Functional diversification and dynamics of CAR-T cells in patients with B-ALL.

Understanding of cellular evolution and molecular programs of chimeric antigen receptor-engineered (CAR)-T cells post-infusion is pivotal for developing better treatment strategies. Here, we construct a longitudinal high-precision single-cell transcriptomic landscape of 7,578 CAR-T cells from 26 patients with B cell acute lymphoblastic leukemia (B-ALL) post-infusion. We molecularly identify eight CAR-T cell subtypes, including three cytotoxic subtypes with distinct kinetics and three dual-identity subtypes with non-T cell characteristics. Remarkably, long-term remission is coincident with the dominance of cytotoxic subtypes, while leukemia progression is correlated with the emergence of subtypes with B cell transcriptional profiles, which have dysfunctional features and might predict relapse. We further validate in vitro that the generation of B-featured CAR-T cells is induced by excessive tumor antigen stimulation or suppressed TCR signaling, while it is relieved by exogenous IL-12. Moreover, we define transcriptional hallmarks of CAR-T cell subtypes and reveal their molecular changes along computationally inferred cellular evolution in vivo. Collectively, these results decipher functional diversification and dynamics of peripheral CAR-T cells post-infusion.

Embryonic endothelial evolution towards first hematopoietic stem cells revealed by single-cell transcriptomic and functional analyses.

Hematopoietic stem cells (HSCs) in adults are believed to be born from hemogenic endothelial cells (HECs) in mid-gestational embryos. Due to the rare and transient nature, the HSC-competent HECs have never been stringently identified and accurately captured, let alone their genuine vascular precursors. Here, we first used high-precision single-cell transcriptomics to unbiasedly examine the relevant EC populations at continuous developmental stages with intervals of 0.5 days from embryonic day (E) 9.5 to E11.0. As a consequence, we transcriptomically identified two molecularly different arterial EC populations and putative HSC-primed HECs, whose number peaked at E10.0 and sharply decreased thereafter, in the dorsal aorta of the aorta-gonad-mesonephros (AGM) region. Combining computational prediction and in vivo functional validation, we precisely captured HSC-competent HECs by the newly constructed Neurl3-EGFP reporter mouse model, and realized the enrichment further by a combination of surface markers (Procr+Kit+CD44+, PK44). Surprisingly, the endothelial-hematopoietic dual potential was rarely but reliably witnessed in the cultures of single HECs. Noteworthy, primitive vascular ECs from E8.0 experienced two-step fate choices to become HSC-primed HECs, namely an initial arterial fate choice followed by a hemogenic fate conversion. This finding resolves several previously observed contradictions. Taken together, comprehensive understanding of endothelial evolutions and molecular programs underlying HSC-primed HEC specification in vivo will facilitate future investigations directing HSC production in vitro.

[Study on Flk1+ Cells during Mouse Early Embryogenesis by Lineage Tracing].

OBJECTIVE: To understand the differentiation of mesoderm-derived Flk1+ cells on different locations of the early mouse embryonic development and to explore the potential of Flk1+ cells to differentiate into mesenchymal lineage, like pericytes during vascular development. METHODS: Based on the Cre-LoxP system conditional knockout study strategy, the Flk1-Cre mice and ROSA26 reporter mice were used for lineage-tracing studies. The fate of the Flk1+ progenitor cells was traced with the GFP+ population. The detection of mesoderm marker Flk1, hematopoietic cell-specific marker CD45, endothelial cell-specific markers CD31, CD144, and Emcn (endomucin), pericyte specific markers PDGFRß and NG2, using the methods of immunohistochemistry, immunofluorescence, and flow cytometry should be combined to solve the concerned problems. RESULTS: Immunohistochemical staining of different fractions of E8.5-10.5 in the early embryogenesis of Flk1-Cre; ROSA26-EYFP mouse lineage showed that there were multiple populations in the Flk1+ cell-derived GFP+ population surrounding hematopoietic sites, such as dorsal aortas, limb buds and yolk sac. In addition to hematopoietic cells, the CD31+/Emcn+ typical endothelial cells distributed specifically along the blood vessel wall, there were many types of cell populations, such as mesenchymal-like cells. The immunofluorescence demonstrated that the cells of this group are neither hematopoietic, non-endothelial cells around the blood vessels, which are NG2+ pericytes. FACS analysis also confirmed that Flk1+ cells contributed to pericytes. In addition, in different hematopoietic sites of the embryo, a small population of CD31+CD140B+ cell populations with a mesenchymal-like morphology was observed in the GFP+ population. CONCLUSION: In the early stages of embryogenesis, mesoderm-derived Flk1+ populations not only contribute to hematopoietic, endothelial, and muscle lineages, but also have a differentiation potential for mesenchymal lineage, like pericytes. The presumably observed CD31+CD140B+ cell population may be a group of endothelial cells differentiated from Flk1+ progenitor cells and undergoing an endothelium-to-mesenchymal transition, EndMT, gradually losing the endothelial surface-specific marker and also starting to express a pericyte surface-specific marker.