The Impact of Graft-Versus-Host Disease on Dendritic Cell Homeostasis and Their Potential Use As Biomarker to Predict the Severity of Chronic Graft-Versus-Host Disease
Authors: Thiant Stephanie , Roy Jean , Trottier Jessica , Giard Marie-Pier , Parat Rachel , Roy Denis-Claude , Guimond Martin 
Published: October 2019

Abstract
PURPOSE: Graft-versus-host disease (GVHD) is the principal complication of allogeneic stem cell transplantation (allo-SCT) and occurs when GVHD T cells recognize differences in major and/or minor histocompatibility antigens expressed by recipient cells. Following cytotoxic regimens, the inflammatory milieu represents a fertile ground for alloreactivity and thymic insults resulting from GVHD are largely responsible for thymic dysfunction and immune-incompetence. Studies have also demonstrated that GVHD insult to the peripheral niche is perhaps the most important factor for limiting thymic independent T cell regeneration. Given that patients with chronic GVHD (cGVHD) are profoundly lymphopenic and that dendritic cells (DCs) play a critical role in T cell homeostasis, we postulated that changes in DC counts could precede the development of clinical signs of acute and/or chronic GVHD.
METHODS: 44 patients that received HLA-matched allogeneic SCT were included in this study. Controls consist of 15 patients that received autologous SCT and 30 healthy donors. Flow cytometric analysis was used to measure plasmacytoid DCs (pDCs), myeloid DC type 1 (mDC1), mDC2 and mDC3. Blood samples were obtained at day 0, 1 month post-SCT and then every other month until 1 year. The percentage and absolute counts of naïve CD4+ or CD8+ lymphocytes (TNA); (CD3+ CD45RA+ CCR7+), central memory (TCM); (CD3+ CD45RA+ CCR7neg), effector memory (TEM); (CD3+ CD45RAneg CCR7neg) and terminal differentiation (TTD); (CD45RA+ CCR7neg) was determined by flow cytometry. B lymphocytes and regulatory CD4+ T cells were also evaluated.
RESULTS: One month after allo-SCT, we found a significant increase in blood DCs followed by a gradual decrease and stabilization by 3 months post-allo-SCT. During the first year, all DC subsets including pDCs, mDC1, mDC2, mDC3 as well as CD4+ and CD8+ T cells were significantly diminished compared to autologous and healthy controls. Plasmacytoid DCs regeneration was in general the most affected and we found a positive correlation between pDC cells count and the number of naïve and central memory CD4+ lymphocytes. Similarly, pDCs counts also correlated with the number of naïve and memory CD8+ T cells. In contrast, effector CD4+ and CD8+ T cell showed a stronger correlation with mDC1. Patients with grade II-IV acute GVHD had lower DC counts compared with mild grade 0-1 aGVHD and a similar trend was also observed during the development of cGVHD, Importantly, loss of pDCs and mDC1 that preceded the development of cGVHD by 1 month was associated with the development of grade 3 cGVHD whereas loss of DCs during cGVHD was associated with milder 0-2 cGVHD. Thus far, our data support a model wherein loss of pDCs and mDC1 could represent potential biomarkers to predict the severity of cGVHD.

Disclosures
No relevant conflicts of interest to declare.