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Background: Acute erythroid leukemia (AEL) or AML-M6 predominantly affects older adults and is rare in childhood. Compared with other AML subtypes, AEL remains relatively understudied because of its rarity. We established LS-CHM, a novel AEL cell line derived from the ascitic fluid of a patient with congenital leukemia. Interestingly, leukemic cells persisted in the ascitic fluid even after successful eradication from the bone marrow and extramedullary sites. Method: Leukemia cells from the ascites fluid exhibited robust proliferation in culture independent of cytokine requirement and were further characterized by flow cytometric immunophenotyping, cytogenetics, cell cycle and doubling time analysis, colony formation, genome and RNA sequencing, myeloid gene next generation sequencing, and cytotoxicity analysis. Results: LS-CHM displayed CD36, partial CD235a, CD31, CD43, and CD71 expression and demonstrated in vitro robust growth and high sensitivity to chemotherapeutic agents. A PDX mouse model showed development of leukemia. Genomic analysis revealed a frameshift BCOR mutation in the absence of additional mutations and downregulated TP53 expression with an exonic non-deleterious mutation. RNA sequencing of LS-CHM cells revealed upregulation of two cohesin complex genes, RAD21 and SMC3, whose high levels are associated with hematopoietic stem cell differentiation into erythroid lineage. Conclusions: LS-CHM represents the first congenital AEL-derived cell line, in contrast to the predominantly adult-origin and often secondary erythroid leukemia cell lines available currently. Thus, LS-CHM provides a unique pediatric and extramedullary AEL model, expanding the existing spectrum of AEL cell lines and offering valuable opportunities for biologic and therapeutic investigations.

ABSTRACTStem cell niches are complex multi‐signalling networks comprised of molecular cues and physical interactions, orchestrated by niche‐resident cells and the extracellular factors they produce. The bone niche specifically houses haematopoietic stem cells (HSCs), a critical cell type responsible for producing all blood and immune cells throughout life. Currently, how niches facilitate an ideal environment with simultaneously coordinating both intrinsic and extrinsic cellular signals is unknown. Studies presented here identify the existence of unique extracellular vesicle (EV)‐defined niches within the haematopoietic system of human individuals. Bridging studies using proteomic signatures, nanoparticle characterization at single‐vesicle resolution and machine learning‐based techniques reveal that EVs can be grouped by blood, bone marrow and trabeculae within a human individual. Stem cell assays demonstrate that these niche‐defined EVs impart functional effects on stem cells/progenitors based on location within the haematopoietic system. Finally, using single‐cell transcriptomic analyses, results identify for the first time how niche‐sourced EVs differentially affect the most primitive human HSCs and progenitors. This study highlights the significance of nanoparticles on human immunity and blood production and provides evidence for a new role for EVs, namely the demarcation of distinct nano‐niches within biological systems.

Several years after the COVID‐19 pandemic, the impact of SARS‐CoV‐2 on immunity and the potential protective role of Bacillus Calmette–Guérin (BCG) vaccination through trained immunity remain a subject of investigation. This study aimed to determine the long‐term impact of SARS‐CoV‐2 on immune cells and the association between BCG vaccination, latent infections and COVID‐19 severity and sepsis progression. We conducted a prospective analysis of patients who recovered from mild/severe/critical COVID‐19 ( n = 97, 3–17 months after COVID‐19) and sepsis patients ( n = 64). First, we assessed the impact of COVID‐19 and its severity on immune cell frequencies and expression of functional markers. Further, we analysed plasma titres of anti‐ Toxoplasma gondii /cytomegalovirus/BCG antibodies and their association with COVID‐19 severity and sepsis outcome. To examine monocyte responses to secondary challenge, monocytes isolated from COVID‐19 convalescent patients, BCG vaccinated and unvaccinated volunteers were stimulated with SARS‐CoV‐2 and LPS. Post‐COVID‐19 patients showed immune dysregulation regardless of disease severity characterised by altered expression of activation and functional markers in myeloid (CD39, CD64, CD85d, CD11b) and lymphoid cells (CD39, CD57, TIGIT). Strikingly, post‐critical COVID‐19 patients showed elevated expression of CD57 in CD8 + T cells compared to other severity groups. A trend toward improved outcomes in BCG‐seropositive COVID‐19/sepsis patients was observed, although this may be confounded by age differences between groups. In contrast, the monocyte response to stimulation appeared unaffected by COVID‐19 severity. These findings highlight the long‐term alterations of immune cells in post‐COVID‐19 patients, emphasising the substantial impact of COVID‐19 on immune function.