A “non-fratricidal” αβ- T Cell Receptor That Targets Survivin Expressed By Hematological Malignancies

Survivin is broadly expressed by hematological malignancies as well as by solid tumors and may be a suitable target for T-cell immunotherapy. Previously, the utility of this target has been challenged by the occurrence of “fratricide” when T cells expressing a high avidity survivin-specific T cell receptor (TCR) killed each other because survivin epitopes can be presented by the T cells themselves (Leisegang M et al, J Clin Invest. 2010 Nov;120(11):3869-77). To overcome this obstacle, we used limiting dilution to isolate a new T-cell clone targeting the HLA-A*02-restricted survivin epitope ELT (survivin_95-104) and its variant LML (survivin_96-10497M) starting from autologous cultures, rather than from the allogeneic cultures previously used for this approach. In ⁵¹Chromium (Cr)-release assays, this T-cell clone, with nanomolar avidity, displayed specific killing against the survivin⁺HLA-A*02⁺ leukemia cells BV173 (39±16% specific lysis, E:T 40:1) and multiple myeloma cells U266 (20±7%) but not against HLA-A*02^– HL-60 cells (2±2%). Furthermore, the colony formation of primary myeloid leukemias was inhibited (>50% reduction) while that of healthy bone marrow (BM) was unaffected. The TCR α- and β-chains were then cloned in an optimized retroviral vector that was used to transduce CD8⁺ T cells which then efficiently expressed the transgenic αβTCR (89±4%, n=6). As compared to non-transduced (NT) T cells, survivin-αβTCR⁺ T cells produced significant lysis of BV173 (46±14% vs 8±6%, E:T 20:1, n=12, p<0.001) and U266 (27±12% vs 14±6%, p=0.003) but not of HL-60 (14±7 vs 14±6 %, p=NS). Blocking the target cells with specific anti-MHC class I antibodies confirmed the HLA-restriction of TCR transgenic T cells. Importantly, transgenic cells recapitulated the function of the original clone by inhibiting colony formation (range 32-78% reduction, n=5) of primary myeloid leukemias while preserving normal clonogenic capacity of healthy BM or cord blood (n=5). When tested in vivo in a xenograft model of established systemic acute leukemia (FFLuc⁺BV173) using bioluminescent imaging, leukemia progression was significantly slower in mice treated with survivin-αβTCR⁺ versus NT T cells (40x10⁶ ± 71x10⁶ vs. 128 x 10⁶ ± 176 x 10⁶ photons/sec by day 28) (p=0.04) and survival improved (n=12/group, p=0.01). This effect was even more pronounced when T cells were transferred to mice with limited leukemia burden (bioluminescent signal by day 40: 8.1 x 10⁶ ± 9 x 10⁶ vs. 195 x 10⁶ ± 85 x 10⁶ photons/sec) (p=0.003, n=10/group). Overall survival was improved by day 80 (p<0.001) and 3/10 mice treated with TCR⁺ T cells completely cleared the leukemia. Crucially, the TCR cloned from our autologous culture system produced no fratricidal activity in ⁵¹Cr-release assays against HLA-A*02⁺ activated T cells (1±2%, E:T 20:1, n=7). Activated T cells were only killed by TCR⁺ T cells when they were also pulsed with survivin peptides (46±12% for LML, 68±14% for ELT, n=7). To elucidate at the molecular level why our “autologous” TCR had selective antitumor activity unlike the fratricidal activity of “allogeneic” TCRs (Leisegang M et al, J Clin Invest. 2010 Nov;120(11):3869-77), we modeled the structure of each TCR-peptide-HLA ternary complex using the Rosetta software. While the overall binding energies of TCR-peptide-HLA interfaces for both TCRs were similar, the “autologous” TCR showed a 48% higher binding energy contribution for the peptide as compared to the fratricidal TCR, whose interaction was primarily with the HLA molecule rather than with the survivin peptide in the HLA-binding groove. In conclusion, we have cloned a novel survivin-TCR with a highly epitope-specific binding mode that can be efficiently expressed in polyclonal T cells and provides antitumor activity in vitro and in vivo without affecting the survival of T cells or normal hematopoietic progenitors. Our results indicate that maximal recognition of the peptide presented in the HLA groove is critical for TCR selectivity.