T Cells, the Human Immune System and the Role of Ras Targeted Immunotherapies

The human immune system is the body’s scanner and defense against infectious organisms and other invaders. It works on two levels: through the innate immune system and the adaptive immune system.

The innate immune system is always active and responds to general threats such as invading microbes or debris from dead cells after injury. The advantage of the innate immune system is that it is always active and responsive. The disadvantage is that it is largely non-specific.

The immune system has its “front line soldiers”  – cells which are non-specific, such as macrophages. These will attack a threat from, for example, cancer cells without knowing anything about them. This is the innate immune system.

The adaptive immune system mounts targeted responses to specific threats. It is slow to be activated, sometimes taking weeks or months for an adaptive immune response to be fully active. Once activated, however, it is extremely effective at eliminating a specific threat such as a virus or, in this case, cancer.

All human cells showcase on their surface a sample of the proteins of which they are made. The proteins are broken up into short peptides (i.e. protein fragments), and displayed on the cellular surface by MHC (Major Histocompatibility Complex) class 1 protein complexes.

Normal cells display a range of normal peptides on their MHC class I complexes, which do not trigger a reaction by CD8+ cytotoxic T cells (cells of the adaptive immune system that can kill other cells if needed).

Cancer cells carry mutations in some genes and thus produce mutated proteins. They display on their surface mostly normal peptides, but also some abnormal peptides. If the CD8+ cytotoxic T cells recognize an MHC class I complex when it presents an abnormal peptide, the T cells will kill the cancer cell. This is how, it is believed, the immune system eliminates most cancer cells. In fact, it is likely we all experience cancer every now and then but are not aware of it because these emergent cancer cells are quickly being cleared out by the immune system.

Sometimes the mutations of the cancer cell do not drastically change the shape of the proteins and the cancer cell displays thus a range of only slightly abnormal peptides on its surface. Cancers with RAS mutations are one such example. CD8+ cytotoxic T cells do not easily recognize these slightly abnormal RAS peptides and therefore are not able to eliminate RAS mutated cancer cells, as for example in pancreatic cancer. Here, the CD8+ cytotoxic T cells need help to learn to recognize RAS mutated peptides. This is where Targovax comes in.

Targovax’s lead peptide-based targeted immunotherapy TG01 is a collection of 7 mutated RAS peptides, which are injected into the skin just like most targeted immunotherapies. TG01 is administered together with Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF), a substance that boosts immune reactions.

T-cells attacking a cancer cell. Targovax is developing immuno-therapies aimed at stimulating the immune system to produce cancer specific T-cells. We expect that their effect will be enhanced further by Checkpoint Inhibitors (CPIs).

The TG01 peptides are then taken up by Antigen Presenting Cells (APCs) and dendritic cells in the skin, which carry the TG01 peptides into the thymus and into lymph nodes - the immune system power hubs. There, the APCs present the RAS peptides of TG01 to naïve T cells on a MHC class II protein complex. Naïve T cells, once confronted with a specific antigen (in this case one of the mutated RAS peptides of TG01), differentiate to become CD4+ CD8+ double-positive T cells. Some of them later become single-positive CD8+ cytotoxic T cells, while others become single-positive CD4+ helper cells.

As their name implies, CD4+ helper T cells are helpful indeed: they secrete growth factors to provide the right environment for CD8+ cytotoxic T cells to proliferate. They live a long time, some a lifetime, and form a lasting memory of the immune system: when the immune system is challenged a second time with the same specific threat, the adaptive immune system will be ready to intervene at short notice. And the CD4+ helper T cells down regulate excessive immune responses in order to prevent auto-immunity.

In short, CD4+ helper T cells are an essential part of a well-calibrated immune response. An important twist to this story is that the MHC class II complexes require peptides of about 17-25 amino acids (a peptide is a short string of amino acids), while MHC class I complexes can recognize peptides of 8-10 amino acids long. MHC class II complexes are needed to generate CD4+ helper T cells and specific CD8+ cytotoxic T cells. Importantly, the RAS mutated peptides in TG01 are sufficiently long as to be recognized by both MHC Class II complexes as well as the MHC class I complexes. The peptides are degraded by the cells and the shorter degradation products can then be recognized by MHC class I complexes. This way, Targovax’s RAS peptide-based targeted immunotherapies activate both CD4+ helper T cells and CD8+ cytotoxic T cells.

Tumors can avoid the attack from the immune system by putting the brakes on it.

Activated CD4+ helper T cells and CD8+ cytotoxic T cells are often not enough. Tumors are very clever at putting the brakes on the immune system. Activated CD8+ cytotoxic T cells are prevented by the tumor from taking action against the cancer cells by various means (outside of our main interest) but there appears to be a solution to this problem. Recently discovered immunomodulatory drugs called Immune Checkpoint Inhibitors (CPIs) remove the brakes from the immune system. It would therefore be advantageous to explore combinations of Targovax’s RAS targeted immunotherapies with CPIs in future clinical trials in additional indications. For TG01’s initial target indication - patients with resected (i.e. surgically removed) pancreatic cancer - there is no tumor left to suppress the immune system, so a monotherapy approach is a logical one. The purpose of targeted immunotherapy in this patient population is therefore to prevent the cancer from coming back.

CD8+ cytotoxic T cells attack and destroy the tumor when the tumor can no longer offer resistance to them, or when CPIs have removed the tumor’s ability to put the brakes on the immune system. The innate immune system then comes into play: clearing away the debris from dead cancer cells and generating an inflammatory microenvironment to attract even more immune cells to the tumor. A virtuous cycle thus starts that leads to the elimination of the tumor.