ICOSL⁺ Plasmacytoid Dendritic Cells As Biomarker and Inducer of Graft-Versus-Host Disease

Acute graft-versus-host disease (GVHD) remains one of the leading causes of death post allogeneic hematopoietic cell transplantation (HCT). Gastrointestinal GVHD (GI-GVHD), the most fatal type of GVHD, would benefit from additional biomarkers that are therapeutic targets. Using state-of-the-art quantitative proteomics we previously identified and validated an increased CD4⁺CD146⁺ T cell population in GI-GVHD patients. This population expressed a Th1 and Th17 phenotype and was induced by Inducible COStimulator (ICOS), a critical costimulatory molecule for the development of pathogenic Th17 (Li W. et al, J. Clin. Invest. Insights, 2016).

ICOS binds its ligand, ICOSL, which is expressed on dendritic cells (DCs) that prime naïve T cells to initiate immune responses. This prompted us to examine ICOSL expression on the two blood DCs subsets that can be identified in human peripheral blood: Lineage^-HLADR⁺CD11c⁺ myeloid DCs (mDCs) and Lineage^-HLADR⁺CD123⁺ plasmacytoid DCs (pDCs). Using the same cohort of patients aforementioned, the frequency of ICOSL was significantly higher on pDCs in 64 GI-GVHD patients when compared to 22 non-GVHD enteritis patients, 35 skin GVHD patients, and 39 patients without GVHD (Figure 1). The numbers and frequencies of total DCs, mDCs and pDCs were similar between groups.

The growth factor fms-related tyrosine kinase 3 ligand (Flt3l) is necessary for the development and differentiation of pDCs, and the transcription factor, Stat3, is required for Flt3l-dependent dendritic cell differentiation in mice. The role of pDCs in acute GVHD is still controversial (tolerogenic or initiator of GVHD depending on the murine model), and confirmatory studies about their functions are necessary before a therapeutic approach based on this mechanism can be contemplated. Based on the patients' data and previous knowledge, we hypothesized that absence of ICOSL signaling in donor DCs would protect against GVHD through Flt3l, Stat3, or both. We first found that knocking out (KO) ICOSL in the donor bone marrow (BM) extended survival compared to wild-type (WT) mice in the major mismatch (B6, H-2^b à BALB/c, H-2^d) experimental HCT model, while recipients of Stat3^KO BM did not show any difference in GVHD mortality (Figure 2A). We also found a significant decrease of Flt3l levels in plasma collected at day 3 from ICOSL^KO BM recipients compared to WT mice (Figure 2B). We then analyzed the recipients' infiltrating intestinal immune cells at day 10 post-HCT for the infiltration of pDCs and pathogenic Th17 cells. We found significantly lower frequencies of intestinal pDCs (CD11b^-CD11c⁺B220⁺CD103⁺) (Figure 2C), and intestinal T cells coexpressing interferon (IFN)g and IL-17 (Figure 2D) in recipients of ICOSL^KO BM compared to recipients of WT BM. Absolute counts of these two populations followed the same trend (data not shown). To confirm these data were not strain-specific, we performed similar analyses in the haplo-identical (B6, H-2^b à B6D2F1, H-2^d) experimental model showing similar outcomes for pDCs and IFNγ⁺IL-17⁺ T cells frequencies and counts in recipients of ICOSL^KO BM compared to recipients of WT BM. Importantly, and in contrast to human T cells, CD146 is not expressed on naïve murine T cells and thus cannot be measured in vivo in acute GVHD models. Transcriptome analyses by Nanostring technology (Immunology panels) comparing 14 days post-HCT of sorted pDCs from ICOSL^KO BM versus WT haplo-identical recipients showed increased expression of key molecules required for development (Itgax, Nos2, Socs1, Tcf4 and Bst2), costimulation (Cd80, Cd48, Cd74 and Cd86), and function (Tyrobp, Ikbkg, Nod2 and Irf7) of pDCs (Figure 3A). Lastly, we found increased T cell activation markers including Prf1, Il17a, eomes in sorted CD4⁺ T cells from ICOSL^KO BM compared to WT recipients (Figure 3B).

We conclude that early quantification of ICOSL⁺ pDCs frequency may allow identification of patients at risk of GI-GVHD development. Targeting ICOSL may represent a new avenue to treat acute GVHD.

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