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Given the complexity of the immune system, many different cellular processes and pathways can be considered for therapeutic intervention when designing cancer immunotherapies. However, before we examine this further, a summary of the immune system as a whole is required. The immune system contains two main arms, the innate and the adaptive. The innate immune system is the body’s first line of defense and acts in a generalized manner. The main aim of the innate immune system is to get the infection under control as quickly as possible hence, although very effective, it lacks a certain finesse. It is composed of different cell types including mast cells, macrophages, neutrophils, dendritic cells (DCs), basophils, eosinophils and natural killer (NK) cells. On the other hand, the adaptive immune system is specialized and acts through targeted killing. It is made up of T cells and B cells, of which there are many different sub-types that have a variety of functions. However, in broad terms B cells are typically involved in immune responses against pathogens while T cells target infected/compromised host cells. T cells can be loosely separated into two groups. CD8+ T cells and CD4+ T cells. CD8+ T cells are called effector cells because their main role is to kill anything that doesn’t belong, while CD4+ T cells are called memory cells because they “memorize” the foreign body so that if there is ever a re-infection, or re-emergence, they can engage more rapidly.
A typical immune response begins with neutrophils and macrophages detecting and recognizing a foreign body. They then become activated and start attacking it, while at the same time sending out signals to recruit and activate other immune cells. Once NK cells are activated they scan the surrounding host cells and destroy any that have been infected/compromised. Meanwhile DCs sample (or eat) bits of the foreign body and migrate to the lymph nodes where they show these pieces to T cells, thus activating them. The T cells then in turn activate B cells and both are subsequently recruited to the site of infection. Once there, B cells secrete antibodies that target the foreign body and initiate further cytotoxic immune responses, CD8+ T cells join the fight by specifically targeting infected/compromised cells, while CD4+ T cells begin the process of memorizing what the foreign body “looks” like. Once the infection has been contained a number of immune cells send out signals to calm down the immune response and induce resolution of inflammation. The destructive immune phase is often referred to as a cytotoxic response, while the resolution of inflammation phase is often referred to as a trophic response. However, it is important to remember that immune responses are highly complex, involve a myriad of pathways and molecules that differ depending on what type of infection or foreign body is detected, and that each cell type can play a number of different roles during immune responses.
Cancers that manage to grow and progress to metastasis do so largely due to their ability to suppress anti-tumor immune responses. These are very similar to what was described above and typically start with an increased recognition of tumor antigens by the innate immune system, particularly by NK cells and DCs. Antigens are defined as any substance that induces an immune reaction. Tumor antigens are specific to tumors and can either be released by the tumor cell or expressed on the cell surface. Recognition of tumor antigens activates NK cells and DCs at which stage the NK cells start killing the tumor cells, while the DCs migrate to the lymph nodes where they activate T cells. The T cells are then recruited to the tumor and initiate cytotoxic anti-tumor immune responses. Tumors can disrupt such immune responses by secreting various agents that; 1) induce immunological tolerance, a process that results in immune cells being unable to recognize and destroy cancer cells; 2) attract immunosuppressive cells which induce trophic responses and hence subdue anti-tumor immune reactions, and 3) alter the attributes of the tumor cells themselves; a process that involves changes in tumor antigens.
Cancer immunotherapies aim to not only overcome immunological tolerance but also enhance cytotoxic anti-tumor responses. A number of different strategies can be employed to achieve this. Adjuvant, or combination, therapies are the most widely known cancer therapies and are used in conjunction with primary therapy to increase the chance of long-term disease-free survival. They are given systemically, allowing the substances to travel and reach tumors throughout the body. Adjuvant therapies include chemotherapy, hormonal therapy and radiation therapy. It is now known that the immune response to these adjuvant therapies can have a great impact on their outcome. For example, chemotherapy and radiotherapy cause damage to the surrounding normal tissues and can result in cytotoxic immune responses or trophic immune responses. Cytotoxic immune responses enhance the tumor killing effects of chemo- and radiotherapies, while on the other hand trophic immune responses have been reported to contribute to relapse and recurrence. Thus, many studies are now assessing the biological pathways behind these immune responses to adjuvant therapies and how these can be modified to support therapeutic efficacy and minimize disease recurrence.
One of the most commonly employed strategies to mount anti-tumor immune responses is the targeting of tumor-associated antigens (TAAs), primarily because these types of therapies are relatively easy to manufacture and typically have low toxicity. The most striking evidence for this type of immunotherapy in breast cancer has been the development of antibodies directed against the HER2/neu protein (Trastuzumab and Pertuzumab). These therapies not only inhibit tumor cell growth, proliferation and survival, but also trigger the recruitment of CD8+ T cells and stimulate cytotoxic immune responses. Cancer cells themselves can also be used to directly elicit CD8+ T cell anti-tumor immune responses. Cells are taken from patients, programmed to express immunogenic molecules (such as antigens) and then re-injected into the patient. Although these therapies are relatively successful in the short term, it is often difficult to determine if the desired immune response has been achieved and if it is long lasting. Given that tumors are constantly working to induce immunological tolerance and suppress cytotoxic immune responses, combination treatments that target TAAs but also enhance and prolong anti-tumor immune responses are more desirable.
An important component of the immune system is its ability to differentiate between normal cells in the body and those that are foreign. If such a system did not exist our immune cells would constantly be attacking normal cells leading to tissue damage and autoimmune diseases. In order to tell the difference between normal and foreign the immune system uses “checkpoints” – molecules on certain immune cells that need to be activated, or inactivated, to start an immune response. Although cancer cells often find a way to use the checkpoints to avoid immune cell attacks they can also be manipulated for treatment of cancer. The most well-known of these treatments target PD-1 or PD-L1, and CTLA-4. PD-1 is a T cell checkpoint protein that acts as an off switch for T cell activation. This is mediated by binding of PD-1 (on the T cell) to PDL-1 (on the target cell) which essentially tells the T cell to leave that cell alone. A number of cancers upregulate expression of PD-L1 which helps them to avoid immune attack. CTLA-4 also acts as an off switch for T cells, and in this case CTLA-4 binding to the CD80 or CD86 molecules mediates the inhibition of T cell function. Therapeutically speaking, blockade of PD-1, PD-L1 or CTLA-4 by antibodies can overcome T cell suppression. Current drugs employing this method include Pembrolizumab, Nivolumab (PD-1 and PD-L1) and Ipilimumab (CTLA-4). These treatments have shown promising results in melanoma patients and are now being tested in other cancers. In breast cancer early reports have demonstrated that blocking the PD-1 pathway has clinical activity in triple negative breast cancer (TNBC) patients.
Dendritic cell (DC) therapies also aim to initiate CD8+ T cell-mediated anti-tumor responses but, in addition, have also been shown to generate CD4+ T cell responses meaning that such therapies can induce immunological memory and potentially control tumor relapse. For this therapy DCs are provided with tumor-specific antigens (such as HER2 or p53); this can be done by removing the DCs from the patient, exposing them to the antigen, and then re-injecting them back into the patient, or by inducing the DCs to take up the antigen systemically. These primed DCs can then initiate anti-tumor T cell responses. These therapies are potent but technically and logistically challenging as they require specialized laboratories. Current work in this field aims to make these therapies more accessible.
Although the therapies described above can be highly effective and can achieve long-term effects in many cases of solid tumors, their application is limited to tumors that present tumor-associated antigens. In addition to this, it appears that the presence of major histocompatibility complex class I (MHC-I) is required. MHC-I is another molecule that mediates activation of cytotoxic T cell responses. Tumor cells that do not express specific antigens and/or MHC I are not recognized by CD8+ T cells and are thus resistant to TAA-mediated immunotherapy. A newly developed therapy utilizing natural killer (NK) cells has shown promising results for targeting cells that are resistant to antigen-based therapies. NK cells are the only innate immune cell type that does not directly destroy pathogens but rather destroys compromised host cells, including tumor cells, and recent studies have shown that NK cells can directly kill MHC-I/antigen-negative tumor cells. NK cells can be easily isolated from patients, expanded and primed to target tumor cells. Such therapies have shown promising results in patients with lymphoma, melanoma, renal cell carcinoma, and breast cancer.
Recently, a very exciting new therapy has been proposed where immune cells, such as macrophages, are loaded with viruses that specifically kill tumor cells and also with magnetic nanoparticles. These pre-loaded immune cells can then be directed from the blood stream into tumor sites using magnetic resonance. Preliminary studies have shown that this therapy increases immune cell infiltration into the tumor and reduces not only tumor burden, but also metastasis. The majority of these studies are preclinical, but clinical trials are now starting in prostate cancer patients. One of the main limitations for cancer immunotherapies is that a vast proportion of immune cells, whether they are re-introduced into the patient or activated in vivo, will migrate to the lung and get stuck there. If successful, magnetic resonance therapy could overcome this problem.