In a recent Cell publication, Cheong et al. uncover how COVID-19 causes IL-6 induced epigenetic reprogramming of human immune stem cells, which causes lasting alterations in the composition and response characteristics of circulating immune cells.1 The study provides important insights into the mechanisms by which SARS-CoV-2 infections impact the human immune system and is an important hook into unraveling the mechanisms of post-acute sequelae of COVID-19 (PASC) commonly referred to as long-COVID.
While vaccination and drugs are reducing the societal impact of acute SARS-CoV-2 infections, between 10 and 40% of patients continue to suffer long after the acute infection has been cleared. The diverse PASC symptoms range from short breath and headaches to cognitive impairment (‘brain fog’) and debilitating fatigue. Not only are no treatments for PASC available but also the underlying molecular mechanisms remain opaque.2
Cheong et al. investigated in patients’ circulating immune cells if detectable changes persisted after clearance of the acute SARS-CoV-2 infection 3 weeks after the first symptoms. They assembled a cohort of COVID-19 convalescent patients, which was sampled between 1–3 and 4–12 months after SARS-CoV-2 infections requiring intensive care unit (ICU) admission and compared these patients to non-infected controls and to patients that had been on the ICU for different reasons. Focusing on peripheral blood mononuclear cells (PBMC) they investigated transcriptional or epigenetic changes using an integrated pipeline of single-nuclei transcriptome analysis and ATAC-seq sequencing, which identifies accessible chromatin regions. Among PBMCs CD14+ monocytes exhibited the most drastic changes. CD14+ monocytes are a group of heterogenous, short-lived antigen presenting cells that help orchestrating immune responses. Among these the authors could distinguish one cluster, M.SC3, which was more abundant even 12 months after the infection. Cells in this cluster resembled intermediate-type monocytes with functions that altogether resemble dendritic cells, the most effective amongst professional antigen presenting cells. In response to stimuli indicating viral infections, post-COVID monocytes showed up to 100-fold increased secretion of proinflammatory cytokines and enhanced transcriptional responses relating to cytokine signaling and monocyte activation. ATAC-seq also revealed a persistent pattern of differentially accessible chromatin which increased in abundance in early convalescent patients and did not return to the low levels observed in healthy individuals even 12 months after the acute infection. Thus, following severe SARS-CoV-2 infections, patients’ CD14+ monocytes carry specific and persistent epigenetic changes that puts them into an alerted state with heightened response characteristics.
Given that monocytes have a lifespan of a single day, the discovery of persistent epigenetic changes is notable and may reflect altered hematopoiesis and inheritance of epigenetic states from hematopoietic stem and progenitor cells (HSPC). To overcome the challenges associated with obtaining bone marrow resident HSPC, Cheong et al. developed a platform to enrich rare circulating HSPCs from PBMC and demonstrated that these faithfully represent the diversity and functional characteristics of their bone marrow-derived counterparts. With this platform, they discovered lasting epigenetic changes in HSPC of post-COVID patients that resembled those observed in mature monocytes. Especially late post-COVID HSPC exhibited skewed hematopoiesis with a significant increase of granulocyte monocyte precursor (GMP) cells. Intriguingly, the stem cells and the mature monocytes shared epigenetic signatures indicating that epigenetic and transcriptional programs are inherited by the mature progeny. The previously identified M.SC3 module activity was similarly increased in stem cells of the same patients.
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Source: Boes, M., Falter-Braun, P. Long-COVID-19: the persisting imprint of SARS-CoV-2 infections on the innate immune system. Sig Transduct Target Ther 8, 460 (2023). https://doi.org/10.1038/s41392-023-01717-9 https://www.nature.com/articles/s41392-023-01717-9 (Full text)