Mapping the alloimmune effector cells in graft versus host disease

Abstract

Graft versus host disease (GVHD) is a life-threatening complication of haematopoietic stem cell transplantation. It is conventionally described as a process mediated by donor immune cells attacking healthy cells of the transplant recipient, causing inflammation in different organs including the skin. However, the cell types involved in the process and their immune mechanisms remain poorly understood. To define the immune landscape of GVHD, I collected paired blood samples and skin biopsies from patients with GVHD (n = 5) at intervals post transplantation (Day 19 – 261) and compared them with transplant controls without GVHD (n = 2) and healthy donors (paired n = 2, skin unpaired n = 2). Single-cell RNA sequencing was performed on peripheral blood mononuclear cells and skin, which was enzymatically split into dermis and epidermis. Using the 10x Genomics platform with 5’ chemistry, a total of 173,168 transcriptomes were obtained and their genotypes (donor or recipient origin) were deconvoluted. T cell receptors were also sequenced to investigate the dynamics of T cell activation across tissues. The cells from each anatomical compartment were defined and compared between conditions. The cellular composition of GVHD-affected tissues was vastly altered, with pronounced infiltration of the skin by donor derived cells from both myeloid and lymphoid lineages. Detailed myeloid cell characterisation identified macrophage and dendritic cell subsets, including resident macrophages and DC3. Inflammatory macrophages and dendritic cells were significantly expanded in GVHD. GVHD-specific features were also evident, which included the presence of plasmacytoid dendritic cells, as well as the expression of an activation signature among dendritic cells. Within the lymphoid compartment, GVHD featured the infiltration of predominately donor-derived, clonally expanded T cells from the blood into the dermis and epidermis. Clonal expansion of recipient-derived T cell was also observed in one case, indicating a potential host versus graft reaction. This study provides a high-resolution analysis of the complex cross-tissue immune reaction in GVHD.