinical and cardiac examination of the CAD-SARS-CoV-2 positive group included blood sampling, echocardiography, and electrocardiography within 24 hours after hospital admission. Phenotyping of platelets was performed by flow cytometry; plasma levels of chemokines were analyzed by ELISA. Respiratory failure of patients was stratified by the Horovitz index as moderately/severely impaired when Horovitz index <200 mm Hg. The clinical end point was defined as Horovitz index <200 mm Hg with subsequent mechanical ventilation within a follow-up of 60 days. CAD-SARS-CoV-2 positive patients display a significant enhanced platelet activation and hyper-inflammation early at time of hospital admission. Circulating platelet/leukocyte co-aggregates correlate with plasma levels of cytokines/chemokines like IL (interleukin)-6, CCL2, and CXCL10 as well as activation of platelets is associated with CCL5 and elevation of pulmonary artery pressure. Furthermore, furin is stored and released from activated platelets. High furin plasma levels are associated with poor clinical prognosis in CAD-SARS-CoV-2 positive patients. Conclusions: Patients with CAD and SARS-CoV-2 infection exhibit elevated systemic platelet activation and enhanced plasma levels of the subtilisin-like proprotein convertase furin, which may contribute to an unfavorable clinical prognosis.