The results of a study conducted by researchers from IFOM, the AIRC Institute of Molecular Oncology, and the University of Turin, in collaboration with physicians and scientists from the Humanitas Research Hospital, have shown that so-called “cold” colorectal tumors, classified as microsatellite stable (MSS), are not simply poorly visible to the immune system.

Rather, these forms of cancer put into action actual strategies to block its activity. More precisely, tumor cells actively produce molecules capable of blocking the “immune synapse”, that is, the functional contact between T lymphocytes and tumor cells. In addition, they reprogram their cell surface through glycosylation, a process that modifies proteins by adding chains of sugars. The research, the results of which were published in the journal Cancer Discovery, was supported by Fondazione AIRC for Cancer Research and the European Research Council.

In 2024 alone and in Italy alone, more than 48,000 new diagnoses of colorectal cancer were estimated. Immunotherapy has greatly improved the prognosis of some patients whose tumors are sensitive to these treatments, but this applies to only about 5% of metastatic cases, namely “hot” tumors, which can be recognised by the immune system also because of microsatellite instability (MSI). The latter is an alteration in which many mutations accumulate, making the affected cells more “visible” to the immune system. For the remaining 95%, namely “cold” tumors with microsatellite stability (MSS) and therefore genetically more “silent”, this therapeutic approach remains largely ineffective. The explanation accepted so far focused precisely on this genetic silence, but it was not sufficient to account for all the cases of resistance observed clinically.

“For years, we have imagined ‘cold’ tumors as simply poorly visible targets,” explains Alberto Bardelli – Scientific Director of IFOM and Full Professor in the Department of Oncology at the University of Turin, and corresponding author of the paper. “The results lead us to reconsider this idea: these tumors do not merely hide, but activate precise molecular mechanisms capable of blocking T lymphocytes even when the latter are able to identify them. This is a distinction that changes the way in which we need to design therapeutic strategies.”

To test this hypothesis, the researchers developed an experimental platform based on tumor organoids derived from patient samples. These are three-dimensional cell cultures in which it is possible to reproduce the original tumor more faithfully than with previous techniques. With this approach, “hot” and “cold” tumors were placed under the same conditions of immune recognition, eliminating the variable related to the different presence of antigens.

Chiara Cattaneo – corresponding author and co-first author of the paper, conducted the experiments at IFOM during her postdoctoral work in Professor Bardelli’s laboratory and is now Group Leader of the Precision Immuno-Oncology Laboratory at the Humanitas Research Hospital. Dr. Cattaneo explains: “Even when antigen recognition is perfectly comparable in the two experimental systems in which we reproduced ‘hot’ and ‘cold’ tumors, MSS tumors retain an ability to resist the response of T cells.”

“This,” continues Sharon Scardellato – co-first author and postdoctoral researcher in Professor Bardelli’s laboratory at IFOM, “does not depend on a difference in antigen presentation, but on intrinsic mechanisms of the tumor that act on the immune microenvironment.”

The cause of the blockade was identified precisely in the so-called secretome, that is, the set of molecules released by the tumor into the surrounding environment. “Cold” tumors release factors that interfere with what is known as the immune synapse, namely the functional contact between T lymphocytes and tumor cells. In addition, they reprogram the surface of nearby cells through alterations in glycosylation. By pharmacologically inhibiting glycosylation, the immune response was completely restored, confirming the functional relevance of this mechanism.

The results have therefore redefined the understanding of resistance to immunotherapy in colorectal tumors and possibly also in other types of cancer. The presence of antigens recognisable by the body’s defenses remains a necessary, but not sufficient, condition: even when the immune system identifies the tumor, the latter can neutralize its action through active mechanisms of suppression.

“The results of this study mark a paradigm shift,” continues Chiara Cattaneo. “We have always focused on making the tumor visible, imagining it as a static target: once discovered by the immune system, it should be attacked. But the tumor is not defenseless: it has its own tools that it can activate in order to neutralise T lymphocytes even when it is recognised. Our platform has unmasked these tools. Now that we know them, we can target them.”

In addition to clarifying a fundamental aspect of the biology of colorectal tumors, the study opens the way to new therapeutic prospects: the platform developed by the researchers can in fact be used to study similar mechanisms in other tumors that are refractory to immunotherapy.

“To make immunotherapy effective against ‘cold’ colorectal tumors, we will need to act on two fronts: increase the visibility of the tumor and deactivate the mechanisms through which it neutralises T lymphocytes,” concludes Bardelli. “Glycosylation is now a concrete molecular target. The next steps will be to validate these results in more complex experimental systems and identify the specific molecules in the secretome responsible for reprogramming the tumor surface.”