In 2018, cancer is predicted to be the cause of 609,640 deaths in the United States alone. Cancer cells are characterized by uncontrollable growth. Since tumor cells are able to evade typical immune response, their multiplication is unchecked, causing detrimental effects to the body. There are many treatments for cancer including surgery, radiation therapy, chemotherapy, hormone therapy, and immunotherapy.
Immunotherapy, which uses the body’s natural defenses against disease to fight cancer, is currently at the forefront of cancer research and treatment. While tumor cells develop mechanisms to avoid attack from immune cells, gathering information on exactly how they do this and working around these tactics in order to mobilize the immune system proves to be effective in the treatment of cancer. Four of the main types of immunotherapy include adoptive cell transfer, monoclonal antibodies, immune checkpoint blockades, and cancer vaccines. These therapies reinforce and manipulate the immune system so that it may attack and destroy tumor cells.
One of the main branches of cancer immunotherapy is adoptive cell transfer. This involves the utilization of the patient’s own immune cells in order to fight cancer. Arguably one of the most promising types of adoptive cell transfer is chimeric antigen receptor (CAR) T-cell therapy. In this type of therapy, T-cells are engineered outside of the body to selectively destroy cells with a specific antigen, like CD19 on cancer cells. These CAR T-cells have three main sections: the antigen recognition domain, the transmembrane domain, and the T-cell intracellular signaling domain. The antigen recognition domain is made from an antibody single-chain variable fragment (ScFV), and it is the portion of the CAR T-cell that locates the cancer cells based on the presence of a specific antigen.
The T-cell intracellular signaling domain is responsible for the activation of the T-cell and its response to multiply and kill the target cell using cytokines. Third generation CAR T-cells are composed of CD3ζ as the intracellular signaling domain as well as two co-stimulatory domains such as 4-1BB and CD28. In order to create the CAR T-cells, its elements are cloned into the plasmid of a lentivirus or a retrovirus. The patient’s blood cells are obtained and treated with anti-CD3/CD28 beads. Then, the virus with the chimeric antigen is used to share its DNA with T-cells. After that, these CAR T-cells are cultured and put into a patient for treatment.
This type of immunotherapy and its ability to fight off blood diseases such as aggressive B-cell lymphoma has been widely studied, eventually leading to FDA approval of two CAR T-cell products, axicabtagene ciloleucel and tisagenlecleucel, in 2017. The development of CAR T-cell therapies was necessitated by a lack of options for cancer patients suffering from relapsed or refractory B-cell lymphoma. The prognosis for such people was not good as that, prior to CAR T-cell therapy, they had an overall response rate of only 26%, a complete response rate of 7%, and a median survival of 6.3 months after traditional treatments in a trial. The term complete response refers to the absence of all symptoms after treatment, and it is synonymous with remission. A similar trial was conducted using chimeric antigen receptor cells, and the results were optimistic. The ZUMA trial collected data to evaluate the efficacy of axicabtagene ciloleucel, or axi-cel. The subjects of this trial, similarly to the aforementioned experiment, suffered from chemorefractory disease. This term is defined by the study as “stable or progressive disease as the best response to the most recent chemotherapy regimen or disease progression or relapse within 12 months after ASTC autologous stem cell transplantation”. The diagnoses of these patients were limited to large B-cell lymphoma, primary mediastinal large B-cell lymphoma, and transformed follicular lymphoma. This test had 108 participants, and the best overall response rate was 82% while the complete response rate was 58%. In addition to this, 12 months after the trial, there was a 59% survival rate. Based on the results from these trials, CAR T-cell therapy is the more viable method of treatment for relapsed or refractory B-cell lymphoma than chemotherapy.
While chimeric antigen receptor T-cell therapy showed early success with the treatment of blood cancers, it faced more challenges with solid tumors. Research is currently being done to test the effectiveness of CAR T-cell treatment in other cancers. For example, one study tests the results of CAR T-cells using human sodium iodide symporter (hNIS) to observe the behavior of CAR T-cells in a cancerous host. In addition to the current uncertainty regarding CAR T-cell therapy outside the realm of lymphomas and leukemias, there are other adverse effects to this method. Patients undergoing this immunotherapy are at a high risk of cytokine release syndrome (CRS). This is a strong inflammatory response that could potentially result in death. Overall, CAR T-cell therapy has shown a lot of potential in the treatment of blood cancers. Scientists are currently working to adapt and apply this success to other cancers with solid and liquid tumors. While there are some risks involved in this method of immunotherapy, it can offer an alternative to patients that have few other options, and the adverse effects can be monitored as long as scientists are aware of the possibility of their occurrence. Another method of cancer immunotherapy involves the use of antibodies produced by B-cells that have the structure to target one specific antigen, also called monoclonal antibodies.
Antibodies are Y-shaped proteins used by the immune system to mark foreign molecules for destruction. Produced by B-cells, these antibodies have the specificity to bind to a species with a particular antigen, and attack by T-cells follows. Antibodies have two variable segments at which they bind with antigens, and they also have an Fc component that serves to notify other immune cells. One can immortalize B-cells using the hybridoma technique developed by Georges J.F. Kohler and Cesar Milstein. This method involves the hybridization of a typical B-cell with a myeloma cell, or hybridoma. Then, the fused cells that produce the proper monoclonal antibodies are selected for use.Bispecific antibodies (BsAbs) are a type of monoclonal antibody used for therapy. Trispecific antibodies (TrABs or TrioMabs) and bispecific T-cell engager antibodies (BiTE) are both types of bispecific antibodies. Trispecific antibodies are composed of an Fc segment and two variable segments to bind to antigens while bispecific T-cell engager antibodies do not have the Fc segment for immune cell recruitment. Initially, BsAbs were produced by reducing and re-oxidizing hinge cysteines in monoclonal antibodies. Additionally, the hybridoma technique was used to create hybrid-hybridomas or quadromas that in turn generate bispecific antibodies. However, these methods have changed over time with the development of new technology in recombinant DNA.
Currently, two bispecific antibodies have been approved for treatment of patients: catumaxomab, a trispecific antibody, and blinatumomab, a bispecific T-cell engager antibody. Catumaxomab was approved by the European Medicines Agency for treatment of malignant ascites in people with EpCAM positive cancer. This means that the patients had cancerous fluid in the peritoneal cavity of the abdomen. The therapy was also given orphan drug status by the FDA for EpCAM positive ovarian and gastric cancer. EpCAM, or epithelial cell adhesion molecule, is normally expressed on some epithelial cells, but overexpressed in cancer cells. Catumaxomab is composed of two arms, one binding to the EpCAM antigen located on the tumor cells, and the other binding to the T-cells’ CD3 antigens. The TrAB also has an Fc domain to bind to the receptors of immune cells such as macrophages. Treatment with catumaxomab helped reduce signs of ascites in patients with EpCAM positive cancer, but it also led to abdominal pain, nausea, and vomiting. The other approved antibody, blinatumomab, is composed of an anti-CD19 and anti-CD3 single chain. This therapy has shown promising results in the treatment of relapsed and refractory B-cell precursor acute lymphoblastic leukemia (ALL). Like CAR T-cells, monoclonal antibodies such as bispecific antibodies offer an alternative for late stage cancer patients who do not have many options. However, there are unintended side effects to therapy, and work needs to be done to improve drug effectiveness.Yet another immunotherapy technique is the use of immune checkpoint blockades. The body’s immune system has natural restraints for itself to prevent overactivity and subsequent tissue damage. Tumor cells exploit molecules designed for this function like CTLA-4, PD-1, and PD-L1 in order to evade immune response and continue replicating uncontrollably.
Immune checkpoint blockade therapy counteracts this mechanism of the immune system so that T-cells may work more effectively to destroy tumor cells. Currently, the FDA has approved six different immune checkpoint inhibitors. The validity of immune checkpoint blockades was first shown through studies with mice, as well as observations in a clinic that showed a correlation between autoimmune conditions and decreased tumor size, melanoma regression combined with vitiligo, for example. Scientists then learned that contact of antigens with the receptors on T-cells itself is not enough to guarantee an immune response. Factors such as costimulation and cytokine support are also necessary. The more information consolidated on the inner workings of the immune system and T-cell response, the more drugs can be made to exploit the body’s natural processes, and this is evident through the examination of immune checkpoint blockades.
Several drugs working to counteract control mechanisms of the immune system are currently approved by the FDA. The first of these treatments targets CTLA-4, which limits the multiplication of T-cells and dampens CD8 and T-cell responses of the immune system. This immune checkpoint inhibitor, ipilimumab, was approved by the FDA in 2011. The therapy was mostly effective for treatment of cutaneous melanoma after complete resection and total lymphadenectomy of the regions of the lymph nodes involved with the tumor. This drug also had some small successes in the treatment of other cancers.Later on, the immune checkpoint inhibitor nivolumab that targets PD-1 was approved by the FDA in 2014. This inhibitor is used to treat melanoma without BRAF mutation. In addition to this, nivolumab is also effective in the treatment of a wider range of cancers, including advanced squamous-cell lung cancer, advanced urothelial carcinoma, and advances hepatocellular carcinoma. Also approved by the FDA in 2014, pembrolizumab is used as a second-line treatment for patients suffering from metastatic melanoma that continued progressing even after treatment with ipilimumab.
Survival rates after two years of treatment with pembrolizumab were 55% compared to the 43% survival rate in ipilimumab. Often the most effective treatment is a therapy combining ipilimumab and nivolumab in order to target CTLA-4 and PD-1 checkpoint pathways of the immune system. While there is a lot of promise in immune checkpoint blockade therapy for the treatment of many cancers, the therapy also has some unforeseen side-effects. Since the treatment targets molecules related to immune checkpoint in general and not specifically anti-tumor T-cells, the increased immune activity sometimes leads to the targeting of healthy body tissue. This can cause dermatological conditions like pruritus and mucositis as well as gastrointestinal distress. Less commonly, hepatotoxicity, pneumonitis, and even neurotoxicity can result. These side-effects tend to be highest with treatments involving CTLA-4 blockades. Immune checkpoint blockade therapy has shown very promising results, especially when several different checkpoint pathways are targeted. However, due to the nonspecificity of the treatments, there are often unintended effects.
Cancer vaccines are another method of immunotherapy being heavily researched currently. One typically thinks of a vaccine as a weak or dead form of a virus used to artificially acquire active immunity in a patient in order to prevent the contraction of a disease. However, cancer vaccines use antigens from tumor cells to promote a response from T-cells and fight off cancer. There are two subcategories of tumor antigens: tumor shared antigens, and tumor specific antigens, and there are currently several different methods of isolating them. Scientists have isolated genes that code for tumor antigens by taking cells with the antigen and transfecting their cDNA. Anti-tumor cytotoxic cells then identified the transfected cells and found which section of the DNA codes for the antigen.
Other methods like tandem mass spectronomy are possible but tedious. Scientists also use genome sequencing to identify tumor specific antigens with somatic mutations, and they predict cancer neoantigens using computer algorithms. Neoantigens are antigens that differentiate cancer cells from healthy cells. Once an antigen has been identified, it must be tested before being used in a cancer vaccine. This is done through T-cell assays where vaccine or tumor antigens are put into the body. Dendritic cells (DCs) then capture these antigens and present them to T-cells which are subsequently activated and proliferate as effector T-cells as well as memory T-cells to attack the cancer and remain in the body for future use. There are three types of cancer vaccines: dendritic cell, peptide, and genetic. Tumor cells have developed mechanisms to evade immune response by causing apoptosis, or programmed cell death, in dendritic cells which act as antigen presenting cells. Therefore, externally introduced dendritic cells can act as an effective therapy against cancer. This type of vaccine has been used to treat diseases like metastatic melanoma, and there were 1000 dendritic cell vaccines by the year 2003. Peptide vaccines use tumor associated antigen (TAA) peptides to treat cancer. Since tumor associated antigen peptides are not consistently shown throughout the entire duration of tumor progression, multipeptide vaccines are more effective than single peptide vaccines. This is because multipeptide vaccines can still bring about a stable immune response even if some TAAs are not presented.
The efficacy of these vaccines can also be improved by modifying the tumor associated antigens. This could involve adding groups like interleukin-2 or synthetic long peptides to increase the peptide’s affinity with major histocompatibility complex (MHC). Genetic vaccines transfect dendritic or somatic cells with DNA coding for the presentation of antigens. This is done by using plasmid or viral DNA. Since it is difficult to generate an adequately large immune response for large animals like humans, many techniques for vaccine delivery have been created to try to improve efficacy. Genetic vaccines can be delivered into the skin or the lymph nodes. In addition to this, they can be delivered using techniques such as “pressure injectors, ultrasounds, liposome, nanoparticles, tattooing, gene gun and electroporation”. Overall, each type of cancer vaccine has its benefits and drawbacks, and this type of therapy is most effective when used in conjunction with another type of immunotherapy like immune checkpoint blockade therapy.
While each category of immunotherapy has its strengths and shortcomings, immunotherapy as a whole is very promising for cancer treatment. The use of the immune system to fight off cancer is intuitive, and as more information is gathered on the ways in which tumor cells evade the response of B and T-cells, more effective immunotherapies can be produced. Since treatments like vaccines are often personalized for a specific patient, they tend to be quite expensive. This additional cost generally proves to be worth it for patients suffering from violent, late stage cancers, however, since there are few alternative treatments at this point. Immunotherapy also tends to have higher response and survival rates than more traditional methods of treatment like chemotherapy.
Drawbacks to immunotherapy include unintended side-effects usually involving inflammation due to the increased activity of the immune system in the case of immune checkpoint blockade therapy or the addition of outside proteins to the body in the case of monoclonal antibodies. As new advancements are made in the field of science such as genome editing, new opportunities for production and understanding of immunotherapies become available. Overall, immunotherapy is a field that warrants further study and provides optimism for the cure of a devastating disease.