High Alloreactive Potential of Central Memory T-Lymphocytes Expressing a Suicide Gene for the Cure of Hematologic Malignancies.

Alloantigen targeting adoptive immunotherapy is a powerful therapeutic approach for the treatment of hematologic malignancies. A compelling example is represented by donor lymphocyte infusions after allogeneic hematopoietic stem cell transplantation. The clinical efficacy of T-cell based therapy relies not only on the ability of T cells to mediate a direct anti-tumor effect (GvT) and a protective immune response against pathogens, but also in their capacity to persist and expand in vivo, providing a long-term protection from disease relapse. Despite undeniable efficacy, the extensive exploitation of donor lymphocytes is limited by the risk of a severe and potentially life-threatening complication: Graft-versus-host disease (GvHD). To solve this double bind, we investigated the therapeutic potential of donor lymphocytes retrovirally transduced to express the suicide gene thymidine kinase of Herpes Simplex virus (TK) in patients affected by hematologic malignancies, and showed that the suicide machinery controls severe GvHD. To maximize the extent and persistence of GvT activity mediated by TK cells, we exploited the positive effects of IL-7 in maintaining the homeostasis of memory lymphocytes. We observed that while stimulation with anti-CD3 antibodies and culture in the presence of high doses of IL-2 generates mainly CD45RA⁻CD62L⁻ effector memory (EM) TK cells with a limited ability to engraft and persist in vivo, stimulation with anti-CD3/CD28 conjugated cell sized beads and culture with low doses of IL-7 result in the generation of CD45RA−/CD62L+ TK cells with central memory (CM) functional phenotype, producing high levels of IL-2 upon stimulation, and expressing persistent high levels of IL-2R alpha and IL-7R alpha, a molecule associated to activation and long term survival memory T-cells, respectively. In mixed lymphocytes cultures (MLR), CM TK cells showed higher proliferative potential and lower sensitivity to apoptosis than EM TK cells, and such alloreactive CM TK cells kept CCR7 and IL-7R alpha expression even after 2ndary MLR. Accordingly, CM cells were more potent than EM cells in mediating allogeneic GvHD, after infusion in NOD/Scid mice, previously transplanted with allogeneic human skin. Newly developed homeostatic CM TK cells combine a high alloreactive potential with the selective sensitivity to GCV-mediated cell death, providing a tool for maximal anti-tumor activity with control of GvHD.