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

Abstract
Abstract 2968

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 gastrointestinal (GI) 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 21 patients who underwent allo-SCT for different hematological malignancies (n=19) and severe aplastic anemia (n=2). The median age of patients was 53 years (range, 16–69). The stem cell source was PBSCs in 19 cases (85%), CB in 2 cases and BM in one case. Ten patients received transplant from a matched-related donor, and 11 patients from a matched-unrelated donor. A reduced-intensity conditioning regimen was used in the majority of cases (n=19; 90%) 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, 16 patients had a histologically proven gastrointestinal acute GVHD. In all cases, biopsies were taken before initiation of systemic corticosteroid therapy. The remaining 5 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+ and CD161+ cells were counted in the intestinal mucosa 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=0.016 and p=0.009 for ROR-gamma-t and CD161 expression respectively).
Given the role of PDCs in triggering Th17-related cytokines, we sought next to determine the proportion of PDCs in intestinal biopsies from these same patients. This analysis showed a significant increase of CD123+ PDCs in the intestinal mucosa of patients with acute GVHD compared with mucosa of patients without acute GVHD (p=0.017). Moreover, we observed a significant correlation between the number of CD123+ PDCs and ROR-gamma-t or CD161 expressing cells in the intestinal mucosa of acute GVHD patients, highlighting the link between PDC and Th17 cells.

Conclusion.
The current study shed some light on the role of Th17 cells in the context of gastro-intestinal acute GVHD. Using well-established specific markers, we show that Th17 cells infiltrate intestinal 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 gastro-intestinal acute GVHD. This is consistent with studies showing that PDCs can drive the differentiation of Th17 cells. Functional analyses are currently ongoing. Although the exact mechanism that links type I IFN production to PDC-mediated Th17 responses is still unclear in acute GVHD, 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.