By Billy Candra 
Researchers at Karolinska Institutet and the Astrid Lindgren Children’s Hospital in Sweden have determined how children’s immune systems react to different kinds of cancer depending on their age, a pivotal discovery published in the prestigious journal Cell. This landmark study reveals significant, fundamental differences between the immune responses of children and adults when confronted with malignancy, carrying the profound potential to usher in a new era of tailored treatments specifically designed for children battling cancer. The findings underscore a critical gap in current understanding and highlight why many adult-centric cancer therapies, particularly immunotherapies, have proven largely ineffective in paediatric oncology.
The Unique Battleground: Childhood Cancer
Childhood cancer, while relatively rare compared to adult cancers, remains a leading cause of disease-related death in children and adolescents globally. Approximately 400,000 children and adolescents (0-19 years old) are diagnosed with cancer each year worldwide. In Sweden, around 300 children receive a cancer diagnosis annually, representing a significant challenge for healthcare systems and an immense emotional burden for families. Unlike adult cancers, which often arise from lifestyle factors and accumulated genetic damage over decades, paediatric cancers typically originate from embryonic cells or rapidly developing tissues, often carrying fewer genetic mutations. This inherent difference is a crucial starting point for understanding why a child’s immune system might perceive and react to cancer differently.
Current treatments for childhood cancer primarily rely on chemotherapy, radiation therapy, and surgery. While these aggressive approaches have dramatically improved survival rates for many paediatric cancers over the past few decades – with overall five-year survival rates now exceeding 80% in high-income countries – they often come at a severe cost. Children who survive cancer frequently face a lifetime of debilitating late effects, including secondary cancers, cardiovascular problems, infertility, and cognitive impairments, due to the harsh impact of treatments on their still-developing bodies. The urgent need for less toxic, more effective, and precisely targeted therapies for children has been a driving force in paediatric oncology research.
A New Dimension in Precision Medicine: The Immune System
"The activation of the immune system is crucial to our ability to fight cancer, but differs between children and adults," explains Professor Petter Brodin, a leading figure in paediatric immunology at the Department of Women’s and Children’s Health, Karolinska Institutet, and a paediatrician at the Astrid Lindgren Children’s Hospital, Karolinska University Hospital. He emphasizes the critical need to decipher the intricacies of the paediatric immune response: "If we’re to properly treat childhood cancer, we need to find out how the child’s immune system is activated and regulated in children with cancer and what factors affect their immune responses."
For decades, precision medicine in oncology has largely focused on the genetic and molecular properties of the tumour itself – identifying specific mutations or protein expressions that can be targeted by drugs. While this approach has yielded significant successes in adult cancers, its application in paediatric solid tumours has been more challenging, partly due to the lower mutational burden in many childhood cancers. Professor Brodin highlights the transformative nature of this new research: "Precision medicine in cancer has mostly focused on the tumour properties. By characterising the immune system, we’re introducing an entirely new dimension that will be instrumental in shaping the future of childhood cancer therapy." This shift in focus from solely the tumour to the dynamic interplay between the tumour and the host’s immune system represents a paradigm change, particularly vital for a population with distinct physiological and immunological profiles.
Unpacking the Study: Methodology and Cohort
The comprehensive study encompassed a cohort of 191 children, ranging in age from newborns to 18 years, all diagnosed with various types of solid tumours at the Astrid Lindgren Children’s Hospital between 2018 and 2024. This extensive six-year collection period allowed for a robust dataset, capturing a wide spectrum of paediatric malignancies and age-related variations. To gain a deep understanding of the tumour-immune interface, the researchers conducted an exhaustive analysis of both tumour tissue and blood samples from each participant.
The methodology involved sophisticated molecular techniques. Genetic mutations within the tumours were identified through advanced genomic sequencing, providing a detailed map of the tumour’s unique genetic landscape. Concurrently, the researchers employed high-throughput transcriptomics and proteomics to ascertain which genes were actively expressed and which proteins were present or absent within the immune cells infiltrating the tumour microenvironment and circulating in the blood. This dual approach allowed them to not only characterize the intrinsic properties of the cancer cells but also to precisely profile the functional state of the child’s immune system in response to the tumour. Such detailed immune profiling, including single-cell analysis to dissect the heterogeneity of immune cell populations, is critical for understanding the complex regulatory networks that govern anti-tumour immunity.
The "Less Foreign" Hypothesis: Why Children’s Tumours Evade Detection
The study’s results unequivocally demonstrate that the immune systems of children and adults react fundamentally differently to cancer. Furthermore, it revealed that various tumour types activate the immune response to varying degrees within the paediatric population itself.
A key finding articulated by Professor Brodin is that "children’s tumours are generally less inflammatory and have fewer mutations, which means that they likely appear less foreign to the immune system and that the immune system therefore doesn’t attack the tumours as forcefully." This "less foreign" hypothesis offers a compelling explanation for the observed differences. In adult cancers, a high mutational burden often leads to the creation of numerous neoantigens – novel proteins produced by cancer cells that the immune system can recognize as abnormal. These neoantigens act as flags, signaling the immune system to launch an attack. With fewer mutations, paediatric solid tumours present fewer such flags, making them less visible and consequently less immunogenic.
The lack of inflammation in the tumour microenvironment further exacerbates this issue. An inflammatory response typically recruits immune cells, such as T cells and natural killer cells, to the tumour site, initiating a cascade of anti-cancer activity. Without this initial inflammatory signal, the immune system remains largely unalerted, allowing the tumour to grow and spread unchecked. This muted immune response is a critical factor differentiating paediatric from adult oncology and underscores why strategies successful in adults may falter in children. Despite these general trends, Professor Brodin also noted, "Having said this, there are large individual variations, which underlines the importance of precision medicine, which is to say the adapting of treatment to individual patients. Our study shows how this can be done in practice." This emphasizes that a one-size-fits-all approach is insufficient, even within paediatric oncology.
The Immunotherapy Conundrum: Why Checkpoint Inhibitors Fail Children
The groundbreaking results from Karolinska Institutet provide a robust explanation for a long-standing clinical observation: children generally do not benefit from immunotherapeutic treatments such as checkpoint inhibitors, which have revolutionized the treatment of several adult cancers. Checkpoint inhibitors, such as PD-1/PD-L1 or CTLA-4 blockers, are a type of biological therapy designed to unleash the immune system’s natural ability to fight cancer. They work by blocking "checkpoints" – proteins on immune cells (T cells) or cancer cells that normally put the brakes on the immune response. By removing these brakes, checkpoint inhibitors allow T cells to remain active and effectively target and destroy tumour cells.
However, as Professor Brodin explains, "This requires the immune cells to be activated against the tumour." He continues, "We show that the child’s immune cells are often initially not activated against the tumour, which means that checkpoint inhibitors won’t work. Children likely need different types of immunotherapies that are more focused on triggering the immune cells to attack the tumour cells from scratch." If the immune system hasn’t even recognized the tumour as a threat in the first place – due to the "less foreign" nature of paediatric tumours and the lack of an initial inflammatory response – then simply removing the brakes on an unactivated system will yield no benefit. It’s akin to pressing the accelerator in a car that isn’t running; no amount of pedal input will make it move.
This insight is crucial for redirecting future research and development in paediatric immuno-oncology. Instead of trying to "unleash" an already dormant immune response, the focus must shift to strategies that can prime or initiate an anti-tumour immune reaction from scratch. This could involve novel approaches such as oncolytic viruses, which selectively infect and kill cancer cells while simultaneously stimulating an immune response; bispecific antibodies, which can bring T cells into close proximity with tumour cells; or cancer vaccines designed to present tumour-specific antigens in a highly immunogenic manner. Even advanced therapies like CAR T-cell therapy, while effective for some paediatric leukemias, target specific antigens (like CD19) and require careful consideration for solid tumours, often needing further modifications to overcome the unique challenges of the solid tumour microenvironment.
Dynamic Tracking of Immune Response: A Clinical Tool for Today
Beyond explaining the current limitations of immunotherapy, the study also offers an immediate, actionable clinical tool. The researchers tracked the immune response over time and during treatment in some of the children, specifically measuring changes in the population of killer T cells – the immune cells primarily responsible for directly destroying tumour cells.
"This is something that we could make clinical use of today to judge the therapeutic effect and adjust the treatment to every individual patient," Professor Brodin states. The ability to dynamically monitor the quantity and activity of killer T cells, potentially through minimally invasive liquid biopsies (blood tests) or repeat tumour biopsies, offers a powerful biomarker. If killer T cell populations are increasing and showing signs of activation, it suggests the treatment is eliciting an effective anti-tumour immune response. Conversely, if they remain low or inactive, it signals that the current therapy may not be working and adjustments are needed.
"We’ll now be testing this on a larger scale as we believe that it can be a useful complement to the genetic analyses of tumours that are already being done in routine care," he adds. Integrating this immune profiling alongside standard genetic analysis provides a more holistic view of the disease and the patient’s response, moving beyond static genomic data to real-time immunological insights. This personalized, adaptive approach to treatment represents a significant step forward in optimizing therapy for each child, aiming to maximize efficacy while minimizing unnecessary toxicity.
Chronology of Progress: A Journey Towards Precision
The journey of cancer treatment has seen remarkable evolution. From the rudimentary surgical excisions and early radiation therapies of the early 20th century, the field progressed to systemic chemotherapy in the mid-20th century, which, despite its toxicity, offered the first hope for disseminated cancers. The late 20th and early 21st centuries witnessed the rise of targeted therapies, drugs designed to hit specific molecular vulnerabilities in cancer cells, often identified through genomic sequencing. Immunotherapy, emerging prominently in the last decade, represents the latest frontier, harnessing the body’s own defence mechanisms.
However, this progression has not been uniform across all patient populations. Paediatric oncology has often lagged behind adult oncology in adopting these innovations, largely due to the unique biological characteristics of childhood cancers and the ethical complexities of conducting clinical trials in children. This Karolinska Institutet study marks a crucial chronological milestone, specifically bridging the gap in immunotherapy research for children. By elucidating the fundamental differences in paediatric immune responses, it sets the stage for a targeted and effective development path for child-specific immunotherapies, rather than merely adapting adult treatments.
Statements and Reactions: A Collective Hope
The publication of this study has been met with significant optimism within the paediatric oncology community. Professor Brodin, who led the study alongside Linda Ljungblad, an oncology resident and researcher at Karolinska Institutet, expressed a profound sense of purpose: "Our ultimate goal is to translate these findings into better outcomes for children with cancer, offering them treatments that are not only more effective but also gentler on their developing bodies." Their collaborative work with the Astrid Lindgren Children’s Hospital paediatric oncology clinic at Karolinska University Hospital highlights the essential synergy between basic research and clinical practice.
Patient advocacy groups, representing children and their families, have also welcomed the findings. A spokesperson for a prominent childhood cancer foundation, speaking on background, noted, "This research validates what many parents have long suspected – that children are not just ‘small adults’ when it comes to cancer. It gives us immense hope that scientists are now truly understanding the unique biology of childhood cancer, which is the first step towards finding cures with fewer long-term side effects." Such sentiments underscore the emotional resonance and practical implications of the study.
The financial backing for this extensive research came from several vital organizations, including the Swedish Cancer Society, the Swedish Childhood Cancer Foundation, the Swedish Research Council, the Knut and Alice Wallenberg Foundation, and Karolinska Institutet itself. Their continued investment in fundamental research is indispensable for breakthroughs of this magnitude.
Broader Impact and Future Implications
The implications of this study extend far beyond the laboratory. Firstly, it will significantly influence drug development strategies. Pharmaceutical companies and biotech firms will now be guided to develop novel immunotherapeutic agents specifically designed to prime the paediatric immune system, rather than merely unleashing it. This could lead to a pipeline of truly innovative, child-centric cancer drugs.
Secondly, it will reshape the design of clinical trials in paediatric oncology. Future trials will likely incorporate comprehensive immune profiling as a standard component, allowing researchers to stratify patients based on their immune landscape and to assess treatment efficacy not just by tumour shrinkage, but by the dynamic changes in immune cell populations. This more nuanced approach promises to accelerate the identification of effective therapies.
Thirdly, the research has potential implications for healthcare policy and diagnostic protocols. As immune profiling becomes more sophisticated and cost-effective, it may eventually become a standard part of diagnostic workups for paediatric solid tumours, allowing for earlier and more precise treatment planning.
Globally, this research serves as a beacon for other nations grappling with childhood cancer. It emphasizes the importance of national and international collaboration in pooling resources and expertise to tackle complex diseases that disproportionately affect vulnerable populations.
Looking ahead, Professor Brodin’s team and collaborators will likely pursue several avenues. This includes expanding the cohort to study specific rare paediatric tumour types, investigating the interplay between the microbiome and immune response in children, and exploring combination therapies that pair conventional treatments with novel immune-priming agents. Further research will also delve into the precise mechanisms by which childhood tumours achieve their "less foreign" status and how these mechanisms can be therapeutically overcome.
In conclusion, the landmark study from Karolinska Institutet represents a pivotal moment in paediatric oncology. By unraveling the age-dependent intricacies of children’s immune responses to cancer, it not only explains the limitations of current immunotherapies but also charts a clear, actionable course towards developing truly tailored, effective, and less toxic treatments. For the thousands of children diagnosed with cancer each year, this research offers a profound message of hope: a future where therapies are precisely tuned to their unique biology, maximizing their chances for not just survival, but a life free from the long-term burdens of aggressive treatment.
(Petter Brodin and two of the other authors are co-founders of Cytodelics AB. Petter Brodin is a member of the Kancera AB executive board and a scientific advisor to Pixelgen Technologies AB, Sention Health AB, Helaina Inc, Scailyte AG and Oxford Immune Algorithmics.)
