The process of hematopoietic engraftment and immune reconstitution after allogeneic hematopoietic cell transplantation (alloHCT) is challenging to study in patients. Dr. Huo and colleagues have performed a single-cell resolution analysis of graft composition and the recovering hematopoietic compartment in transplant patients and recently published this work in Science Immunology.1 For patients undergoing alloHCT, the goals are to cure their underlying condition, avoid transplant-related toxicities (including infection and graft-versus-host disease [GVHD]), and recover normal immune function. Precision strategies that improve immune recovery are not available in clinical practice, largely because targeted pathways that may enhance early post-transplant hematopoiesis have been difficult to identify.
In this observational study, the authors carefully selected 10 patients undergoing transplantation for severe aplastic anemia. All received granulocyte colony-stimulating factor (G-CSF)-mobilized allogeneic stem cell grafts. They collected samples of the in-going peripheral blood stem cell graft, as well as post-transplant bone marrow and peripheral blood samples, enriched for CD34+ hematopoietic stem cells (HSC), then performed single-cell reverse transcription RNA sequencing. Bone marrow samples were collected on days 1, 14, 30, 60, and 90 after transplant, and another sample was taken more than six months post-transplant. Peripheral blood was collected at all time points except for day 1, but d21 analysis was added. This remarkably detailed time series allowed researchers to analyze the reconstituting immune system at a resolution that has not previously been possible.
In Brief
With these data, the authors were able to describe the reconstitution trajectory of HSCs and multipotent progenitors into mature blood cells after transplant. As expected, the first cells to reconstitute in the bone marrow were the megakaryocyte and erythroid precursors. They also observed that neutrophil progenitors and precursors were highly prevalent in the peripheral blood during the first 30 days after transplantation — likely reflecting emergency granulopoiesis. In addition to using their sequencing data to define cellular subsets, the researchers developed a profile of the transcriptome that gives valuable insights into the moderators of immune reconstitution. This dataset will be a valuable resource for others in the field.
A detailed exploration of the emerging granulocyte population revealed an accumulation of neutrophil progenitors with a gene expression profile consistent with regulatory capacity. These potentially immunosuppressive cells (the S100Ahigh Neu2 immunoregulatory neutrophil progenitor subset) were further studied in an independent cohort of 32 transplant patients. Intriguingly, patients who did not develop acute GVHD received a higher absolute number of this specific neutrophil population within the donor graft than those who developed acute GVHD. While the clinical dataset here is small, and more work is needed, this research highlights the value of single-cell sequencing approaches for developing readily testable hypotheses that may lead to both valuable biologic insights and new treatment strategies.
Competing Interests
Dr. Markey indicated no relevant conflicts of interest.
