Mobilising T cells against Cancer
The first step in manufacturing T cell immunotherapies against cancer is T cell activation.
The immune system is our military, defending our body from invasion — both external (infection) and internal (cancer).
Our army of white blood cells course through our veins; they are immune cells with various roles that patrol our body in search of invaders. The most powerful of these soldiers are perhaps our T cells. T cells can identify their target with high specificity, rally other immune cell troops, and deliver lethal attacks.
T cell Immunotherapies
Cancer treatment has evolved from surgery (excision of tumour) to radiation (use of high-energy waves to damage cells), chemotherapy (use of drugs to damage cells), targeted therapy (more precise use of drugs to attack cancer cells), and immunotherapy (use of your own immune system to fight cancer). T cell therapy is a subset of the relatively new class of cancer immunotherapy which hopes to achieve even more potency and precision (and hence safety) in fighting cancer.
The principle of T cell immunotherapies is to stimulate the inherent ability of T cells to fight cancer. When rogue and potentially malignant cells occur in our bodies, our immune system is usually capable of eliminating these threats. In cancer patients, their cancer has somehow evaded their immunosurveillance. Understanding these immune evasion tactics have won some people Nobel Prizes.
The current array of T cell therapies involve isolating T cells from patients or donors then stimulating the cancer-specific T cells ex vivo for infusion of the enhanced T-cells back into the patient. Be it Tumour-Infiltrating Lymphocyte (TIL) therapy, Virus-specific T (VST) cell therapy, T Cell Receptor (TCR) T cell therapy, or Chimeric Antigen Receptor (CAR) T cell therapy, these therapies have been developed and are evolving thanks to our understanding of how T cells work.
Origins — T cell development
T cells, like all our blood cells, originate from the bone marrow. They continue to mature in the Thymus, hence their name “T” cells.
The thymus is where we generate our diversity of T cells — each T cell has a slightly different T cell receptor (TCR) that can recognise one of many different patterns of foreign particles known as antigens. These antigens could belong to a virus or a cancerous mutant protein as long as it does not belong to our healthy cells too.
After completing their training in the thymus, these fresh recruits also known as naive T cells patrol the body waiting to encounter its target antigen for the first time.
Target Acquired — T cell Activation
When our body’s cells become damaged, infected or mutated, they display signs of their internal problems on their surface to alert passing T cells. These signs are displayed on proteins called Major Histocompatibility Complex (MHC) molecules which have a groove that can bind enemy or mutant antigens. T cells inspect these MHC molecules with their TCRs.
MHC molecules come in two types: MHC Class I molecules are expressed on most cells to alert T cells of intracellular enemies, while MHC Class II molecules are expressed on specialized antigen presenting cells (APCs) which have an additional job of phagocytosing extracellular antigens and then presenting them to T cells.
3 Signals for T cell Activation
In order to activate T cells to perform its effector functions, the T cell requires 3 main signals — for both in vivo and ex vivo activation.
Signal 1 — Antigen Receptor Engagement
- When the T cell encounters its target antigen on the MHC molecule of another cell, the TCR complex binds strongly to the MHC-antigen complex to begin the formation of this immunological synapse.
- On the T cell side, the TCR complex comprises the antigen-binding chains (TCR-α and TCR-β), signaling subunits (CD3 and ζ chains) and CD4 or CD8 co-receptors.
- In general, T helper cells (CD4-positive) recognise antigens on MHC Class II molecules and Cytotoxic T cells (CD8-positive) recognise antigens on MHC Class I molecules.
Signal 2 — Co-stimulation
- The information that the antigen — receptor engagement has occurred needs to be relayed to the intracellular compartment of the T cell. This intracellular signaling supports T cell survival and proliferation.
- This second signal involves the interaction of the B7 molecules (CD80 and CD86) of the target cell to the CD28 co-stimulatory molecule on the T cell.
- This second signal is a verification step to ensure the T cell is recognizing a foreign antigen. Without this signal, the T cell becomes anergic and will soon die as it assumes Signal 1 was a self-antigen.
There are many other co-stimulatory factors that have been discovered that are important to relay the TCR signal into the inside of the cell. Apart from CD28, another molecule 4-1BB (aka CD137) was discovered by Dr. Dario Campana. The 4–1BB molecule shows weaker TCR signalling but the resultant activated T cells are more resistant to exhaustion. With better understanding of the signalling effects of the various co-stimulatory molecules, we may, in the future, be able to select an assortment of molecules for our CAR design to customize the fighting styles of CAR-T cells.
Signal 3 — Signalling Molecules
- Once a naive T cell receives signals 1 and 2, they upregulate receptors that allow it to receive the third signal — cytokines. The most important cytokine is IL-2 which activates the T cell’s proliferation pathways.
Non-specific T cell Activation — A Special Case
- In the manufacturing of CAR-T cells, non-specific T cell activation can be achieved with just antibodies that bind and activate CD3 (Signal 1) and CD28 (Signal 2) of the T cells.
- This allows most of the T cells to become activated and proliferate regardless of their TCR targets.
- Once we have expanded enough T cells, we can introduce the Chimeric Antigen Receptor (CAR) into the T cells. In the case of the three approved CAR-T therapies, the CAR has been designed to target the CD19 molecule.
Launch Attack — Activation leads to Effector Functions
A naive T cell that achieves activation can proliferate and become effector T cells. The activated T cells produce an army of clones all with TCRs that can recognise the enemy.
We have two main T cell battalions: T Helper cells release signals to orchestrate the immune response by rallying other immune troops to the battlefield. And their comrades, Cytotoxic T cells, deliver lethal injections into the enemy cells.
After the eliminating the threat, most effector T cells undergo apoptosis (cell death) but some persist as memory T cells waiting to be activated if the enemy dares to appear again.
Medical history is dotted with doctors who would and could try anything on patients who had few other options. Only in retrospect can we spot the few ideas that have survived the trials of modern science. In the case of Coley’s Toxins, he did not know why infecting patients with bacteria was helping treat cancer. We now know that he was activating the immune system which has the intrinsic ability to recognise and eliminate cancer cells.
Our immunotherapies against cancer are built on our understanding of immune activation. We have come far from the first immunotherapies of the 1900s but as the circle of knowledge of our immune system expands, so does the circumference of darkness that surrounds it. With more basic science, our power to tinker with the very molecular mechanisms of life grows.
Murphy, K., Travers, P., Walport, M., & Janeway, C. (2008). Janeway’s immunobiology. New York: Garland Science.