Dissecting the Graft-versus-Leukemia Effect: Identification of Target Antigens
The curative effect of allogeneic hematopoietic stem cell transplantation (allo-HSCT) for chronic myelogenous leukemia (CML) and other hematologic malignancies is primarily based on the generation of the donor-derived graft-versus-leukemia (GvL) immune response. In order to better exploit the beneficial effects of GvL responses while avoiding the treatment-related toxicities often associated with allo-HSCT, greater insight into the mechanistic basis of GvL is required. To elucidate the mechanisms of effective anti-tumor immunity, we have been dissecting a remarkably effective immunotherapy, donor lymphocyte infusion (DLI) following allogeneic stem cell transplant, in which 75% of treated patients consistently achieve durable remission. Starting from our unexpected observations that DLI-responsive patients develop a brisk B cell response associated with remission, I previously used these patient sera to probe a CML cDNA expression library and identified a series of highly immunogenic CML antigens, a subset of which are expressed in CML stem cells and have no donor-host polymorphisms, suggesting that it may be possible to elicit anti-tumor responses through vaccination against leukemia antigens. Our published and preliminary studies have established similar findings in patients with other diseases that are responsive to DLI, including multiple myeloma and CLL. Our studies of this effective tumor response suggest that development of polyclonal coordinated cellular and humoral tumor immunity is required, and we have begun to test out this concept in two IRB-approved clinical trials which seek to enhance GvL and eradicate residual disease through the use of a GM-CSF based cellular vaccine. My current research goals are to further advance our mechanistic understanding of the GvL responses. Ongoing laboratory studies seek to further characterize this potent adaptive immune response by: (a) serologic profiling of patients who demonstrate effective clinical immunity using a novel custom leukemia antigen array, (b) determining the relationship between antigen specific T and B cell immunity following DLI, (c) by understanding how innate immune stimuli can drive the development of this effective adaptive immunity, and (d) characterizing candidate populations that might represent the malignant progenitor cells of CLL that might be targeted by potent immunity to develop curative responses. In parallel, we want to utilize what these information to develop effective clinical trials. To achieve this, we are developing standardized assays to monitor anti-leukemia immunity of patients enrolled on current vaccine trials and comparing their responses to those observed following DLI. Through tight interactions between clinic and laboratory, our goal is to both dissect current therapies and devise new methods to target residual disease and enhance GvL. Elucidation of these mechanisms could provide novel strategies by which tumor immunity against leukemia can be enhanced in the allogeneic setting, as well as potentially in the non-transplant setting.In addition to the generation of anti-tumor immunity, allo-HSCT also results engraftment of donor cells with normal hematopoietic function. In parallel with my studies to understand and therapeutically enhance GvL effects, I have also developed a program at DFCI to treat adult patients with severe hemoglobinopathies using nonmyeloablative transplantation. These studies are aimed at generating curative responses while minimizing toxicity. To measure the functional outcome of donor engraftment, I have developed a novel erythroid lineage specific chimerism assay based on quantitation of RBC single nucleotide polymorphisms. Our studies revealed that initial donor engraftment can be achieved in these patients without treatment-related toxicity and have revealed that donor engraftment can overcome the ineffective erythropoiesis that is inherent to sickle cell disease. However, our observation of delayed graft rejection led me to devise a novel immunosuppressive conditioning regimen, that has been tested in a collaborative group of 6 adult transplant centers that I have formed. I am currently an External Review Committee Member for Patient Eligibility Review of a national multicenter trial to evaluate the safety and efficacy of a reduced intensity regimen for the treatment of children with SCD (BMT-CTN prot. #0601), and am one of the founding and organizing committee members of STRIDE, which seeks to develop a national trial to test these regimens in young adults.