Plasmacytoid Dendritic Cells (PDC) and Th17 Immune Response Contribution in Skin Acute Graft-Versus-Host-Disease (GVHD)
Authors: Malard Florent , Bossard Céline , Chevallier Patrice , Guillaume Thierry , Delaunay Jacques , Brissot Eolia , Mosnier Jean-François , Saas Philippe , Moreau Philippe , Gaugler Béatrice , Mohty Mohamad 
Published: October 2019

Abstract
Abstract 4103

Background.
Acute GVHD after allogeneic stem cell transplantation (allo-SCT) is an exaggerated immune response against alloantigens involving dysregulation of inflammatory cytokine cascades. Previous studies established an important role of Th1 cells in acute GVHD pathophysiology. However, the identification of proinflammatory Th17 cells which contribute to autoimmune diseases pathophysiology, raised the issue of the role of Th17 cells in human acute GVHD. Indeed, the contribution of Th17 cells in acute GVHD was assessed in GVHD mouse models with conflicting results. In addition, the role of the PDC subset (the professional type I IFN-secreting cells), which play an important role in triggering Th17-related cytokines and autoimmune diseases, is not yet established in the acute GVHD setting. This report investigated the role of Th17 cells and their interaction with PDC in cutaneous biopsies taken from patients with or without acute GVHD.

Patients and Methods.
Studies described in this report were performed in a single centre series of 38 patients who underwent allo-SCT for different hematological malignancies (n=37) and severe aplastic anemia (n=1). The median age of patients was 52 years (range, 17–70). The stem cell source was PBSCs in 27 cases (71%), CB in 6 cases and BM in 5 cases. 11 patients received transplant from a matched-related donor, and 27 patients from an unrelated donor. A reduced-intensity conditioning regimen was used in the majority of cases (n=29; 76%) Immunohistochemistry was performed on deparaffinized tissues sections using an indirect immunoperoxydase method. A quantitative evaluation of antigens expression was performed by counting the number of positive cells in the whole biopsy at 200 magnifications for each sample.

Results.
In this cohort, based on standard pathology criteria, 29 patients had a histologically proven skin acute GVHD. In all cases, biopsies were taken before initiation of systemic corticosteroid therapy. The remaining 9 patients did not have histological signs of acute GVHD (and did not develop clinical signs of acute GVHD) and thus, were used as controls. In order to identify the Th17 cell population, biopsies were tested for expression of the CD161 and CCR6 markers, and ROR-gamma-t, the key transcription factor that orchestrates the differentiation of Th17 cells. Significantly higher numbers of ROR-gamma-t+, CD161+ and CCR6+ cells were counted in the skin of patients with acute GVHD compared with intestinal mucosa of patients without acute GVHD, mainly found in the lamina propria but also in the epithelium of altered glands (p=0.001, p<0.0001 and p=0.01 for ROR-gamma-t, CD161 and CCR6 expression respectively).Given the role of PDCs in triggering Th17-related cytokines, we sought next to determine the proportion of PDCs in cutaneous biopsies from these same patients. This analysis showed a significant increase of BDCA2+ PDCs in the skin of patients with acute GVHD compared with skin of patients without acute GVHD (p=0.03). Moreover, we observed a strong expression of the type I IFN-inducible protein Mx1 in the skin of patients with acute GVHD compared with skin of patients without acute GVHD, reflecting the high production of type I IFN by the BDCA2+ PDCs.

Conclusion.
The current study shed some light on the role of Th17 cells in the context of cutaneous acute GVHD. Using well-established specific markers, we show that Th17 cells infiltrate skin biopsies from patients with acute GVHD. In addition, Th17 infiltration was paralleled by the infiltration of PDCs, suggesting a potential new pathophysiological link between PDCs and Th17 response in the context of cutaneous acute GVHD. This is consistent with studies showing that PDCs can drive the differentiation of Th17 cells. Functional analyses are currently ongoing. These data raise the prospect of future innovative approaches to optimize immunosuppression regimens for the treatment or prophylaxis of acute GVHD by targeting PDCs and the Th17 response.

Disclosures:
No relevant conflicts of interest to declare.