By Edgar Wex
Our immune system is the biological army of cells that defend us from invaders, and they are constantly being used and regulated to protect us. One potent enemy of it is cancer: a very common disease that is characterised by several hallmarks, mainly genetic mutations causing too much cell division, forming a tumour. Cancer cells themselves are not too powerful, and instead rely on manipulation of the immune system through various molecular weapons that make them invisible, and trick the immune system into helping it grow. However, this occurs very rarely, and 99.9% of the time, the immune system kills cancer cells hundreds of times a day like swatting a fly - and this is how they do it!
Being the second most complex biological system in the body (the brain being the first), the immune system contains a huge number of signaling pathways that help regulate the cells within it. These can be inflammatory (to increase cell activity and/or cell division) or suppressive (to send death signals or prevent further cell activity). These pathways are crucial to our survival, as they allow the immune system to adapt to various situations; for example, to facilitate defense where an aggressive response is needed, but also while minimising damage to the body.
The crucial mechanism by which deadly immune cells recognize friend from foe is the use of checkpoint molecules. These molecules act like ‘don’t eat me!’ signs, and are expressed on healthy cells - when they bind to an immune checkpoint receptor on an immune cell, cytotoxicity (attack) will not occur. However, pathogens which lack these checkpoint molecules indicate outside invasion, and are (usually) found by the immune system, and destroyed.
When a cancer cell is detected, the adaptive immune system steps into the spotlight; an Antigen-presenting Cell (APC), such as a dendritic cell, will recognise Death-associated Molecular Patterns (DAMPs) released by dying cancer cells. This will stimulate the dendritic cell to pick up tumour antigens. These antigens are the “tags” cells present to identify them. The dendritic cell then takes the tumor antigen and displays it on a peptide-MHC complex, called MHC II. The name ‘MHC’ means the ‘Major Histocompatibility Complex,’ which is a group of genes coding for recognition proteins - these code for proteins that display peptides (processed antigens) on the cell surface, for lymphocyte receptors. The MHC proteins help display these peptides to communicate with lymphocytes and help them decide what to attack. Then, the dendritic cell will then express a chemokine receptor (chemokines are signaling molecules that give cells instructions on where to go), which will bind to chemokines and direct the dendritic cell, carrying the peptide “tag” into a lymphatic vessel.
Once in a lymph node, it will present the antigen along with B7 (a costimulatory molecule) to a matching T lymphocyte. There are a huge number of these T lymphocytes in lymph nodes which wait there for instructions - they must interact with a dendritic cell to develop into their full form; an effector T cell. Note that there are two types of B7; CD80 and CD86 - their functions are very similar. CD means ‘Cluster of Differentiation’, and the numbers associated with CD molecules assign them within a standardized system for naming cell surface proteins to keep track of them and their uses. The numbers only signify the order of discovery.
This T cell expresses CD3 (the T cell receptor that recognizes the antigen) and CD28 (a costimulatory molecule) to join with the dendritic cell’s B7 - creating the most important of the many interactions that occur between the two cells at this time. This will prime (1st activation) the naive T cell to become an effector T cell (one that has survived development and is ready to perform its function). Once enough cell division has occurred, the T cells will begin expressing the higher affinity co-inhibitory molecule CTLA-4, which displaces CD28, binds B7 and prevents further cell division, and thus potential for autoimmunity (when the body attacks itself).
After this, in order to fight cancer properly, the T cell (if it is the helper T cell phenotype), should be exposed to IL12 (IL meaning interleukin, another system of naming signaling molecules). This will allow it to differentiate into a Th1 T cell, and create a Th1 immune response, which is the most powerful/deadly type, desired for anti-cancer immune activity. If it becomes one of the other, less powerful types (Th2 and Th17) it won’t want to attack cancer, and if it becomes a Th3 cell, it will actually inhibit the immune response and help the cancer.
The activated T cells may become memory cells, used to make more cells later in case the cancer resurfaces, or migrate to the tumour via blood vessels, while being guided by chemokines. Helper T Cells (CD4 positive) can also secrete IL2 to increase clonal expansion (when the cells reproduce to increase population size) and differentiation of Cytotoxic T cells (CD8 positive), or differentiate into Follicular Helper T cells (Tfh) through a process called the germinal center reaction. These Tfh cells can help B cells produce higher affinity antibodies, and to develop and differentiate into plasma or memory B cells. Plasma cells will release tumour antigen-specific antibodies, which will bind to the cells, recruiting complement (a cascade of proteins which lead to membrane puncturing and cell death), and inducing antibody-dependent cellular cytotoxicity (ADCC) by tagging cancer cells, so that other immune cells can recognize them.
At the tumour site, Helper T cells must recognize their specific antigen again - this is provided by local APCs, also on MHC II. This step is necessary, as if all effector T cells were continuously releasing inflammatory molecules we would all be dead due to autoimmunity. The Helper T cells then perform their function (promoting other immune cell functions) and indirectly activate Cytotoxic cells through APCs. Once these are activated, they will kill any cell that expressed their specific antigen on MHC I, through the release of perforin and granzymes to puncture the cell membrane and induce apoptosis. They also express FasL (Fas Ligand), which binds to the Fas molecule on a target cell to aid apoptosis, and release some pro-inflammatory molecules like TGFa (transforming growth factor alpha), TGFb, and interferon gamma (IFNy).
Other than activating Cytotoxic T cells and B cells, the Helper T cells will also release cytokines to act back on the APC to further activate it. For example, macrophages can be made into more effective tumour killers through secretions like IFNy, and upregulation of CD40L production, which ligates the APC’s CD40, will also stimulate it. Activated macrophages will in turn release inflammatory cytokines like tumour necrosis factor (TNF) and IL1 to improve leukocyte ability. An interacting dendritic cell will also directly activate a Cytotoxic cell.
Another prominent type of immune cell involved here is the Natural Killer cell; a cytotoxic lymphocyte which specializes in virally-infected or cancerous cells. It does this by recognizing the absence of MHC I - this is expressed on the vast majority of healthy cells (an exception would be the red blood cell/erythrocyte, for example) - this is a sort of passport check for cells. While cells function normally, they perform proteolysis (the breakdown of proteins in the cytosol into peptides or amino acids); these proteins can be attached to MHC proteins, and are sent off for expression on the cell surface. If the cell is healthy and functional, the antigen will not alert immune cells, while if it is damaged or infected/foreign, an alarm is raised. One of the most simple methods of immune evasion is to cease the expression of MHC I. This means normal lymphocytes cannot bind to it and recognize the antigen (as their receptors require MHC), and so the cell is effectively invisible. This ‘invisibility’ is arguably the biggest part of what makes cancer cells so dangerous, as without it the immune system would have no problem dealing with it.
This is where the Natural Killer (NK) cells come in. These are another type of cytotoxic lymphocyte, however is unusual in that it is not antigen-specific, meaning its function is more similar to that of an innate immune system cell. Natural Killer cells contain activating receptors, which will alert the cell if danger is present, and inhibitory receptors - Killer Immunoglobulin-like Receptors (KIRs) - which bind to MHC I. Due to the inhibitory override, if an NK cell binds to a cell displaying MHC I, it will not activate, and the cell will be left alone. But if a cell fails to express MHC I (and also healthy non-MHC-expressing cell checkpoints, like CD47), a Natural Killer cell will be alerted. Cancer cells with no MHC (no inhibition) and sufficient activating ligands, therefore, will trigger an immune response. Killing is done in a similar way to Cytotoxic T cells; that is, through release of perforin, granzymes, and FasL expression. NK cells can also use their antibody receptor (FcyRIIIA) to bind to antibody Fc stems - this binding event has a large enough activation effect that it will overpower inhibition through MHC, and will lead to antibody dependent cellular cytotoxicity (ADCC). This is where the Fab regions (the two upper arms of the Y shape of antibodies) bind to antigen, and the Fc stem (the bottom of the Y) sticks out, to attract lymphocytes to the site and perform anti-cancer function.
As we all know, cancer is a formidable disease, causing millions of deaths yearly - if the immune system is so good, why can’t it kill all cancer cells? While the vast majority of the time, cancer cells are discovered and killed by immune surveillance, there are unfortunate mutations which sometimes allow them to stay under the radar and continue to grow. Being so incredibly complicated and large, the immune system, with its countless pathways and interactions, is unfortunately liable to a lot of things that can go wrong. However, it also means that there are a lot of opportunities to intervene, with immunotherapeutic treatments, which try to help the immune system fight cancer.
Citations
Image credit: Flickr @ nihgov - https://flic.kr/p/xuSZkh
Other images: Edgar Wex
https://www.news-medical.net/life-sciences/Antibody-Dependent-Cell-Mediated-Cytotoxicity-(ADCC).aspx
What did you learn?
How is a T cell activated so that it can fight cancer?
T cells are first primed by an antigen presenting cell, in the lymph node. The T cell needs to receive specific tumour antigen on MHC II, and a co-stimulatory molecule, B7, by interacting with a dendritic cell. It will then undergo clonal expansion. After some time, CTLA-4 displaces CD28 from B7, and stops further activation. Effector T cells can become a memory cell, activate other cells, or migrate to the tumour site. At the tumour, T cells need further antigen exposure, and interact with APCs again.
What do Natural Killer cells do?
Natural Killer cells selectively kill cells that have lost expression of MHC I, and are trying to hide from the immune system. It achieves this by having an inhibitory receptor that binds MHC I, and an activating receptor that recognizes signs of danger. This is particularly useful in cancer as many mutations occur, and MHC loss is quite common. NK cells can also bind to antibody Fc stems to perform ADCC, killing cancer cells.