Megakaryocytes (MKs), the platelet progenitor cell, play important roles in hematopoietic stem cell (HSC) maintenance and immunity. However, it is not known whether these diverse programs are executed by a single population or by distinct subsets of cells. Here, we manually-isolated primary CD41+ MKs from the bone marrow (BM) of mice and human donors based on ploidy (2N-32N), performed single-cell RNA sequencing analysis. We found that cellular heterogeneity existed within three distinct subpopulations possessing gene signatures related to platelet-generation, HSC niche interaction, and inflammatory responses, respectively. In situ immunostaining of mouse BM demonstrated that platelet-generation and HSC-niche related MKs were physically in close proximity to blood vessels and HSCs, respectively. Proplatelets, which could give rise to platelets under the blood shear forces, were predominantly formed on platelet-generation subset. Remarkably, the inflammatory responses subpopulation, consisting generally of low-ploidy LSP1+ and CD53+ MKs (≤8N), represented approximately 5% of total MKs in the BM. These MKs could specifically respond to pathogen infections in mice. Rapid expansion of this population was accompanied by strong upregulation of a pre-existing PU.1 and IRF-8-associated monocytic-like transcriptional program involved in pathogen recognition and clearance, as well as antigen presentation. Consistently, isolated primary CD53+ cells were capable to engulf and digest bacteria and to stimulate T cells in vitro. Together, our findings uncover new molecular, spatial, and functional heterogeneity within MKs in vivo and demonstrate the existence of a specialized MK subpopulation that may act as a new type of immune cell.

Graft-versus-host disease (GVHD) is a major obstacle to the success of allogeneic hematopoietic stem cell transplantation (allo-HSCT). Although many indicators have been used to predict GVHD, the prediction of GVHD remains very difficult. Determining how to predict the occurrence of acute GVHD (aGVHD) as exactly as possible has been a huge challenge so far. Systemic release of inflammatory cytokines during the conditioning regimen and early-phase post-HSCT plays a crucial role in the generation of aGVHD; additionally, the conditioning regimen causes the damage to the intestinal tract. Diarrhea is a common symptom of intestinal tract damage during the conditioning regimen. We therefore performed a prospective study to investigate the relationship between diarrhea related to the conditioning regimen, systemic release of inflammatory cytokines during the conditioning regimen and the early-phase post-HSCT, and the development of aGVHD. This study demonstrated that duration of diarrhea was >5.5 days, with a maximal volume >8.72 mL/kg and a mean volume of diarrhea for days -3 to 0 >7.94 mL/kg were risk factors of grade II to IV aGVHD. Diarrhea with any 1 risk factor was defined as severe diarrhea. Furthermore, this study first confirmed that the correlation between diarrhea and aGVHD was related to the serum levels of tumor necrosis factor (TNF)-α and interleukin (IL)-6 during the conditioning regimen and during the early phase after transplantation. Our study demonstrated that diarrhea related to the conditioning regimen could be used as a marker for the prediction of aGVHD and further explain the possible mechanism underlying the linkage described here. Therefore, for the patients with severe diarrhea related to the conditioning regimen, low-dose glucocorticoid may be used to reduce the levels of inflammatory cytokine release by a damaged intestinal tract, and possibly further reduce the occurrence of aGVHD. For these patients, prophylaxis of aGVHD may be need to be adjusted individually.

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