Chapter 40: Immune Cell Therapy: Genetically Engineered T Cells

GENERAL ASPECTS OF T-CELL IMMUNOTHERAPY

• The potent antitumor activity of T cells against many hematologic malignancies is illustrated by the graft-versus-leukemia effect of both allogeneic hematopoietic stem cell transplantation and donor lymphocyte infusion (see Chap. 39).

• Autologous T cells recognizing tumor neoantigens are found in the microenvironment of many cancers.

• These tumor-specific T cells are often downregulated by the expression of the tumor cells of a range of immune checkpoint molecules.

• Attempts to expand tumor-reactive T cells in vivo using interleukin (IL)-2 infusions have met with some success in selected solid cancers such as melanoma but have not been very successful in the hematologic malignancies.

• Attempts to expand in vitro and subsequently reinfuse tumor-infiltrating T cells (TILs) has had considerable success in melanoma and some other solid tumors but is logistically highly demanding. Continued trials are in progress.

• The use of ex vivo expanded Epstein-Barr virus (EBV)-specific T cells has proven efficacious in EBV-driven lymphoproliferative disorders.

• Infusion of bispecific T-cell engagers (BiTEs) and trispecific T-cell engagers (TriTEs) enables a wide range of previously nonspecific T cells to be recruited to tumor cells, leading to tumor cell death. They are effective in a number of hematologic malignancies.

• An alternative strategy to recruit a wider pool of autologous T cells to tumor cell killing is to genetically engineer T cells, harvested by leukapheresis, so that they recognize a tumor-selective antigen. This can be done by transfecting these cells either with a specific T-cell receptor (TCR) or with a chimeric antigen receptor (CAR).

CHIMERIC ANTIGEN RECEPTORS

• A CAR is an artificial type I transmembrane protein that has an amino-terminal extracellular domain (ectodomain), a transmembrane domain, and a carboxyterminal intracellular domain (endodomain) (Figure 40–1).

• The ectodomain contains the antigen-binding moiety conveying the novel specificity and is usually a single-chain variable fragment (scFv) of a monoclonal antibody raised against the target antigen.

• The endomain is the signaling domain and is usually made up of CD3ζ (signal 1) and a co-receptor signaling moiety (signal 2) such as CD28 or 41BB. These second signals are required for full activation and for in vivo proliferation of the transduced T cells after reinfusion into the patient.

• Transfection of CARs is most commonly performed with retroviruses, either γ-retroviruses or lentiviruses.

• Alternative strategies for transgene insertion are under development.

• After transfection, the transduced T cells are expanded in vitro (Figure 40–2) prior to cryopreservation. The manufacturing period can take several weeks, and there is then a further interval of up to 2 weeks between cryopreservation and reinfusion, during which time the quality control checks are performed.

• Lymphodepleting chemotherapy must be given before the T-cell infusion to facilitate T-cell engraftment. A combination of cyclophosphamide and fludarabine is usually given.