T-Bet Is Critical for the Development of Acute Graft-Versus-Host Disease By Regulating Hematopoietic Antigen Presenting Cells

Graft-versus-host disease (GVHD) remains to be a major obstacle for the efficacy and continuing success of allogeneic hematopoietic stem cell transplantation in the treatment of various malignant and non-malignant diseases. Activation of antigen presenting cells (APCs), both host and donor origin, plays a crucial role in priming alloreactive donor T cells to induce and intensify acute GVHD (aGVHD). Beyond its critical effects on T cells, the T-box transcription factor T-bet also regulates activity of APCs, including dendritic cells (DCs) and B cells. However, the effect and mechanism of T-bet in regulating APCs in the development of aGVHD has not been investigated. To evaluate the role of T-bet in modulating APC function and aGVHD development, we compared the severity of aGVHD in WT versus T-bet^-/- recipients using several well-defined, clinically relevant murine models of allogeneic bone marrow transplantation (allo-BMT). We observed that T-bet^-/- recipients developed much milder aGVHD than their WT counterparts, reflected by significantly higher rate of survival, lower clinical scores, and better donor BM-derived B- and T-cell reconstitution. In T-bet^-/- recipients, donor T cells significantly reduced IFN-γ production, proliferation and migration, and caused less damage in aGVHD target organs, such as liver and gut. By using various BM chimeras as the recipients, we further observed that T-bet expressed on recipient hematopoietic APCs was primarily responsible for donor T-cell response and pathogenicity in causing aGVHD. Additionally, we evaluated the role of T-bet in donor APCs by transplanting WT or T-bet^-/- BM together with WT T cells into lethally irradiated allogeneic recipients. We observed that recipients of T-bet^-/- BM developed attenuated aGVHD compared with those of WT BM, suggesting that T-bet also contributes to the function of donor APCs in the induction of GVHD.

Given DCs are the most potent hematopoietic APCs, we subsequently focused on recipient DCs. DCs in T-bet^-/- recipient produced less IFN-γ, expressed higher levels of Trail, but not FasL or TNF, to induce significantly higher levels of apoptosis on donor T cells prior to their massive proliferation. To test whether Trail/DR5 interaction is responsible for the induction of donor T cell apoptosis and subsequent reduction of aGVHD in T-bet^-/- recipients, we compared the ability of WT or DR5^-/- T cells in inducing aGVHD in WT versus T-bet^-/- recipients after allo-BMT. While WT T cells induced severe aGVHD in WT recipients, they failed to do in T-bet^-/- recipients. In contrast, DR5^-/- donor T cells were capable to induce severe aGVHD in the recipients regardless of T-bet expression. These data suggests that Trail/DR5 interaction is a major signaling pathway responsible for donor T-cell apoptosis induced by T-bet^-/- APCs, through which alleviates the development of aGVHD. In conclusion, we demonstrate that T-bet up-regulates IFN-γ production and down-regulates Trail expression on recipient DCs, which promotes donor T-cell activation and mitigates T-cell apoptosis, respectively. Thus, T-bet plays a critical role in the development of aGVHD by regulating the activity of hematopoietic APCs, particularly DCs. Taken together with our previous findings, we propose that T-bet is a potential therapeutic target for the control of aGVHD through regulating T-cell activation and differentiation as well as APC functions.