Open Access

Catrin Birgit Heim

• Metastatic rhabdomyosarcoma (RMS) is one of the most challenging tumor entities in pediatric oncology caused by treatment resistances and immune escape. Novel chimeric antigen receptor (CAR) immunotherapies as specific, effective and safe treatment provide antitumor cytotoxicity by soluble factors and ligands/receptor signals. Besides its intrinsic potential as innate immune cell the ErbB2-sprecific CAR-engineered natural killer (NK)-92 cell line NK-92/5.28.z also provides CAR-mediated cytotoxicity, resulting in a high lytic capacity against 2D and 3D RMS cell structures in vitro. Also in a xenograft model using immune deficient NOD/Scid/IL2Rγ-/- (NSG) mice inhibited NK-92/5.28.z the tumor growth as long as the cells were administered and therefore prolonged the survival of the animals. The NK-92/5.28.z were distributed by the blood circulation and subsequently infiltrated the tumor tissue. Due to the malignant origin of the NK-92 cell line the cells must be irradiated prior to the use in patients. While the irradiation hampered the proliferation of NK-92/5.28.z cells, the cytotoxicity against RMS cells in vitro is retained for at least 24 hours. In the xenograft model irradiated NK-92/5.28.z cells inhibited the tumor growth but to a lower extent than untreated cells, as irradiated cells have only a limited life span in vivo no durable persistence and remission was achieved. Therefore, combinatorial approaches were focused and while blocking of the PD-1/PD-L1 axis did not resulted in a significantly enhanced tumor cell lysis, the combinatorial treatment with proteasome inhibitor bortezomib exhibited a significant enhanced cytotoxicity against RMS cells at least in vitro. Bortezomib itself induces caspase mediated apoptosis and also the upregulates the expression of TRAIL receptor DR5. The corresponding ligand TRAIL is expressed on the surface of the NK-92/5.28.z and pursuing experiments with purified TRAIL and bortezomib revealed a synergism. NK-92/5.28.z as an off-the-shelf product is therefore feasible for the therapy of metastatic RMS, but it might be necessary to support the cytotoxicity by additive agents like proteasome inhibitor bortezomib to archive durable remission. Another cell population suitable for RMS CAR-immunotherapy are cytokine induced killer (CIK) cells, a heterogenous cell population generated from autologous PBMCs consisting of T, NK and T-NK cells. Lentivirally transduced ErbB2-specific CAR-CIK cells were previously shown to inhibit the tumor engraftment in a RMS xenograft model. However, lentiviral transduced adoptive immunotherapies bear risks for the transfer in patients, therefore the Sleeping Beauty Transposon System (SBTS) as a non-viral method, which integrates the CAR coding DNA by a cut-and-paste mechanism from a minicircle (MC) into the CIK cells genome is more feasible for the generation of CAR-CIK cells. The Sleeping beauty transposase mRNA and the MC were transferred in the cell by nucleofection, different factors influence the transfection efficiency and viability of the CIK cells in this harsh procedure. In preliminary experiments with MC Venus, a MC encoding eGFP, the highest transfection efficiency with the best proliferative capacity was achieved with cells on day 3 of CIK culture and without the addition of autologous monocytes as feeder cells. For the CAR construct the protocol was further improved by adjusting crucial factors, for this construct the best results were achieved on day 0, without irradiated PBMCs as feeder cells and cultivation in X-Vivo10 medium supplemented with human fresh frozen plasma. The X-Vivo10 medium enhanced the percentage of NK- and T-NK cells significantly compared to CAR-CIK cells cultured in RPMI. Since the gene transfer by SBTS resulted in CAR-CIK cells stably expressing a CAR in all subpopulations, resulting in a significantly enhanced cytotoxicity against RMS cells in vitro, these cells were compared to lentiviral transduced CAR-CIK cells in vitro and in vivo. While the SBTS CAR-CIK cells were superior to viral CAR-CIK cells in 2D short-term assays, the viral cells showed higher lytic capacity in 3D spheroid long-term assays. In a RMS xenograft model lentiviral CAR-CIK cells significantly prolonged the survival of mice and persisted, whereas SBTS CAR-CIKs did not favor the overall survival compared to untreated controls and also did not persist. Phenotypic analysis revealed a highly cytotoxic CD8+ and late effector memory dominant phenotype for SBTS CAR-CIK cells supporting short-term cytotoxicity but also more prone for exhaustion, while viral CAR-CIK cells showed a more balanced phenotype for memory and cytotoxicity. Therefore, the SBTS is feasible for the ErbB2-CAR gene transfer in CAR-CIK resulting in a stable CAR-expression with high short-term cytotoxicity, but these cells are also more prone to exhaustion and the protocol might be adapted further to prevent this limitation for in vivo application. This work underlines the hard-to-treat characteristics of metastatic RMS, but also shows some approaches for further evaluation like the combination of NK-92/5.28.z cells with bortezomib and the feasibility of the generation of CAR-CIK cells via SBTS.