Early PD-1 and CTLA-4 Expression on Peripheral Blood T Cells and Subsequent Chronic Critical Illness and 28-Day Mortality: A Prospective Cohort Study
This prospective cohort study demonstrates that early upregulation of PD-1 and CTLA-4 expression on peripheral T cells, particularly the proportion of CD4+ PD-1+ T cells, is strongly associated with the development of chronic critical illness and 28-day mortality in critically ill adults, suggesting these immune checkpoints serve as promising biomarkers for early risk stratification.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
When a person survives a life-threatening injury or severe infection, the body often enters a prolonged state of struggle. Instead of recovering quickly, the patient remains stuck in the intensive care unit, with organs that refuse to function normally. Doctors call this chronic critical illness. It is a difficult condition to predict, and once it sets in, the outlook is often poor. For years, researchers have known that the immune system plays a central role here. After the initial shock of illness, the body's defenses can become confused, sometimes turning off too early or failing to fight off new infections. Two specific proteins on the surface of immune cells, known as PD-1 and CTLA-4, act like brakes on the immune system. When these brakes are pressed too hard, the immune cells stop working effectively. The question facing doctors has been whether they could spot these "brakes" being pressed very early in a patient's stay, perhaps within the first day, to predict who would fall into this long, dangerous trap of chronic illness.
A team of researchers at the First Affiliated Hospital of Shihezi University in China set out to answer this question. They followed a group of eighty-one adults who were admitted to their intensive care unit between July 2025 and July 2026. The team did not wait for patients to get worse; instead, they acted immediately. Within twenty-four hours of admission, they drew blood from every patient. They looked closely at the white blood cells, specifically the T cells, which are the generals of the immune system. Using a specialized machine that counts and identifies cells, they measured how many of these T cells carried the PD-1 and CTLA-4 proteins on their surfaces. They also checked standard medical markers, such as how well the heart was pumping and how much inflammation was present in the blood.
The researchers then watched these patients for two weeks. They defined a patient as having chronic critical illness if they remained in the intensive care unit for at least fourteen days and still had failing organs. By the end of the two-week period, they could clearly see which patients had developed this condition and which had not. The results were striking. The patients who went on to develop chronic critical illness had significantly higher levels of these "brake" proteins on their T cells right from the start. Specifically, the team found that a higher proportion of CD4+ T cells (a major type of immune cell) carrying the PD-1 protein was the strongest single indicator of who would develop the chronic condition. Patients who developed the illness also had weaker heart function and higher levels of general inflammation compared to those who recovered more quickly.
The study went further to see if these early signs could predict who would survive the first month. The researchers found that the level of PD-1 on CD4+ T cells was not just a marker for chronic illness, but also a sign of higher risk of death within twenty-eight days. When they combined this immune marker with a standard measure of heart pumping strength, known as the left ventricular ejection fraction, and a common score for illness severity called the APACHE II score, they created a very powerful tool for prediction. This combination correctly identified patients who would develop chronic critical illness with high accuracy. It is important to note that this was a single study at one hospital, and the researchers described their predictive models as exploratory, meaning they need to be tested in larger groups of people before they can be used as a standard medical tool. However, the data clearly showed that the immune system's state in the first day of intensive care is deeply connected to the heart's health and the patient's long-term survival.
The researchers also looked for a direct link between the immune "brakes" and the heart's condition. They found that in patients who developed chronic critical illness, higher levels of PD-1 on T cells were associated with a larger volume of blood in the heart's main pumping chamber before it squeezed. This suggests a connection between the immune system's overactive suppression and the heart's struggle, though the study did not prove that one caused the other. The team was careful to state that they did not test how well the immune cells were actually working, only how many carried these proteins. Therefore, while the presence of these proteins suggests the immune system is struggling, it does not definitively prove the cells are exhausted.
Ultimately, this work offers a new way to look at the earliest moments of critical care. It suggests that the path to a long, difficult recovery is often paved by immune changes that happen within the first day. By measuring these specific proteins on T cells, doctors might one day be able to identify patients who are at high risk for chronic critical illness and heart failure much earlier than current methods allow. The study concludes that the interplay between the immune system and the heart is a critical factor in survival, and that checking these immune markers early could help guide better care for the most vulnerable patients.
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