Activation of DR3 Signaling Expands Recipient Derived Regulatory T Cells and Enhances Donor Immune Reconstitution Derived from Allogeneic Hematopoietic Stem Cells
Authors: Nishikii Hidekazu , Kim Byung Su , Pierini Antonio , Baker Jeanette , Schneidawind Dominik , Pan Yuqiong , Alvarez Maite , Negrin Robert S. 
Published: October 2019

Abstract
CD4+ Foxp3+ regulatory T cells (Treg) are a subpopulation of T cells which regulate the immune system, maintain the tolerance of self-antigens and enhance immune tolerance after transplantation. It was also reported that recipient derived Treg could provide immune privilege to allogeneic stem cells (HSC) after transplantation. However, the precise interaction with Treg and HSC has not been fully elucidated. In this study, we investigated the role of recipient derived Treg in the engraftment and immune reconstitution following transplantation of purified allogeneic HSC and the effectiveness of Treg expansion following activation of DR3 (Death receptor 3, also called as TNFRSF25) signaling in this model.
We first tested the effect of Treg depletion using Foxp3-DTR mice in allogeneic HSC transplantation. In this system, FACS-sorted purified HSC (Lin-cKit+Sca1+ population) derived from WT-FVB mice (CD45.1+/H2kq+) were injected into lethally irradiated B6-Foxp3-DTR mice (CD45.2+/H2kb+) with or without pre-treatment of diphtheria toxin (DT). On day 0 and day 28 after transplantation decreased frequencies of Foxp3+ cells in residual recipient derived CD4+ T cells were observed in peripheral blood from the DT treated mice (P<0.001 on day 0, P<0.002 on day 28). Although total myeloid chimerism was comparable between control and DT-treated mice, the frequency of donor derived immune cells including CD4+ T cells (P<0.01 on day 56), CD8+ T cells (P<0.01 on day 56), and B220+ B cells (P<0.001 on day 56) was significantly decreased in DT-treated mice. These data suggested that recipient derived Treg play an important role in allogeneic immune reconstitution after transplantation.
DR3 is a member of the TNF receptor superfamily and we previously reported the expansion of Treg by the activation of this signaling pathway (Kim et al, ASH abstract 2013). We next tested whether activation of DR3 signaling by its agonistic antibody would affect the donor immune reconstitution after allogeneic HSC transplantation. The frequencies of Foxp3+ cells in CD4+ T cells were significantly increased in thymus, spleen, peripheral blood, and bone marrow 4 days after antibody injection (P<0.01). Isolated Treg derived from antibody treated mice showed stronger suppressive function in the mixed lymphoid reaction compared with those from isotype antibody treated mice.
The mice treated with antibody on day -4 were transplanted with purified allogeneic HSC on day 0. Antibody treated mice showed a higher frequency of donor derived CD4+ T cells (P<0.001 on day 28), CD8+ T cells (P<0.05 on day 28), and B220+ B cells (P<0.05 on day 28) in this allogeneic HSC transplantation model.
In summary, our data suggest that recipient derived CD4+Foxp3+ Treg play an important role in donor immune reconstitution and the activation of DR3 signaling in recipient mice enhances donor immune reconstitution by expansion of recipient derived Treg.
H.N and BS-K contributed equally to this work.

Disclosures
No relevant conflicts of interest to declare.