Single-cell transcriptome analysis reveals atypical monocytes circulating ahead of acute graft-versus-host disease clinical onset

Acute graft-versus-host disease (aGVHD) represents the rejection of the skin, gut and liver tissues of the recipient of an allogenic hematopoietic stem cell transplantation (HSCT) by the donor T-cells. The onset of aGVHD is often rapid and its evolution unpredictable. We undertook the single-cell RNA sequencing of peripheral blood mononuclear cells (PBMCs) collected before aGVHD clinical onset in three patients and at aGVHD diagnosis in one, using four HSCT recipients who remained free of aGVHD as controls. This analysis unveiled the presence in the circulation of particular subpopulations of monocytes and cytotoxic T cells (CTLs) in pre-aGVHD samples up to 18 days before clinical disease. These pre-aGVHD monocytes were characterized by an upregulation of the M2 polarity marker CD163, and the transmembrane protein SIGLEC1/CD169, while their CTL counterparts stood out for the upregulation of the CXCL10 receptor CXCR3 and the antigenic stimulation marker CD70. The occurrence of CD163/CD169-co-expressing monocytes upstream of aGVHD onset was validated using RNAseq data from an independent cohort and by flow cytometry of blood samples. These findings point to potential early diagnostic tools and preventive therapeutic strategies for aGVHD.