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Dysregulation of the myeloid cell compartment is a feature of severe disease in hospitalized COVID-19 patients. Here, we investigated the response of circulating dendritic cell (DC) and monocyte subpopulations in SARS-CoV-2 infected outpatients with mild disease and compared it to the response of healthy individuals to yellow fever vaccine virus YF17D as a model of a well-coordinated response to viral infection. In SARS-CoV-2-infected outpatients circulating DCs were persistently reduced for several weeks whereas after YF17D vaccination DC numbers were decreased temporarily and rapidly replenished by increased proliferation until 14 days after vaccination. The majority of COVID-19 outpatients showed high expression of CD86 and PD-L1 in monocytes and DCs early on, resembling the dynamic after YF17D vaccination. In a subgroup of patients, low CD86 and high PD-L1 expression were detected in monocytes and DCs coinciding with symptoms, higher age, and lower lymphocyte counts. This phenotype was similar to that observed in severely ill COVID-19 patients, but less pronounced. Thus, prolonged reduction and dysregulated activation of blood DCs and monocytes were seen in a subgroup of symptomatic non-hospitalized COVID-19 patients while a transient coordinated activation was characteristic for the majority of patients with mild COVID-19 and the response to YF17D vaccination., (© 2022 The Authors. European Journal of Immunology published by Wiley-VCH GmbH.)

Infection with SARS-CoV-2 is associated with thromboinflammation, involving thrombotic and inflammatory responses, in many COVID-19 patients. In addition, immune dysfunction occurs in patients characterised by T cell exhaustion and severe lymphopenia. We investigated the distribution of phosphatidylserine (PS), a marker of dying cells, activated platelets and platelet-derived microparticles (PMP), during the clinical course of COVID-19. We found an unexpectedly high amount of blood cells loaded with PS + PMPs for weeks after the initial COVID-19 diagnosis. Elevated frequencies of PS + PMP + PBMCs correlated strongly with increasing disease severity. As a marker, PS outperformed established laboratory markers for inflammation, leucocyte composition and coagulation, currently used for COVID-19 clinical scoring. PS + PMPs preferentially bound to CD8 + T cells with gene expression signatures of proliferating effector rather than memory T cells. As PS + PMPs carried programmed death-ligand 1 (PD-L1), they may affect T cell expansion or function. Our data provide a novel marker for disease severity and show that PS, which can trigger the blood coagulation cascade, the complement system, and inflammation, resides on activated immune cells. Therefore, PS may serve as a beacon to attract thromboinflammatory processes towards lymphocytes and cause immune dysfunction in COVID-19., (© 2021 The Authors. Journal of Extracellular Vesicles published by Wiley Periodicals, LLC on behalf of the International Society for Extracellular Vesicles.)

Disease manifestations in COVID-19 range from mild to severe illness associated with a dysregulated innate immune response. Alterations in function and regeneration of dendritic cells (DCs) and monocytes may contribute to immunopathology and influence adaptive immune responses in COVID-19 patients. We analyzed circulating DC and monocyte subsets in 65 hospitalized COVID-19 patients with mild/moderate or severe disease from acute illness to recovery and in healthy controls. Persisting reduction of all DC subpopulations was accompanied by an expansion of proliferating Lineage-HLADR+ cells lacking DC markers. Increased frequency of CD163+ CD14+ cells within the recently discovered DC3 subpopulation in patients with more severe disease was associated with systemic inflammation, activated T follicular helper cells, and antibody-secreting cells. Persistent downregulation of CD86 and upregulation of programmed death-ligand 1 (PD-L1) in conventional DCs (cDC2 and DC3) and classical monocytes associated with a reduced capacity to stimulate naïve CD4+ T cells correlated with disease severity. Long-lasting depletion and functional impairment of DCs and monocytes may have consequences for susceptibility to secondary infections and therapy of COVID-19 patients., Competing Interests: The authors have declared that no competing interests exist.

COVID-19, the disease caused by SARS-CoV-2 infection, can assume a highly variable disease course, ranging from asymptomatic infection, which constitutes the majority of cases, to severe respiratory failure. This implies a diverse host immune response to SARS-CoV-2. However, the immunological underpinnings underlying these divergent disease courses remain elusive. We therefore set out to longitudinally characterize immune signatures of convalescent COVID-19 patients stratified according to their disease severity. Our unique convalescent COVID-19 cohort consists of 74 patients not confounded by comorbidities. This is the first study of which we are aware that excludes immune abrogations associated with non-SARS-CoV-2 related risk factors of disease severity. Patients were followed up and analyzed longitudinally (2, 4 and 6 weeks after infection) by high-dimensional flow cytometric profiling of peripheral blood mononuclear cells (PBMCs), in-depth serum analytics, and transcriptomics. Immune phenotypes were correlated to disease severity. Convalescence was overall associated with uniform immune signatures, but distinct immune signatures for mildly versus severely affected patients were detectable within a 2-week time window after infection., Competing Interests: CZ has received consulting fees from Novartis and Janssen Pharma. They are unrelated to the contents of the manuscript. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Chu, Sabath, Fibi-Smetana, Sun, Öllinger, Noeßner, Chao, Rinke, Winheim, Rad, Krug, Taher and Zielinski.)