Acute Myeloid Leukemia: Chemotherapy-Induced Senescence Makes Cancer Cells More Visible to the Immune System

Chemotherapy does not act on cancer cells only by eliminating them: in some cases, cells that survive treatment can enter a particular state known as cellular senescence, in which they stop growing but remain alive and metabolically active, profoundly changing their behavior.

The results of the study showed in particular that, in a subgroup of samples from patients with acute myeloid leukemia (AML), the process induced by chemotherapy had an important consequence: leukemia cells became more recognizable to T cells, the immune system cells capable of identifying and attacking abnormal cells.

The research was coordinated by Raffaella Di Micco, group leader of the Senescence in Stem Cell Aging, Differentiation and Cancer Unit at the San Raffaele Telethon Institute for Gene Therapy (SR-Tiget) and Associate Professor of Pathology at the IUSS University School for Advanced Studies in Pavia, and was also supported by Fondazione AIRC.

The findings, published in Nature Communications, were obtained through laboratory experiments on samples derived from patients with AML. The researchers observed that when leukemia cells enter senescence after chemotherapy, the expression of HLA molecules increases on their surface. These act somewhat like molecular “display windows”: they expose small fragments of cellular proteins, thereby helping T cells recognize that the cell is abnormal and become activated against it.

The data also made it possible to identify an epigenetic mechanism, meaning a system that regulates gene activity without changing the DNA sequence. This mechanism could explain why the phenomenon occurs in some samples, referred to as “senescence high”, or high-senescence samples, but not in others, called “senescence low”, or low-senescence samples. In the latter, in particular, some programs involved in the immune response appear to remain more strongly suppressed.

This led the researchers involved in the study to a second insight: if this “brake” contributes to making some leukemia cells less recognizable to the immune system, acting on the epigenetic mechanism could increase their immunological visibility. The researchers therefore tested, in laboratory experiments, a drug already approved for clinical use that is capable of acting on this mechanism, obtaining greater exposure of HLA molecules and improved activation of T cells. The results do not yet indicate a new treatment for patients, but they open up the possibility of studying senescence as a potential biomarker of different immune responses to chemotherapy and, in the future, exploring new combination treatment strategies.

Acute myeloid leukemia is an aggressive blood cancer that remains difficult to treat, progresses very rapidly, and is extremely heterogeneous. It is characterized by the accumulation in the bone marrow and blood of immature myeloid cells known as blasts, which interfere with the normal production of blood cells. It can occur at any age, but is more common in older adults.

In Italy, approximately 50 new cases of AML per million inhabitants are estimated each year. Globally, according to analyses based on the Global Burden of Disease 2021, approximately 145,000 new cases were estimated in 2021.

Increasing knowledge of the genetic and molecular characteristics of AML has made it possible in recent years to refine disease classification and introduce targeted therapies. Chemotherapy, however, remains the treatment of choice for many patients and, under certain conditions, may be followed by an allogeneic hematopoietic stem cell transplant.

One of the main challenges is represented by leukemia cells that survive treatment and may contribute to persistence or recurrence of the disease. Understanding what happens to these cells and how they subsequently interact with the immune system is therefore a central topic in AML research.

At San Raffaele, this line of research had already led to the identification of mechanisms by which, in relapses after transplantation, leukemia cells can reduce the expression of HLA molecules and escape immune recognition. The new findings have made it possible to investigate the opposite phenomenon: what happens to the immunological visibility of leukemia when the cells enter senescence in response to chemotherapy.

The researchers studied samples from 21 patients with newly diagnosed acute myeloid leukemia who had not yet received chemotherapy, exposing the leukemia cells in the laboratory to cytarabine, a chemotherapy drug commonly used to treat AML. Not all samples responded in the same way:

• in 15 of the 21 samples, a proportion of the cells developed features consistent with senescence after treatment: these samples were classified as “senescence high”;
• in the other 6, classified as “senescence low”, the response was much less pronounced.

Senescence should not be confused with cell death or quiescence. A senescent cell stops proliferating, generally in a stable manner, but remains alive and metabolically active, altering the activity of numerous genes and its relationship with the surrounding environment. In oncology, this phenomenon can have different effects:

• in the short term, the arrest of proliferation may contribute to tumor control and, as the results of this study suggest, may make some cancer cells more recognizable to the immune system;
• however, if senescent cells persist in the body for a long time, they can contribute to the development of a chronic inflammatory environment that, under certain conditions, promotes treatment resistance and disease progression.

In the new study, the researchers focused primarily on what happens in the early stages after treatment.

In the “senescence high” samples, chemotherapy was found to activate molecular programs linked to inflammation and interferons and to increase the presence of HLA class I and II molecules on the surface of leukemia cells.

The effect was not only molecular. In co-culture experiments, in which leukemia cells and T cells from the same patient were grown together to observe their interaction, chemotherapy-treated “senescence high” leukemia cells induced greater proliferation of CD4+ and CD8+ T cells than “senescence low” cells.

The results of additional experiments confirmed the role of HLA molecules: when the researchers blocked their function, T cells became less activated against the leukemia cells, demonstrating that the increased visibility of these cells depends specifically on HLA molecules.

“Our results indicate that therapy-induced senescence is not simply a state in which a leukemia cell stops proliferating. In some samples, it also changes the interaction between the tumor and the immune system, increasing the ability of leukemia cells to present antigens and be recognized by T cells,” explains Raffaella Di Micco. “However, it is important to emphasize that not all samples respond in this way: understanding the origin of this difference is precisely one of the most interesting aspects of the study.”

To understand why some leukemias develop this response while others do not, the researchers also analyzed epigenetic mechanisms, which regulate gene activity without modifying DNA. These include the PRC2 protein complex, of which EZH2 is a key component.

“In the ‘senescence low’ samples,” explains Simona Fusco, co-first author of the article, “we observed greater activity of this inhibitory system before treatment. Some of the genes involved in the immune response were therefore more strongly ‘suppressed’ and less available for activation. This may help explain why, after chemotherapy, these cells increase HLA expression to a lesser extent and remain less recognizable to the immune system.”

To determine whether it was possible to act on this mechanism, the researchers treated “senescence low” cells in the laboratory with tazemetostat, a drug already approved for clinical use that selectively inhibits EZH2. The treatment reduced the epigenetic “brake” in DNA regions involved in immune and inflammatory responses and increased the expression of HLA class I and II molecules. The treated cells also became more capable of stimulating the proliferation of CD4+ and CD8+ T cells.

This result suggests that pharmacologically modifying certain epigenetic mechanisms could, in the future, represent a possible strategy for increasing the immunological recognition of some leukemia cells.

“These data mainly tell us that the immune effects of chemotherapy can vary from patient to patient,” adds Di Micco. “The next step will be to understand whether the ability of leukemia cells to enter senescence could also become a biomarker, useful for identifying patients with different immune responses to chemotherapy.

At the same time, we want to further investigate the possibility of combining chemotherapy with immunotherapies based on engineered T cells and/or drugs capable of acting on the epigenetic mechanisms that regulate the visibility of leukemia cells to the immune system. These are approaches that still require validation in larger groups of patients, as well as dedicated preclinical and clinical studies, before any application in clinical practice can be considered.”

The research received support from several national and international organizations: Fondazione AIRC per la Ricerca sul Cancro, Fondazione Telethon, Human Frontier Science Program, American Society of Hematology, New York Stem Cell Foundation, European Research Council (ERC), European Hematology Association, Leukemia Research Foundation, and the Italian Ministry of Health.