Bacillus Calmette-Guérin (BCG) vaccine is the live attenuated vaccine form of Mycobacterium bovis used to prevent tuberculosis and other mycobacterial infections. Now, using zebrafish “avatars,” a model developed by the Cancer Development and Innate Immune Evasion lab at the Champalimaud Foundation (CF), researchers demonstrate that the vaccine may be used to help destroy bladder cancer cells.
The findings are published in the journal Disease Models and Mechanisms in an article entitled “Macrophages directly kill bladder cancer cells through TNF signaling in an early response to BCG therapy,” and led by Rita Fior, Mayra Martínez-López—a former PhD student at the lab now working at the Universidad de las Américas in Quito, Ecuador.
“The Bacillus Calmette–Guérin (BCG) vaccine is the oldest cancer immunotherapeutic agent in use,” the researchers wrote. “Despite its effectiveness, its initial mechanisms of action remain largely unknown. Here, we elucidate the earliest cellular mechanisms involved in BCG-induced tumor clearance. We developed a fast preclinical in vivo assay to visualize in real time and at single-cell resolution the initial interactions among bladder cancer cells, BCG and innate immunity using the zebrafish xenograft model. We show that BCG induced the recruitment and polarization of macrophages towards a pro-inflammatory phenotype, accompanied by induction of the inflammatory cytokines tnfa, il1b and il6 in the tumor microenvironment.”
The idea behind zebrafish Avatars (zAvatars), which are still an experimental model, is to take tumor cells from a cancer patient and inject them into zebrafish embryos. The tumors will then grow inside the embryos, effectively turning them into Avatars of that specific cancer patient. The various treatment options available for that patient can then be tested on the zAvatars, and, in a matter of days rather than several weeks, or even months, that it takes using the traditional testing in mice, it will be possible to determine the best treatment for that patient.
Fior had a new idea for applying the model. “When I joined Rita’s lab, as a PhD student” (in 2019), Martínez-López recalls, “we were discussing several projects I could undertake, and Rita mentioned that the BCG vaccine was being used in bladder cancer patients.” Martínez-López remembered getting the BCG against tuberculosis when she was a child in South America, and had later worked on the disease. “But it was the first time I heard of the TB vaccine as a cancer treatment”, she points out.
“BCG immunotherapy is still rather empirically used” says Martínez-López. “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.”
The treatment consists of instilling the BCG vaccine directly into the bladder. When the treatment works, the 15-year survival rate for patients with so-called “non-muscle-invasive” (early-stage) bladder cancer is 60% to 70%. However, in 30% to 50% of the cases, bladder tumours are unresponsive to BCG treatment. In these cases, the whole bladder has to be removed.
Until now, the way the BCG vaccine works as an immunomodulator to clear bladder tumors was not fully known.
The team’s idea was that immune cells, and in particular macrophages resident in the bladder, were involved. And they were able to determine what happens right after BCG is injected into zebrafish Avatars. To do this, they used so-called light sheet microscopy and confocal imaging, which allowed them to see the macrophages interacting with the tumour cells, at single cell resolution and in real time.
The researchers further showed that, when macrophages were depleted in the zAvatar, the anti-tumor effects of the BCG vaccine were completely blocked, suggesting that macrophages are crucial for the initial anti-tumor response.
“What Mayra saw”, says Rita Fior, “was that if you inject BCG, you have a marked increase of the amount of macrophages going into the tumor.”
“Not only did we unravel the mechanisms involved in the first steps of the vaccine’s anti-tumoral action, we also demonstrated that the zebrafish Avatar model is a powerful preclinical tool for drug discovery in oncology”, concluded Martínez-López.
