Research Hotspots and Trends of Natural Killer Cells in Breast Cancer Immunotherapy: A Bibliometric Analysis

Breast cancer remains the most common cancer diagnosed in women worldwide, a disease that has long been fought with surgery, radiation, and drugs designed to kill rapidly dividing cells. In recent years, a different kind of weapon has joined the arsenal: the immune system itself. Among the many soldiers in the body's immune army, natural killer cells are unique. Unlike other immune cells that must be trained to recognize a specific enemy before they attack, natural killer cells are ready to strike immediately. They patrol the body, identifying and destroying abnormal cells, including cancer cells, without needing prior instruction. They also act as a bridge, helping to coordinate the broader immune response. When these cells are working well, they can keep tumors in check; when they are suppressed or absent, cancer often grows unchecked. For patients with aggressive forms of breast cancer, particularly those that do not respond to hormone therapy or targeted drugs, understanding how to harness these natural killers offers a new path forward.

A team of researchers set out to map the entire history of scientific effort dedicated to using these natural killer cells to treat breast cancer. They did not conduct a new laboratory experiment or a clinical trial. Instead, they performed a vast digital excavation, gathering and analyzing research papers published over the last thirty years. By examining who wrote these papers, where they were written, what topics they covered, and how often they were cited by others, the team constructed a detailed picture of how this field has grown and where it is heading. Their work reveals a story of rapid expansion, shifting alliances between nations, and a clear evolution from basic discovery to complex clinical strategies.

The story begins with a slow start. Between 1996 and 2005, the field was in its infancy, with researchers publishing only a handful of papers each year. The work was sporadic, focused on understanding the basic biology of these cells. However, a steady climb began in 2006, and by 2016, the field entered a period of explosive growth. In the most recent decade, the number of published studies has surged, with nearly one hundred papers appearing in a single year by 2025. This exponential rise mirrors the growing confidence in immunotherapy as a viable treatment for solid tumors. The data shows that the United States has long served as the central hub of global collaboration, connecting researchers across the world. While China has produced the highest number of papers in total, the American network remains the most interconnected, acting as the primary bridge between different research groups.

As the volume of research grew, the focus of the questions changed. In the early years, scientists were primarily asking how natural killer cells function and how they could be triggered to kill cancer. The research was fundamental, seeking to understand the mechanics of the immune response. Over time, the conversation shifted toward the specific challenges of breast cancer. Researchers began to realize that the environment inside a tumor often acts as a shield, protecting the cancer from immune attack. A major turning point in the literature occurred around 2018, when large clinical trials demonstrated that combining immune checkpoint inhibitors—drugs that release the brakes on the immune system—with chemotherapy could significantly improve outcomes for patients with triple-negative breast cancer, the most aggressive subtype. This success validated the idea that natural killer cells are not just bystanders but critical players in the fight against this specific disease.

The analysis of the literature also highlighted a distinct pattern in how different countries contribute to the science. While China leads in the sheer number of publications, the research coming from the United States tends to be cited more frequently, suggesting a higher average impact per study. European nations form a tight-knit network of collaboration, while other regions, such as Argentina and Canada, produce smaller volumes of work that are nonetheless highly influential. At the institutional level, a few universities and research centers have emerged as the primary drivers of progress. Sun Yat-sen University in China has risen to become a new global core, while institutions like Memorial Sloan Kettering in the United States continue to anchor the field with high-quality, translational research.

The most significant finding of this review is the clear trajectory of the research itself. The field has moved from simple observations of cell behavior to a sophisticated understanding of the tumor microenvironment. Scientists now know that natural killer cells do not operate in a vacuum; they are constantly interacting with other cells, such as macrophages, which can either help or hinder the immune attack. The current frontier of research is focused on overcoming these barriers. One of the most promising avenues is the development of engineered natural killer cells, known as CAR-NK cells. These are cells that have been genetically modified to better recognize and destroy cancer, offering a potential advantage over other cell therapies because they carry a lower risk of severe side effects. Although CAR-NK therapy has entered clinical trial stages, it remains in an exploratory phase regarding its application in solid tumors like breast cancer.

Another major theme emerging from the data is the power of combination therapy. The research suggests that natural killer cells work best when paired with other treatments. For instance, certain chemotherapy drugs can damage cancer cells in a way that makes them more visible to the immune system, effectively turning the tumor into a target for natural killer cells. This synergy between traditional chemotherapy and modern immunotherapy is becoming a key research focus. The literature also points to the importance of precision medicine. Not all patients respond to these treatments in the same way, and researchers are now working to identify specific markers that can predict who will benefit most from natural killer cell therapies.

Despite the progress, the study notes that challenges remain. The tumor microenvironment is complex and often suppresses the immune system, making it difficult for natural killer cells to function effectively once they reach the tumor site. Furthermore, while the number of studies is growing, the field still needs more international collaboration to fully realize its potential. The researchers conclude that the future lies in refining these combination strategies, improving the engineering of cell therapies, and developing better ways to measure how well these treatments are working in individual patients. The journey from the first basic studies to the current clinical trials has been long, but the path forward is now clearly defined, pointing toward a future where natural killer cells play a central role in curing breast cancer.