Monocytes As an Early Risk Factor for Acute Graft-Versus-Host Disease after Allogeneic Hematopoietic Stem Cell Transplantation
Authors: Sun Huimin , Hao Sha , Dong Fang , Wu Linjie , Zhao Xueying , Cheng Tao 
Published: December 2024

Acute graft versus host disease (aGVHD) is a major complication after allogeneic hematopoietic stem cells transplantation (allo-HSCT), and contributes to high morbidity and mortality. Given the severe impact of aGVHD on patient outcomes, to precisely predict the occurrence of aGVHD is of vital importance for the early intervention and potentially improving survival rates. Current pretransplant clinical risk factors for aGVHD mainly include human leukocyte antigen (HLA) compatibility, the ages and genders of recipient and donor, and conditioning regimen intensity. However, our current understanding about the development and progression of aGVHD after allo-HSCT remains limited. To identify the potential biomarkers for prevention and treatment of aGVHD during the early hematopoietic reconstruction after transplantation, we involved scRNA-seq data of total nucleated cells (TNCs) of peripheral blood (PB) and bone marrow (BM) from 3 healthy controls (HCs) and 6 patients, as well as scRNA-seq data of TNCs from patient paired G-CSF-mobilized peripheral blood (Donor), and meticulously performed a comparative analysis on sequencing data. Patients included in this study were diagnosed as aplastic anemia (AA) and underwent allogeneic granulocyte colony-stimulating factor (G-CSF)–mobilized peripheral blood stem cells transplantation (allo-PBSCT) after bone marrow (BM) conditioning. Three of them developed different grades of aGVHD and receiving surging immunosuppressive treatment, while the rest did not show any clinical manifestations of aGVHD within 6 months after allo-PBSCT.
Prior to the onset of aGVHD, monocytes in peripheral blood of patients with aGVHD experienced an obvious rise on day 21 post-transplantation, and the differentiation preference of HSPCs towards monocytes in aGVHD group occurred prior to day 21, accounting for the abnormal regeneration of monocytes before the onset of aGVHD. Additionally, our findings demonstrated that monocytes from patients with aGVHD exhibited heightened functional activation 21 days post-transplantation, as indicated by gene ontology analysis. Concurrently, cell-cell interaction analysis revealed a strengthened regulatory network linking monocytes to CD8 effector T cells within the aGVHD patients. These observations suggest that monocytes may play a crucial role in inducing T cell activation and proliferation in the context of aGVHD.
To validate the function of PB monocytes on day 21 in aGVHD patients, we collected PB samples from AA patients around 21 days after allo-PBSCT, and isolated monocytes to co-culture with T cells from HCs for 5-7 days. As expected, monocytes from aGVHD patients were noted to be more capable of inducing T cell proliferation, thus confirming functional superiority of day 21 PB monocytes from aGVHD patients in inducing the proliferation of T cells.
To further verify the clinical value of the enrichment of PB monocytes on day 21 in monitoring aGVHD onset, we progressively conducted two clinical cohort analysis. We collected the results of blood routine examination of patients 60 days post-transplantation and compared the median cell percentage within three days for each post-transplantation timepoint, observing significant monocyte enrichment on day 21 in aGVHD patients, which aligns well with the results from the transcriptome analysis. These findings suggest that aGVHD-associated aberrant accumulation of PB monocytes on day 21 after allo-PBSCT is close to be a universal phenomenon.
Our study introduced several innovations to known findings: First, we comprehensively described early hematopoietic reconstruction dynamics under potential aGVHD condition after HSCT. Second, prior to onset of aGVHD, we found that monocytes in PB of patients with aGVHD experienced an obvious rise and activation on day 21 post-transplantation. Third, aGVHD-associated monocyte enrichment could be easily validated by blood routine examination and thus readily to be applicable in clinical practices. Collectively, our study indicates that monocytes could be a crucial early clinical risk factor for the development of aGVHD, and this insight could potentially guide the timing of monitoring efforts, recommending assessments at the pivotal juncture of around day 21 post-transplantation, shedding fresh light on the significance of early hematopoietic regeneration in relation to the onset of aGVHD.