The chances of success of immunotherapy depend on the degree of heterogeneity of the cancer tumor

Recovery rates from melanoma skin cancer have increased significantly in recent years thanks to immunotherapy treatments that successfully harness the cells of the immune system to fight cancer. However, despite success, many patients do not respond to treatment for unknown reasons. Following a series of studies that showed a positive correlation between the success of the treatment and the "tumor mutational burden" in the tumor (Tumor Mutational Burden), the leading hypothesis in the field today is that the more mutations there are in the cancer cells, the higher the chances of success of the treatment

Some people will certainly testify that they are excellent at "multi-tasking", but studies show that our ability to engage in several tasks at the same time is quite poor. Now it turns out that multitasking is a challenge not only for the human brain, but also for our immune system. Weizmann Institute of Science scientists have shown that as melanoma cancer cells divide into more subtypes - where each subtype is a "task" in itself - the ability of the immune system to fight them effectively drops. Moreover, the chances of success of innovative treatments designed to improve the ability of the immune system to fight cancer also drop as the cancer tumor is more heterogeneous. These findings, published today in the scientific journal Cell, may help tailor treatment for cancer patients, and open the door to the development of personalized cancer vaccines.

Recovery rates from melanoma skin cancer have increased significantly in recent years thanks to immunotherapy treatments that successfully harness the cells of the immune system to fight cancer. However, despite success, many patients do not respond to treatment for unknown reasons. Following a series of studies that showed a positive correlation between the success of the treatment and the "tumor mutational burden" in the tumor (Tumor Mutational Burden), the leading hypothesis in the field today is that the more mutations there are in the cancer cells, the higher the chances of success of the treatment. Moreover, there are now companies offering expensive treatments aimed at increasing the number of mutations in patients (through radiation exposure or inhibiting DNA repair mechanisms) to make them better candidates for immunotherapy. However, although this concept has gained traction among doctors, no causal relationship has been shown so far, and there are definitely cases where the mutation load is low, but the response to treatment is good and vice versa. On the other hand, there are studies that show a negative correlation between treatment success and the degree of tumor heterogeneity, that is, how many subtypes of cells it contains. In order to decide the issue, Prof. Jordana Samuels from the Department of Molecular Biology of the Cell and her research partners developed a unique experimental system that makes it possible to check precisely which factors influence the success of the treatment.

The research team, led by Dr. Yohai Wolff and Dr. Osnat Bartok from Prof. Samuels' laboratory, first took mouse melanoma cells and exposed them to a central factor in the formation of this cancer - medium-wave ultraviolet radiation, with the aim of increasing both the number of mutations in these cells and the difference between the cells (heterogeneity). Surprisingly, after being injected into healthy mice, the irradiated cells multiplied at a faster and more aggressive rate than the original melanoma cells, even though they had a higher mutation load. Also, contrary to the popular opinion in the field, despite the load of mutations, these cells responded less to immunotherapy compared to the original cells. The researchers hypothesized that the reason for this lies in the increase in heterogeneity that occurred simultaneously with the increase in the number of mutations.

Since the two variables - mutation load and heterogeneity - are intertwined, the researchers looked for a way to isolate them. To do this, they took individual cells from the "aggressive" cell culture and created a new cell culture from each individual cell. Thus they created 22 cultures, all of which have low heterogeneity, while the mutation load is random. The researchers were amazed to find that when they injected the new cell cultures into healthy mice, the growth rate was extremely low, and the tumors were rejected even without immunotherapy. In other words, although the number of mutations was random, the result was similar in all cases. To make sure that the rejection of the tumors was indeed due to the action of the mice's immune system and not from other factors, the researchers injected the cell cultures into mice with a weakened immune system and saw that this time they grew rapidly.

"Since immunotherapy targets the T cells of the immune system which are able to eliminate the tumor cells, we did not limit ourselves to mice with a weakened immune system, and we also injected the cells into mice specifically engineered for a low level of T cell activity. The results were similar," says Dr. Wolff. "We then extracted T cells from the various tumors and saw that their activity levels were much higher in the homogenous tumors compared to those created from the mother culture The researchers were surprised to discover that while in homogeneous tumors T cells entered the core of the tumor, in heterogeneous ones they remained outside the tumor and a greater presence of T cells of a different type was recorded which suppress the action of the immune system.

In the bottom line, we showed that the number of cell subtypes and their position on the phylogenetic tree is a much more powerful and relevant measure than the mutation load when you want to predict therapeutic success in melanoma cancer"

A phylogenetic tree that maps the composition of the cells in the heterogeneous culture and the development of the cell subtypes on the timeline. Based on this diagram, the researchers assembled cellular "cocktails" of varying levels of heterogeneity

"We showed what happens in two extreme situations - a very homogeneous situation and a very heterogeneous situation, but what about intermediate situations?" asks Dr. Bartok and answers: "For this purpose, we created a phylogenetic tree that maps the composition of the cells in the heterogeneous culture and the development of the cell subtypes on the timeline We received a tree with six branches, and based on it we composed in a controlled manner cellular 'cocktails' of varying levels of heterogeneity that we injected into the mice." According to the hypotheses, the more homogeneous tumors were relatively easily rejected by the immune system of the mice, while the heterogeneous tumors grew faster - and the rate of growth increased as the heterogeneity increased.

"Bottom line, we showed that the number of cell subtypes and their position on the phylogenetic tree is a much more powerful and relevant measure than the mutation load when you want to predict therapeutic success in melanoma cancer," says Prof. Samuels. "In fact, when we used the index to analyze data obtained from melanoma patients who underwent immunotherapy, we received unequivocal results: a high correlation with treatment success. For us, this is just the beginning - the experimental system we developed will serve us for many years to come, and will help us develop protocols for personalized vaccines for cancer patients ".

Also participating in the study were Prof. Eitan Rupin from the United States National Institutes of Health (NIH), Prof. Leah Eisenbach from the Department of Immunology and Dr. Yishai Levin from the Israeli National Center for Personalized Medicine named after Nancy and Steven Grand, both from the Weizmann Institute of Science, Prof. Martin L. Miller from the British Cancer Research Institute at the University of Cambridge, Prof. Eli Pikarski from the Lautenberg Center for immunology and cancer research at the School of Medicine of the Hebrew and Hadassah University, Prof. Aryeh Edmon from the Faculty of Biology at the Technion and Prof. Charles Swanton from the Department of Medical Oncology at University College London.

More of the topic in Hayadan:

• Israeli researchers have uncovered a central mechanism that causes metastatic melanoma to be resistant to immunotherapy
• Researchers at Tel Aviv University succeeded in developing a vaccine against melanoma skin cancer
• Researchers were able to shrink cancerous tumors in mice by manipulating the reward region of the brain