Antileukemic effect of interleukin-2-transduced murine bone marrow after autologous transplantation

Myeloablative chemotherapy or radiation therapy supported by autologous stem cell transplantation (SCT) for the treatment of hematologic malignancies such as acute leukemia, lymphoma, and myeloma is associated with high rates of relapse. The reasons for this are 1) autologous transplantation lacks the in vivo graft-vs.-tumor (GVT) effect associated with allogeneic SCT, which is effective in controlling or eliminating residual malignant cells remaining in the body after high-dose therapy, and 2) contaminating malignant cells in the autologous graft are reinfused into the body. Some researchers have attempted to administer immunomodulatory cytokines to simulate a GVT effect, and although this has shown some efficacy, it has several disadvantages. These include high toxicity associated with systemic administration, a short in vivo half-life, and insufficient levels reaching the site of residual disease. As an alternative, we investigated whether delivery of the cytokine interleukin (IL)-2 to the bone marrow can exert an antileukemic effect while avoiding the problems associated with systemic administration. We describe the delivery of IL-2 to the bone marrow by transplantation of syngeneic bone marrow, retrovirally transduced with the gene for IL-2, into lethally irradiated mice. We were able to efficiently transduce murine bone marrow with the IL-2 gene without adversely affecting clonogenic output from hematopoietic progenitors, and we were able to achieve expression of the transgene in transplanted animals. However, IL-2 transduction inhibited hematopoietic reconstitution in lethally irradiated mice. Marrow transduced with high-titer, high-expressing IL-2 retrovirus failed to engraft, and a low-titer, low-expressing IL-2 retrovirus also demonstrated reduced engraftment, although engraftment was sufficient to support survival of transplanted mice. Long-term, low-level expression of the IL-2 transgene was detectable in these mice and was effective in exerting an antileukemic effect. Mice transplanted with control marrow and challenged with leukemic cells suffered 100% mortality within 70 days, whereas mice transplanted with IL-2-transduced marrow exhibited 50% survival over the 175-day duration of this study. The work shows that delivery of immunomodulatory cytokines to the bone marrow can be achieved by transplantation of genetically modified hematopoietic cells. Furthermore, low-level IL-2 expression can exert an antileukemic effect. These data suggest that this may be an effective immunotherapeutic strategy to reduce relapse after autologous transplantation, but the selection and expression of the cytokine must be carefully considered to minimize adverse effects on hematopoiesis.