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Abstract Extracellular vesicle (EV) secretion is an important mechanism used by cells to release biomolecules. A common necroptosis effector—mixed lineage kinase domain like (MLKL)—was recently found to participate in the biogenesis of small and large EVs independent of its function in necroptosis. The objective of the current study is to gain mechanistic insights into EV biogenesis during necroptosis. Assessing EV number by nanoparticle tracking analysis revealed an increased number of EVs released during necroptosis. To evaluate the nature of such vesicles, we performed a newly adapted, highly sensitive mass spectrometry‐based proteomics on EVs released by healthy or necroptotic cells. Compared to EVs released by healthy cells, EVs released during necroptosis contained a markedly higher number of unique proteins. Receptor interacting protein kinase‐3 (RIPK3) and MLKL were among the proteins enriched in EVs released during necroptosis. Further, mouse embryonic fibroblasts (MEFs) derived from mice deficient of Rab27a and Rab27b showed diminished basal EV release but responded to necroptosis with enhanced EV biogenesis as the wildtype MEFs. In contrast, necroptosis‐associated EVs were sensitive to Ca2+ depletion or lysosomal disruption. Neither treatment affected the RIPK3‐mediated MLKL phosphorylation. An unbiased screen using RIPK3 immunoprecipitation‐mass spectrometry on necroptotic EVs led to the identification of Rab11b in RIPK3 immune‐complexes. Our data suggests that necroptosis switches EV biogenesis from a Rab27a/b dependent mechanism to a lysosomal mediated mechanism.

Abstract Risk stratification of COVID‐19 patients is essential for pandemic management. Changes in the cell fitness marker, hFwe‐Lose, can precede the host immune response to infection, potentially making such a biomarker an earlier triage tool. Here, we evaluate whether hFwe‐Lose gene expression can outperform conventional methods in predicting outcomes (e.g., death and hospitalization) in COVID‐19 patients. We performed a post‐mortem examination of infected lung tissue in deceased COVID‐19 patients to determine hFwe‐Lose’s biological role in acute lung injury. We then performed an observational study (n = 283) to evaluate whether hFwe‐Lose expression (in nasopharyngeal samples) could accurately predict hospitalization or death in COVID‐19 patients. In COVID‐19 patients with acute lung injury, hFwe‐Lose is highly expressed in the lower respiratory tract and is co‐localized to areas of cell death. In patients presenting in the early phase of COVID‐19 illness, hFwe‐Lose expression accurately predicts subsequent hospitalization or death with positive predictive values of 87.8–100% and a negative predictive value of 64.1–93.2%. hFwe‐Lose outperforms conventional inflammatory biomarkers and patient age and comorbidities, with an area under the receiver operating characteristic curve (AUROC) 0.93–0.97 in predicting hospitalization/death. Specifically, this is significantly higher than the prognostic value of combining biomarkers (serum ferritin, D‐dimer, C‐reactive protein, and neutrophil–lymphocyte ratio), patient age and comorbidities (AUROC of 0.67–0.92). The cell fitness marker, hFwe‐Lose, accurately predicts outcomes in COVID‐19 patients. This finding demonstrates how tissue fitness pathways dictate the response to infection and disease and their utility in managing the current COVID‐19 pandemic.