Many researchers have attempted to find ways to use functional T cells to selectively target and destroy cancer cells. However, most of these have been of limited, if any, clinical use due to the difficulty of maintaining and expanding a population of T cells in vivo and selectively targeting them to the tumour.

These approaches generally rely on genetic modification of T cells. These cells can be modified to express chimeric antigen receptors (CARs), which are combinations of the antigen recognition domain of a specific antibody with T cell-specific co-stimulatory and intracellular signalling domains

Carl H. June from the University of Pennsylvania, Philadephia, Pennsylvania, USA and his colleagues have engineered T cells to express CARs in which the antigen recognition domain is directed to the CD19 protein, which is expressed only in normal and malignant B cells.

This construct, which also includes the CD137 signalling domain and an intracellular domain of the CD3-z protein, was first found to have potent anti-tumour activity in a pre-clinical model. June and his co-workers have now used a pilot clinical trial in three patients with advanced chronic lymphocytic leukaemia (CLL) to show that these "CAR" T cells (CART19 cells) can also have clinical activity against B cell cancers.

Each of the three patients recruited had undergone multiple courses of chemotherapy and biological therapies for their leukaemia, and two were diagnosed with p53-deficient CLL, a type of leukaemia that often has a poor outcome. All patients were treated first with chemotherapy to deplete their own T cells, and then with infusions of the engineered CART19 cells on three successive days. Endpoint assays were performed after four weeks and the patients were monitored monthly for six months and then quarterly until two years post-treatment.

The patients' specific immune responses and T cell functions were tested through analysis of the levels of 30 different cytokines and related molecules in blood and serum, and CART19 cells were monitored and tracked through the patients' blood and bone marrow using quantitative PCR.

In all patients, blood levels of CART19 cells expanded from 1000- to 10,000- fold within one month after treatment; the cells also trafficked successfully to the bone marrow, although in fewer numbers. Cell numbers persisted for at least six months, and levels of 11 of the 30 immune-related molecules tested were modulated as CART19 cell numbers peaked. Peak cell numbers, observed between one and three weeks after infusion, also coincided with clinical toxicity that was generally short-term and reversible.

Most importantly, the infusions of CART19 cells demonstrated profound anti-leukaemic effects, with two patients experiencing complete responses with no observed circulating leukaemia cells and one a partial response; all responses lasted more than eight months. The researchers calculated that, on average, each CART19 cell targeted and destroyed at least 100 leukaemia cells. Most importantly, a proportion of the cells were found to persist as memory cells, maintaining a potential long-term response.

All subsequent clinical side effects were minor and transient apart from hyper-gammaglobulinaemia. This results from the depletion of normal B cells and is thus an on-target side effect, as the CART19 cells target the CD19 protein found on normal as well as leukaemic B cells. Levels of plasma cells were also found to be depleted in all patients throughout the trial, which is consistent with the observation that most of these cells express the CD19 antigen.

June and his co-workers concluded that, even though this pilot clinical trial was very small, it provided enough evidence to suggest that this type of therapy has the potential to benefit many patients with refractory CLL and that larger trials should be carried out. One feature of the therapy that is of particular utility is that it is not restricted by the particular MHC molecules that a patient carries.

Reference

Kalos, M., Levine, B.L., Porter, D.L., Katz, S., Grupp, S.A., Bagg, A. and June, C.H. (2011). T Cells with Chimeric Antigen Receptors Have Potent Antitumor Effects and Can Establish Memory in Patients with Advanced Leukemia. Science Translational Medicine 3, 95ra73. doi: 10.1126/scitranslmed.3002842