Transforming Bladder Cancer Care: The Impact of BCG and TB Vaccine Innovations

Bacillus Calmette-Guerin (BCG) Vaccine: From Tuberculosis Prevention to Bladder Cancer Treatment

What is Bacillus Calmette-Guerin?

Bacillus Calmette-Guerin (BCG) is a live attenuated vaccine derived from a weakened strain of the Mycobacterium bovis bacterium, which is responsible for tuberculosis (TB). Developed by Calmette and Guérin in 1921, it has become the most widely administered vaccine globally. Its primary function is to prevent severe forms of tuberculosis in children, such as TB meningitis, and to provide protection against pulmonary tuberculosis, the most prevalent form in adults. In addition to tuberculosis, the BCG vaccine offers protection against leprosy and Buruli ulcer, both caused by mycobacteria-related organisms. Recently, BCG has gained attention for its successful repurposing as a treatment for bladder cancer.

BCG as a Vaccine

Administering a weakened form of bacteria stimulates a dual immune response in the body. The innate immune system responds first, rapidly deploying macrophages, natural killer cells, and dendritic cells to neutralize the weakened bacteria. Subsequently, the adaptive immune system activates, producing specific T cells and B cells that recognize and eliminate tuberculosis bacteria in future encounters. This coordinated response is crucial for the vaccine’s long-lasting protection against severe tuberculosis forms.

The Transition of BCG to Cancer Immunotherapy

Historical Background

In 1929, Dr. Pearl’s autopsy study at Johns Hopkins unveiled a surprising correlation between tuberculosis infections and lower cancer rates, sparking interest in a potential link. This curiosity was further fueled by late 1800s observations of cancer regression in patients with bacterial infections, leading William B. Coley to experiment with bacterial products, including BCG, as a cancer treatment in 1893. These early explorations laid the foundation for immunotherapy.

In the late 1950s, Lloyd Old’s pioneering studies demonstrated that BCG could enhance immune responses against tumors in mice, underscoring its indirect anti-cancer properties. Burton Zbar’s research in the 1970s validated BCG’s tumor-inhibiting effects, particularly when injected directly into tumors, emphasizing the necessity of close contact between BCG and tumor cells for optimal effectiveness.

The late 1960s and 1970s marked promising early clinical trials of BCG for various cancers, including leukemia and melanoma. A notable breakthrough occurred in 1975 when BCG effectively treated melanoma in the bladder, prompting further investigations into its potential for bladder cancer therapy. However, while BCG initially showed promise, its efficacy in treating other cancers proved limited, leading to its replacement by more effective therapies.

The FDA approved BCG for bladder cancer immunotherapy in 1990, making it the first cancer immunotherapy to gain approval. Today, it is a standard treatment for early-stage bladder cancer, exemplifying the potential of immunotherapy in leveraging the body’s immune system to combat this disease.

Mechanism of Action in Bladder Cancer Treatment

Currently, a single dose of Mycobacterium bovis BCG immunotherapy is administered directly into the bladder via a catheter, triggering a robust immune response targeting and destroying cancer cells. Initial beliefs suggested that BCG activated the immune system to combat cancer; however, recent studies have revealed that BCG exerts direct toxic effects on cancer cells.

BCG induces cancer cell death through programmed self-destruction, known as apoptosis, as well as a more chaotic form of cell death termed necrosis. Additionally, it generates harmful molecules that overwhelm cancer cells through oxidative stress, leading to damage and cell death.

When introduced into the bladder, BCG bacteria attach to both cancerous and healthy cells, facilitated by a protein called fibronectin, which is more prevalent on cancer cells. Once attached, BCG invades the cell, prompting the cell’s mechanisms to recognize the intruder. Antigen-presenting cells (APCs) are then recruited to the site, recognizing BCG as a foreign entity and displaying its fragments on their surface, alerting the immune system.

This triggers T cells to multiply and differentiate into specific types capable of directly attacking the cancer cells displaying BCG fragments. The activated immune cells release a surge of cytokines, creating a “cytokine storm” that attracts additional immune cells like macrophages and natural killer cells to the tumor site, ultimately leading to the destruction of cancer cells.

Research conducted in 2020 indicated that immune cells recognizing BCG fragments on tumor cells play a significant role in the therapy’s effectiveness, resulting in increased T cell presence within tumors.

Side Effects of Bacillus Calmette-Guerin

As a vaccine, BCG is generally safe and not linked to severe complications. However, it is contraindicated for individuals with weakened immune systems, including those with congenital immunodeficiency or HIV, as well as adults living in areas with low TB prevalence.

As an immunotherapy, BCG may cause several uncomfortable side effects, including:
– Irritated bladder
– Blood or debris in urine
– Pain or discomfort during urination
– Flu-like symptoms 24-48 hours post-treatment
– Increased frequency and urgency of urination

Rare side effects may include:
– Cough
– Skin rash
– Fever and chills
– Joint or muscle pain
– Extreme fatigue
– Nausea or vomiting

Unanswered Questions and Future Directions

Understanding Mechanisms of Action

While researchers have made significant strides in understanding the mechanisms behind BCG immunotherapy, many aspects remain unclear. Ongoing studies aim to elucidate its complexities further, potentially enabling tailored treatments for patients.

Effectiveness Across Patient Populations

Despite BCG’s notable success, a substantial proportion of bladder cancer patients do not respond to the treatment. Scientists are investigating the reasons behind this variability and exploring ways to enhance treatment efficacy.

Exploring Additional Applications

Currently, BCG is primarily utilized for tuberculosis and bladder cancer, yet researchers are exploring its potential for treating other diseases, aiming to optimize its immunotherapy applications.

References

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Moliva JI, Turner J, Torrelles JB. Immune Responses to Bacillus Calmette-Guerin Vaccination: Why Do They Fail to Protect against Mycobacterium tuberculosis? Front Immunol. 2017;8:407. doi:10.3389/fimmu.2017.00407
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Guallar-Garrido S, Julián E. Bacillus Calmette-Guerin (BCG) Therapy for Bladder Cancer: An Update. Immunotargets Ther. 2020;9:1-11. doi:10.2147/ITT.S202006
Han, J. et al. (2020) ‘Mechanisms of BCG in the treatment of bladder cancer-current understanding and the Prospect’, Biomedicine & Pharmacotherapy, 129, p. 110393. doi:10.1016/j.biopha.2020.110393.
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BCG into the bladder (2023) for non muscle invasive bladder cancer | Cancer Research UK. Available at: https://www.cancerresearchuk.org/about-cancer/bladder-cancer/treatment/non-muscle-invasive/bcg (Accessed: 14 June 2024).