Cancer vaccines mobilize the body’s own defense mechanisms against malignant cells. Unlike traditional vaccines that primarily prevent infectious diseases by targeting external pathogens, cancer vaccines aim to train the immune system to recognize and eliminate cancerous cells. They aim to enhance the body’s natural immune response to fight existing cancer or prevent its development in certain contexts. This therapeutic strategy seeks to turn the immune system into a targeted tool against disease.
The Immune System’s Role in Cancer
The human immune system has a surveillance system designed to identify and eliminate abnormal cells, including those that could become cancerous. Specialized immune cells, such as T cells, constantly patrol the body, capable of recognizing subtle differences on the surface of unhealthy cells compared to healthy ones. When these aberrant cells are detected, the immune system initiates a coordinated attack to destroy them.
However, cancer cells are adept at evading this natural defense. They can develop various strategies to escape immune detection and destruction. This evasion can occur by downregulating the expression of specific molecules that immune cells use for recognition, essentially hiding from the immune system. Cancer cells may also create an immunosuppressive environment around tumors, or produce signals that actively switch off or exhaust immune cells, preventing them from mounting an effective attack. Understanding these evasion mechanisms is important for developing therapies that can overcome them.
How Cancer Vaccines Operate
Cancer vaccines stimulate the immune system to specifically recognize and target cancer cells. They introduce specific molecules, known as tumor antigens, to the immune system. These antigens are proteins found on cancer cells that are either unique to them or present in much higher quantities than on healthy cells.
There are two broad categories of cancer vaccines based on their purpose. Preventive, or prophylactic, vaccines aim to stop cancer from developing in the first place, often by targeting viruses known to cause certain cancers. Therapeutic vaccines, in contrast, are designed to treat existing cancer by stimulating an immune response against established tumors. Once introduced, these vaccines help antigen-presenting cells, a type of immune cell, to capture and display the tumor antigens to T cells. This process allows T cells to identify and attack cancer cells.
Current Cancer Vaccine Applications
Several cancer vaccines are currently approved and in use. Preventive vaccines play a role in averting virus-associated cancers. The Human Papillomavirus (HPV) vaccine, for instance, prevents infections by specific HPV types, which are a primary cause of cervical, anal, and certain head and neck cancers. Similarly, the Hepatitis B Virus (HBV) vaccine protects against HBV infection, thereby reducing the risk of developing liver cancer. These preventive vaccines work by generating an immune response against the viruses themselves, not directly against cancer cells.
For existing cancers, therapeutic vaccines are available. Sipuleucel-T (Provenge) is an approved therapeutic vaccine used to treat prostate cancer that has spread. This vaccine is customized for each patient, using their own immune cells which are activated outside the body to recognize prostate cancer antigens before being reinfused. Another example is Bacillus Calmette-Guérin (BCG), a weakened bacterium initially used for tuberculosis vaccination, which is effective in treating early-stage bladder cancer when delivered directly into the bladder. Additionally, Talimogene laherparepvec (T-VEC), an oncolytic virus, is approved for advanced melanoma that cannot be surgically removed, working by infecting and killing cancer cells while stimulating an immune response.
Advanced Approaches in Cancer Vaccine Science
Research in cancer vaccine science is continually advancing, exploring strategies to enhance efficacy and broaden applications. One promising area involves personalized cancer vaccines, often referred to as neoantigen vaccines. These vaccines are tailored to an individual patient’s tumor, targeting unique mutations (neoantigens) found only on their cancer cells and not on healthy cells. This tailored approach aims for specific immune responses with minimal impact on healthy tissues.
Oncolytic viruses are another advanced approach, utilizing viruses engineered to selectively infect and destroy cancer cells while leaving healthy cells unharmed. As these viruses replicate within and lyse cancer cells, they can also stimulate an immune response by releasing tumor antigens and danger signals, effectively turning the tumor into an in-situ vaccine. Furthermore, messenger RNA (mRNA) vaccine platforms, gaining prominence due to their role in infectious disease vaccines, are being explored for cancer. mRNA vaccines can rapidly deliver genetic instructions for tumor antigens, prompting the body’s cells to produce these antigens and trigger a strong immune response.
Combining cancer vaccines with other immunotherapies, such as immune checkpoint inhibitors, is also a key focus. Checkpoint inhibitors work by “releasing the brakes” on immune cells, allowing them to better attack cancer. When combined with vaccines, this approach aims to generate an initial immune response via the vaccine and then sustain that response by overcoming the tumor’s immunosuppressive mechanisms. These combination strategies seek to leverage synergistic effects to improve patient outcomes and overcome resistance to single therapies.