Modified 25 October 2023 by

How does CAR T cell therapy work?

Whilst the treatment is simply one infusion given to the patient, the overall process of CAR-T therapy is highly specialised as each treatment is customised for each patient. The entire process, from the design of each therapy to the delivery in the patient, is comprised of various stages- as follows:

In the clinic
In the lab
In the body

During the manufacturing process in the lab, the patients’ T-cells are made into CAR-T cells- before being infused back into the patient. To understand how the CAR-T cells are able to fight cancer cells once they’re inside the patients’ body, it’s important to know more about how and why the CAR-T cells are made this way and what makes them unique in fighting cancer specifically for the patient.

Why are CAR-T cells made to fight cancer?

During an immune response to cancer cells, usually antigens are broken down into smaller protein units. Then they’re presented on the surface of antigen-presenting cells (such as B cells) by a protein called MHC (major histocompatibility complex)-so that they can be detected by T-cells.

CAR-T cells remove the need for antigen-presentation by MHC proteins and are instead facilitated by CAR-T cell receptors. The CAR-T cell receptors, which are designed to specifically target the tumour cell antigens, enable novel T cell specificity for particular types of cancer cells whereby recognition is independent of the MHC protein.

How are CAR-T cells made in the lab?

Making CAR-T cells:

T cells, taken from the patient, are infused with DNA that specifically codes for a CAR protein that will target the patients’ cancer cells. Inside the T cell, the CAR protein is formed and expressed on the surface of the T-cell like a synthetic receptor-making it a CAR-T cell (as described below). The cancer targeting CAR-T cells are then multiplied before re-infusing them into the patient.

A receptor is a type of protein which is naturally found on many different cells of the body. It acts as a hook for other specific proteins called antigens, found on the surface of foreign disease causing cells. Once the receptor and antigen bind together, a specific effect is produced inside the cell. Similarly, when CAR-T cell receptors bind with antigens on cancer cells, it stimulates a series of immune cell reactions that eventually kill cancer cells.

Structure of CAR-T cells:

CAR-T cells are made from T-cells donated specifically by the patient to be treated, referred to as autologous cells. Alternatively, many studies are looking at employing T-cells that are donated by healthy volunteers which are then modified into CAR-T cells for cancer patients, referred to as allogeneic cells.

The design of a unique CAR receptor is an important step, as it allows T-cells to recognise and attack specific cancer cells. Classical CAR receptors are made up of three different regions, each with a particular function:

Outside the T cell (extracellular domain: CAR and spacer)
Across the boundary of the T cell (transmembrane domain)
Inside the T cell (intracellular domain)

The region of the CAR which is on the outside of the cell (extracellular domain) is made from part of an antibody called single-chain variable fragments(scFV) and a spacer. The role of the extracellular domain is to direct the CAR-T cell towards a specific cancer antigen target. The scFv antigen-binding region is able to detect and bind with specific antigens on cancer cells, such as CD19/BCMA. When this happens, the activated CAR-T cells prompt signals from the internal part of the receptor (intracellular domain), which triggers the CAR-T cell to release tumour cell killing molecules.

An optimal CAR-T cell therapy depends on the strength of the interaction between the antigen on the cancer cell and the CAR, in addition to the specific complementary bond between the cancer cell antigen and the CAR. Therefore, there is much emphasis on the design and development of

the CAR extracellular domain, which impacts the efficacy of the therapy.

The spacer which connects the transmembrane domain to the antigen-binding region, gives flexibility to the scFV. The length of the spacer, which can be long or short, also enhances the ability of the CAR to recognise the binding region of specific cancer antigens.

The intracellular part of the CAR-T cell receptor plays an important functional role. It can consist of at least two or more proteins (depending on the version of CAR-T cell) involved in sustaining a cascade of signals required for an efficient antitumour response. Not only does the intracellular region influence the activation of the CAR-t cell, but it also has an impact on how long CAR-T cells last after they’ve been infused into the patient and how well they proliferate inside the patient.

The novel structure and design of CAR-T cells enable several cancer cell antigens to be targeted at the same time. This is beneficial for tumours that express varying levels and types of surface antigens on their cells (antigen heterogeneity) and allow the therapy to be adapted when tumour variants sometimes appear. In addition, after infusing the patient with CAR-T cells, the structure of CAR-T cells can be manipulated so that the immune response can be stopped early if needed, particularly in the event of severe side effects developing.

CAR-T cell targets

Many blood cancers are caused by cancerous white blood cells-called B cells. Whilst the job of T-cells is to recognise and destroy cells with foreign antigens, they can’t always recognise the antigens on cancerous B-cells. Approved CAR-T cells have been modified to be able to specifically recognise and kill cancerous B-cells which express one of two different antigens:

Cluster of Differentiation 19 (CD19)
B-cell maturation antigen (BCMA)

The most common CAR-T cell targets are CD19, which are specifically expressed on B cells. The B-cell antigen CD19 has, to date, proven very effective in trials for acute lymphoblastic leukaemia (ALL) and some types of lymphoma. CD19 positive cancers include B-cell cancers with high priority medical needs such as:

B-cell acute lymphoblastic leukaemia (B-ALL)
B-cell chronic lymphocytic leukaemia (B-CLL)
B-cell Non-Hodgkin lymphoma (B-NHL)

BCMA was discovered to play a role in the progression of multiple myeloma (MM) in 2004 and has led to many clinical trials investigating their value as an appropriate target for CAR-T cell therapy. BCMA is the only target other than CD19 to be used to develop an approved CAR-T cell therapy- Abecma and Carvykti.

There are numerous ongoing clinical trials which are investigating alternative cancer targets for CAR-T cell therapy, in addition to CD19 and BCMA.

What happens when CAR-T cells are inside the patient?

Upon infusion, full activation of the CAR-T cells and producing an anti-tumour response relies on three separate signals:

CAR binding with a tumour cell antigen
Signalling from the intracellular domain
Release and cascade of tumour cell-killing toxins/enzymes

Once the region of the CAR-T cell receptor situated on the outside of the cell binds to the tumour cell surface antigen, it activates the CAR-T cell to generate a response that will eventually kill cancer cells. A series of signals from the internal part of the CAR-T receptor upon activation, leads to the release of toxins and enzymes from the CAR-T cell. These molecules act on the tumour cell(s) to subsequently cause tumour cell death. The activated CAR-T cells continue to detect cancer cells and eliminate them, hence the therapy is sometimes referred to as a “living drug”.

How does it work?

How does it work?