Donor Plasmacytoid Dendritic Cells Home to the Thymus of the Transplant Recipient and Limit Graft Versus Host Diseases through Production of Vasoactive Intestinal Polypeptide
Authors: Wang Yitong , Li Yiwen , Li Jianming , Li Jingxia , Waller Edmund K. 
Published: October 2019

Abstract
Introduction: In allogeneic hematopoietic stem cell transplant (AHSCT), donor T cells promote hematopoietic engraftment, reconstitute T-cell immunity and mediate potent beneficial antitumor effects, such as graft versus leukemia (GVL) as well as detrimental graft-versus-host disease (GVHD). We have shown that purified bone marrow pDC to a donor graft composed of purified HSC and T cells significantly improved long-term leukemia-free survival without increasing the risk of GVHD (Lu BLOOD 2012). Vasoactive intestinal peptide (VIP) is a neuropeptide/neurotransmitter, which acts as a major anti-inflammatory factor in animal models of inflammatory and autoimmune diseases. VIP is produced by T cells and dendritic cells under conditions of inflammation (Li Cancer Research 2016). VIP-signaling, thus represents a newly appreciated co-inhibitory pathway involved in T cell activation and expansion and persistence of antigen-specific T cells, but the role of VIP produced by donor dendritic cells in allo-BMT is unclear. Our previous data has shown that production of VIP in pDC improves survival in a murine allo-BMT model. Since the thymus plays a critical role in regenerating naive T cells in which allo-reactive donor T cells undergoes central deletion. We hypothesized that donor pDCs that home to thymus modulate the negative selection of allo-reactive T cells and iTreg production through VIP signaling.
Methods and Results: Our previous data has shown that the mice receiving HSC, T cells and WT pDC had a significantly higher survival (71%) compared to those receiving VIP-KO pDC (31%). On day 15 after transplant, recipients of WT pDC, VIP-KO pDC and no pDC developed ~98% chimerism, without significant differences among the three groups. Local production of VIP in pDC inhibited activation and Th1 immune polarization of donor T cells.
Recipient spleens were harvested on day 15 after transplant for analysis of cytokine production by donor T cells. The percentage of CD8+ donor T cells producing IL17 was significantly higher in recipients of VIP-KO pDC compared to recipients of WT-pDC (Fig 2.A-B). The ratio of foxp3+ CD4+ donor T-reg to IL17+ CD4+ T cells from recipients of WT pDC was almost 3 times higher than the recipients of VIP-KO pDC (Fig 2.C-D).
To visualize production of VIP by donor pDC, B10.BR mice were transplanted with 5,000 stem cells, 1M T cells and 50,000 pDCs from VIP-GFP (VIP promoter and GFP reporter) or GFP mice. On day 7 post-transplant, the thymus was examined with confocal microscopy with GFP (green), anti-PDCA-1-Alexa Fluor 568 (red) and DAPI (blue). A superimposed profile of the thymus showed that donor GFP pDCs homed to thymus (Fig.2-A), and that donor pDC in the recipient thymus produced VIP (Fig.2-B).
Conclusion: Expression of VIP in donor pDCs inhibited activation and Th17 immune polarization of donor CD8+ T cells after allo-BMT. Foxp3+ expression tended to be higher among CD4+ donor T cells from recipients of WT pDCs compared with recipients of VIP-KO pDCs. Thus, VIP-producing donor pDC in thymus could be very critical to contribute to negative selection of allo-reactive donor T cell or facilitate the generation of Foxp3+ nTreg. This data supported a new mechanism by which GvHD maybe regulated and central tolerance maintained. Ongoing experiments aim at defining the role of VIP production in the thymus by donor pDC in positive and negative T cell selection.

Disclosures
Waller: Celldex: Research Funding; Novartis Pharmaceuticals Corporation: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Cambium Medical Technologies: Consultancy, Equity Ownership; Kalytera: Consultancy; Pharmacyclics: Other: Travel Expenses, EHA, Research Funding.