Credit: CORBIS

One strategy that has been used in an attempt to enhance the ability of the immune system to kill tumour cells is the ex vivo introduction of tumour-specific-antigen receptors into activated T cells (ATCs), followed by their administration to patients. However, these cells have limited survival and antitumour activity in vivo, partially because of the lack of effective costimulation of these T cells by the tumour cells. Malcolm Brenner and colleagues reasoned that one way to enhance the costimulation of such T cells, and perhaps increase their survival and antitumour activity in vivo, is to express a tumour-specific-antigen receptor in cytotoxic T lymphocytes (CTLs) that are specific for Epstein–Barr virus (EBV), as these cells are extensively costimulated by EBV-infected B cells in vivo. EBV infection is widespread — approximately 33–50% of normal children, approximately 80% of patients with neuroblastoma and more than 90% of adults in Western populations carry the virus.

The authors created a chimeric antigen receptor (CAR) that contained a signal-transducing endodomain from the native T-cell receptor and an antigen-binding domain that was directed to the disialoganglioside GD2, which is present on most neuroblastoma cells. The CAR was then introduced into autologous ATCs and EBV-specific CTLs derived from each of 11 patients with relapsed or refractory advanced neuroblastoma who had persistent EBV infection. Ex vivo characterization of these cells showed that CAR-CTLs, but not CAR-ATCs, could kill autologous EBV+ B cells, and that both CAR-CTLs and CAR-ATCs could kill GD2+ neuroblasts.

CAR-CTLs and CAR-ATCs were then infused into the patients from whom they were derived to examine their in vivo survival and antitumour activity. To follow the survival of transduced cells in vivo, the authors used cells that expressed one of two CAR vectors tagged with distinguishing non-coding sequences; this allowed each cell population to be identified by PCR. After only 24 hours, the PCR signal from CAR-CTLs was much higher than that for CAR-ATCs. CAR-CTLs persisted for more than 6 weeks in vivo, whereas CAR-ATCs were not detectable beyond 3 weeks. Re-culturing of the cells 4–24 weeks after infusion showed that the CAR-CTLs proliferated in response to EBV+ B cells and retained cytotoxicity against GD2+ neuroblasts.

Can the CAR-CTLs induce a tumour response in vivo? Evaluable tumours were present in 8 of the 11 patients; of these, 4 showed evidence of regression or tumour necrosis after infusion of CAR-CTLs and CAR-ATCs. One patient experienced complete regression that has continued for more than 1 year, and one experienced tumour necrosis that led to a stable residual tumour 1 year after treatment. Furthermore, up to 24 months of follow-up revealed no adverse events that were attributable to the infused modified T cells.

The persistence and efficacy of virus-specific CTLs directed to a neuroblastoma-associated antigen indicates that these cells could be used to treat neuroblastoma, and this model should be of value in developing T-cell-based therapy for different tumours.