Some CAR T therapies are approved and some are being investigated in clinical trials.

Adoptive Immunotherapy for Several Cancer Types

 

Autologous CAR T cell therapies are created from the patient’s own T cells. The production processes used to create CAR T cell therapies are specific to each product, which makes each CAR T cell therapy different.

CAR T cell therapy involves the infusion of T cells that have been genetically engineered to express a chimeric antigen receptor (CAR) to reprogram the T cells. The CAR combines the specificity of a monoclonal antibody with the cytotoxic and memory functions of T cells.1

CAR specificity comes from the extracellular domain, which is derived from the antigen-binding site of a monoclonal antibody.2,3 The intracellular domain attempts to recapitulate the normal series of events by which T cells are activated and incorporates stimulatory and co-stimulatory domains, such as CD28 or 4-1BB (CD137), to augment CAR T cell survival and proliferation.1,4-6

Since CAR T cells carry their own co-stimulatory signaling, they may be less susceptible than unmodified T cells to the negative regulation from tumors.1 Unlike T cell receptors, CAR T cells do not rely on dendritic cell antigen processing and presentation.7 CAR T cells may multiply and differentiate into central or effector memory cells,6,8,9 which may be associated with enhanced survival for CAR T cells.6,8 At the same time, CAR T cells are susceptible to tumor evasion mechanisms such as the IL -4 and TGF-ß cytokines, which promote tumor growth and limit effector T-cell function.2

Anatomy of a chimeric antigen receptor (CAR)

Image of CAR T cell highlighting the target antigen, extracellular space/hinge region, transmembrane domain, and intracellular co-stimulatory domain
ig1 ig2 ig3 ig4

1.Target Antigen

A variety of antigen types have been investigated in several cancer types.10 Potential antigen targets have included CD19,10-12 tumor necrosis factor receptor superfamily member 17 (TNFRSF17, or BCMA),13 CD20, CD22, CD23, ROR1, and the kappa light chain.14

2.Extracellular Antigen-binding Domain

The extracellular antigen-binding domain is derived from a monoclonal antibody and recognizes a specific antigen. A single-chain variable fragment is generated by linking the variable region heavy and light chains. This domain allows the T cell to bind to the antigens and does not rely on dendritic cell antigen processing and presentation. Binding of the targeting domain to the target antigen triggers T cell activation.10,15

Extracellular Spacer/Hinge Region

Connecting sequence between the extracellular antigen-binding domain and the transmembrane domain. Differences in the length and flexibility of the resulting CAR can affect CAR T cell function.16,17

3.Transmembrane Domain

Structural anchors that may influence CAR expression on the T cell surface or the function of CAR T cells—potentially by affecting CAR T stability or signaling.2,7

4.Intracellular Co-stimulatory Domain

These domains provide the co-stimulatory signal required for full T cell activation. Tumor cells can induce anergy (lack of response to antigen under optimal conditions of stimulation); CAR design aims to provide sufficient co-stimulatory signals for T cell activation. Signaling through the co-stimulatory domain may increase survival, proliferation, and function of CAR T cells.7
These topics are discussed in more detail below in CAR T cell Expansion and Persistence.

Intracellular Signaling Domain

The intracellular portion of the CAR also includes the signaling domain, typically the TCR complex CD3 zeta.
 

How CAR T cell therapy is thought to work

Evidence suggests that CAR T cell therapies stimulate a T cell response against antigen-expressing cells, including normal and malignant cells. The external targeting domain binds to the antigen, activating the CAR T cell. Once activated, CAR T cells release cytokines and other soluble mediators that may play a role in the killing of antigen-expressing target cells and normal cells.10

 

CAR T cell expansion and persistence

Expansion and persistence of CAR T cells in the body are linked to several important factors.6

Figure of T cell and chimeric antigen receptor co-stimulatory domains 4-1BB and CD28

Adapted from Kawalekar OU, et al. Immunity. 2016;44:380-390.6

Co-stimulatory signaling

Co-stimulatory signaling is thought to influence T cell expansion, metabolic profile, persistence, and subset composition.6 All approved CAR constructs currently available use 1 of 2 co-stimulatory domains: CD28 or 4-1BB.16 Investigational co-stimulator domains currently in preclinical and clinical studies include inducible co-stimulator (ICOS), OX40, CD27, CD28, and 4-1BB.14

CD28 co-stimulatory signaling in preclinical studies has been associated with6,18-20:

  • Higher proportion of effector memory T cells, which rapidly differentiate into short-lived effector T cells
  • Short persistence

4-1BB co-stimulatory signaling in preclinical studies has been associated with6,18,19:

  • Higher proportion of central memory T cells, which are less differentiated and more able to proliferate than effector memory cells
  • Slower, sustained effector function
  • Greater persistence6

T cell selection and composition

CD4 and CD8 T cells are functionally distinct subsets that differ in their ability to proliferate and persist in the body.21

A mix of effector and memory cell subtypes is essential to immune response.22 Distributions of T cell subtypes can vary widely among patients due to disease and treatment-related factors.8,23

CAR T Products Are Manufactured From Less Differentiated T Cells24

Less differentiated T cells are believed to have the proliferative potential needed for long-term persistence24

Figure that shows how less differentiated T cells have the proliferative potential needed for long-term persistence Figure that shows how less differentiated T cells have the proliferative potential needed for long-term persistence

Ex vivo expansion

CAR T cells are expanded, or grown, outside the body to an appropriate therapeutic dose.25 The impact of CAR T cell dose on safety outcomes is unknown and may differ between products and disease states.

Learn about CAR T cell therapy administration and monitoring >
Learn more about toxicities associated with CAR T cell therapy >

Explore more with the informational modules of CAR T Academy.

For downloadable content on MOA, visit CAR T Academy

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