New research suggests that a type of immune cell traditionally known for coordinating the body’s defenses may also directly kill damaged, infected, or abnormal cells, and may have both beneficial and harmful effects as we age.

The immune system is highly complex, and scientists are learning that immune cells do not always fit neatly into fixed categories. A new review of research on cytotoxic CD4+ T cells, or CD4 CTLs, highlights how one group of immune cells may play an important but complicated role in aging and age-related diseases.

T cells are commonly divided into two major groups: CD4+ T cells and CD8+ T cells. CD4+ T cells are traditionally described as “helper” cells because they coordinate immune responses by sending signals to other immune cells. CD8+ T cells, meanwhile, are generally recognized for directly attacking and killing infected or abnormal cells.

However, researchers say some CD4+ T cells can also act as killers.

These CD4 CTLs can produce powerful molecules such as perforin and granzymes, which allow them to destroy targeted cells. They share some of these characteristics with CD8+ T cells and natural killer (NK) cells.

A potential benefit in aging

CD4 CTLs appear to develop or increase after the immune system encounters the same antigen repeatedly or over long periods. This can happen during chronic viral infections or in the environment surrounding tumors.

Researchers have also found evidence that these cells can target senescent cells, damaged or aged cells that have stopped dividing but remain in the body. The accumulation of senescent cells is associated with aging and several age-related conditions.

By recognizing and eliminating some of these cells, CD4 CTLs could potentially help the body maintain healthier tissues and provide additional protection against infections and cancer.

The harmful side of the equation

The same immune activity that can protect the body may also cause damage when it is poorly controlled.

Researchers have linked abnormal CD4 CTL activity to several diseases involving excessive or misdirected immune responses, including autoimmune disorders, neurodegenerative diseases, and severe COVID-19.

In these situations, CD4 CTLs may attack healthy tissues or contribute to damaging inflammation. This makes them something of a double-edged sword: useful when properly directed, but potentially harmful when their activity becomes inappropriate.

Why this matters as we age

Older adults face increased risks of infections, cancer, autoimmune diseases, and neurodegenerative conditions. At the same time, the immune system changes significantly with age, including changes in the number and behavior of different T-cell populations.

The researchers propose that the age-related expansion or altered activity of CD4 CTLs could be one factor connecting the aging immune system with several age-related diseases.

However, the researchers emphasize that much remains to be learned. Scientists still need to determine exactly which antigens these cells recognize, where they operate in aging tissues, how their killing mechanisms are controlled, and when their activity becomes beneficial or harmful.

Potential for future treatments

A better understanding of CD4 CTLs could eventually open new possibilities for treating or preventing diseases associated with aging.

For example, researchers may one day be able to find ways to strengthen the beneficial activity of these cells, such as their ability to remove infected, cancerous, or senescent cells, while limiting harmful immune attacks against healthy tissues.

The review therefore points to CD4 CTLs as an emerging area of research that could provide important insights into the relationship between aging, immunity, and disease.

The key message: CD4+ T cells are more versatile than traditionally believed. Some can directly kill targeted cells, and their activity may help protect the aging body in some circumstances while contributing to disease in others. Understanding this balance could become increasingly important as scientists work to develop new approaches to healthy aging.