The fight against cancer has been revolutionized in recent years by the rise of immunotherapy. However, not all tumors respond equally to these therapies. There are so-called “hot” tumors, which have an active immune microenvironment with T cell infiltration and pro-inflammatory molecule expression, and “cold” tumors, which are immunologically silent and easily evade the immune system. One of the major challenges in current oncology is turning those cold tumors into hot ones. And this is where an often-overlooked player, cytosolic DNA (cyDNA), takes on an unexpected starring role.

In a recent study published in Cell Reports, Mosley and collaborators (2025) explore the role of cyDNA as a key modulator of antitumor immunity, particularly in the context of colorectal cancer (CRC). This DNA, released into the cytoplasm after genomic damage, is detected by intracellular sensors such as cGAS, which upon activation generate potent immune-stimulating signals through the STING pathway. The study shows that not all cyDNA is equal: its composition, size, subcellular origin, and sequence determine whether it will be a strong immune activator… or not.

One of the study’s central comparisons was between tumors with microsatellite instability (MSI) and those with chromosomal instability (CIN). Although both produce cytosolic DNA as a consequence of genomic instability, MSI tumors generate cyDNA that is much more effective in activating the cGAS-STING pathway. This pathway stimulates the production of type I interferons (IFN-α, IFN-β) and chemokines such as CXCL10, which are key to attracting cytotoxic T lymphocytes into the tumor.

What makes the cyDNA from MSI tumors different? The researchers identified several key features:

Higher content of microsatellites: repetitive genetic regions that are especially prone to replication errors when DNA repair systems fail, as in MSI.

Longer DNA fragments: which allow more efficient binding to the cGAS protein, essential for triggering immune signaling.

Greater proportion of mitochondrial DNA (mtDNA): which, due to its bacterial evolutionary origin, is recognized as “foreign” by the immune system and triggers stronger responses.

Moreover, the study demonstrated that conventional therapies like radiotherapy or oxaliplatin not only damage tumor DNA but also change the quality of the resulting cyDNA, enhancing its ability to stimulate the immune system. Radiation, for instance, increased the size of cyDNA fragments and favored the release of mtDNA into the cytoplasm. This suggests that radiotherapy, when applied strategically, not only kills cancer cells but turns the tumor into a source of immune-activating signals.

In a fascinating experimental phase, the researchers synthesized cyDNA fragments enriched in microsatellites and used them to activate dendritic cells (DCs), which were then transferred into mice with CIN tumors. The result was striking: there was greater infiltration of CD8⁺ T cells, activation of the tumor microenvironment, and signs of specific immunity without an unwanted increase in immunosuppressive markers like PD-L1. In other words, they managed to turn a cold tumor into a hot one using only signals derived from specific DNA sequences.

This finding completely changes the way we understand immunotherapy. Until now, many strategies have relied on stimulating the immune response through tumor antigens (neoantigens). However, this study shows that there are neoantigen-independent mechanisms—like cGAS-STING activation by cyDNA—that can be equally or even more effective in stimulating the immune system.

At the molecular level, it was shown that microsatellites induce stronger cGAS binding and more efficient oligomerization, which is crucial for the production of the cyclic messenger 2’3’-cGAMP, the actual activator of STING. These fragments, even if they do not represent functional genes, act as damage-associated molecular patterns (DAMPs) capable of reprogramming the behavior of immune cells interacting with the tumor.

Figure 1. Cytosolic DNA from MSI tumors strongly activates the STING pathway, leading to a robust immune response. In contrast, DNA from CIN tumors triggers weaker STING activation, resulting in a less effective immune response.

 

The study also highlights that the long-held view of cGAS as a sequence-independent DNA sensor may need revision. The presence of certain repetitive patterns, secondary DNA structures, or even methylation and other modifications could modulate its affinity for cGAS—and therefore its ability to trigger immunity.

Looking ahead, this approach could enable the development of DNA-based STING agonists, designed with specific sequences like microsatellites optimized to induce a powerful immune response without necessarily relying on personalized tumor antigens. In fact, it could become an ideal adjuvant therapy for patients with resistant tumors, in combination with radiotherapy, checkpoint inhibitors (such as anti-PD-1), or therapeutic vaccines.

 

Main Reference:

Mosley, S. R., Chen, A., Doell, D. N. W., Choi, S., Mowat, C., Meier-Stephenson, F., Meier-Stephenson, V., & Baker, K. (2025). Cytosolic DNA composition is determined by genomic instability mechanism and regulates dendritic cell-mediated anti-tumor immunity. Cell reports, 44(2), 115177. https://doi.org/10.1016/j.celrep.2024.115177

 

Other References:

Dekker, E., Tanis, P. J., Vleugels, J. L. A., Kasi, P. M., & Wallace, M. B. (2019). Colorectal cancer. Lancet (London, England), 394(10207), 1467–1480. https://doi.org/10.1016/S0140-6736(19)32319-0

Tennstedt, P., Fresow, R., Simon, R., Marx, A., Terracciano, L., Petersen, C., Sauter, G., Dikomey, E., & Borgmann, K. (2013). RAD51 overexpression is a negative prognostic marker for colorectal adenocarcinoma. International journal of cancer, 132(9), 2118–2126. https://doi.org/10.1002/ijc.27907

Kloor, M., & von Knebel Doeberitz, M. (2016). The Immune Biology of Microsatellite-Unstable Cancer. Trends in cancer, 2(3), 121–133. https://doi.org/10.1016/j.trecan.2016.02.004