Leukemia arises from uncontrolled proliferation of a specific type of hematopoietic (blood) cell that is critical for a functional immune system. As a result, when patients are given very high doses of chemotherapy to eradicate leukemic cells most normal cells are killed as well, necessitating a transplant of hematopoietic stem cells from a donor to reconstitute the patient’s bone marrow and circulating hematopoietic cells. In some cases, the transplanted T cells from the donor can also recognize and eliminate the hematopoietic cells, including leukemia, from the recipient, thus preventing relapse.
In a study recently published in the journal Blood, Drs. Robson Dossa and Marie Bleakley (Clinical Research Division) describe how they took advantage of the ability of donor T cells to bind and eliminate specific cells—in this case hematopoietic cells—from the recipient for TCR-mediated immunotherapy. This strategy could benefit transplanted patients in the future by lowering the risk for relapse.
The authors tested HA-1, a minor histocompatibility antigen as a T cell target. HA-1 is expressed specifically in hematopoietic and leukemic cells in some individuals and can be recognized by specific T cell receptors (TCR). 15-20% of the transplanted patients would be eligible for HA-1 immunotherapy. Several candidate T cell receptor (TCR) clones binding to HA-1 were identified, cloned in lentiviral vectors and tested in vitro in primary T cells isolated from HA-1 negative donors.
HA-1 TCR2 was selected as the optimal TCR clone among five other candidates based on the level of expression and cytotoxic activity against HA-1-expressing cell lines. HA-1 TCR2 expression in CD8+ T cells allowed specific recognition and killing of both myeloid and lymphoid leukemic cells expressing the HA-1 antigen. These anti-HA-1 TCR-expressing T cells also expressed the CD8 co-receptor, allowing the modified TCRs to be functional in both CD4+ and CD8+ T cells. As a consequence, HA-1 TCR2 efficiently triggered CD4+ T cell activation, as indicated by release of interleukin-2 and interferon gamma cytokines, cell proliferation and HA-1+ leukemia cell killing. The presence of CD4+ T lymphocytes in the immunotherapy infused product is of importance as they can support CD8+ T cell function and persistence in vivo.
The HA-1 TCR2 construct was next modified for future clinical trials. First, a safety switch, the suicide gene iCasp9, was added. iCasp9 expression is activated upon AP1903 drug administration and allows inducible elimination of the modified expressing cells. As pointed out by Dr. Bleakley, “life-threatening toxicities would justify the use of the safety switch, such as very severe graft versus host disease, or sudden unexpected severe organ toxicity.” A CD20 epitope was also included in the construct facilitating cell tracking by flow cytometry. The new constructs were confirmed to be functional in vitro.
Finally, the data were also validated in human T cells clinical product transduced with the HA-1 TCR2 lentiviral construct along with the CD8 co-receptor and iCasp9. The modified human T cells efficiently expressed HA-1 TCR2 while presenting an effector and non-exhausted phenotype, conserved specific cytolytic activity against HA-1+ tumor cells and remained sensitive to the AP1903 drug. Characterization of the clonal diversity of the T cell product by TCR sequencing indicated the ability of the T cells to persist and expand in vivo. These experiments address safety concerns and validate further use of this anti-HA-1 construct for future clinical trials.
When asked about future directions, Dr. Bleakley answered: “we are very excited that the clinical trial to test the HA-1 TCR T cell immunotherapy product that we developed is very close to opening (days to weeks) and that we will actually start treating patients in early 2018. We consider the trial to be exciting because any evidence of efficacy will be a proof-of-principle for this particular target antigen, HA-1, and for the minor histocompatibility antigens as a class of targets for immunotherapy. Patients who relapse after transplantation generally have a very poor prognosis. If it is effective, minor H antigen-targeted immunotherapy could provide patients who develop post-transplant relapse with a new effective treatment and hope of survival. One other thing that we are very interested in is how the HA-1 TCR CD4+ T cells perform in patients. We believe that this is the first time that introduction of a CD8+ co-receptor* along with a HLA class I-restricted TCR into CD4+ T cells has been evaluated in a clinical trial (*to enable full activation of the CD4+ T cells when stimulated by antigen through the HLA-class I restricted TCR, and thereby help the antigen-specific CD8+ T cells in the cell product).”
Funding was provided by the Damon Runyon Cancer Research Foundation, Richard Lumsden Foundation, Alex’s Lemonade Stand Foundation, Cure4Cam Childhood Cancer Organization, the Leukemia and Lymphoma Society, Unravel Pediatric Cancer, the Bezos Family Foundation and the National Cancer Institute (National Institutes of Health).
Dossa RG,Cunningham T,Sommermeyer D,Medina-Rodriguez I,Biernacki MA,Foster K,Bleakley M. 2017. Development of T cell immunotherapy for hematopoietic stem cell transplantation recipients at risk of leukemia relapse. Blood.