Background Coronavirus induced disease 2019 (COVID-19) can be complicated by severe organ damage leading to dysfunction of the lungs and other organs. The processes that trigger organ damage in COVID-19 are incompletely understood. Methods Samples were donated from hospitalized patients. Sera, plasma, and autopsy-derived tissue sections were examined employing flow cytometry, enzyme-linked immunosorbent assays, and immunohistochemistry. Patient findings Here, we show that severe COVID-19 is characterized by a highly pronounced formation of neutrophil extracellular traps (NETs) inside the micro-vessels. Intravascular aggregation of NETs leads to rapid occlusion of the affected vessels, disturbed microcirculation, and organ damage. In severe COVID-19, neutrophil granulocytes are strongly activated and adopt a so-called low-density phenotype, prone to spontaneously form NETs. In accordance, markers indicating NET turnover are consistently increased in COVID-19 and linked to disease severity. Histopathology of the lungs and other organs from COVID-19 patients showed congestions of numerous micro-vessels by aggregated NETs associated with endothelial damage. Interpretation These data suggest that organ dysfunction in severe COVID-19 is associated with excessive NET formation and vascular damage. Funding Deutsche Forschungsgemeinschaft (DFG), EU, Volkswagen-Stiftung

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Adoptive transfer of third-party mesenchymal stromal cells (MSCs) has emerged as a promising tool for the treatment of steroid-refractory graft-versus-host disease (GVHD). Despite numerous in vitro studies and preclinical models, little is known about their effects on the patients' immune system. We assessed immune alterations in the T-cell, B-cell, natural killer cell, dendritic cell, and monocytic compartments of steroid-refractory GVHD patients 30, 90, and 180 days after MSC (n = 6) or placebo (n = 5) infusion, respectively. Infused MSCs were bioactive as suggested by the significant reduction in epithelial cell death, which represents a biomarker for acute GVHD. There were several indications that MSCs shift the patients' immune system toward a more tolerogenic profile. Most importantly, infusion of MSCs was associated with increased levels of regulatory (forkhead box P3 (FOXP3)+ and interleukin (IL)-10+) T-cells, reduced pro-inflammatory IL-17+ T(Th17)-cells, and skewing toward type-2 T-helper cell responses. Furthermore, IL-2, which has been recently shown to exert a positive immune modulating effect in GVHD patients, was higher in the MSC patients at all evaluated time points during 6 months after MSC-infusion. Overall, our findings will contribute to the refinement of monitoring tools, for assessing MSC treatment-efficacy and increase our understanding regarding the MSCs' in vivo effects.

Key Points ALPS DNT cells and their putative precursors reveal high proliferative activity in vivo, which is associated with hyperactive mTOR signaling. Rapamycin therapy controls mitotic activity and abnormal differentiation of ALPS DNT cells and reduces CD4+ or CD8+ precursor DNT cells.