Targeting CD134 Results in Selective Depletion of Alloreactive Human T Cells without Loss of Virus-Specific and Leukemia-Specific Effectors.
Authors: Ge Xupeng , Sykes Megan , Brown Julia , Boussiotis Vassiliki A. 
Published: April 2020

Abstract
Graft versus host disease (GvHD) remains a frequent and severe complication of allogeneic stem cell transplantation (SCT) and is mediated by recognition of recipient alloantigens by donor T cells. Numerous interventions have been employed against GvHD which target the number and function of T cells including pharmacological non-specific immunosuppression, depletion of donor T cells or positive selection of donor CD34+ stem cells. While these strategies reduce the risk of GvHD, the benefits are often offset by prolonged immunodeficiency and increased risk of infection and disease relapse. Thus, research efforts have focused on developing a method to selectively deplete alloreactive T cells without compromising other immune responses. One approach to reducing alloreactivity is to remove donor T cells that are activated after ex-vivo co-culture with recipient antigen-presenting cells (APC). Clinical studies have been performed using anti-CD25-conjugated immunotoxin to deplete alloreactive T cells after incubation with haplo-identical PBMC ex-vivo. Although this approach is promising, post-transplant infections, disease relapse and residual GvHD remain a problem. These observations mandate further experimentation to improve our strategies for allodepletion. We undertook studies to identify an inducible surface activation marker expressed selectively on alloreactive effector T cells following culture with HLA-mismatched allostimulators. Costimulatory molecules play a major role in activation of the effector T cells involved in GvHD. In contrast to CD28, which is constitutively expressed, TNF receptor (TNFR) superfamily members are induced and therefore their expression can also be used as an indicator of T cell activation during GvHD, in experimental animal models and allogeneic human transplantation. Here, using peripheral blood mononuclear cells as responders and HLA-mismatched dendritic cells as allostimulators, we observed that compared to other activation markers the TNFR superfamily member, CD134, was superior because of its completely negative base-line expression and rapid upregulation upon activation. Depletion of CD134+ cells from responder populations dramatically reduced specific alloreactivity as determined by reduction of helper T cell precursor (HTLp) frequencies below the threshold predicting development of clinical GvHD and reduction of cytolytic T cell precursor (CTLp) frequencies by 92% against original allostimulators, while retaining responses to third party alloantigens. However, effector CD8+ T cells expressing perforin and granzyme B were preserved in the CD134 allo-depleted populations. Detailed analysis revealed that, by this approach, functional CMV-specific effectors were retained as determined by their identification with CMV-specific pentamers and assessment of their functional responses by CMV-specific ELISpot. Effectors specific for WT1 leukemia antigen were also retained. In addition, Foxp3+ Treg cells were preserved in the allodepleted populations. These observations suggest that CD134-based selective allodepletion may allow allogeneic SCT without requirement for global T cell depletion or non-specific immunosuppression and without loss of pathogen-specific and leukemia-specific immunity.