Key Points Allogeneic CD19-CAR VSTs are well tolerated by patients with relapsed B-cell malignancies post-HSCT. At periods of CD19-CAR VST persistence, these cells demonstrate antitumor activity.

Purpose Graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (alloSCT) has been associated with low numbers of circulating CD4+CD25+FoxP3+ regulatory T-cells(Tregs). Because Tregs express high levels of the IL-2 receptor, they may selectively expand in vivo in response to doses of IL-2 insufficient to stimulate T-effector T-cell populations, thereby preventing GvHD. Experimental Design We prospectively evaluated the effects of ultra low-dose (ULD) IL-2 injections on Treg recovery in pediatric patients after alloSCT and compared this recovery with Treg reconstitution post alloSCT in patients without IL-2. Sixteen recipients of related(n=12) or unrelated(n=4) donor grafts received ULD-IL-2 post HSCT (100,000–200,000 IU/m2 3×weekly), starting