Several groups are involved in attempts to induce T cell immunity toward pediatric tumors. Dr. Crystal Mackall (National Cancer Institute) presented data from a recently completed Phase II trial of consolidative immunotherapy that incorporated autologous adoptive cell transfer and dendritic cell (DC) vaccination for patients with high risk and recurrent pediatric sarcomas. The data demonstrated favorable overall survivals using autologous T cell transfer following cytoreductive chemotherapy, but immune responses to peptides targeted with DC vaccines were modest(19
). As a result, future studies are utilizing tumor lysate-based vaccination, and newer approaches to DC maturation have been developed. A new Phase II trial for Ewing sarcoma, rhabdomyosarcoma or neuroblastoma combines regulatory T cell (Treg) depletion, which in preclinical models has improved the effectiveness of adoptive immunotherapy, with rhIL-7 administration to induce peripheral expansion of T cells and augment immune responses utilizing tumor lysate-based vaccination. She also presented data demonstrating that rhIL-7 is capable of safe, transient T cell expansion in humans associated with increased T cell repertoire diversity(20
Dr. Stephan Grupp (Univ. of Pennsylvania) presented a similar approach for consolidative immunotherapy that exploits lymphopenia to expand T cells collected at diagnosis from neuroblastoma patients after tandem autologous stem cell transplants (SCTs). In a phase II trial, CD3/CD28 costimulated/expanded T cells were given to patients at day+12 after SCT, and patients showed robust recovery of CD4+
T cell counts. In a follow-up randomized pilot study, T cells were given at day+2 or day+90 to assess T cell recovery and responses to pneumococcal vaccination given 12 and 60 days post-SCT. The cohort that received T cells on day+2 had protective pneumococcal responses, but some patients developed a self-limited engraftment syndrome clinically similar to graft-versus-host-disease (GVHD). Survivin is a potentially universal cancer antigen, and is an anti-apoptotic protein expressed in high levels in neuroblastomas. The group has observed survivin-specific T cells in neuroblastoma patients(21
), and future studies will aim to optimize a survivin-based, tumor-directed vaccine into this platform.
Dr. Helen Heslop (Baylor College of Medicine) discussed how Type 3 latency EBV+
lymphomas are very immunogenic such that adoptively transferred donor-derived T cell lines specific for EBV, given after allogeneic SCT as a means for targeting EBV-associated lymphoproliferative syndrome, have generated sustained CRs in 11/13 patients. Further, complete protection was observed when EBV-specific T cells were prophylactically administered to 101/101 high-risk patients. These T cells can persist for up to 8 years in patients. However treating immunocompetent patients is a greater challenge since their tumors have mechanisms of evasion in place. Her group manufactured polyclonal EBV-specific cytotoxic T cells and administered them to patients with advanced nasopharyngeal carcinoma in two phase I trials, and saw an overall response rate of 50% in patients with active disease. More recently, they used EBV-specific T cells that target the subdominant antigens LMP1 and LMP2 after chemotherapy or autograft in 24 patients with Type 2 latency EBV+
lymphoma, with 12/13 of the high risk patients remaining in remission up to 4.5 years after therapy and 9 of 11 patients with active disease having clinical responses(22
Dr. Richard O’Reilly (Memorial Sloan Kettering) presented substantial data demonstrating that WT1 is highly expressed in a variety of adult and childhood malignancies including gliomas, acute and chronic leukemia, Wilm’s tumor, desmoplastic small round cell tumor, and rhabdomyosarcoma. Moreover, several groups have demonstrated that WT1 is immunogenic and that immune responses toward WT1 can lyse tumor cells. Hence, Dr. O’Reilly has constructed a pool of pentadeca- (15-mer) peptides that span the Wilm’s tumor antigen (WT-1) that can sensitize donor-derived T cells for treatment of WT-1+
). His group has utilized epitope mapping to select and define 26 relevant epitopes that elicit a robust immune response. Using a xenograft tumor model with a WT-1-expressing leukemia or solid tumor, they have shown that adoptive transfer of WT1-EBV transgenic T cells selectively causes regression of established tumors as well as prevents growth of new tumors when leukemias are infused at the time of the lymphocyte infusion. A combined phase I/II trial is underway to look at infusing these transgenic T cells after a T cell-depleted allogeneic SCT for patients with WT-1 expressing leukemias or myelodysplastic syndrome.
Dr. Bryon Johnson (Medical College of Wisconsin) presented a murine model of neuroblastoma that is under study to optimize approaches for consolidative immunotherapy. Using a syngeneic SCT platform, his group implants a luciferase-expressing tumor (AGN2a) on day-8, supplements the graft with added T cells on day+0 of transplant, then gives a vaccine consisting of irradiated AGN2a cells genetically modified to express the immune stimulatory molecules CD54, CD80, CD86 and CD137L on days+2, 7 and 14 post-SCT. They have found that vaccinating during the first 2 weeks post-SCT induces anti-tumor immunity capable of eliminating established tumors. He also has used T cells sensitized to tumor antigens, and demonstrated that these T cells improve anti-tumor efficacy. The tumor responses mediated by T cells sensitized to tumor antigens are CD8+
dependent and are improved after CD4+
depletion in part due to the loss of Tregs, but long-term CD8+
memory responses were also lost(24
). Selective elimination of Tregs similarly enhances tumor immunity but does not compromise CD8+
memory, suggesting that this may be a viable approach for generating potent long-term anti-tumor responses against neuroblastoma(25
Dr. John Ohlfest (Univ. of Minnesota) presented data on targeting brain tumor stem cells (BTSCs) that form the therapy-resistant portion of gliomas. His group has found that BTSCs exhibit a heterogeneous expression of MHC class I or NK cell ligands and thus are difficult targets for immunotherapy(26
). He emphasized the substantial heterogeneity present within clinical tumors, and the potential for targeting of a smaller, and perhaps more homogenous, population of BTSCs as a means for immunologic targeting of the subset responsible for tumor growth. He presented work aimed at developing T cell-based therapies for brain tumors. By combining administration of CpG nucleotides with a tumor lysate vaccine and intratumoral gamma interferon gene transfer in a murine model of glioblastoma multiforme, his group has demonstrated that gliomas can be made immunogenic, resulting in improved trafficking of effector cells, enhanced tumor cell lysis and overall survival(27
). He is now moving these observations into a more clinically relevant canine model.
Dr. Alex Huang (Case Western Reserve) presented studies using intravital 2-photon microscopy to identify critical chemokines that regulate T cell trafficking to lymph nodes during the early stages of T cell priming and memory generation. Using genetically engineered tumors, Dr. Huang also demonstrated the enhancing effect of inflammatory chemokines in primary tumor rejection and in the generation of systemic anti-tumor immune responses that could ultimately be targeted in the clinical setting(28
Collectively, T cell-based therapies seem to be effective at expanding both CD4+ and CD8+ subsets, as well as promoting responses to infectious vaccines, but generation of Tregs appear to inhibit anti-tumor responses. Future studies will need to consider Treg depletion or inhibition of Treg function to be more effective. As observed with moAb trials, T cell-based therapies may function better in the setting of MRD than in patients with gross residual disease. We are also finally developing strategies to move these therapies past the blood-brain barrier to target brain tumors in a specific fashion.