On a muggy midsummer morning in Brooklyn in 1916, 4-year-old Eugene was practicing somersaults, flipping and rolling, as nearby, his mother stitched…
How the Classic TB Vaccine Treats Bladder Cancer – Zebrafish Avatars Reveal Mechanism

Thanks to biotechnology, immunotherapy has become standard of care along many a cancer patient’s journey, with many targeted drugs now available. One of the oldest and most successful immunotherapies is simpler: a tamed version of a classic vaccine, against the infectious disease tuberculosis (TB).
“BCG” is the “treatment” vaccine’s technical name, for Mycobacterium bovis Bacillus Calmette-Guérin. Oncologists have used BCG to treat early stage bladder cancer for decades.
A research team led by Rita Fior at the Champalimaud Centre for the Unknown, Lisbon, Portugal, figured out how BCG decimates cancer cells. Their report appears in Disease Models and Mechanisms. First author is Mayra Martínez-López, who was a PhD student at the lab and is now at the Universidad de las Américas in Quito, Ecuador.
Immunotherapy Began with Coley’s Toxins
Retooling vaccines to kill cancer cells is a classic tale in the history of medicine.
The idea to redirect an immune response against a pathogen to fight cancer goes back to 1891, when Manhattan bone surgeon William Coley became intrigued by a man’s neck tumor that melted away after he contracted a nasty Streptococcus skin infection. After Dr. Coley found a few more cases, he began to experiment by rubbing bacteria-oozing goop into skin breaks in a few cancer patients. And every so often, tumors shrank.
Writes Edward F. McCarthy in the Iowa Orthopaedic Journal:
“Over the next forty years, as head of the Bone Tumor Service at Memorial Hospital in New York, Coley injected more than 1000 cancer patients with bacteria or bacterial products. These products became known as Coley’s Toxins. He and other doctors who used them reported excellent results, especially in bone and soft-tissue sarcomas.
Despite his reported good results, Coley’s Toxins came under a great deal of criticism because many doctors did not believe his results. This criticism, along with the development of radiation therapy and chemotherapy, caused Coley’s Toxins to gradually disappear from use. However, the modern science of immunology has shown that Coley’s principles were correct and that some cancers are sensitive to an enhanced immune system. Because research is very active in this field, William B. Coley, a bone sarcoma surgeon, deserves the title ’Father of Immunotherapy.’”
Coaxing BCG to Tame TB and then Cancer
The novel vaccine strategy builds on the accumulation of natural mutations when DNA copies itself as cells divide.
Biologist and physician Léon Charles Albert Calmette and veterinarian, bacteriologist, and immunologist Jean-Marie Camille Guérin, working in France, coaxed Mycobacterium into a form that could induce an immune response against TB. The experiments took more than 13 years, as they meticulously transferred 230 cultures of the bacterial species that would take their name, Mycobacterium bovis Bacillus Calmette-Guérin – BCG – named for the cattle respiratory pathogen Mycobacterium bovis.
With transfer, the inventors assessed the ability of the changeling microbes to cause TB. Eventually, a strain emerged that seemingly protected against a host developing the associated infectious lung disease.
By 1921, BCG was tame enough to be a TB vaccine. Decades later, it was redirected against bladder cancer, taking advantage of the mechanism of “immunological reactivity.” This was the forerunner of immunotherapy.
Beginning in1972, Canadian urologist Alvaro Morales tested topical BCG to treat superficial bladder cancer, publishing research using animal models in 1976.
The approach worked. Today, the vaccine is introduced directly into the bladder.
Urologists quickly adopted the protocol for patients who’d had tumors at the bladder opening removed, and the cancer not yet invading the muscle layer. A six-week “induction” of weekly infusions of hundreds of millions of bacteria would lead to a rest period. Then “maintenance therapy” every 3 months, for one to three years, would continue if the cancer progressed.
For many patients, BCG stops the cancer. Fifteen-year survival rate is 60% to 70%. In the 30% to 50% of cases in which BCG doesn’t work, the bladder is removed.
Transparent Fish Host Cancers
In the new investigation, zebrafish “avatars” were used as stand-ins for human bladder cancers.
The tiny fish are a favorite animal model because their skin is transparent, revealing what’s going on. And the see-through animals revealed that huge, blobby macrophages, which fight infection, also attack bladder cancer cells. The wandering macrophages induce apoptosis – a form of cell death – in the cancer cells, and then engulf and dissolve the resulting mess.
The zebrafish models are called zAvatars because cancer cells from a patient are injected into fish embryos. The definition of “avatar,” aside from being a film series, is “something that embodies something else.”
Human bladder tumors grow inside the fish embryos, effectively becoming avatars of the cancer patient. Then researchers test treatments on the zAvatars. This takes just a few days, compared to the weeks or months required to test treatments in mice. The fishy strategy is both fast and patient-specific.
Fish Reveal How BCG Unleashes an Immune Response Against Cancer
The researchers had already used zAvatars to test treatments for colorectal cancer when Fior suggested testing BCG to reveal how the vaccine kills bladder cancer cells.
Martínez-López remembered getting the BCG vaccine against TB growing up in South America, and had later worked on the infectious disease. “But it was the first time I heard of the TB vaccine as a cancer treatment,” she recalled. No one really knew how it worked, but macrophages, known to be in the bladder, were a likely first step in an immune response against the cancer.
The researchers filmed activation of the macrophages using light sheet microscopy, which produces a high-resolution image of biological material coupled with confocal imaging. And they observed the blobby macrophages envelop individual cancer cells, in real time.
By marking the macrophages green and the cancer cells red, the researchers watched macrophages zeroing in on the cancer cells soon after BCG injection. The giant immune cells release tumor necrosis factor (TNF), which directs more specific arms of the immune response against the cancer cells. When that happens, the macrophages change from red to yellow, indicating that they are spewing TNF.
Once the zAvatar runs out of macrophages, the tumor-killing effect halts.
“Mayra saw that if you inject BCG, you have a marked increase of macrophages going into the tumor,” said Fior. Added Martínez-López, “Not only did we unravel the mechanisms involved in the first steps of the vaccine’s anti-tumor action, we also demonstrated that the zebrafish Avatar model is a powerful preclinical tool for drug discovery in oncology.”
CODA
The bigger picture of the zAvatar story is that a place remains for century-old observations and ideas that can perhaps now be fleshed out with molecular explanations. And so an old vaccine against an infectious disease can still hold its own, for some patients, against engineered cancer treatments, such as monoclonal antibody-drug conjugates and the immune checkpoint inhibitors that dominate advertising, especially for us older folk.
Summed up Martínez-López, “BCG immunotherapy is still rather empirically used. However, since it works for many people, it has become a gold standard treatment. Surprisingly, it is a very effective immunotherapy, even when compared to so many fancy immunotherapies that are being developed.”