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STEMIMMUNE Report Summary

Project ID: 322093
Funded under: FP7-PEOPLE
Country: Italy

Final Report Summary - STEMIMMUNE (Improving adoptive T cell transfer immunotherapy for cancer with T memory stem cells)

Adoptive cell transfer with tumour-specific T lymphocytes is currently under test in the clinic for the treatment of solid tumours such as melanoma. However, the poor persistence in vivo of adoptively-transferred cells is the major caveat to successful immunotherapy. It is now accepted that CD8+ T cells derived from lymphocytes at early stages of differentiation better mediate tumour rejection due to their increased persistence, survival and proliferation in vivo. We originally identified the least differentiated human memory T cell subset, the T stem cell memory (TSCM), capable to self-renew and simultaneously generate more differentiated progeny. These TSCM are also endowed with superior immune reconstitution capacity and anti-tumour immunity (Figure 1A; Gattinoni L, Lugli E et al. Nat Med, 2011;17:1290-97). However, the genetic program at the basis of the persistence and superior effector function of these cells, especially under chronic stress, still needs to be defined. By studying immune reconstitution in haploidentical, T-repleted bone marrow transplantation (Figure 1B) and by studying their gene expression architecture, our original aim was to determine the following:
1. Determine whether TSCM cells are major contributors to the process of immune reconstitution as suggested by non-human primate and humanized mouse models.
2. Identify the genetic program responsible for the persistence of TSCM cells in vivo under conditions of continuous stress, i.e. the lymphopenia induced by the conditioning regimen
3. Genetically-modify tumour-specific T cells with candidate genes to improve persistence in humanized mouse tumour models.

We determined that TSCM are the most abundant circulating T-cell population in the early days following haploidentical transplantation combined with post-transplant cyclophosphamide (pt-Cy; a chemotherapeutic treatment capable to remove allogeneic T cells; Figure 1B) and precede the expansion of effector cells. Transferred naive, but not TSCM or conventional memory cells preferentially survive pt-Cy, thus suggesting that posttransplant TSCM originate from naive precursors. Moreover, donor naive T cells specific for exogenous and self/tumor antigens persist in the host and contribute to peripheral reconstitution by differentiating into effectors. Similarly, pathogen-specific memory T cells generate detectable recall responses, but only in the presence of the cognate antigen.

Gene expression profiling of T cell subsets isolated from healthy individuals identified transcription factors that are associated improved effector functions of the memory T cell compartment. Given the paucity of TSCM ex vivo, we optimized a cell culture protocol to generate large numbers of TSCM from purified naive CD8+ T cell precursors (Figure 1C). Gene expression data were validated by real time PCR and Western blot, while co-IP experiments identified molecular partners involved in the regulation of transcription. Overexpression and downregulation of specific targets were able to define their role in TSCM cell development. Experiments are currently ongoing to identify the genes that are regulated by such transcription factors in human TSCM cells. Repopulation experiments in highly immunodeficient mice will follow. We expect these transcription factors to regulate the function of T cells following adopitve transfer and thus improve immune reconstitution in lymphopenic hosts. In addition, more potent T cells capable to persist in the long term and endowed with improved effector functions will benefit cancer immunotherapy with adoptively-transferred T cells.

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